residual stress measurement

Department of Mechanical Engineering 1

Final Project Presentation Measurement of Residual Stress Distribution And

Fatigue Life Assessment of Butt Welded Joint

Pankaj Kumar : (1BG11ME034)

Guide: Co-Guide:

Mr. Harish A. Mr. Karthik N.

Asst. Prof. Asst. Prof.

Mechanical Dept. Mechanical Dept.

B.N.M.I.T B.N.M.I.T


Aim of the Project

Measurement of Residual Stress Distribution and Estimation of Fatigue life of a Butt Welded Joint before and after surface treatment.

Department of Mechanical Engineering

Introduction Welding process induces residual tensile stress that is detrimental to

fatigue life.

Residual tensile stress act to stretch or pull apart the surface of the material.

With enough load cycles and high enough tensile stress, a metal surface will initiate crack. Residual stress directly affects a components fatigue life.

It is well known that when a welded component fails in fatigue the failure location is usually at the weld. Residual stress directly affects a components fatigue life.

Detrimental residual tensile stress decreases the fatigue life while beneficial residual compressive stress increases the fatigue life.


Both types will be thoroughly investigated through the use of X-ray diffraction in this project work.

This project work will investigate how detrimental residual stresses are created through the welding process.

This paper will explain through theory and residual stress measurements how welding stresses can be modified by surface modification process such as shot peening for a more beneficial condition from a fatigue standpoint.

To complete the technical study, multiple carbon steel components were welded and exposed to various manufacturing processes.

Introduction


Introduction

Figure: Residual Stress Distributions after Welding


Project Objectives1] To review the literature on fatigue life of welded joint and on

surface enhancement treatment.

2] Experimental investigation of induced residual stress in butt welded joint before and after surface modification process of shot penning method.

3] Estimation of fatigue life in butt welded joint before and after surface modification process.

4] Analyzing the Effect of residual stress modification on fatigue life of the welded component.


Objective 1: To review the literature on fatigue life of welded joint and on surface enhancement treatment. Literature Summary: Residual stresses are a consequence of the welding process and are an important consideration in structural assessment. The welding process induces residual tensile stress that is detrimental to fatigue life. Tensile stress act to stretch or pull apart the surface of the material . With the enough load cycles at a high enough tensile stress a metal surface will initiate the crack. It is commonly observed that complex fabricated structures subject to fatigue loading fail at the welded connections. Some problems can be corrected by improved detail design but fatigue performance can also be improved using post-weld improvement methods.

Methods and Methodology


Method and Methodology RESIDUAL STRESS MODIFICATION:Residual stress can significantly affect engineering properties of materials and structural components, notably fatigue life, distortion, dimensional stability, corrosion resistance etc.

The residual stress analysis is a compulsory stage in the design of structural elements and in the estimation of their reliability under real service conditions Significant improvement in the fatigue life can be obtained by modifying the residual stress levels in the material.

Some of the different methods of residual stress modification methods are a.) Shot peening b.) Hammer peening c.) Ultrasonic method d.) Heat treatment.


Shot peening: Shot peening is a cold work process used to finish metal parts to prevent fatigue and stress corrosion failures and increase product life for the part. In shot peening, small spherical shot bombards the surface of the part to be finished. The shot acts like a peen hammer, dimpling the surface and causing compression stresses under the dimple.



Figure: Wheel blasting machine for Shot peening



Objective 2: Experimental investigation of induced residual stress in butt welded joint before and after surface modification process.

Experimental details :

a.) Material : SS304L and Mild steel

b.) Type of joint : Butt weld joint.

c.) Type of welding : TIG

d.) NDE technique : XRD method



Figure: XRD equipment setup for measurement of residual stress in given specimen




Figure: XRD machine setup


Objective 3: Estimation of fatigue life in butt welded joint before and after surface modification process.

Estimation of fatigue life of the butt welded component is carried out with the help of rotating fatigue testing machine TM7001.

The dumbbell shape specimen of required size is gripped on to a motor at one end to provide the rotational motion whereas the other end is attached to a bearing and also subjected to a load or stress.

When the specimen is rotated about the longitudinal axis, the upper and the lower parts of the specimen gauge length are subjected to tensile and compressive stresses respectively. Therefore, stress varies sinusoidal at any point on the specimen surface. The test proceeds until specimen failure takes place.



The number of cycle for which the specimen withstands without any failure is noted down.

This cycle is known as fatigue strength of given specimen. The revolution counter is used to obtain the number of cycles to failures corresponding to the stress applied.

Increasing of the load applied to the fatigue specimen results in a reduction in number of cycles to failure.



Objective 4: Analyzing the Effect of residual stress modification on fatigue life of the welded component.

Residual stress directly affects a components fatigue life. Detrimental residual tensile stress decreases the fatigue life while beneficial residual compressive stress increases the fatigue life.

Surface modification process such as shot peening reduces the residual tensile stress and induces residual compressive stress in the welded component.

Hence the hardness and fatigue life of the welded component increases with the decrease of detrimental tensile stress.



Literature Review Sylvain Chataingner et al: Steel structures are mainly prone to two types of

degradation: corrosion and fatigue particularly in the case of welded structures. The presented work aims at investigating two treatment methods to increase the fatigue life expectancy of welded steel joints. It includes both numerical and experimental investigations and is interested in the use of shot peening. As far as experimental investigations are concerned, for both treatment methods and for untreated samples, the stress concentration coefficients are determined, surface residual stresses are measured using X-ray diffraction and fatigue tests are conducted. The results allow explaining for both methods the observed improvements in fatigue behaviour. Numerical investigations are concerned and this allows the determination of residual stresses due to welding operation and their comparison with the former experimental measures.


M Hasegawa et al: The shot peening method, which requires simple equipment and treatment, is extensively employed as a method to improve fatigue strength. However, conventional steel ball shot material has a low specific gravity, so high speed projection is required in order to improve fatigue strength; thus, the accompanying noise and high air flow are problems. Accordingly, in order to improve these aspects, high hardness and high specific gravity shot material has been developed with 1.7 times the hardness (HV1400) and approximately 1.9 times the specific gravity (approximately 14) compared with a conventional steel ball.

Mark S. Molzen et al: The welding process induces residual tensile stress that is detrimental to fatigue life. Tensile stresses act to stretch or pull apart the surface of the material. With enough load cycles at a high enough tensile stress, a metal surface will initiate a crack. Significant improvements in fatigue life can be obtained by modifying the residual stress levels in the material. Two methods of performing this are through heat treating and shot peening. Both will be thoroughly analyzed in this paper through the use of x-ray diffraction. X-ray diffraction is the most accurate and best-developed method to characterize the residual stress in polycrystalline material.

Literature Review


Manoj Saini et al: Dissimilar metal joints between austenitic stainless steels and carbon steels containing low amounts of carbon are being extensively utilized in many high-temperature applications in energy conversion systems. In steam generating power stations, the parts of boilers that are subjected to lower temperatures as in the primary boiler tubes and heat exchangers are made of ferritic steel(mild steel) for economic reasons. Other parts, such as the final stages of the super heaters and repeaters operating at higher temperatures where increased creep strength and resistance to oxidation are required, are constructed with austenitic stainless steels. Therefore, transition welds are needed between the two classes of materials.Dissimilar metal weld is also used to anlysis of strength of joint.Beacause due to residual stress induces in the weld.We can find the amount of residual stress in two different metal.

Literature Review


Work Carried Out

Literature survey is carried out on fatigue life of welded joint and on surface enhancement treatment process.

Selection of material and specification required according to ASTM standard. Mild steel and stainless steel plates are machined to the required shape and size. Machined MS and SS304L materials are joined by butt welding process and

specimens of MS to MS and MS to SS are prepared according to ASTM standard.

Shot peening is done on the specimens surface to reduces tensile stress and to induces compressive stress in the specimens.

Residual stress on the specimen is measured before and after shot peening operation.

Fatigue strength is measured for both shot peened and without shot peened specimen and compared.

Hardness of the specimen near the welding zone is measured before and after shot peening process.


Results and Discussion

Sl. No Specimen Stress before shot peening(Psi) Stress after shot peening(Psi)

1.

MS to MS

4

-186.9

2.

MS to SS

3.75

-910.8

Table 1] Experimental result of residual stress of the specimens before and after shot peening

Sl. No Specimen Load(N) Speed(rpm) Number of cycles to

failure before shot

peening

Number of cycles to

failure after shot

peening

1.

MS to MS

25

2500

35,475

49,942

2.

MS to SS

25

2500

45,372

61,266

Table 2] Experimental result of fatigue life of the specimens before and after shot peening


Results and Discussion

Sl. No Specimen Load (kgf) Hardness before shot

peening(HRC)

Hardness after shot

peening(HRC)

1.

MS to MS

60

52

63

2.

MS to SS

60

57

65

Table 3] Experimental result of hardness on the heat affected zone in the specimens before and after shot peening


Results and DiscussionDiscussion:

The obtained results of all the specimens were tabulated.

It is observed that (from table 1) the residual stress in the specimen before shot peening is positive i.e. tensile but it becomes negative i.e. compressive after shot peening. This indicates that the shot peening reduces the residual tensile stress and induce residual compressive stress.

Table 2 shows that the fatigue life of component increases after the shot peening process. Also the fatigue life of MS to SS specimen is greater than that of MS to MS specimen before and after shot peening process.

Table 3 shows the hardness value of two specimens at the heat affected zone. It is observed that the shot peening process increases the hardness of material. Also the hardness of the MS to SS specimen is greater than that of MS to MS specimen before and after shot peening.


Conclusions Conclusions are drawn based on the results obtained from experimental methods are summarized as follows:-

Welding process creates residual tensile stress which is detrimental to fatigue life and is required to be removed or compensated.

The similar (MS to MS) and dissimilar (MS to SS304L) butt welded joints were prepared according to the ASTM (American Society for Testing and Materials) standard and the induced tensile stresses were measured by X-Ray diffraction method.

Shot peening method which is employed for surface modification, uses S330 shot balls of diameter ranges between 0.7mm to 1.20mm to strike the surface of the specimen for duration of 25 seconds. This Surface modification process induces compressive residual stresses.

After shot peening process the induced compressive residual stresses in the specimen measured by using X-Ray diffraction method.


The fatigue test was carried out for both the specimens before and after shot peening process for load 25N, numbers of cycles to failure were noted down.

The effect of surface enhancement method has improved the fatigue life of the welded specimen by 28.97% in the case of M.S-M.S welded joint and by 12.31 % in the case of MS-SS304L.

The effect of surface enhancement method on the surface hardness of the welded specimen are found to be increased by 17.46 % in the case of M.S-M.S welded joint and by 35.03% in the case of MS-SS304L

Conclusions


Scope for future work

Here, there are few research work are projecting as scope for future researches to improve welded joint fatigue life.

Improvement of fatigue strength of welded joint by using other surface enhancement process like burnishing process, surface grindings process, heat treatment process can be carried out and compare the fatigue strength obtained from the above mention process.

Improvement of the surface hardness and fatigue strength of welded joints by surface modification process such as shot peening process by using different shot material by varying peening condition.


References

1. Mark S. Molzen and Doug Hornbach, Evaluation of Welding Residual Stress Levels Through Shot Peening and Heat Treating.

2. Sylvain Chataigner, Lamine Dieng, Kevin Guit and Michel Grasset, Improving welded joint fatigue life using shot peening or grinding, France, Jan 2013.

3. M Hasegawa and H Suzuki, Improvement of the fatigue strength of aluminium alloy welded joints by high hardness and large specific gravity shot peening, Welding International, Volume 19, 2010.

4. Manoj Saini, Navneet Arora, Chandan Pandey and Husain Mehdi, Mechanical properties of bimetallic weld joint between sa 516 grade 65 carbon steel and SS 304 l for steam generator application, 2014.

5. Xiaohua Cheng1, Henry J. Prask, Thomas Gnaeupel Herold, Vladimir Luzin4, and John W. Fisher, Neutron Diffraction Measurements for Residual Stresses in AL-6XN Stainless Steel Welded Beams.

6. M.H. (Johnny) Johnson, Fatigue life improvement Techniques for welds, Process Barron Alabama.

7. Vinod M. Bansode and N. D. Misal, Fatigue life estimation of a butt welded joint by s-n approach, College of Engineering, Pandharpur, India, 2012.


References8. Baptista R., Infante V., and Branco C.M., Experimental and Numerical

Analysis of Fatigue Life Improvement Techniques in Welded Joints of Stainless Steels, Lisbon, Portugal.

9. B. Mvola, P. Kah and J. Martikainen, Dissimilar ferrous metal welding using advanced gas metal arc welding processes, Finland, 2014.

10. N. S. Rossini, M. Dassisti, K. Y. Benyounis and A. G. Olabi, Methods of Measuring Residual Stresses in Components, Ireland.11. Rafailov G., Sariel J., Dahan I., Reuven R., and Szanto M., Prediction and XRD measuring of residual stress in Machined welded parts, Beer-Sheva. 12. Y. Kudryavtsev and J. Kleiman, Fatigue of Welded Elements: Residual Stresses and Improvement Treatments, Canada.


Acknowledgements The journey of this research endeavour has been a great learning experience. We feel truly blessed to have had the support of many people, who have made this journey of research meaningful and worthwhile. We take this opportunity to express our gratitude to all those who helped us during the course of this work. We have extremely thankful to Prof. T.J. Ramamurthy, Director and Dr. K. Ranga, Dean, for constantly encouraging us to do our best. We express our heartfelt thanks to our beloved principal, Dr. M.S. Suresh for his encouragement all through our graduation life and providing us with the necessary infrastructure, without which we could not have demonstrated this project. We express our deep sense of gratitude to Dr. N.C. Mahendra Babu, Head of the Department, Dept. of Mechanical for his constant guidance and cooperation which consequently resulted in getting the project work completed successfully. We are thankful to Dr. Mukesh Patil, Project Coordinator, for his valuable guidance and support leading to the successful completion of the project. We are thankful to our project guide Mr. Harish A., Asst. Professor, Dept. of Mechanical, for extending his valuable insight and suggestions offered during the course of this project. He had a very important role in the successful completion of the project. We are indebted to the staff of the Department of Mechanical Engineering, BNM Institute of Technology for their sincere cooperation, encouragement and constructive criticism. We would also like to thank our non-teaching staff for their efforts to support us in our venture.


Thank You