The Effect of Residual Stress on Fatigue crack growth rate in AISI 304LN Stainless Steel

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ScienceDirect Materials Today: Proceedings 4 (2017) 677–686

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5th International Conference of Materials Processing and Characterization (ICMPC 2016)

The Effect of Residual Stress on Fatigue crack growth rate in AISI 304LN Stainless Steel Amit Prakasha,b *, H. N. Bara, S. Sivaprasada, S. Tarafdera, J. P. Dwivedib 0F

b

a Materials Science and Technology Division, CSIR – National Metallurgical Laboratory, Jamshedpur – 831007, India Department of Mechanical Engineering, Indian Institute of Technology (Banaras Hindu University), Varanasi-221005, India

Abstract Fatigue crack growth rate (FCGR) was studied in SMAT- exposed AISI 304LN stainless steel. Compact Tension (CT) specimens were taken for this study. Specimens were exposed to SMAT at various exposure timefor generating varying compressive residual stresses and FCGR study was done on those specimens at R=-1. It was seen that exposure to SMAT, produced microcracks within the material that increased the FCGR of the material with increased exposure time. ©2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference of Materials Processing and Characterization (ICMPC 2016). Keywords:fatigue crack growth rate (FCGR); surface mechanical attrition treatment (SMAT); residual stress; AISI 304LN

1. Introduction Fatigue cracking normally initiates at the surface and can be mitigated by inducing surface compressive residual stress. The magnitude of this type of residual stress and its distribution decides the extent of mitigation. Any residual stress relaxation in a component during operation, affects the desired benefits. Residual stresses can have a significant influence on the fatigue life of engineering components.Tensile mean stress should be avoided to improve fatigue resistance for those components. In particular, nearsurface tensile residual stresses tend to accelerate the initiation and growth phases of the fatigue process while compressive residual stresses close to a surface may prolong fatigue life. Residual stresses can be introduced unintentionally into components by, for example, forging,

* Corresponding author. Tel.: +91-657-234-5024; fax: +91-657-234-5213 E-mail address: [email protected] 2214-7853©2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference of Materials Processing and Characterization (ICMPC 2016).

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bending, welding, machining or heat treatment operations which may be employed during manufacture or by forces and thermal gradients imposed during use. They can also be generated deliberately by such processes as autofrettage, shot peening, laser shock peening, low plasticity burnishing and a variety of surface treatments. During the service life of a component, initial residual stress field induced by the manufacturing process may change, causing relaxation and redistribution due to a variety of mechanisms. Exposure to elevated temperatures and repeated cyclic loading can also cause gradual changes in the residual stresses over time, even if no single fatigue cycle induces local yielding. The number of cycles required to form fatigue micro-cracks depends on the amplitude of alternating stress and not the mean stress, while the fatigue growth rates of the cracks depend on both the stress amplitude and mean stress. This indicates that residual stresses have a significant influence on fatigue crack growth and relatively little influence on fatigue crack nucleation. Additional applied tensile stresses near the surface are concentrated by scratches, notches and sharp changes in cross-section etc. and a crack may be initiated. Hence, residual stresses needs due consideration. A crack will not initiate unless and until the applied load overcomes the compressive surface residual stresses which is a highly favourable condition. The effect of residual stress on fatigue crack propagation is of great practical significance and has been the focus of much research studies. W. Z. Zhuang et al. [1] proposed an elasto-plastic finite element analytical model for the estimation of residual stress relaxation during cyclic loading considering the magnitude and distribution of the residual stress, the degree of cold working required, the applied alternating and mean stresses, and the number of applied loading cycles. J. E. LaRue et al.[2] determined the crack opening stress during crack propagation through the initially stressed aluminium alloy using finite element analysis, from which the effective stress intensity factor range ∆Keff and the fatigue crack growth were predicted. Experimental investigations were done by I. Cerny et al. [3] on 2.4 mm thick aircraft V-95 Al-alloy sheets (a type of a 7075 alloy), clad with a 7072 Al-alloy (Al–Zn1) to see the effect of severe shot peening on fatigue crack growth. D. Kirk [4] used the X-ray diffractometry to examine the variation of residual stress across shot peened surfaces, the transformation of austenite to martensite on peening and the decay of surface residual stress during fatigue testing of peened specimens. It was also inferred that masking gives rise to tensile surface residual stresses adjacent to the peened area. The compressive residual stress and fatigue strength of AISI 304 austenitic stainless steel was evaluated at various shot peening conditions by L. Singh et al. [5]. Later, in one of their work [6], regression analysis of shot peening process for performance characteristics of AISI 304 austenitic steel was done using astatistical tool, MINITAB 14. Works have also been reported by various researchers to study the effect of surface mechanical attrition treatment (SMAT) on the properties of material. T. Balusamy et al. [7] studied the influence of SMAT on the corrosion behavior of AISI 304 stainless steel. They also studied [8] the effect of SMAT on the mechanical and electrochemical properties of Pb-Sn alloy. T. Roland et al.[9] investigated the effect of a nanocrystalline surface layer on the fatigue behavior of a 316L stainless steel. It was shown that the mechanical properties after SMAT could be improved significantly by the use of a short post-annealing treatment which caused a recovery at the grain boundaries leading to a reduction of the internal stress. Residual stress analysis was also done on iron and steel after SMAT by Nishanth S. Prabhu et al. [10] by Magnetic Barkhausen emission technique. In this work, an attempt has been made to study the effect of residual stresses induced by surface mechanical attrition treatment (SMAT) on fatigue crack growth rate behaviour of AISI 304LN Stainless steel compact-tension (CT) specimens. 2. Basic Theory 2.1. Fatigue crack growth rate Fatigue is the weakening of a material caused by repeatedly applied loads. It is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values that cause such damage may be much less than the strength of the material typically quoted as the ultimate tensile stress limit or the yield stress limit. Microscopic cracks will begin to form at the stress concentrators such as persistent slip bands (PSBs), grain interfaces etc.,when the loads are above a certain threshold value. A crack, after reaching a critical size, will propagate suddenly, and the structure will fail without any prior indication. Some macroscopic irregularities like holes, sharp corners etc. will also lead to elevated local stresses where fatigue cracks

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can initiate.The thickness of the CT specimen for FCGR study may be varied but it should be able to withstand buckling effects. Compressive stresses have great impact on Fatigue crack growth rates especially at low intensity factors and low stress ratios. Under fatigue, a crack can propagate at a stress level well below the critical stress value. This crack growth phenomenon is also called “Subcritical crack growth”. Fatigue analysis based on Mechanics of materials approach: Number of cycles required for failure is a function of the stress difference (S) between the maximum and minimum stress levels (S =  = max - min) and the average stress level av . There is a threshold value of , below which fatigue phenomenon is not observed. This threshold value for a given material, depends on a number of parameters including surface roughness of the body and surrounding environment. Fatigue analysis based on Fracture mechanics approach: The work done by Paris and Erdogan in 1960, is the basis of the theory of fatigue crack growth. When a crack length remains constant, stress intensity factor (K) oscillates between Kmax& Kmin with an average value of Km = Kmean =  



 . When crack starts to propagate, values of

&

,

&Km start to vary with time, even when values of

remain unchanged.

In fatigue crack growth analysis, crack growth rate is assumed to be a function of K =  

-

and Km =

. = f (K, Km).

i.e.

(1)

where ‘a’ is the crack length and ‘N’ is the number of load cycles. In the log-log scale, the crack growth rate is linearly dependent on K. Thus equation (1) takes the form: log

= log C + m.log (K)

(2)

In equation (2), log C is the y- intercept of the straight line variation of log against log(K) and “m” is the slope of this straight line (as shown in figure1). Values “C” and “m” depend on the mean stress intensity factor Km. From equation (2), log

= log C + m.log (K) = log C + log (K)m

log 

= log [C(K)m] = C(K)m.

“C’ & “m” here, are material properties. When crack length “a” is in inches, unit of K is KSi√ .

(3) (4)

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At low K value, crack propagation is extremely slow. A growing crack of some length has to be arrested by gradually decreasing K until below the threshold. Example, by decreasing the stress amplitude. Crack growth rate is closely related to the crack tip opening displacement. i.e.

= C.

or,

=C



(5)

 

where E is Young’s modulus &

(6) is the cyclic yield stress.

Paris law can be used to quantify the residual life (in terms of load cycles) of a specimen given a particular crack size. Defining the crack intensity factor as: K = Y√

(7)

where,  is a uniform tensile stress perpendicular to the crack plane and Y is a dimensionless parameter that depends on the geometry, the range of the stress intensity factor follows as: K = Y√ (8) where,  is the range of cyclic stress amplitude. Figure 1 shows the schematic plot of the typical relationship between the crack growth rate and the range of the stress intensity factor.

Fig. 1. Schematic plot of the typical relationship between the crack growth rate andthe range of the stress intensity factor.

2.2. Surface mechanical attrition treatment (SMAT) It is a surface severe plastic deformation (S2PD) process which effectively induces localized plastic deformation which results in grain refinement down to nanometer scale. It is a method which enables surface nanostructuring, grain refinement and enhancement of hardness and corrosion resistance of metallic materials. SMAT produces large compressive residual stress and to greater depth compared to conventional shot peening. 3. Experimentations 3.1. Material The material selected for this investigation is a grade AISI 304LN stainless steel. It is used in the fabrication of primary heat transport (PHT) piping of the advanced heavy water reactors (AHWR) of nuclear power plants. 304LN SS is known to be strain rate sensitive even at ambient temperature.

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Table 1 Chemical composition of the investigated steel (in wt %) Steel

C

Si

Mn

P

S

Cr

Ni

N

Fe

304LN

0.03

0.54

1.8

0.028

0.014

18.55

9.50

0.08

Bal

The chemical composition (weight %) of the experimental material measured by Optical Emission Spectrograph is given in Table 1. 3.2. Hardness Measurement Macro-hardness (Vickers) measurement was done on surfaces of the specimens with the help of a universal hardness tester (Make: Reicherter Stiefelmayer, Model: UH-3) using a load of 30 kgf and dwell time of 10 secs. Ten indentations were taken to estimate the average hardness values. The average hardness value of the used samples was found to be 170 Hv. Micro hardness measurements were taken on Virgin, 15 minutes and 30 minutes SMAT exposed samples. The equipment used was a Vickers micro hardness tester (OmniTech, Pune, Model MVH-AUTO). Indentation load was selected as 300 grams with a dwell time of 10 seconds. Some measurements were taken on the SMAT exposed surface and some on its cross section so as to see the effect of SMAT on surface and subsurface area. 3.3. Specimen Preparation 12 nos. of full CT specimens were prepared. Initially the sample thickness was maintained as 20 mm. But later it was decided to reduce the thickness to 6 mm keeping in view the fact that the effect of SMAT would not be much deeper from the surface. The dimensions of the 20mm thick CT specimen and a SMAT exposed specimen are shown in Figure2 (a).

(a)

(b)

Fig.2.CT specimen (a) showingvarious dimensions (b) SMAT exposed sample (30 min.)

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3.4. Surface mechanical attrition treatment (SMAT) SMAT was performed using the Surface nano crystallization equipment (Model: SNC1, Chengdu SNC Advanced Technology Co. Ltd., Chengdu, China). 5 mm dia. balls of SS 316L were used. SMAT was done for 15 & 30 minutes on both sides of the specimens. The distance between the sample surface and the balls were maintained at 30-35 mm. The sample pot was evacuated during the entire period of treatment. Frequency of vibration of the machine was set to 50 Hz. Templates were made to mask the samples so that the SMAT exposure is restricted to the desired portion only as shown in Figure 2 (b). 3.5. Residual stress measurement Portable X ray stress analyzer, Model: XStress 3000, Finland was used to measure the residual stresses induced into the material surface by SMAT. Test parameters were taken as follows: diameter of Collimator: 3 mm, Voltage: 25kV, Current: 5mA, Exposure time: 25 sec. and Source used was Cr Kb. 3.6. FCGR Test (As per ASTM Designation E647 - 08) This test determines fatigue crack growth rate from near-threshold to Kmaxcontrolled instability.FCGR expressed as a function of crack tip stress intensity factor range, da/dN versus ΔK, tells about a material’s resistance to stable crack extension, under cyclic loading. FCGR’s are functions of ΔK and stress ratio (R) and are used in design & evaluation of structural components.Tests were carried out based on “K decreasing” by shedding force by a series of decremental steps as the crack grows. Initial ΔK was selected as 25 MPa √m.Notched specimens were loaded in cycles with 15 Hz frequency. Crack size was measured along with the number of fatigue cycles. Data obtained were subjected to numerical analysis to get the test results. Fatigue machine (Model: 8501, Make: Instron) as shown in Figure 3, was used to generate the FCGR curves.

Fig. 3. FCGR test on Fatigue testing machine Instron 8501

3.7. Optical/SEM images Small specimens were cut from virgin material, SMAT exposedsamples for 15& 30 min. They were polished using emery papers from Grit size 80 to 1500. Finally, cloth polishing was done. The samples were etched with nitro hydrochloric acid (aqua regia), which wasmade by mixing concentrated nitric acid and hydrochloric acid in a volume ratio of 1:3. Optical images (cross section) were captured using Leica DFC 295 for the three samples. Morphology of thesespecimens were also studied using scanning electron microscope (SEM) [Make: Jeol, Japan; Model-JSM-840A].

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4. Results and Discussion Figure 4shows the optical micrographs of the virgin as well as SMAT exposed specimens. All the images were taken from the cross-section of the specimens. Additional twinning may be seen in the images although the extent of twinning is same for both the SMAT exposed specimens for 15 & 30 mins. This twinning might be due to the plastic deformation occurred during SMAT.

(a)

(b)

(c)

Fig. 4. Optical images (a) Virgin; (b) SMAT exposed for 15 mins; (c) SMAT exposed for 30 mins

Table 2 Residual stress values (MPa); (a) Virgin; (b) SMAT exposed for 15 minutes; (c) SMAT exposed for 30 minutes (a) Residual stress (MPa) Side A

Residual Stress (MPa) Side B

(b)

1

2.1 ± 13

2

Position

Residual stress (MPa) Side A

Residual stress (MPa) Side B

23.1 ± 11.2

1

-197.4 ± 21.5

-179.9 ± 11.5

19.8 ± 7.2

1.2 ± 2.3

2

-326.8 ± 18

-314.2 ± 22.5

3

21.2 ± 5.1

29.2 ± 9.4

3

-290.4 ± 15.1

-360.6 ± 20.1

4

-8 ± 6.3

-9.1 ± 6.3

4

-307 ± 4.2

-312.7 ± 14.5

5

12.5 ± 9.9

76.3 ± 4.4

5

-320.5 ± 21.4

-335.4 ± 2.2

6

-3.6 ± 1.5

-90.3 ± 5.5

6

-290.3 ± 14.6

-272.9 ± 7.9

7

43.3 ± 12.4

-37.5 ± 7.8

7

-307.5 ± 20.8

-332.9 ± 14.4

Position

(c) Residual stress (MPa) Side A

Residual stress (MPa) Side B

1

-285.6 ± 14.9

-233.9 ± 11.8

2

-418.7 ± 16.2

-388.1 ± 14.6

3

-395.3 ± 22.2

-402.6 ± 12.4

4

-382.4 ± 21.6

-413.8 ±7.5

5

-334.3 ± 12.3

-389.4 ±22.9

6

-404.2 ±10.5

-406.9 ±18.7

7

-392.6 ± 19.7

-413.3 ± 9.4

Position

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FCGR tests were carried out as per ASTM standard E 647. Tests were done with decreasing K values. The cyclic loading was started with K = 25 MPa √ . Higher values may have been chosen to have higher crack propagation rate but it might have produced some undesirable effects. Sudden jerk or load might have caused the crack to propagate at higher rate which would have been detrimental for the test. CT specimens were SMAT exposed 3-5 mm away from the notched tip as shown in figure 2(b)so that it did not get damaged. Table 2 shows the values of residual stress in virgin specimen and that generated after SMAT exposure on both the faces (named as Side A &B) of the CT specimen. Figure 5shows the variation of micro-hardness values along the depth of the specimen. Higher hardness values can be seen on the surface of the specimen and it reduces along the depth.304LN stainless steel being highly ductile (70% approx.), shows significant plasticity. So enough time is available to stabilize the plastic zone, in front of the crack tip. Plastic zone arrests the rate of crack growth.

Micro-Hardness vs. Surface Depth

Micro-Hardness (Hv)

400 350

SMAT Exposed (30 mins)

300

SMAT exposed (15 mins)

250 200 150 100 50 0 0

0.2

0.4

0.6

0.8

1

Surface Depth (mm)

Fig. 5. Graph showing the variation of micro-hardness along the depth on the SMAT exposed specimens

Different CT specimens with different time of exposure, which have been tested for crack initiation and propagation, have been shown in figure6. Values of slopes “m” for FCGR curves have been tabulated separately as in Table no. 3. (a)

(b)

(c)

Fig. 6. FCGR curves for CT specimens (a) virgin; (b) SMAT exposed (15 mins); (c) SMAT exposed (30 mins)

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Table 3 Values of slopes (m) of various FCGR tests Specimen

Slope (m)

Virgin

3.66478

SMAT for 15 mins

4.23878

SMAT for 30 mins

4.77201

The slopes, in case of 15 and 30 minutes SMAT exposed samples, show that the fatigue crack growth rates have increased with the time of exposure, which is not desirable. This might be due to the use of larger ball size of 5 mm diameter with some dent marks present on some of the used balls. This may have led to stress concentration at some places of the SMAT exposed area which further led to formation of micro cracks as revealed by SEM images as shown in Figure 7. (a)

(b)

(c)

Fig. 7. SEM images showing the generated micro-cracks after SMAT (a) Virgin; (b) SMAT exposed (15 mins); (c) SMAT exposed (30 mins)

5. Conclusion It is concluded that the compressive residual stresses were induced in CT specimens made of AISI 304LN stainless steel due to surface mechanical attrition treatment (SMAT). However, SMAT parameters must be optimized to ensure that beneficial microstructural changes take place on the surface of the treated material. The surface and subsurface hardness increased, may be due to work hardening by SMAT. A hard and comparatively brittle layer is formed on the surface due to SMAT and the Fatigue crack growth rates varied in different specimens with different exposure time. As the exposure time is increased, compressive residual stresses are increased along with the hardness. Micro cracks developed which resulted in high fatigue crack growth rate in CT specimens exposed for longer duration. Acknowledgements I am grateful to Dr. S. Srikanth, Director, CSIR- National Metallurgical Laboratory, Jamshedpur, Jharkhand, for providing the necessary Laboratory facilities.Special appreciation goes to Prof. A. K. Agrawal, HOD – Department of Mechanical Engineering, IIT (BHU), Varanasi, for his support and guidance.

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References [1] Wyman Z. Zhuang, Gary R. Halford, Investigation of residual stress relaxation under cyclic load, International Journal of Fatigue, Vol 23, 2001, S31–S37. [2] J.E. LaRue, S.R. Daniewicz, Predicting the effect of residual stress on fatigue crack growth, International Journal of Fatigue, Vol 29, 2007, 508–515. [3]Ivo Cerny, Jiri Sis, Dagmar Mikulova, Short fatigue crack growth in an aircraft Al-alloy of a 7075 type after shot peening, Surface & Coatings Technology Vol 243, 2014, 20-27. [4] D. Kirk, Residual stresses and retained austenite in shot peened steels, Materials and Energy Science Dept., Coventry Polytechnic, UK. [5] L. Singh, R.A. Khan, M.L. Aggarwal, Influence of Residual Stress on Fatigue Design of AISI 304 Stainless Steel, The Journal of Engineering Research, Vol. 8 No. 1, 2011, 44-52. [6] Lakhwinder Singh, R.A. Khan, M.L. Aggarwal, Regression analysis of shot peening process for performance characteristics of AISI 304 austenitic stainless steel, International Journal of Modern Engineering Research (IJMER), Vol. 3, Issue. 5, 2013. [7] T. Balusamy, T.S.N. Sankara Narayanan, K. Ravichandran, Il Song Park, Min Ho Lee, Influence of surface mechanical attrition treatment (SMAT) on the corrosion behaviour of AISI 304 stainless steel, Corrosion Science, Vol 74, 2013, 332–344. [8] T. Balusamy, M. Venkateswarlu, K.S.N. Murthy, S. Vijayanand, T.S.N. Sankara Narayanan, Effect of Surface Mechanical Attrition Treatment (SMAT) on the Surface and Electrochemical Characteristics of Pb-Sn Alloy, Int. J. Electrochem. Sci., Vol 9, 2014, 96 – 108. [9] T. Roland, D. Retraint, K. Lu, J. Lu, Fatigue life improvement through surface nanostructuring of stainless steel by means of surface mechanical attrition treatment, Scripta Materialia, Vol 54, 2006, 1949–1954. [10] Nishanth S. Prabhu, J. Joseyphus, T. S. N. Sankaranarayanan, B. Ravi Kumar, Amitava Mitra, A. K. Panda, Residual Stress Analysis in Surface Mechanical Attrition Treated (SMAT) Iron and Steel Component Materials by Magnetic Barkhausen EmissionTechnique, IEEE Transactions onMagnetics, Vol. 48, No. 12, 2012. [11] E. Macheranch and K. H. Kloos, Origin, measurements and evaluation of residual stresses, Residual Stresses Sci. Technol., DGM Inform, 1987, 3–26. [12] Sandeep Dwivedi, Radha Krishna Lal, V. K. Choubey, J. P. Dwivedi, V. P. Singh, S. K Shah, Study of Residual stresses in I sectioned bars of Non-Linear work hardening materials under torsion, Materials Today Proceedings, Vol. 2, 2015, 2046-2055. [13] I. V. Singh, B. K. Mishra, Sachin Kumar, A. S. Shedbale, Nonlinear Fatigue crack growth analysis of a center crack plate by XFEM, Advanced materials manufacturing & characterization, Vol.4, Issue 1, 2014. [14] P. J. Withers and H. K. D. H. Bhadeshia, Residual stress part 2—Nature and origins, Materials Sci. Technol., Vol. 17, 2001, 366–375. [15] D. Dye, H. J. Stone, R. C. Reed, Intergranular and interphase microstress, Curr. Opin. Solid State Mater. Sci., Vol. 5, 2000, p. 31. [16] I. C. Noyan, J. B. Cohen, Residual stress Measurement by diffraction and interpretation. [17] A. J. Bottger, R. Delhez, E. J. Mittemeijer, Proceedings of 5th European Conference on Residual stresses. [18] R. C. McClung, A literature survey on the stability and significance of residual stresses during fatigue, 2007. [19] ASTM designation E-647, Standard test method for measurement of Fatigue crack growth rates. [20] T. Kundu, Fundamentals of fracture Mechanics. [21] David Broek, Elementary Engineering Fracture Mechanics.