Analysis of the Effects of Surface Preparation on Fracture Crack Propagation

Transcription

1 Analysis of the Effects of Surface Preparation on Fracture Crack Propagation by Timothy George Andrews A Project Submitted to the Graduate Faculty of Rensselaer Polytechnic Institute in Partial Fulfillment of the Requirements for the degree of MASTER OF ENGINEERING SCIENCE Approved: Balkrishna Annigeri, Project Advisor Ernesto Gutierrez-Miravete, Project Advisor Rensselaer Polytechnic Institute Hartford, Connecticut April, 2014 (For Graduation May 2014) 1

2 . Copyright 2014 by Timothy George Andrews All Rights Reserved ii

3 CONTENTS LIST OF TABLES... iv LIST OF FIGURES... v LIST OF SYMBOLS... vii ACKNOWLEDGMENT... viii ABSTRACT... ix 1. Introduction Descriptions of Methods of Applying Residual Stress Shot Peening Laser Shock Peening Cold Working Holes Analysis of Surface Treatment Effect on Crack Growth Analysis Method Shot Peening Analysis Laser Shock Peening Cold Hole Working Analysis Conclusions References iii

4 LIST OF TABLES Table 1. NASGRO and FRANC3D stress intensity results for shot peen analysis Table 2. NASGRO and FRANC3D Stress intensity results for laser peen analysis iv

5 LIST OF FIGURES Figure 1. Schematic of Residual Stress and Operating Tensile Stress Figure 2. Schematic of Shot Peening Operation Figure 3. Residual Stress from a Shot Peening Operation in 2024 Aluminum, based on experimental data. [1]... 3 Figure 4. Schematic of Laser Shock Peening Figure 5. Residual Stress in Laser Shock Peened IN718. [2]... 5 Figure 6. Residual Stress in a 4% cold worked hole in 5083 Aluminum. [3]... 7 Figure 7. Graphical Representation of the Residual Stress around a Cold-Worked Hole. [3]... 7 Figure 8. Geometry of Cracks used in NASGRO for Shot Peen, and Laser Peen Analysis Figure 9. Stress Applied in Shot Peening Model Figure 10. Crack growth in unpeened Al 2024, comparing NASGRO analysis with test data. [7] Figure 11. Crack growth in shot peened Al 2024, comparing NASGRO analysis with test data. [7] Figure 12. FRANC3D analysis of a x0.200 corner crack Figure 13. FRANC3D Analysis of Shot Peened Al Figure 14. Stress Applied in Laser Peening Model Figure 15. Crack growth in laser peened Al 2024, comparing NASGRO analysis with test data from Everett. [7] Figure 16. Stress intensity in laser peened Al 2024 FRANC3D analysis Figure 17. Crack growth analysis using 58% of published residual stress Figure 18. Crack growth analysis results using FRANC3D with 58% of published residual stress Figure 19. FRANC3D analysis of a 0.050"x0.200" crack in a laser peened specimen with 58% of published residual stress Figure 20. ANSYS model of LPB test specimen Figure 21. Applied and resultant stress in specimen Figure 22. Crack propagation of cold worked holes. [3] v

6 Figure 23. Crack propagation of cold worked holes, using 75% of residual stress from [3] vi

7 LIST OF SYMBOLS Symbol Units Description a inch Crack Depth C inch -1 Empirically derived material property in Forman s model c inch Crack half-length f Crack opening function in Forman s model K KSI-root-inch Stress intensity ΔK KSI-root-inch Stress intensity range ΔKth KSI-root-inch Threshold stress intensity Kc Kmax N n P q R KSI-root-inch Critical stress intensity at which crack becomes unstable KSI-root-inch Maximum stress intensity during the load cycle Number of elapsed stress cycles Empirically derived material property in Forman s model Empirically derived material property in Forman s model Empirically derived material property in Forman s model Stress ratio (σmin/σmax) σ KSI Stress vii

8 ACKNOWLEDGMENT I would like to thank Dr. Balkrishna Annigeri for his guidance and patience. I also thank Lars Vestergaard for his assistance and training in the use of NASGRO and Dr. Ernesto Gutierrez-Miravete for his assistance. I thank my wife, Sandra Andrews, for her patience and encouragement. Finally, I thank United Technologies Corporation for their assistance throughout my graduate program. viii

9 ABSTRACT Shot peening and other forms of surface treatment can beneficially affect the crack propagation properties of materials used in aerospace applications. Shot peening has been used for many decades as the most common method of surface treatment, but other methods include laser peening and cold working of holes. These methods all work by applying a layer of compressive stress on the surface, which acts to counter applied tensile stress. The magnitude and depth of the compressive stress layers are publicly available for many materials in the literature. The effect on crack growth can be analyzed using commercially available crack growth analysis software, such as NASGRO and FRANC3D. The analysis of the crack growth is done by using superposition to add the residual stress in the surface layer to an applied stress. The analysis results are compared to test results in the literature. The analytical results did not match the test data well enough for engineering purposes for the surface treatments using simple NASGRO models. The possible reasons for this are that the material properties in the software code might not match exactly with the test specimens, and that the crack propagation might not be semi-elliptical as the NASGRO code assumes. In order to match analytical results to the test data, more analysis was required, using FRANC3D to determine the stress intensity around the crack front. This resulted in a better match with the shape of the crack curve, but did not accurately predict the crack growth lives within reasonable margins of error. To better model the crack growth lives, further analysis was done using reduced residual stress, below the residual stresses documented in the literature. This analysis showed much closer match with test data. ix

10 1. Introduction Cracks in aircraft parts can lead to catastrophic failures, and therefore it is important to reduce crack growth. In order to do this, there are several methods of applying residual compressive stress to the surfaces of parts where cracks may form, including the following: 1. Shot peening 2. Laser shock peening 3. Cold hole working These methods all apply forces to the surfaces of metal parts in order to cause a compressive plastic stress. The compressive stress acts against applied tensile stress occurring in operation. Because the plastic stress partially offsets the tensile operating stress, the crack experiences lower effective tensile stress and grows less quickly, resulting in longer part life. This is shown schematically in Figure 1. Figure 1. Schematic of Residual Stress and Operating Tensile Stress. 1

11 2. Descriptions of Methods of Applying Residual Stress 2.1 Shot Peening Shot peening bombards a surface of a part with small spherical shot made of metal, glass, or ceramic materials, typically in the size range of to (0.1 mm to 1.0 mm) [1]. These particles hit the surface hard enough to induce a compressive stress to the surface. The depth of the compressive stress layer is typically about This depth is enough to significantly increase the fatigue life of the part, and to slow the growth of shallow cracks that are formed. Below the layer of compressive stress, there is a layer of residual tensile stress, which is caused by the need to balance the layer of compressive stress. A schematic of the shot peening process is shown in Figure 2. A typical residual stress pattern in 2024 Aluminum plate caused by shot peening is shown in Figure 3, from Ludian et al. [1]. Figure 2. Schematic of Shot Peening Operation. 2

12 Figure 3. Residual Stress from a Shot Peening Operation in 2024 Aluminum, based on experimental data. [1] Shot peening can be done at different intensities. The plot in Figure 3 shows residual stress parallel to the treated surface three different levels: 0.1T coverage, 1T coverage, and 4T coverage. Coverage at 1.0T refers to peening for the amount of time needed to cover the entire specimen. Coverage at 0.1T refers to peening for 0.1 times the amount of time needed to cover the entire specimen, and 4T coverage is four times the amount of time needed to cover the entire specimen. The plot shows that the specimen can be peened with significant residual compressive stress at one tenth the time required for full coverage, which can result in significant cost savings while still retaining much of the advantage of the surface treatment. Shot peening is the most common and least expensive process for surface treatment. This process has been used in industry for decades and is well understood. The process has some imprecision due to the randomness of the location of the impacts of the shot. 3

13 Because of this, there is some possibility that some areas will have more residual stress than other areas, and thus the process will not be as repeatable and predictable as other forms of applying residual stresses. 2.2 Laser Shock Peening Laser shock peening is a process that applies a compressive stress layer much deeper than shot peening [2]. In this process, an ablative layer is applied to the surface of the part. This layer is usually black paint or black tape. A layer of water is run over the black ablative layer. A high energy laser is aimed at the part, and the ablative layer absorbs the energy of the laser, heating the water into plasma. The beam is sent in pulses of tens of nanoseconds, creating rapid expansion of the ablative layer. This rapid explosion creates a shock wave that runs through the part, and this creates a compressive stress layer, analogous to shot peening, but penetrating deeper into the material. The layer of water contains the explosion and forces the shock wave into the part. A schematic of laser shot peening is shown in Figure 4. Laser peening creates a compressive layer about four times deeper than shot peening. This makes it much more effective at preventing crack growth. Because shot peening creates a compressive layer only about deep, shot peening will greatly improve the resistance to fatigue cracks forming, and will reduce the growth of very shallow cracks. However, once a crack grows deeper than the compressive layer, the shot peening process leads to increased stresses and increased crack growth rates, because the tensile layer below the compressive layer increases stress, and accelerates crack growth. Laser shock peening creates a much deeper compressive layer, so cracks up to about are slowed due to the compressive layer. Typical residual stress parallel to the treated surface for laser shock peened 2024 Aluminum, from Dorman et al. [2] is shown in Figure 5. 4

14 Residual Stress (KSI) Figure 4. Schematic of Laser Shock Peening. 0-5 Residual Stress in Laser Peened Al Residual Stress Depth (inch) Figure 5. Residual Stress in Laser Shock Peened IN718. [2] 5

15 The laser shock peening process is more expensive and much less widely used than shot peening. The process is slower and not as well understood. However, lasers can be pointed accurately to ensure full coverage and is more predictable than shot peening. 2.3 Cold Working Holes Cold working is a process for strengthening the area around holes. This process uses a split sleeve that is inserted into an existing hole. The split sleeve pushes against the sides of the hole, and expands the size of the hole, typically about 2% to 4% expansion. As the hole diameter is increased, the metal surrounding the hole is deformed plastically. This deformation results in a compressive stress layer around the hole that will resist fatigue, and reduce the growth of any cracks that form. The depth of the compressive layer is typically on the order of about A typical residual stress profile in the circumferential direction for a cold-worked hole from Pasta [3] is shown in Figure 6. A graphical representation of the stress around the hole is shown in Figure 7. A compressive stress around holes is valuable because there is often high stress on the surface of holes. Like other forms of surface treatment, cold working holes results in a tensile residual stress deep into the part. Because stresses in operation of aircraft parts often are very high around the surface of holes, and the stresses die out quickly a short distance from the holes, cold-working holes offers a good solution to fatigue and crack propagation. Thus the residual compressive stress from the cold working exists in the locations of high operating stress, which results in significantly increased fatigue and crack propagation life. 6

16 Figure 6. Residual Stress in a 4% cold worked hole in 5083 Aluminum. [3] Figure 7. Graphical Representation of the Residual Stress around a Cold-Worked Hole. [3] 7

17 3. Analysis of Surface Treatment Effect on Crack Growth 3.1 Analysis Method For the analysis portion of this project, the different methods of surface treatment were analyzed using the NASGRO code version 6.2 [4]. Residual stress fields were found in existing literature, based on test data. These residual stress fields were overlaid on the applied stress to determine the resultant stress. This stress was mapped onto an analytical model and run through a NASGRO analysis with a starting crack size in order to determine the crack propagation life until failure. This crack life was then compared against crack growth data in existing literature. NASGRO is a commercially-available code that uses linear elastic fracture mechanics (LEFM) to analyze crack propagation. LEFM uses the elastic stress and crack length to determine the rate of crack growth. The method was first developed in 1921 by A. A. Griffith [5], who observed that the parameter needed for a crack to grow in glass fibers was proportional to the stress multiplied by the square root of the crack depth. This method was later improved by G. R. Irwin, who developed the stress intensity factor [5]: (1) Where Y is a geometry factor When the stress intensity factor is greater than the material s threshold of crack propagation, the crack grows, until the stress intensity equals the material s fracture toughness, at which point the crack becomes unstable and failure occurs. LEFM can be used to determine the rate of growth of a crack using the linear elastic stress and the length of the crack. NASGRO uses this method by determining the crack growth for every stress cycle, and repeating until the crack becomes unstable, or until the crack no longer grows, or until the predetermined number of cycles has been completed. 8

18 The rate of crack growth is determined in NASGRO using Forman s model. NASGRO calculates the crack growth rate as [4]: (2) The values of C, n, P, and q are found in commercially available software such as NASGRO for many commonly used materials. The analysis in this project will use the available libraries in NASGRO to compare with test data found in the literature. In commercial applications, a rigorous testing process would be needed to determine these parameters before an analysis system can be useful for product development. In order to better correlate the analysis to test, the FRANC3D code version 6.0 [6] was also used. For this study, FRANC3D was used to model a crack using finite element modeling in order to determine the stress intensity. This can be more accurate than NASGRO because FRANC3D can determine stress intensity along the crack front, rather than just at the crack tip and the crack depth, as NASGRO does. 3.2 Shot Peening Analysis A crack propagation analysis was done to analyze the effect of shot peening. This analysis was then compared to test data found in the literature; test data was found for Al 2024 in Everett et al. [7]. Analysis was done in NASGRO for initial crack length. The analysis was done in NASGRO using Al 2024 (NASGRO material ID M2EI11AA11). The geometry of the specimen was a solid block; dimensions are shown in Figure 8. The loading on the specimen was in tension, using a cyclic load with a maximum applied stress of 13.3 KSI, and a minimum applied stress of zero. The stresses applied to the NASGRO model are shown in Figure 9. 9

19 Figure 8. Geometry of Cracks used in NASGRO for Shot Peen, and Laser Peen Analysis. The analysis results were then compared to test data from [7]. Crack growth results of the analysis are shown in Figure 10 for unpeened specimens and in Figure 11 for peened specimens. This resulted in a close match between the NASGRO analysis and the test data for the unpeened specimens. The difference between the analytical results and the test data could be due to differences in material properties between the analytical model and the test specimens. The correlation with the test data for the peened specimens was not close. In both the test data and the analysis, the shot peened specimens showed significantly more resistance to crack propagation than did the unpeened specimens. 10

20 Crack Depth (inch) Residual Stress (KSI) Stress Applied in Shot Peening Model Residual Stress Applied Stress Resultant Stress Depth (inch) Figure 9. Stress Applied in Shot Peening Model NASGRO Analysis of Unpeened Al NASGRO Analysis Test Data Cycles Figure 10. Crack growth in unpeened Al 2024, comparing NASGRO analysis with test data. [7] 11

21 Crack Depth (inch) NASGRO Analysis of Shot Peened Al Cycles NASGRO Analysis Test Data Figure 11. Crack growth in shot peened Al 2024, comparing NASGRO analysis with test data. [7] Because the correlation between the NASGRO analysis and the test data did not match within reasonable amounts, a different approach was needed. This approach was to use FRANC3D to more accurately determine stress intensity. Using FRANC3D, a model was created of the cross-section of the specimen, with four crack models of sizes between x0.050 and x FRANC3D calculated the stress intensity factors for each crack size. The model and the stress intensity for a crack x0.200 is shown in Figure 12. The plot on the right shows the stress intensity from the shotpeened surface (labeled as A on the plot) to the depth of the crack (labeled as B on the plot). 12

22 Figure 12. FRANC3D analysis of a x0.200 corner crack. For the x0.200 crack size, FRANC3D showed K(a)=4.132 KSI-root-inch, and NASGRO showed K(a)=3.226 KSI-root-inch. The maximum stress intensity of KSI-root-inch was not at a surface, but in the interior of the specimen. This indicates that the crack would not grow as a quarter-elliptical crack, as NASGRO would assume, but rather would grow into the interior of the specimen. Table 1 shows the values of K(a) and K(max) for the analyzed crack sizes from the FRANC3D analysis, and K(a) from the NASGRO analysis. The table shows that the maximum stress intensity is close to or equal the K(a) value for the smaller crack sizes, but for deeper cracks, the maximum stress intensity is much larger than the K(a) value. Crack Size K(a) NASGRO K(a) FRANC3D K(max) FRANC3D.050 x x x x Table 1. NASGRO and FRANC3D stress intensity results for shot peen analysis. 13

23 Crack Depth (inch) Using the maximum FRANC3D stress intensity values from Table 1, the crack growth rate was calculated by scaling the crack growth rate with the K(max) results from the FRANC3D results to the K(a) values from the NASGRO results. The results of this analysis are shown in Figure 13. The shape of the crack growth curve using the maximum stress intensity values closely matched the shape of the test data. The likely reason for the difference between the NASGRO results and the FRANC3D results is that the cracks in the test specimens do not likely grow as quarter-elliptical cracks, as NASGRO assumes, but rather once the cracks grow below the compressive surface layer, the cracks grow further into the interior of the specimen. The magnitude of the crack growth using the FRANC3D, however, does not match the test data. The difference in the magnitude of the crack growth may be due to variation in the material properties between the test specimens and the analytical material model, and also due to differences in residual stresses between those in the analytical models, measured by Ludian et al. [1], and the test specimens used by Everett et al. [7]. NASGRO and FRANC3D Analysis of Shot Peened Al Cycles NASGRO Analysis Test Data FRANC3D Analysis using Kmax Figure 13. FRANC3D Analysis of Shot Peened Al

24 Residual Stress (KSI) 3.3 Laser Shock Peening Laser shock peening analysis was done using the same methodology, specimen size, and loading as the shot peening analysis in the previous section. The residual, applied, and resultant stresses used in this model are shown in Figure 14. The results comparing the test data and the NASGRO analysis are shown in Figure 15. Stress Applied in Laser Peening Model Depth (inch) Residual Stress Applied Stress Resultant Stress Figure 14. Stress Applied in Laser Peening Model. The NASGRO analysis showed no crack growth. The maximum stress at the depth of the crack was in compression and the ΔK was below the threshold for crack growth; therefore there was no crack growth. A FRANC3D model was run to confirm that the stress intensity was very low at the initial crack size. The FRANC3D model showed that the stress intensity was negative; the FRANC3D results are shown in Figure 16. Two possible reasons for the difference in crack propagation between the test data and the NASGRO analysis are that there may have been some differences in material properties, and that the residual stresses may have been different between the specimens measured by Dorman et al. [2] and those tested by Everett et al. [7]. The residual stresses in the 15

25 Crack Depth (inch) NASGRO model were taken from [2], and the test specimens in [7] may have had different residual stresses. NASGRO Analysis of Laser Peened Al Cycles NASGRO Analysis Test Data Figure 15. Crack growth in laser peened Al 2024, comparing NASGRO analysis with test data from Everett. [7] Figure 16. Stress intensity in laser peened Al 2024 FRANC3D analysis. 16

26 Crack Depth (inch) The test specimens could have had less residual stress than was measured in [2]. Lower residual stress was modeled using NASGRO; the NASGRO model was modified so that the residual stress was reduced to 58% of the published residual stress. The results are shown in Figure 17. While these results do show some crack growth, the crack growth does not match the test data. NASGRO Analysis of Laser Peened Al 2024 Using 58% of Published Residual Cycles Baseline NASGRO Analysis Test Data 58% Residual NASGRO Figure 17. Crack growth analysis using 58% of published residual stress. In order to more accurately analyze the lower residual stress, FRANC3D was used to model four different crack sizes, as was done for the analysis of the shot peened specimens in the previous section. The FRANC3D stress intensities are listed in Table 2. The crack growth was then calculated by scaling the crack growth rate with K(max) from the FRANC3D results to the K(a) values from the NASGRO results. The crack growth using this approach, using 58% of the published residual stress, is shown in Figure 18. This much more closely matches the test data. The likely reason for the difference between the NASGRO results and the FRANC3D results is that the cracks in the test specimens do not likely grow as quarter-elliptical cracks, as NASGRO assumes. The stress intensities listed in Table 2 show that for larger crack sizes, the K(max) is much greater than the K(a), so the maximum stress intensity is not at the depth of the 17

27 Crack Depth (inch) crack, but rather in the interior of the specimen. Figure 19 shows the FRANC3D stress intensity for the x0.200 crack size. For this crack size, the maximum stress intensity is not at the depth of the crack. This indicates that a crack would grow more into the interior of the specimen than the K(a) value would imply, and that the crack would grow much faster, because the K(max) is much higher than the K(a). Crack Size K(a) NASGRO K(a) FRANC3D K(max) FRANC3D.050 x x x x Table 2. NASGRO and FRANC3D Stress intensity results for laser peen analysis. NASGRO Analysis of Laser Peened Al 2024 Using 58% of Published Residual Stress Cycles Baseline NASGRO Analysis Test Data 58% Residual FRANC3D Figure 18. Crack growth analysis results using FRANC3D with 58% of published residual stress. 18

28 Figure 19. FRANC3D analysis of a 0.050"x0.200" crack in a laser peened specimen with 58% of published residual stress. 3.4 Cold Hole Working Analysis In order to analyze cold hole working and compare the results to test data, a literature search was done. A study was done by S. Pasta [3] in 2006 on cold worked holes in specimens of 5083-H321 aluminum. These specimens were 200mm x 50mm (7.874 x1.969 ) samples with a 5 mm (0.197 ) circular cold worked hole in the center, pulled in tension in an Instron machine at different force levels: 30 kn, 32 kn, and 35 kn, with holes worked to 2.5% and 4% expansion. For the purposes of this project, the 30 kn (6,740 pounds) force was analyzed, using the holes worked to 4% expansion. The holes were tested for crack propagation, with a through crack applied to the side of the hole in the specimens tested. The specimen was modeled in NASGRO in order to determine the crack propagation life of the part, for both the unworked and worked holes. The crack propagation life was then compared to tests that had been done by Pasta [3]. In order to analyze this specimen in NASGRO for this project, it was necessary to determine the stress in the specimens. This was done using a finite element model in 19

29 ANSYS [8]. The model measured one quarter of the specimen, with symmetrical boundary conditions. The model is shown in Figure 20. The stresses from this model were taken from the edge of the hole to the edge of the specimen. Figure 20. ANSYS model of LPB test specimen. In Pasta [3], the cold worked hole residual stress was measured. This residual stress was added to the stress from the ANSYS model, and this stress is shown in Figure 21. The applied stress from the ANSYS model (green curve in Figure 21) was used to model the untreated hole, and the resultant stress (blue curve) was used to model the cold-worked hole. Theses stresses were applied to a NASGRO model of a through crack. It was necessary to adjust the material properties because the NASGRO library did not specifically have properties for 5083-H321 aluminum; the closest material the NASGRO library was 5083-O. Test data obtained from Wu [9] show Paris Law values for Al 20

30 Stress (KSI) 5083-H321 of c=7.87e-8 inch, and n=1.719, and these values were used in the NASGRO model. 80 Stress in Cold Work Hole NASGRO Analysis Residual Stress Applied Stress Resultant Stress Depth (inch) Figure 21. Applied and resultant stress in specimen. The results of the NASGRO analysis are shown in Figure 22, compared with test results from Pasta [3]. The crack propagation life for the untreated hole in the test data was 30,000 cycles before the crack became unstable, but the NASGRO model show only 9,800 cycles. This difference is likely due to material property differences. The cold-worked hole test data shows a crack propagation life of 109,000 cycles, but the NASGRO model showed infinite life. The crack did not grow in the NASGRO analysis because the stress intensity was below the threshold. 21

31 Crack Depth (in) Analysis and Test Results for Cold-Worked Holes Untreated (Test) Cold Worked 4% (Test) E E E+05 Cycles Untreated (NASGRO) Cold Worked 4% (NASGRO) Figure 22. Crack propagation of cold worked holes. [3] While the NASGRO analysis and the test in Pasta [3] showed that cold working the hole had a significant effect on the crack propagation life, the absolute numbers appear significantly different between the NASGRO model and the test results. A possible reason that the crack growth analysis does not match the test data is that the published residual stress measurements might not accurately represent the actual residual stress. The analysis was run using residual stress multiplied by a factor of The results are shown in Figure 23. The crack growth matched much more closely to the test using this adjusted residual stress. 22

32 Crack Depth (in) Analysis and Test Results for Cold-Worked Holes Using 75% of Published Residual Stress E E E+05 Cycles Untreated (Test) Cold Worked 4% (Test) Untreated (NASGRO) Cold Worked 4% (NASGRO) Figure 23. Crack propagation of cold worked holes, using 75% of residual stress from [3]. 23

33 4. Conclusions Surface treatment has a significant effect on the rate of crack growth in metal specimens. In order to improve the life of aircraft parts, surface treatment is an effective tool. Specifically, the analysis has shown the following: Shot peening results in an improved crack propagation life, but not as much as some other treatment methods. Because other treatments can be more expensive, shot peening is often the most cost effective way to improve crack propagation life. Laser shock peening and cold-working holes are more effective than shot peening. These processes are more expensive, and in some cases are more difficult than shot peening. Material properties in the commercially available NASGRO library might not necessary correlate well with test data. For this reason, rigorous testing must be done in order to develop material property libraries before analysis can be used for engineering of aircraft parts. Material properties that have been correlated to test data for untreated specimens do not necessarily correlate well to treated specimens. Therefore, in order for these processes to be useful for engineering aircraft parts, fracture mechanics testing must be done on treated specimens in order to have correlated crack growth data libraries. Analytical models using residual stress from measured specimens may not closely match crack growth test data. Residual stress fields must be correlated to test data before analysis can be used for engineering purposes. 24

34 5. References 1. Ludian, T. and Wagner, L., Coverage Effects in Shot Peening of Al 2024-T4, Proceedings 9 th International Conference of Shot Peening (ICSP9), Sept 6-9, 2005, Paris, France, pp Dorman, M.; Toparli, M. B.; Smyth, N.; Cini, A.; Fitzpatrick, M. E. and Irving, P. E. (2012). Effect of laser shock peening on residual stress and fatigue life of clad 2024 aluminum sheet containing scribe defects, Materials Science and Engineer: A, 548 pp Pasta, S., Fatigue Crack Propagation from a Cold-Worked Hole, Engineering Fracture Mechanics, June 2007, pp Beek, Joachim M, et al, NASGRO Fracture Mechanics and Fatigue Crack Growth Analysis Software Reference Manual Version 6.21 Final, January Neal-Sturgess, C.E., A Direct Derivation of the Griffith-Irwin Relationship using a Crack tip Unloading Stress Wave Model, arxiv Volume, DOI, Fracture Analysis Consultants, FRANC3D ANSYS Tutorial Version 6, November Everett, Jr, R. A.; Matthews, W. T.; Prabhakaran, R.; Newman, Jr., J. C.; Dubberly, M. J., The Effects of Shot and Laser Peening on Fatigue Life and Crack Growth in 2024 Aluminum Alloy and 4340 Steel, NASA/TM ARL-TR-2363, December ANSYS, Inc., ANSYS Mechanical Release 12.1, November Wu, Weidong, Fatigue Crack Propagation Behavior of Welded and Weld- Repaired 5083 Aluminium Alloy Joints, Thesis, School of Aerospace and Mechanical Engineering, University College, The University of New South Wales, June