Y-Stage

Collection Fiber

ExcitationFiber

Laser Probe

X-Stage

Z-Stage

Tripod Mount

Objective

0 5000 10000 150000

500

1000

1500

2000

2500

NRSCA

RSCA

Untreated

Motivation

Objective

Al2O3 for carbon fiber

composite reinforcement

SCAs for improved dispersion

Future Work

References and

Acknowledgements

Stress mapping methods with applied loading

Stress analysis and mechanical behavior of HCFRPs under

tensile loading

Mechanical Analysis of Hybrid Carbon Fiber Reinforced with Alumina Nanoparticles via PiezospectroscopyAlex Selimov

1, Ryan Hoover

1, Valentina Villegas

1, Albert Manero

1, Peter Dackus

2, Declan Carolan

2, Ambrose Taylor

2, and Seetha Raghavan

1

1Mechanical and Aerospace Engineering, College of Engineering and Computer Science, University of Central Florida, Orlando, FL 32816, USA

2Department of Mechanical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K.

Al2O3

R

R

R

O

O

O Si X

Epoxy m

atrix

Hydrolyzable reactivegroups (O) which reactwith Alumina molecules(R) to form covalentbonds.

Organofunctionalgroup (X) whichbonds to theepoxide group inthe polymermatrix

To investigate the stress sensitive properties of hybrid reinforced carbon fiber composites and manufacturing steps for their improvement.

4A2

4T2

2A

2E

Ground State

R2~14430cm-1

R1~14403cm-1

Excitation of chromium impurities in aluminum oxide causes

emission of stress sensitive R-lines

Cr3+

Wavenumber(cm-1)

Inte

nsi

ty (

Arb

itra

ry)

R1 Δv

R2 Δv

Unstressed stateStressed state

Alumina when excited with a laser has a characteristic emission pattern which relates to the energy state of

the chromium ion impurities present in the crystal structure. The spectral emissions of alumina shift with

applied stress. This is a linear relationship and is described by the following equation:

Where Δv is the shift in wavenumber of the spectrum, σii is the applied hydrostatic stress, and Πii is the

hydrostatic PS coefficient which is measured experimentally. Using this relationship, applied stress to

alumina can be calculated. This allows for the understanding of material behavior in multicomponent

systems in which alumina is used.

Schematic for manufacturing of HCFRP samples through RIFT

Carbon Fiber stack

Loaded resin

Vacuum Reservoir

Epoxy Resin

Alumina Particles Vacuum is used to inject resin to create void free composite

Stress sensitivity is enabled in carbon

fiber samples through the inclusion

of alumina nanoparticles within the

epoxy resin used to bind the carbon

fiber sheets. The samples tested are

manufactured using a method known

as RIFT (resin infusion under

flexible tooling) which provides a

matrix free of voids and enables high

volume fraction of filler.

Schematic explaining Silane Coupling Agents (SCAs)

Silane coupling agents

(SCAs) are chemicals which

act as bridges between

organic and inorganic

materials. In the case of the

HCFRPs investigated, SCAs

were analyzed for their effect

on dispersion which

ultimately affects mechanical

behavior.

Forc

e (k

N)

0

2

4

6

8

10

12

14

16

18

20

0

0.5

1

1.5

2

2.5

3

PS stress map

DIC strainmap

Time

Stre

ss (M

Pa)

σa

Simplified schematic of PS equipment explaining various systems required.In order to take optical stress measurements using Piezospectroscopy, various experimental systems are

required to be operated in conjunction. This includes simultaneous spectral collections using

spectrograph and CCD devices and application of loading using an electromechanical load frame.

Load profile applied to carbon fiber samples with labeled stress and strain

measurement points

Load was applied to the samples in 1.05kN steps. Each step was held to enable PS stress measurements.

Strains were captured only during the ramping periods due to laser interference with the DIC camera. This

enabled a full stress and strain profile of the samples with the applied loading patterns.

Improvement of tensile strength due to hybrid reinforcements

The tensile strength of hybrid

materials is improved due to the

presence of a secondary inhibitor to

crack propagation. In hybrid

materials both the fiber and

nanoparticles deflect cracks

preventing early failure. In these

materials the failure mode tends to be

total failure of the material due to the

large presence of inner microcracks.

Epoxy matrixCarbon fiber reinforcement

Alumina nanoparticle reinforcement

Deflection of crack by alumina nanoparticles Deflection of

crack by carbon fiber

Experimental Parameters for HCFRP sample tested

The experimental parameters for the carbon fiber samples are outlined in the table above. The samples

had end tabs applied post manufacturing. These tabs were made of aluminum and were attached to the

carbon fiber using Loctite® EA 9394 . Sample dimensions were 10x100x4mm and the central

mapping dimensions were 10x50mm. The spectral dimensions were 32x16 resulting in a higher

resolution in the x-direction.

The experimental setup is shown above with the labeled DIC and PS collection

systems. Data was collected on both sides of the samples in conjunction with applied

loading in order to determine stress and strain patterns.

Combined experimental setup for DIC and PS

measurements

Dispersion maps of 12wt% carbon fiber sample with various SCA

treatments

0

8 Biaxial strain

(e1+e2) (m

m/m

)

-400

0

400

Comparison of stress and strain maps from PS and DIC for 10wt% reveal earlier detection of high stress zones with PS

Comparison of PS and DIC stress and strain maps reveal an advantage of using alumina particle reinforcements to enable stress sensitivity. Columnar stress concentrations are seen which highlight the ability of

PS to detect internal load transfer behavior.[1] An explanation for the origins of these stress concentrations are provided below. It must be noted that while DIC is not able to detect this sub-surface behavior, the

agreement of final stress and strain maps validate initial stress map results.

Intensity concentrations relate to location of stress concentration zones

for sample.Columnar particle concentrations are seen through the intensity maps, where higher intensity

correspond to a higher local concentration of particles. Higher concentration of composite

inclusions are known to relate to higher average matrix stress.[2] This stress is amplified through

loading which results in the visible stress concentration zones. This relationship enables

predetermination of failure zones based on particle concentrations.

Intensity distributions for different surface treated filler

R1 Intensity (AU)

Pix

el C

ou

nts

Initial work has been conducted on

the improvement of filler

dispersion through the use of the

silane coupling agent surface

treatments. It is seen that reactive

silane coupling agent treatments

(RSCA) improve dispersion when

compared to the untreated sample

at 12wt% of alumina filler. This is

seen by the narrower and lower

intensity peak of the RSCA

sample. A legend is shown below.

Untreated NRSCA RSCA

High intensity hot spots indicate areas of

high alumina concentration

Improved dispersion in

RSCA sample seen from

lower intensity peak

position combined with

small distribution width

Schematic representation of hybrid carbon

fiber composite structure reinforced with

alumina nanoparticles.

Addition of stress-sensitive

particulate reinforcements allows for

early crack detection.

Laser source.

Intensity (Counts) Stress(MPa)

0

12

Inten

sity (C

ou

nts x 1

03)

High resolution local stress measurements will allow for the detection of

particle-matrix debonding in HCFRPs with different particle treatments

Future work will involve the

dispersion characterization of

hybrid carbon fiber samples

with reinforcement surface

treatment at various volume

fractions. Mechanical properties

of each composite will be

characterized along with failure

behavior in order to understand

optimal manufacturing methods

for these materials.

Early detection of crack

Measurement of crack growth until composite failure

Surfacetreatmentsprovide bettermechanicalproperties andcrack resistance

Columnar stress concentrations seen in stress maps indicating sub-

surface load transfer behavior.

Validation of stress maps with DIC through matching of stress and

strain behavior

PS shift calibrated with DIC biaxial strain in small region

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Biaxial Strain e1+e2 (mm/m)

Peak s

hif

t (c

m-1

)

Using a small homogenous region, the PS response of the sample can be calibrated to

strain by plotting the peak shift versus biaxial strain. The beginning of yielding can be

seen through the lowered peak shift response at higher strains. As the sample returns

to zero load there is a residual positive shift which indicates a reduction in the

compressive residual stress in the particles due to a breakdown of the matrix-particle

interface.

Yielding proposed to result

from breakdown of particle-

matrix interface

Residual positive peak

shift shows relaxation

from compressive

residual stress.

Weight % Single point collection time

Total number of stress points

Spectral Resolution

Max load achieved

10wt% 300ms 512 312µm 18.8kN

Speckle pattern on backside for DIC

strain measurements

Front side used for PS stress

measurements. Side used had

higher intensity response.Stress sensing capabilities enabled through the use of next generation hybrid

carbon fiber composites

Carbon fiber composites are highly suitable materials for aerospace applications due to their light weight

and high strength. However, the improvement of carbon fiber composites is limited due to the necessary

tradeoffs in mechanical properties when balancing the ratio of fibers to epoxy matrix. The reinforcement of

carbon fiber composites with ceramic particles provides hybrid properties which will be characterized for

high strength and stress sensing applications.

[1] Freihofer, G., Dustin, J., Tat, H., Schülzgen, A., & Raghavan, S. (2015). Stress and structural damage sensing piezospectroscopic

coatings validated with digital image correlation. AIP Advances, 5(3), 037139.

[2] Clyne, T. W., & Withers, P. J. (1995). An introduction to metal matrix composites. Cambridge University Press

This material is based upon work supported by the National Science Foundation under Grant No. CMMI 1130837. This work is also

supported by the University of Central Florida s EXCEL program and Duke energy. The authors of this work would like to thank

Yangyang Qiao of Dr. Gou s Research group for his help in obtaining the DIC strain measurements. We would also like to thank Dr.

Gregory Freihofer for providing continuing support for the experimentation and analysis performed in this work.

Homogenous

region for

calibration

-300

-230

-140

-70

0

70

140

230

300

35 MPa

315 MPa

630 MPa

5 10 15 20 25 30

0

2

4

6

8

10

12

142000

4000

6000

8000

10000

12000

14000

16000

Peak shift columnar stress concentrations clearly noticed corresponding to intensity concentrations

Columnar particle concentrations detected through intensity distributions

5 10 15 20 25 30

02

46

8

1012

14

Δ𝑣 = 1

3Π𝑖𝑖𝜎𝑖𝑖

PS stress

map

DIC strain

map

Carbon fibercoupon in tension

Photo-luminescent data collection

DIC camera forstrain measurements

R1 Intensity (AU)

Pix

el C

ou

nt Good dispersion

Poor dispersion