Elevons - NCAIRncair.in/newsletters/NewsletterV4.2.pdfin Lakshmi Precision Screws Ltd. He has joined NCAIR as a Research Assistant. Manan Panchal He received his Master's degree in

Oct 2014 . Newsletter . Volume 4 . Issue 2

CHALLENGES IN MODELLING MULTIPHASE FLOWS

IN COMBUSTOR NOZZLES

6 FEATURED ARTICLE

Elevons

12R & D ARTICLE

Multi-Scale Modeling Using Continuum Approach to Study the

Interfacial Stress in CFRP

Effect of Laser Assisted Machining on Titanium alloys

20R & D ARTICLE

CONTENTS Editorial, NCAIR News Updates 1-2

Important Announcements 4

FEATURED ARTICLE

Challenges in modelling multiphase flows in combustor nozzles – Prof. Shivasubramanian Gopalakrishnan 6

R&D UPDATES

Multi-Scale Modeling Using Continuum Approach to Study the Interfacial Stress in CFRP 12

Effect of Laser Assisted Machining on Titanium alloys 20

Damage Initiation in Fiber Composite 25

TECHNOLOGY UPDATES

Resins (Part II) : Properties and Selection Criteria 30

AEROSPACE NEWS BRIEFS

HAL Faculty Chair at IIT Bombay 34

Opportunities in the aero industry supply chain 34

Aircraft engine components to be 3D printed 34

Robots to play a greater role in building airplanes 35

Acknowledgement

We extend our sincere gratitude to the faculty members of IIT-Bombay for their support. We are also thankful to the students and staff of NCAIR, for their valuable articles and other support.

Editors : Prof. Suhas Joshi and Prof. Asim Tewari Asst. Editors : Dr. Sarbani Banerjee Belur Ms. Vani K. Sreedhara Contact details : [email protected] Articles that fall under the purview of NCAIR Newsletter are always welcome.

Editorial

Welcome to the present edition of the NCAIR ‘Elevons’ newsletter.

The NCAIR newsletter is an outcome-based quarterly report of the centre and its various activities. Research is the prime focus of this centre. It is through the newsletter that we draw attention to various ongoing and completed R&D activities in the arena of aerospace manufacturing.

We welcome you to Volume 4, Issue 2 of the NCAIR newsletter. The important aim of this newsletter is to keep the readers informed of the ongoing developments, and innovations in research being a part of NCAIR. The newsletter is a collection of various review and contributed articles from academia, industry NCAIR Ph.D./M.Tech. students and staff.

Apart from the various research articles, the newsletter also serves the dual purpose of being a notice board where important announcements related to NCAIR are put up. Various training programs, workshops, events and seminars slated for being held in the coming months have been covered in this newsletter.

In this issue of the newsletter, we present some interesting contributions related to aerospace applications and manufacturing processes. The featured article of this issue discusses the challenges faced in modeling multiphase flows in combustor nozzles. Typical operating characteristics of a fighter jet may go from cruising at lean power to full afterburners within seconds due to mission considerations. This will result in rapid increase in fuel flow in the intake line and if the fuel is sufficiently superheated, this could result in a vapor lock and a flame–out of the engine. Hence, comprehension of the vaporisation dynamics of superheated fuels in combustors is of great importance.

Research and development is an important aspect of NCAIR. This is reflected in the various R&D updates, which have articles on multi-scale modeling using continuum approach to study interfacial stress in CFRP, effect of laser assisted machining on titanium alloys and damage initiation in fiber composite.

The technology update section focuses on recent technological advancements in the arena of aerospace manufacturing globally. In this issue, we present a review on resins (part -2). This part of the article focuses on the properties of resins and their selection criteria. The newsletter also carries important news briefs in the aerospace domain.

Hope you will find this newsletter interesting. Please feel free to provide us with any feedback, including things that you would like to see being featured in the forthcoming editions through email: [email protected]

This newsletter is also available online at www.ncair.in. Happy Reading !!!!!

Kind Regards,

Suhas S. Joshi & Asim TewariEditors

1.

NCAIR News updAtes

technical Writing workshop, ‘Capable Communicative Engineers’, april 5-6, 2014, iit BombayThis was a two-day workshop as a part of the induction training programme for students and staff at NCAIR. 20 students (MTech and PhD) participated in this workshop and they were from the Mechanical Engineering department of IIT Bombay. This workshop was more of a hands-on training where students learnt to collate, evaluate and synthesize information collected from various sources. There were reading, writing and presentation exercises as well. Several softwares such as Turnitin, Docear, MATLAB, their use and application were demonstrated to the students. Scientific search engines such as Scopus and Sciencedirect and Mind maps which help in organising research were discussed in this workshop.

technical communication workshop at National aerospace laboratories (Nal), Bengaluru, July 3- 4, 2014NCAIR conducted a workshop on 'Technical Communication' on July 3rd and 4th, 2014 at the National Aerospace Laboratories (NAL), Bengaluru. The workshop was aimed at enhancing technical writing skills of participants and emphasized on the need to publish research. Dr. Asim Tewari, Associate Professor, Department of Mechanical Engineering, IIT-Bombay, taught the course with assistance from NCAIR employees Ms. Swetha Manian and Mr. Taha Khot.

2.

1.1, 1.2 - Participants at the workshop : Capable Communicative Engineers - NCAIR workshop, April 5-6, 2014.

2.1 - Prof. Asim Tewari (center) flanked by Mr. H. N. Sudheendra, Chief Scientist, Head, Advanced Composites Division (ACD) to his right and Dr. Ramesh Sundaram, Sr. Principle Scientist, Dy. Head, ACD, NAL, Bengaluru to his left; along with Ms. Swetha Manian-Sridhar and Mr. Taha M. Khot and the attendees of NAL-NCAIR Technical Communication Workshop.

1.1

1.2

2.1

The workshop empowered the participants with the know-how to conduct organized research, introduced them to various resources and also helped the participants realize the nuances of the English language and its appropriate practice in technical writing. The workshop was well attended and received by the participants. They found the content very relevant and useful and an eye-opener to various aspects in many cases. With the pre-requisites to writing addressed, the participants commented that an advanced level workshop would now be useful to understand some aspects in depth.

aerospace meet, along with Sandvik Coromant at Bengaluru, July 5, 2014In our endeavour to increase aerospace manufacturing capability through correlated training, technology transfer, R&D and shared infrastructure, NCAIR along with its partner Sandvik Coromant organized a half-day industry meet at The Lalit Ashok, Bengaluru on 5 July, 2014.

Program was inaugurated by Mr. Shailesh Prabhune, President, Sandvik Coromant India. The other dignitaries present at the event were Dr. Bala Bharadvaj, Director, R&T, Engineering, Operation and Technology, Dr. Suhas Joshi, Professor, Dept. of Mech. Engg, IIT-Bombay, Dr. Asim Tewari, Associate Professor, Dept. of Mech. Engg, IIT-Bombay, Dr. Harish Barshilia, Chief Scientist and Joint Head, Surface Engineering Division, NAL, Mr. N. C. Vyas, Executive Director, Design, HAL.

The program was well received. There were more than 50 participants from over 30 companies participating in the industry meet. In the program, eminent dignitaries from the Aerospace industry talked about challenges and opportunities in the Aerospace sector in India. The program was capped off by a panel discussion which allowed the participants to interact with the dignitaries. Many participants bought forth a number of issues facing them leading to a very lively deliberation among those present.

3.

2.2 - NAL-NCAIR Technical Communication workshop, July 3-4, 2014. Participants at the course.

3.1 - Inauguration of the Aerospace meet along with Sandvik Coromant, July 5, 2014 at Bangalore.

3.2 - Participants at the Aerospace meet.

3.3 - On the dias- Right to Left: Dr. Harish Barshilia, NAL, Dr. Asim Tewari, IIT-Bombay, Dr. Bala Bharadvaj, Boeing India, Mr. Shailesh Prabhune, Sandvik Coromant, Mr. N.C. Vyas, HAL, Mr. Jose Varghese, DMG Mori Seiki. Speaker: Dr. Suhas Joshi, IIT-Bombay

2.2

3.1

3.2

3.3

New Colleagues at NCAIR Kashyap Mohan

He has pursued his Bachelor of Technology degree in Metallurgical Engineering and Material Science from Indian Institute of Technology, Mumbai. After graduating he worked as Quality Assurance Engineer in Lakshmi Precision Screws Ltd. He has joined NCAIR as a Research Assistant.

Manan Panchal

He received his Master's degree in Mechanical Engineering (CAD/CAM) from Sardar Vallabhbhai National Institute of Technology (SVNIT), Surat, Gujarat, India. He has joined NCAIR as a Project Research Associate.Technology (MNNIT), Allahabad, Uttar Pradesh, India. He has joined NCAIR as a Project Research Associate.

Jigar Goda

He has pursued his Bachelor of Technology degree in Mechanical Engineering from K. J. Somaiya college of Engineering, Mumbai. He has also worked as a composite lead of a formula student team, Orion Racing India for 2 years. Recently he has joined NCAIR as a Research Assistant.

NCAIR IIT Bombay M.Tech students graduated 2014Omnath PawarAnalysis of hole quality in drilling of fiber metal laminates (FMLs) Glare 5 and Glare 6. Partial support for this project was provided by National Aerospace Laboratories (NAL)

Nikesh NayakExperimental analysis of high speed drilling of Inconel 718 superalloy using coated carbide drills. Partial support for this project was provided by National Aerospace Laboratories (NAL)

Ranjan DasModelling and Analysis of progression of tool wear in drilling. Partial support for this project was provided by National Aerospace Laboratories (NAL)

Narendra Singh Analysis of Machining Stability in flank milling of impeller blades. Partial support for this project was provided by DMG-Mori Seki

Kapil WanaskarAnalysis of machining stability in flank milling of Impeller Blades with decreasing thickness. Partial support for this project was provided by Sandvik Coromant

Ankita AgrawalExperimental and Numerical Analysis of workpiece stability in flank milling of Ti6Al4V. Partial support for this project was provided by DMG-Mori Seki, SVNIT Surat

Jainil BhattModeling and simulation of resin infusion in advanced aerospace composites manufacturing

Vinayak KhandareModeling and simulation of process induced deformations in fiber reinforced polymer matrix composites

Jigar PatakStrain path modelling in single point incremental sheet metal forming (SPIF)

Rakesh ChaudhariSynthesis and deformation of single crystal and micropillars of Ti6Al4V

Forthcoming during November 1-2, 2014: Technical Writing Workshop for upcoming engineers

4.

PublicationsShashikant Joshi, Asim Tewari, Suhas S Joshi, Microstructural characterization of chip segmentation under different machining environments in orthogonal machining of Ti6Al4V, Transactions of ASME, Journal of Engineering Materials and Technology. Paper accepted. Revision pending July 18, 2014.

Shashikant Joshi, Pravin Pawar, Asim Tewari, Suhas S Joshi , Influence of β-phase fraction on deformation of grains in and around shear bands in machining of titanium alloys, Materials Science & Engineering: A Paper accepted for publication.

VisitsMr. Praveen Roy from DST visited NCAIR on May 28, 2014.

Dr. Rajesh Raghavan, Sandvik Coromant visited NCAIR on July 9, 2014.

Mr. Ravindra Latey, Technical Manager, Sandvik Coromant visited NCAIR on July 21, 2014.

FacilityState of the art facility of Infinite focus system for form and roughness will be added to the Institute facility list of IIT-Bombay for which NCAIR has exclusive access. This would be situated in the Micromachining and Micrometrology facility, Department of Mechanical Engineering, IIT Bombay from September 2014.

5.

4.1, 4.2 - Visit of Mr. Praveen Roy, DST, Govt. of India to NCAIR office on May 28, 2014

4.1 4.2

FEATURED ARTICLE

Challenges in modeling multiphase flows in combustor nozzlesDr. S. Gopalakrishnan, Department of Mechanical Engineering, Indian Institute of Technology, Bombay.

In high performance jet engines, such as those found in military aircrafts or those proposed for supersonic civilian applications, the proposition to utilize jet fuel as a heat sink may positively impact both the aircrafts thermal management and combustor performance. The high fuel temperature greatly raises the potential for the fuel to be in the superheated state which can result in better atomization and improved mixing, thus yielding better combustor performance and emissions. Typical operating characteristics of a fighter jet may go from cruising at lean power to full afterburners within seconds due to mission considerations. This will result in rapid increase on fuel flow in the intake line and if the fuel is sufficiently superheated this could result in a vapor lock and a flame–out of the engine. The consequence of this could be disastrous for a military jet. A similar situation could occur for civilian jets when they taxi on a hot tarmac and then proceed to full power for take–off power rapidly. Hence, comprehension of the vaporisation dynamics of superheated fuels in combustors is of great importance. A typical block diagram of an aerocombustor system is shown in Fig. 1.

Similarly, the rise in popularity of direct injection systems for gasoline engines necessitates the understanding of the complex phenomena of the fuel spray and its vaporization. The spray structure in the combustion chamber is affected by the external breakup of the jet which itself depends upon the internal flow characteristics in the fuel injector nozzle. The fuel in the injection system can acquire heat from the relatively hot surroundings and from compression during pumping, raising its temperature and the vapor pressure. If the pressure downstream of the injector is less than the vapor pressure, the fuel will likely flash boil causing vapour locks. Though this will not result in disastrous consequences as in aero-combustors, it will definitely lead to poor performance and a rejection of vehicles based on such designs. A model figure for a typical GDI engine is given in Fig. 2.

Fig. 1 Block diagram of an aerocombustor system

The rise in popularity of direct injection systems for gasoline engines necessitates the understanding of the complex phenomena of the fuel spray and its vaporization. The spray structure in the combustion chamber is affected by the external breakup of the jet which itself depends upon the internal flow characteristics in the fuel injector nozzle.

6.

Flash-boiling is a phenomenon similar to cavitation, which is also known to occur in gasoline injectors. Both are a transition from liquid to vapor due to a drop in pressure. In contrast to nucleate boiling, the enthalpy for vaporization is not provided at walls during the phase change process, but is instead provided by inter-phase heat transfer.

A gross distinction between cavitating and flash-boiling nozzle flow is simply that the enthalpy of the cavitating flow is below the saturated liquid enthalpy at the downstream pressure, while the enthalpy of the flash-boiling flow exceeds the saturated liquid enthalpy at the downstream pressure. Even though only a very small fraction of the liquid mass changes phase while still in the nozzle, this small amount of mass can occupy a large volume and greatly affect the nozzle flow.

Any numerical investigation in the physics of the atomization consists of two distinct sections, the external jet break up and the internal nozzle flow. The calculations of the nozzle flow calculations serve as inputs to external jet break up models. These external spray models have been developed with the best information available, but the open literature has very little to offer about the details of the internal flashing flow. Important questions remain about the velocity of the fluid leaving the nozzle and the fraction of vapor present at the nozzle exit.

When a hot fluid has a vapor pressure that falls between the upstream and downstream pressure in a nozzle, the discharge of the nozzle may be sensitive to the effects of inter-phase heat transfer. This heat transfer will take place on small length scales and will be affected by interfacial and turbulent dynamics. Neither the details of the small-scale temperature fluctuations, the amount of interfacial area, nor the small scale velocity features are known. Despite these complexities, the limits of thermal equilibrium and frozen flow have been useful for very long and very short nozzles, respectively. An intermediate closure that addresses the finite rate of heat transfer between phases would provide wider applicability to nozzle geometries. If the analyses could further be extended to multiple dimensions, then multi-dimensional CFD techniques could be applied to studying flash- boiling nozzles.

The rate of heat transfer and its role as a limiting factor in phase change depends largely upon the temperature of the fluid. Pressure-driven phase change can be viewed as

Fig. 2 A typical model figure for a GDI engine

A gross distinction between cavitating and flash-boiling nozzle flow is simply that the enthalpy of the cavitating flow is below the saturated liquid enthalpy at the downstream pressure, while the enthalpy of the flashboiling flow exceeds the saturated liquid enthalpy at the downstream pressure.

7.

a spectrum with cavitation at the cold end of the spectrum and flash-boiling at the hot end. In some cavitating flows, the time scales of heat transfer can be assumed to be much faster than the time scales governing acceleration due to pressure. Consequently, for small, high-speed cavitating flows, thermal equilibrium assumptions have produced successful cavitation models [6]. Under such conditions, the vapor density of the cold fluid is very small and is not significant when compared to the liquid density. Thus little energy transfer is required to produce vapor. In contrast, for hot liquid the phase change is more like a boiling process. The difference between the saturated vapor density and saturated liquid density decreases at higher temperature. Consequently, the liquid must provide more energy per unit volume of vapor. Thus flashing nozzle simulations require additional modeling of finite-rate heat-transfer processes.

Empirical observations are also essential to validate computational models. In experiments such as Reitz [5], the mass flow rate through a short nozzle was clearly a function of upstream liquid temperature. As the temperature of the upstream liquid approached the vapor temperature at the upstream conditions, mass flow rate decreased. When heated to a point just below the upstream vapor temperature, the flow rate dropped abruptly.

However, the complex physics are only the first obstacle to creating CFD simulations of phase change. Depending on the speed and size of the channel flow, the rate of heat transfer can range from slow, e.g. the thermal equilibrium limit, to very fast, namely the frozen-flow limit. When the rate of phase change is extremely fast, numerical stiffness problems can occur. Unless an implicit model of heat transfer is closely coupled to conservation of mass and momentum equations, the resulting scheme may be limited to very small time steps. For application to transient, three-dimensional flow, severe stability constraints would render an explicit model prohibitively expensive.

This article gives a summary of the earlier work by the author [4] on the development of a new fully three dimensional CFD solver which models the thermal non-equilibrium in the phase change process as a finite rate process and the Homogeneous Relaxation Model (HRM) was developed. In this model, the thermal inertia of the liquid fuel is modelled as a relaxation of the instantaneous quality (mass / void fraction) to the equilibrium state over a finite empirical characteristic time.

In some cavitating flows, the time scales of heat transfer

can be assumed to be much faster than the time scales

governing acceleration due topressure. Consequently, for

small, high-speed cavitating flows, thermal equilibrium

assumptions have produced successful cavitation models [6].

8.

FEATURED ARTICLE

The Homogenous Relaxation Model is based on a linearized expansion proposed by Bilicki and Kestin [1]. The general model form originates with refrigeration modeling by Einstein [3]. It has been used by numerous others for one-dimensional two-phase flow. The model represents the enormously complex process by which the two phases exchange heat and mass. The model form determines the total derivative of quality, the mass fraction of vapor.

Equation 1 describes the exponential relaxation of the quality, x, to the equilibrium quality, x ̅, over a timescale, Θ. The equilibrium quality is a function of the enthalpy and the saturation enthalpies at the local pressure, as given by Eq. 2 with bounds at zero and unity.

The quality, the mass fraction of vapor, is calculated from each cell’s void fraction, α for densities falling inside the saturation dome.

The void fraction in the two-phase region is, in turn, a function of the local density as well as the saturated vapor and liquid densities at the local pressure.

The timescale in Eqn.1 is empirically fit to data describing flashing flow of water in long, straight pipes. The work of Downar-Zapolski et al. [2] provides two correlations, one recommended for relatively high pressures, above 10 bar, and a different correlation for lower pressures. In the low-pressure form, for upstream pressures below10 bar, the best-fit values suggested by Downar- Zapolski et al. for flashing water appear in Eqn. 5. The empirical parameters include Θ0 and the two exponents. These values are Θ0 = 6.51 . 10−4 [s], a= −0.257, and b = −2.24.

The variable α represents the volume fraction of vapor and Ψ is a dimensionless pressure difference between the

(1)

(2)

(3)

(4)

(5)

The Homogenous Relaxation Model is based on a linearized expansion proposed by Bilicki and Kestin [1]. The general model form originates with refrigeration modeling by Einstein [3]. It has been used by numerous others for one-dimensional two-phase flow. The model represents the enormously complex process by which the two phases exchange heat and mass.

9.

local static pressure and the vapor pressure, as defined in Eqn. 6. The absolute value is used in the present work since the pressure in the domain can fall below the saturation pressure. A slightly different fit is suggested for upstream pressures above 10 bar, as given by Eqn. 7.

The dimensionless pressure ϕ, defined in Eqn. 8, differs from the definition in Eqn. 6 by including the critical pressure pc . The coefficient values in the high-pressure correlation, Eqn. 7 are Θ0 = 3.84 • 10−7 [s], a = −0.54, and b = −1.76. Another correlation that was explored was one with a mixed character. In this mode the indices for void fraction and non-dimensional presssure were from the high-pressure correlation while Θ0 was of the low pressure correlation.

An experimental investigation using an axisymmetric nozzle and water as the working fluid was done by Reitz [5]. The downstream pressure was kept at 101 kPa and the inlet pressure was fixed at 787 kPa. He varied the inlet temperature and noted the effect on mass flow rate. The mass flow rate gradually decreased until an upstream temperature of about 430K was reached, whereupon the mass flow discontinuously dropped. Reitz reported that the mass flow rate dropped below the measurement range of his flow meter. The validity of the CFD model was tested with Reitz’s experiment, see Fig. 3.

The finite rate process modeled by HRM is shown to obtain results very close to that experimentally observed. Though, there is a slight disagreement about the exact temperature at which the drop-off in flow occurs, the model succeeds in predicting the mass flow rate in the flashing nozzle over a range of temperature. In the simulations, the critical temperature for vapor lock corresponds closely to the temperature at which the vapor pressure equals the upstream static pressure. At the upstream pressure of 787 kPa, the saturation temperature of water is 443 K. Under superheated conditions, where the incoming fluid has a vapor pressure in excess of the upstream pressure, the simulation predicts that the entire nozzle fills with vapor.

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(7)

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Fig. 3 Comparison of mass flow rates between experiments and simulations

10.

FEATURED ARTICLE

At elevated temperatures, the interphase heat transfer is the main mechanism which provides enthalpy for phase change. This causes the fluid to be in thermodynamic nonequilibrium when the flow–through time is comparable to the relaxation time. The thermal nonequilibrium has been modeled and tested as a finite rate heat process. The Homogenous Relaxation Model (HRM) has been successfully demonstrated as a model for the phase change process in flash–boiling flows.

The lack of experimental data for validation is a concern; experimental data for internal flows using realistic fuels are not available at present. It is anticipated that, due to the importance of flash-boiling, more experimental investigations of internal nozzle flow with fuels will soon be published. At that time, further validation of models similar to ones discussed in the article will be possible.

The finite rate phase change model is based on an empirical timescale relationship. The coefficients for this equation are based on experimental data obtained for water. Though these provide a good starting point for calculations using hydrocarbons as working fluids, it is expected that these will need to be fine tuned once experimental data using such fluids becomes available.

Dissolved gases and impurities in the working fluid start as nucleation sites for phase change process. The number of these germination sites can vary with the quality of the fluid used and physical parameters such as pressure and temperature. A sophisticated nucleation model which considers thermophysical variables is expected to improve the fidelity of the CFD calculations.

rEfErENCES

[1] Bilicki, Z., and Kestin, J. Physical aspects of the relaxation model

in two-phase flow. Proc. Roy. Soc. London A. 428 (1990), 379–397.

[2] Downar-Zapolski, P., Bilicki, Z., Bolle, L., and Franco, J. The

non-equilibrium relaxation model for one-dimensional flashing

liquid flow. IJMF 22, 3 (1996), 473–483.

[3] Einstein, A. U¨ ber Schallschwingungen in teilweise dissocierten

Gasen. Sitzung Berl. Akad. Physik Chemie (1920), 380–385.

[4] Gopalakrishnan, S. Modeling of thermal non-equilibrium in

superheated injector flows. PhD thesis, University of

Massachusetts, Amherst, 2005.

[5] Reitz, Rolf D. A Photographic Study of Flash-Boiling Atomization.

Aerosol Science and Technology 12 (1998), 561–569.

[6] Schmidt, D.P., Rutland, C.J., and Corradini, M.L. A Fully

Compressible Model of Cavitating Flow. Atomization and Sprays

9 (1999), 255–276.

The lack of experimental data for validation is a concern; experimental data for internal flows using realistic fuels are not available at present. It is anticipated that, due to the importance of flash-boiling, more experimental investigations of internal nozzle flow with fuels will soon be published.

11.

R & d updAtes

Multi-scale modeling using continuum approach to study the interfacial stress in CFRPJITESH VASAVADA

PhD student, IIT Bombay

introductionCarbon Fiber Reinforced Polymers (CFRP) are one of the widely used composite materials which contain reinforcements in epoxy and matrix. The carbon fiber provides strength and rigidity to the composite, while the matrix provides support for the reinforced material by maintaining their relative position.

During processing of composites, due to their different coefficients of thermal expansion (CTE), the volumetric shrinkage of the two entities is different. However, when processed in the temperature range of 150°C to 300°C, during curing, the volumetric shrinkage of matrix is significantly higher than for fiber. It leads to uneven shrinkage and development of residual stresses along the interfaces of the fiber and material. The residual stresses are from external load and temperature gradients. In this article, the thermal stresses reinforced as residual stresses, are modeled using a multi scale modeling approach involving Representative Volume Element (RVE) method in FEM. In the following sections of this article, results of the modeling process are presented.

MethodologyIn this work, the methodology adopted is shown in the form of a flow diagram in Fig.1. The micro scale concerns the fiber matrix interaction and the meso scale takes into account ply to ply interaction. As shown in Fig. 1, mismatch in the CTE of fiber and matrix is a necessary parameter to study in the micro level. In this level, curing of composite occurs as the polymer matrix undergoes the temperature cycle. During cooling, shrinkage of matrix is higher than fiber which generates driving force for residual stresses. Two types of stresses are observed. The compressive stresses which are produced in the fiber during cooling in both longitudinal and transverse directions, and the tensile stresses which are observed in the matrix.

In the meso scale (refer Fig. 1), stresses are generated at the interaction between two plies. This is due to anisotropy of the laminate. Residual stresses at the meso scale arise due to different longitudinal and transverse CTE. For example, 0° ply imposes a mechanical constrain on the 90° ply during curing and vice-versa. This is due to the difference in the directions of contraction of the two plies. Residual stresses present after curing are also analyzed.

During processing of composites, due to their

different coefficients of thermal expansion (CTE), the volumetric

shrinkage of the two entities is different. However, when

processed in the temperature range of 150°C to 300°C, during

curing, the volumetric shrinkage of matrix is significantly

higher than for fiber.

12.

The residual stresses induced after curing are analyzed using the Representative Volume Element (RVE) method. The RVE has been modeled using COMSOL Multiphysics finite element tool. Cure kinetics is analyzed in the Transport of Diluted Species Module, and the stress analysis is done using Solid Mechanics Module in COMSOL Multiphysics.

Cure KineticsThe cure kinetics of resin has two parts: autocatalytic kinetics at low conversion [1, 4] and diffusion limited nth order kinetics at higher conversion [1, 4]. The autocatalytic model, which is valid up to α < 0.5, is given as:

where, K1 and K2 are the temperature dependent constants and ma and na are the temperature independent constants. E01 and E02 are the activation energies relating with K1 and K2 respectively. R is the universal constant. The nth order model is applicable to higher conversion (α > 0.5) is given as

Micro Scale Undergo temperature cycle

during curing

During cooling, shrinkage of matrix is higher than fiber

Undergo temperature cycle

during curing

Different contraction in each

direction

Fiber Matrix interaction

Compressive stresses in fiber

Tensile stresses in matrix

Residual stresses

Mismatch of CTE of fiber and matrix

Different CTE in longitudinal and transverse direction

Micro Scale

Composite

Ply to ply interaction

Fig. 1 Flow chart of multi-scale approach

(1)

(2)

(3)

(4)

Multi-scale modeling using continuum approach to study the interfacial stress in CFRP

13.

where,

In above equations, parameter “n” is the reaction order. Keff is the overall effective reaction rate. K0 and E0 are pre-exponential factor and activation energy, respectively. The values of all constants are given in the Table 1. The properties of fiber and matrix are given in Table 2. Thermal cycle used for the simulation and corresponding degree of cure curve are shown in Fig. 2 and Fig. 3, respectively.

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Fig. 2 Thermal Cycle imposed to the domains

Fig. 3 Degree of cure during 10000s


14.

Table 1. Kinetic parameter for autocatalytic model at low conversion and for the nth order model at high conversion [3]

Parameter

K01(1/s)

K02(1/s)

E01 (J/mol)

E02 (J/mol)

ma

na

K0(1/s)

E0 (J/mol)

n

C

αc

Value

2.7321×105

3.8231×105

7.2776×104

6.6934×104

1.07

2.43

29.10

3.58×104

0.0403×T(K)+19.48

69

0.0092×T(K)-3.14

Autocatalytic model (conversion range 0-0.5)

Nth order model (conversion range 0-0.5)

Table 2. Fiber and Matrix Properties

Parameter

Young’s Modulus (GPa)

Poison’s Ratio

Coefficient of Thermal Expansion (1/K)

Matrix Properties

Young’s Modulus (GPa)

Poison’s Ratio

Coefficient of Thermal Expansion (1/K)

Volumetric Shrinkage (%)

Value

235

0.2

10×10-6

4.67

0.4

100×10-6

6

Fiber Properties


15.

Stress analysisRVE approach was used to represent single ply and double plies with a volume fraction of 0.6. Fiber is assumed to be isotropic, continuous, linearly elastic and uniformly placed while the matrix is assumed to be isotropic, linearly elastic and homogeneous throughout the analysis. The matrix was assumed to be linearly elastic. However, the results presented here are at the last time step when matrix is fully cured. As discussed in the previous section, CTE of matrix was taken as a function of degree of cure and volumetric shrinkage. At the last time step, the residual stresses were present due to shrinkage of the matrix. During cooling, the fiber experienced a negative hydrostatic stress, while, the matrix experienced a positive hydrostatic stress.

Distribution of stresses in the radial direction from the center of the fiber is plotted in the Fig. 6 and Fig. 7. Each graph contains two curves in which one curve represents stress distribution from the center to the free edge and another represents stress distribution from center to the interface of two plies. Here (see Fig. 4(a)), the free edge is the edge which is free from any constrain that represents the single ply. An interface edge is the common edge between the two plies. A comparison of maximum stress and hydrostatic stresses at the free edge and the corresponding stresses at the interface is shown in Fig. 6 and Fig. 7. Due to presence of second ply, maximum shear and hydrostatic stress increased significantly at the interface compared to the stresses at the free edge. It is examined from the result that shear stress plays a dominant role during the processing of the composites.

Fig. 4 Schematic diagram shows the considered edge in the analysis of stress distribution in radial direction (a) and at the interface (b)

RVE approach was used to represent single ply and double

plies with a volume fraction of 0.6. Fiber is assumed to be

isotropic, continuous, linearly elastic and uniformly placed while the matrix is assumed

to be isotropic, linearly elastic and homogeneous throughout

the analysis. The matrix was assumed to be linearly elastic.


(a) (b)

16.

Fig. 5 Distribution of maximum shear stress in single ply (a) and double ply (b)

Fig. 6 Variation of maximum shear stress at free edge and at the interface

Fig. 7 Variation of Hydrostatic stress at free edge and at the interface


17.

A comparison of stresses at the interface between two plies and at the free edge of single ply is shown in the Fig. 9 and Fig. 10. It is observed that at the free edge of the single ply, stresses in the transverse direction increase gradually, reach maximum and then decrease. However, in the longitudinal direction, the stresses increase rapidly, remain constant and decrease to the initial level. Here (Fig. 4(b)), transverse direction is in the plane, perpendicular to the fiber axis while, longitudinal direction is in the plane parallel to the fiber axis. The stresses increase in transverse direction at the free edge due to the interaction of the fiber. Maximum interaction of the fiber and matrix occurs at the center of free edge which causes the stresses to reach maximum. Fig. 5 and Fig. 8 show the distribution of maximum shear stresses and hydrostatic stresses respectively in single ply and double plies.

The behavior of the stresses at the interface between two plies is different as compared to the free edge of single ply which is shown in Fig. 9 and Fig.10. The presence of another RVE with a 90° orientation increases the maximum shear stress compared to the stresses at the free edge (refer Fig. 10). The maximum shear stress increased at the middle of the interface due to interaction of 0° orientation ply. However, the interface has no significant effect on hydrostatic stresses (Fig. 9).

Fig. 8 Distribution of hydrostatic stress in single ply (a) and double ply (b)

Fig. 9 Variation of hydrostatic stress at free edge along the transverse, along the longitudinal

direction and at the interface


18.

Conclusions• After the cooling, compressive hydrostatic stresses are present in the fiber

while tensile hydrostatic stresses are present in the matrix.

• Due to the presence of the second ply, hydrostatic and maximum shear stresses increase significantly as compared to that of in a single ply. These stresses could possibly be responsible for damage initiation in composites.

• It is seen that maximum shear stress is significantly increased due to the different orientation of the fiber in each ply which may be a major cause of delamination.

• Finite element analysis using RVE approach seems to be a good methodology for the analysis of interfacial stresses.

recommendation• More detailed understanding of the physics behind the formation of shear

stresses is required to model in FEM. This can further be validated by making a small experiment setup.

• By making this model more realistic, stresses at micro-scale and meso scale can be studied. Delamination and debonding can be predicted in a different loading condition by the use of this methodology.

rEfErENCES

[1] Musa R.Kamal (1974), “Thermoset Characterization for moldability analysis”, Polymer

Engineering and Science, 14 (3), pp.231-239.

[2] M.E.Ryan (1984), “Rheological and heat transfer consideration for the processing of reactive

system”, Polymer Engineering and Science, 24 (9), pp.698-706.

[3] Y.Eom, L.Boogh, V.Michaud,P, Sunderland, J. Anders (2000), “Time-cure-temperature

superposition for the prediction of instantaneous viscoelastic properties during cure”,

Polymer Engineering and Science, 40 (6), pp.1281-1292.

[4] Bhaskar Patham (2012), “Multiphysics Simulation of Cure Residual Stresses and Spring back

in a Thermoset Resin Using a Viscoelastic Model with Cure-Temperature – Time

Superposition”, Journal of Applied Polymer, 129 (3), pp.983-998.

Fig. 10 Variation of maximum shear stress at free edge along the transverse, along the

longitudinal direction and at the interface


19.

R & d updAtes


SAGAR TELRANDHE

PhD student, NCAIR, IIT Bombay

PAVAN SUTAR

M.Tech. student, IIT Bombay

PROF. SUSHIL MISHRA

Dept. of Mech. Engg., IIT Bombay

introductionWith growing use of titanium alloys in the present times in the aerospace sector, it is important to study the challenges faced in manufacturing of titanium alloys. The overall outstanding mechanical properties make titanium alloys difficult to machine accurately as it has high strength and very low thermal conductivity. During machining of titanium alloys, tool or insert rapidly gets damaged as compared to that in steel and aluminum alloy machining. The primary reason of this rapid tool damage is the accumulation of heat in the cutting zone. Titanium is known for its low thermal conductivity which arrests the heat to be taken away through chips. The localized temperature rise also leads to the formation of shear bands which generates continuous saw-tooth chips. This saw-tooth chip geometry is the reason behind the force variation on the tool/insert leading to vibration in machining. This vibration on tool has adverse effect on it. Hence, there is a need to find ways of improving the machinability of titanium alloys so that the tool life can be improved and eventually the cost of production can be reduced.

Need for Hybrid machiningIn most cases, many researchers tried to go through the conventional ways like optimizing tool geometry, tool material, operating parameters, etc. [1-3]. However, these methods do not have a direct impact on localized heated zone during machining of titanium alloys. To address this problem, different cooling media were used in which pressurized coolants were passed in the heat affected zone during machining, thereby, reducing the rapid temperature rise. The cooling medium used was liquid nitrogen and other non-reactive conventional coolants [4, 5]. However, with coolants there is a problem of recycling as well as direction accuracy over a precise region, where tool and workpiece comes in contact. Therefore, the design of the nozzle becomes a critical issue.

laser assisted machining in turning The concept of hybrid machining has given a new edge to titanium alloy machining. In this type of metal cutting, the region in the work piece, where the tool has to pass, is heated locally, by assuring very less heat transfer in the remaining portion (refer Fig.1). Hence, the thermal gradient reduces when the tool begins to remove layers during the machining process. The heat input can be provided through various sources like gas torch, plasma, induction heating and laser beam heating. Amongst all of these, laser beam heating has an edge over the other methods, as it proves to be well controlled, rapid and gives an instant rise in temperature over the localized region. The effect of integrating laser in titanium machining process has been discussed briefly in this work. The machining process is limited to turning, as it is easy to understand with a simple metal cutting process.

The concept of hybrid machining has given a new edge

to titanium alloy machining. In this type of metal cutting, the region in the work piece,

where the tool has to pass, is heated locally, by assuring

very less heat transfer in the remaining portion.

20.

During turning, the position of the laser beam, relative to the tool is critical because it requires only the non-machined surface to get heated. The tool and machine surface need to be spared completely. The tool–beam distance along with cutting speed determines the time interval between the laser heating and machining operation and hence, the temperature distribution at the cutting zone. Larger reduction in the cutting force is achieved with the laser spot positioned closer to the cutting tool when cutting commercially pure titanium [6]. This tool-beam distance has great influence in LAM, since shorter spacing leads to overheating of material and sometimes also leads to sticky or hot chips on machined surfaces which, deteriorates the overall quality.

Effect of laser power on cutting forcesHigher laser power reduces the cutting forces when machining at medium or larger cutting speeds [6]. Laser heating reduces the yield strength of material to a certain extent. Thus, a relatively softened material comes under tool for machining.

As shown in Fig.2, the cutting force reduces only slightly with increasing

Fig.1 Laser assisted machining setup for turning operation


21.

cutting speed in case of conventional machining. However, through the laser assisted machining, there is a significant reduction in force [Fig.3]. This is due to material softening through laser heating.

Effect of laM on chip morphology:A detailed study of chip geometry gives an insight of intrinsic details during machining [7]. Large pitch and thickness variation occurs in case of conventional machining (refer Fig. 3a), which is a sign of greater force variation. However, through laser assisted machining, this variation was found to be less (refer Fig. 3b).

Fig. 2 Comparison of cutting force between conventional and laser assisted machining [6]

Fig. 3 Comparison of chip morphology between conventional machining and LAM (a) Conventional machining (b) LAM


22.

Effect of laM on residual stresses

The value of compressive residual stress on the circumferential surface is higher for conventional machining. It decreases with the use of higher laser power [7]. As the reduction is significant, it is useful in case of subsequent machining or finishing operations. This is because the shear stress for cutting material needs to overcome comparatively lower compressive stress value.

Conclusions• The LAM has been an effective method, as it provides lower force values,

and lower force variation.

• The lower variation of forces has been explained through chip morphology.

• The compressive residual stresses decrease with the use of LAM.

advantages and disadvantages

rEfErENCES

1. E.O. Ezugwu, Z.M. Wang, “Titanium alloys and their machinability- a review” , Journal of

Materials Processing Technology, vol. 68, (1997), 262-274.

2. A.R. Zareena, S.C. Veldhuis, “Tool wear mechanisms and tool life enhancement in ultra-

precision machining of titanium”, Journal of Materials Processing Technology vol.212, (2012),

560–570.

Fig. 4 Comparison of residual stress between conventional machining and LAM

for Ti6Al4V alloy [7]

Advantages of LAM

Improvement in tool life (lesser tool wear)

Reduction in chatter due to vibration

Reduction in cutting forces

Disadvantages of LAM

Need to be operated in a closed environment

Precise focusing of laser beam and its intensity are basic requirements

More costlier than conventional machining


23.


3. E.O Ezugwua, J Bonneya, Y Yamane “An overview of the machinability of aeroengine alloys”,

Journal of Materials Processing Technology, vol.134 (2003), 233–253.

4. Suresh Palanisamy, Stuart D. McDonald, Matthew S. Dargusch “Effects of coolant pressure

on chip formation while turning Ti6Al4V alloy”, International Journal of Machine Tools and

Manufacture, vol. 49,(2009),739–743.

5. Christian Machai, Dirk Biermann “Machining of β-titanium-alloy Ti–10V–2Fe–3Al under

cryogenic conditions: Cooling with carbon dioxide snow”, Journal of Materials Processing

Technology, vol. 211 (2011), 1175–1183.

6. G. Germain, P.DalSanto, J.L.Lebrun “Comprehension of chip formation in laser assisted

machining”, vol. 51 (2011), 230–238.

7. T. Braham-Bouchnak , G. Germain , A. Morel , J. L. Lebrun“The influence of laser assistance

on the machinability of the titanium alloy Ti555-3”, The International Journal of Advanced

Manufacturing Technology, vol. 68 (2013),2471-2481.

24.

R & d updAtes

SAMIR AGRAWAL

PhD Student, IIT Bombay

PROF. SUSHIL MISHRA

Dept. of Mech. Engg., IIT Bombay

Damage Initiation in Fiber Composite

introductionThe term damage is representative of the volume of numerous cracks at a material point. Thus damage means the stiffness reduction which is irreversible in nature. Damage growth does not always lead to full failure of a material point. There are two types of approaches for the design of the system, the damage approach and a non damage approach. In non damage approach, the composite is considered to have failed when the first sign in reduction in property of material is observed or when the damage initiation takes place. In damage approach, the progressive stiffness reduction is considered to estimate the composite failure.

The relation [1, 2] between the nominal stress and effective stress under uniaxial loading is given as follows.

(1)

Where σ = P/AO is Cauchy nominal stress and σ ̃ = P/Aeff is the effective stress and d is the damage variable. The stress strain relationship in damaged model is represented as

σ = S(d) : εe and σ ̃ = Soε : εe where stiffness tensor S is given by

Where, D = 1-(1-d1)(1-d2) νo12 νo21 and d1 ,d2 and d3 are the damage developed in fiber direction, transverse direction and shear direction, Eio and νijo are the elastic modulus and Poisson’s ratio in respective directions. With an assumed damage population growth model (stiffness softening model) based on the experiment and behaviour of composite, the stiffness can be calculated at various time steps. For the property estimation, various methods can be used like Eshelby method, Mori Tanaka method, Differential method, Self consistency method which are based on the interaction between the matrix and particle. If the distribution of inclusion is also taken into account than other method like generalised self consistency method or H-S method can be used for property estimation. Once the degraded properties are known at each time step, various failure modes like matrix cracking, fiber breakage or delamination can be predicted. Various failure criteria like matrix failure, delamination and fiber breakage are described in detail below.

The term damage is representative of the volume of

numerous cracks at a material point. Thus damage means the

stiffness reduction which is irreversible in nature. Damage

growth does not always lead to full failure of a material point.

25.

failure Model and damage CriteriaThe coordinate system for the composite is shown in figure (1). The direction is as follows1- Along the fiber direction 2- Across fiber direction 3- Transverse Direction

Based on the coordinate system given above, normal and shear stress and strain along the various directions can be defined in case of complex loading condition. Various models for the composite failure are listed below

1) Hashin Damage Model [3]

Note: X, Y, Z denotes the tensile and compressive strength in 1,2,3 direction and Sij denotes the shear stress

Type of Damage

Tensile Matrix Failure (σ22 + σ33 > 0)

Compressive Matrix Failure (σ22 + σ33 < 0)

Tensile Fiber Breakage (σ11 ≥ 0) Compressive Fiber Breakage (σ11 < 0)

Inter Lamilar Tensile Failure (σ33 > 0)

Inter Lamilar Compressive Failure (σ33 < 0)

Damage Criteria Explanation of Criteria

Applicable when sum of the two transverse principal stresses is positive, although either can be negative.

Applicable when sum of the two transverse principal stresses is negative, although either can be positive.

Applicable when the longitudinal principal stress is positive.

Applicable when the longitudinal principal stress is negative.

Delamination can occur if principal stress perpendicular to laminate is positive.

Laminate compressive failure can occur if principal stress perpendicular to laminate is negative.

HASHIN DAMAGE MODEL


Fig. 1 Coordinate system in fiber composite

26.


2) Hou Failure Model [4]

HOU DAMAGE CRITERIA

Type of Damage

Matrix Cracking

Matrix Crushing in Transverse loading

Delamination

Explanation of Criteria

Applicable when a quadratic interaction between transverse principal stress and other shear stress (in plane and out of plane) is positive and transverse principal stress is positive.

Applicable when a quadratic interaction between transverse principal stress and in plane shear stress is positive and transverse principal stress is negative.

Applicable when a quadratic interaction between the principal stress perpendicular to laminate and out of plane shear stress is positive.

Note: em is the damage factor Yt is the Yield strength in tension Sij is respective shear strength σij is the induced stress

Damage Criteria

27.

3) Christensen’s Strain Model [5]

4) Tsai – Wu Failure Model [6]This is again the maximum stress criteria and the anisotropy of the material is also taken into account. In this criteria, the failure surface in stress space is described by a tensor quadratic polynomial which is given below-

F1σ1+ F2σ2 + F3σ3+ F11σ12+ F22σ22 + F33σ32+ F44σ232+ F55σ132+ F66σ122+2 F13σ1σ2+2 F13σ1σ3+2 F23σ2σ3 >= 1

Where, σI (i = 1,2,3) and σij (i,j = 1,2,3) are the normal and shear stress component. The strength parameter account for the failure is defined as

Where, Xt and Xc are the longitudinal tensile and compressive strength,

CHRISTENSEN’S STRAIN MODEL

Damage Criteria Explanation of Criteria

Applicable when a single quadratic equation with all the principal stress (individually positive or negative) are positive. Constants are to be obtained experimentally.

Applicable when the linear interaction between the three principal stresses is positive.

Applicable when the linear interaction between the three principal stresses is negative.

Type of Damage

Matrix Failure

Fiber Breakage: (Tensile): ( σ1 - ν12σ2 − ν13σ3 > 0)

Fiber Breakage (Compressive): ( σ1 - ν12σ2 − ν13σ3 > 0)


28.

Yt and Yc and Zt and Zc the transverse and normal tensile and compressive strength. S12, S23, and S13, are the shear strength in 1-2, 2-3 and 1-3 plane. The left hand side of equation is called failure index and when the failure index exceed unity damage is predicted.

5) Feng Model [7]

ConclusionsWhile designing the component made out of isotropic material prediction of damage is quite easier but in case of anisotropic material, like composites, with different modes of failure, the damage model needs to be different which takes care of the anisotropy of the composite material. Various models listed here can be used in the methods for estimating properties. Further, with a suitable property degradation model, the models can predict failure in a composite under complex loading conditions.

rEfErENCE

[1] Ever J. Barbero , Daniel H. Cortesm. "A mechanistic model for transverse damage initiation,

evolution and stiffness reduction in laminated composites". Elsevier 2010.

[2] J. F. Chen, E.V. Morzov, K.Shankar. “A combined elastoplastic damage model for progressive

failure analysis of composite material and structure”. Elsevier 2012.

[3] Hashin Z. “Failure criteria for unidirectional fiber composites”, ASME Journal of Applied

Mechanics, Vol. 47 (2), 1980, pp 329-334.

[4] Hou JP, Petrinic N, Ruiz C, Hallett SR. “Prediction of impact damage in composite plates”,

composite Sci . Technology 2000:60(2):273-81.

[5] Christensen, R. M. “Stress Based Yield/Failure Criteria for Fiber Composites,” Int. J. Solids

Structures, 34, 529-543.

[6] Tsai, S. W. and Wu, E. M. “A General Theory of Strength for Anisotropic Materials,” J. Comp.

Mater. 5, 58-80.

[7] Feng W. W. “A Failure criteria for composite material”. Journal of composite material, Vol 25,

Jan 1991, pp 88-100.

FENG MODEL

A1Jl + AllJ12 + A2J2 - 1 >= 0

A5J5 + A55J52 + A4J4 - 1 >= 0

Type of Damage

Matrix Failure

Fiber Breakage

Explanation of Criteria

Applicable when quadratic equation of strain invariant is positive.

Damage Criteria

Note: : Strain Invariant is defined as follows: J1 = ε1 + ε2 + ε3 J2 = ([( ε1 - ε2)2 + (ε3 - ε2)2 + (ε1 - ε3)2] / 6)+ ε42 + ε52 + ε62 in which ε1, ε2, ε3, ε4, ε5, and ε6 are the ply strains. J4 = ε42 + ε52 J5 = ε1


29.

teCHNOLOGY updAtes

Resins - their properties and selection criteria (Part II)

SWETHA MANIAN

Research Assistant

NCAIR, IIT-Bombay

In the previous edition of the newsletter, we had discussed about what resins are, their types and some basic chemistry. In this article we shall discuss some of the important properties and performance characteristics of the resin and also present an overview of their selection criteria.

the performance indicatorsWhile it is difficult to rate a resin on its performance characteristics alone, other features such as processability and profitability help in determining the choice and suitability of a resin for an application. Performance properties of the resins such as high flame retardance, better dimensional stability, low water absorption, high temperature resistance and better thermal and mechanical properties are some of the desired features to select one resin over the other. Processability of the resin is determined by its nature of curing, storage, and transportation requirements. On the other hand, the ease and cost of manufacturing, maintenance requirements and the ease of logistics dictate its profitability. But processability and profitability of a resin are interlinked since a better processed resin will have a better shelf-life and better out-life. This will eventually lead to lower maintenance and handling cost of resins, thereby increasing its profitability.

Characteristics of epoxy resinDiglycidyl ether of bisphenol-A (DGEBA) (Figure 1), the most commonly used epoxy resin, is a polymer formed by the reaction of bisphenol A and epichlorohydrin as discussed in part 1 of this article series. It is still very extensively used in various high-performance structural materials. While epoxy does have many advantages, it has the biggest disadvantage of moisture absorption, which occurs by diffusion through it and also by immediate surface adsorption.

Epoxy is usually reacted with amine functional group (-NH2) group, which is used as a 'hardener' for the resin. Figure 2 shows the general epoxy-amine reaction.

Fig. 1 Epoxy Resin

R O C

CH3

CH3

O CH2 CH

OH

CH2 O C

CH3

CH3

O R

n

30.

The hydroxyl group (-OH), which is formed during this reaction, is disrupted by water entering the polymer network. In composites, this water is transported through the layers by capillary action causing the fiber-resin matrix to swell, thus plasticizing the polymer and ultimately reducing its mechanical strength. This inherent problem of epoxy resin has driven engineers and scientists to find newer resins that can overcome such defects.

New-age resinsRelatively younger class of thermoset resins like bismaleimide (BMI) and benzoxazine (BOZ) are slowly gaining acceptance in the aerospace industry as they exhibit a combination of suitable features. These resins are more promising and exhibit better characteristics over epoxy and find their use in various applications including composites for aerospace materials. BMI is well known for its low cost materials, excellent mechanical properties, thermal stability, solvent resistance, and electrical insulation properties over a wide range of temperature but it does have some cons such as poor processability and the poor toughness. BOZ, on the other hand, possesses good glass transition temperature, low water absorption values, and good dielectric properties in addition to zero shrinkage. The only challenge BOZ poses is the temperature it requires for self-curing (minimum ≥ 190°C), which could be tackled using catalysts for curing. BOZ is also compatible with various other thermoset resins. The reason that this class of polymer can be an excellent resource for the aerospace industry is that many shortcomings associated with the traditional phenolic resins, like release of condensation by-products, are over come. It does however retain the good thermal properties and the flame retardance property of phenolics. They have excellent mechanical properties and molecular design-flexibility and undergo near-zero volumetric changes or expansion polymerization.

Polymer blendsTable 1 shows the comparison of some of the resins currently in use or as a promising contender for aerospace applications. Recently, a lot of research is being done on various blends of polymers, especially BMI and BOZ. Benzoxazines are copolymerized with an epoxy resin in order to modify their performance. The addition of epoxy to the polybenzoxazine network greatly increases the crosslink density of the thermosetting matrix and strongly influences its mechanical properties by bringing by a combination of the better properties. For this reason, copolymerization is good to incorporate the better properties of resins. It leads to the increase in the flexural stress, and flexural strain over those of the polybenzoxazine homopolymer, with

Fig. 2 Epoxy Amine reaction Note: R and R’ designate the lengthy organic

chains (carbon chains) separating functional groups.

R-NH2 + R'-CH2-CH-CH2 R-N-CH2-CH-CH2-R'

O H OH

Hydroxyl group


The addition of epoxy to the polybenzoxazine network

greatly increases the crosslink density of the thermosetting

matrix and strongly influences its mechanical properties by

bringing by a combination of the better properties.

31.


only a minimal loss of stiffness. With BOZ, the processing is the same as epoxy, minus the issues with voids during cure that must be managed with phenolics and also lower heat of reaction than epoxies. It is a possibility that, such custom tailoring of resins will be the norm in composites in the next couple of decades.

ConclusionChallenges in the aerospace industry have created the need for extreme-performance resins that combine very high heat and chemical resistance with good mechanical properties. Current research in the field is on going to find which resin best fits this need while satisfying the requirement of the triple bottom line of performance, processability and profitability. It is being observed that a polymer blend exhibits improved properties as against a single resin-fiber system, while continuing to maintain the inherent positives of the either of the single systems. Hence a blend of such resins with the right set of complimentary properties and other reactive co-monomers has helped in formulating resins with desirable properties. Fig. 3 is a Venn representation of a suitable resin which would meet the triple bottom line criteria of profitability, processability and performance.

BiBliograPHy

[1] Jain, R, A. K. Narula, and V. Choudhary, “Studies on curing and thermal behavior of diglycidyl

ether of bisphenol-A and benzoxazine mixtures.,” J. Appl. Polym. Sci., vol. 106, no. 5, pp.

3327–3334, 2007.

[2] Thomas M. Donnellan, “Relationships in a Bismaleimide Resin System. Part 1: Cure

Mechanisms”, Polymer Engg. And Science, Vol. 32, No. 6, pp 409 – 414, March 1992.

[3] Dana A. Powers, "Interaction of Water with Epoxy", Sandia Report SAND2009-4405, Sandia

National Laboratories Albuquerque, New Mexico 87185 and Livermore, California 94550,

July 2009.

[4] Maureen A. Boyle, C. J. Martin, “Epoxy Resins.pdf.” Hexcel Corporation. http://home.

Table 1: Resin comparison

Resin

Epoxy

Benzoxazine

Bismaleimide

Cyanate Ester

Advantage

Excellent fatigue properties. Easy processability.

Easy processability. Good thermal properties.

Non-flammability. High thermal stability.

Good dimensional stability at high temperature.

Limitations

Moisture absorption leading to reduced mechanical strength. Poor thermal stability.

Long cure cycle but can be reduced using appropriate catalysts.

Relatively poor processability.

Brittle. Poor impact resistance.

Processability

BOZ

BMI

?

Epoxy

Performance Profitability

Fig. 3 Venn representation of a suitable resin that will meet the triple bottom line criteria

32.

engineering.iastate.edu/~mkessler/MatE454/Constituent%20Materials%20Chapters%20

from%20ASM%20Handbook/%284%29%20Epoxy%20Resins.pdf

[5] http://www.epoxyproducts.com/chemistry.html; last retrieved on August, 10, 2014.

[6] Hatsuo, Ishida and Douglas J. Allen, "Mechanical characterization of copolymers based on

benzoxazine and epoxy." Polymer, Volume 37, Number 20, pp 4487 - 4495, 1996.

[7] American Society for Testing and Materials, (ASTM) STP 658, Advanced Composite Materials

– Environmental Effects, 1978

[8] Guanglei, Wu, Kaichang Kou, Longhai Zhuo, Yiqun Wang, Jiaoqiang Zhang, “Preparation and

characterization of novel dicyanate/benzoxazine/bismaleimide copolymer.” Thermochimica

Acta, vol. 559, pp. 86-91, May 2013.

[9] Vanaja, A., R.M.V.G.K Rao, “Synthesis and characterisation of epoxy–novolac/bismaleimide

networks.” European Polymer Journal, vol. 38, Issue 1, pp. 187–193, Jan 2002.


33.

HAL Faculty Chair at IIT Bombayhttp://www.iitb.ac.in/en/breaking-news/hal-faculty-chair-iit-bombay

In an endeavour to promote applied research, development and academic work in the field of new and emerging aerospace technologies, Hindustan Aeronautics Limited (HAL), has entered into a Memorandum of Understanding (MoU) with Indian Institute of Technology Bombay (IIT Bombay), to establish a Faculty Chair (HAL Chair) at the Institute.

The MoU was signed on May 21, 2014 at IIT Bombay by Prof. Ravi Sinha, Dean (Alumni & Corporate Relations), IIT Bombay and Mrs. S. Thenmozhi, General Manager, HAL Hyderabad, in the presence of Prof. Khakhar and Chairman of HAL, Dr. R. K. Tyagi.

The MoU aims at bringing together the best from both the organisations in the field of aerospace systems technology and its application. This would facilitate skilled scientific and technical people from both IIT Bombay and HAL to undertake applied research and tackle multi-disciplinary problems in the field of aerospace technologies.

Opportunities in the aero industry supply chainhttp://www.theengineer.co.uk/in-depth/opportunities-in-the-aero-

industry-supply-chain/1018962.article

Global demand for aircrafts is surging. The order books for new aircrafts are at their record levels. The aerospace industry on the whole – OEM’s, integrators, SME’s - is expanding its manufacturing capacity to keep up with the demand. In the present times, modern aircraft designs, materials and manufacturing processes have all become more complex. Today, the manufacturers focus on light weight, energy efficient and innovative aircrafts.

All this has translated into larger demand for skilled engineers and the demand is expected to increase in the future. The engineering skills sets most sought after are: system integration, design and analysis, fatigue and dynamic modelling, composite fabrication and assembly, manufacturing engineering,

AeROspACe News BRIeFs

power electronics, etc. Skills in higher maths or physics are also a big advantage.

The future outlook is positive. There will be plenty of opportunities available to get involved in a technology driven, challenging and fascinating industry.

Aircraft engine components to be 3D printedhttp://www.flightglobal.com/news/articles/analysis-ge-ponders-3d-

printing-for-ge9x-turbine-blade-401898/

Critical aircraft engine components such as Low-pressure turbine (LPT) blades and fuel nozzles will soon be manufactured using 3D-printing or additive manufacturing technology. Making a foray into this technology is GE Aviation which on July 15, 2014 announced that its factory in Alabama would be the first to manufacture engine parts using additive manufacturing at a production level. Fuel nozzles for the CFM Leap-1 engines will be produced at this facility. However, GE wants to take this technology further and is now contemplating using it to build the LPT blades for its most advanced jet engine-the GE9X.

LPT manufacturing specialist, Avio Aero, which was acquired by GE last year, has set up a 20,000 sq.ft dedicated additive manufacturing facility and is now evaluating and validating its manufacturing process. The technology which Avio is working on is the Electron Beam Welding process, a kind of a 3D-printing process in which the LPT blade is 'grown' in an additive manner, section after section. Titanium aluminide (Ti6Al4V) is a material extensively used to make engine components as is extremely strong, light in weight and can withstand extreme temperatures. However, it is very difficult to work with, whether in casting or in machining. 3D printing is therefore seen as a promising technology to manufacture these parts efficiently.

34.

Robots to play a greater role in building airplaneshttp://www.boeing.com/boeing/Features/2014/07/bca_777_

fuselage_07_14_14.page

Robots will soon be used extensively to assemble fuselages for the Boeing 777 airplane. They will be employed for drilling 60,000 holes and inserting fasteners per 777 fuselages, which are essentially very labor intensive and time consuming processes. Boeing envisages that using this technology, known as Fuselage Automated Upright Build or FAUB, will not only make the assembly process efficient, thereby reducing costs, but also improve the quality of building the aircraft. In the FAUB process, automated and guided robots will drill and fasten the fuselage panels together, which currently is done by hand. Another benefit of FAUB is that it will improve worker safety and reduce injuries half of which occur in the assembly phase of production.

Currently in the testing phase, the FAUB process is expected be used on the 777 and 777X aircrafts over the next few years. The system has been designed by KUKA Systems for Boeing and is amongst the latest in the Advanced Manufacturing Strategies for the 777 program.

35.

37.

ELEVONS are aircraft control surfaces that combine the functions of the elevator (used for pitch control) and the aileron (used for roll control), hence the name.

National Centre for Aerospace Innovation and Research (NCAIR), 2nd Floor, Pre-engineered building, Opp. Power house, IIT-Bombay, Powai, Mumbai-400076

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Documents

Elevons - NCAIRncair.in/newsletters/NewsletterV4.2.pdfin Lakshmi Precision Screws Ltd. He has joined NCAIR as a Research Assistant. Manan Panchal He received his Master's degree in