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Residual stress and fatigue life in laser shock peened open hole samples
International Journal of Fatigue 44 (2012) 8–13
Contents lists available at SciVerse ScienceDirect
International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue
Residual stress and fatigue life in laser shock peened open hole samples Servando D. Cuellar a, Michael R. Hill a,⇑, Adrian T. DeWald a,b, Jon E. Rankin a a b
Department of Mechanical and Aerospace Engineering, University of California, One Shields Avenue, Davis, CA 95616, United States Laser Science and Technology, Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94550, United States
a r t i c l e
i n f o
Article history: Received 22 April 2012 Received in revised form 15 May 2012 Accepted 8 June 2012 Available online 16 June 2012 Keywords: Residual stress Laser shock peening Fatigue Contour method
a b s t r a c t This study investigates the effects of various laser shock peening patterns on the residual stress distribution and fatigue performance of beta-solution-treated and over-aged (BSTOA) Ti–6Al–4V open hole fatigue samples. The residual stress produced by various laser shock peening patterns was measured using the contour method. Additional samples were laser peened with similar patterns and fatigue tested to establish a correlation between the residual stress distribution and fatigue performance. A description of each of the patterns is presented along with the corresponding residual stress measurements and fatigue testing results. The results clearly show that the laser shock peening pattern can have a large impact on the residual stress produced. A laser shock peening pattern with multiple concentric rings of spots around the circumference of the hole produced the most favorable residual stress distribution in the vicinity of the hole and also resulted in the best fatigue performance. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Laser shock peening (LSP) is an emerging tool for enhancing the mechanical performance of metallic materials. The LSP process is capable of introducing deep subsurface compressive residual stress [1], which is beneficial in extending the fatigue life of metallic components, preventing stress corrosion cracking [2], and reducing fatigue crack growth rates [3]. Laser shock peening is currently being applied to fan and compressor blades of commercial and military aircraft engines to increase fatigue performance and to protect against failures due to foreign object damage (FOD). The compressive residual stress produced by the LSP process is the result of plastic deformation caused by a laser induced shock wave. Prior to LSP treatment, the workpiece surface is covered with an ablative layer (e.g. paint or tape) [4]. Next, a laminar flow of water (or sometimes glass) [5], called the inertial tamping layer, is directed to the surface of the ablative layer. Then a laser pulse (having duration between a few and several nanoseconds) is fired at the surface of the part. The laser beam passes through the transparent inertial tamping layer and is absorbed by the ablative layer, forming a high-pressure plasma at the interface between the ablative and tamping layers. The expansion of the plasma is confined by the inertial tamping layer, which allows high pressure to build on the surface of the workpiece. The pressure on the surface causes a shock wave to travel through the material and if the pressure is high enough, the shock wave induces plastic deformation in the workpiece. The plastically deformed material is left in a state of ⇑ Corresponding author. Tel.: +1 530 754 6178; fax: +1 530 752 4158. E-mail address: [email protected] (M.R. Hill). 0142-1123/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijfatigue.2012.06.011
compressive residual stress as a result of geometric compatibility with nearby unprocessed material and stress equilibrium. Previous studies published in the literature have shown that laser irradiance, pulse duration, and number of successive treatment layers are the parameters that most strongly affect the depth and magnitude of the compressive stress imposed by LSP [6]. There has recently been an increase in LSP research demonstrating the influence of these parameters for specific metals [6–9]. Such studies show it is possible to tailor the distribution of residual stress produced by laser shock peening (such as deep compressive residual stress or high magnitude near surface compressive residual stress) to meet the needs of the specific application. One LSP parameter that has not been extensively documented in the literature is the LSP pattern or coverage area. It is known that the residual stress field due to LSP is extremely sensitive to geometric features [10] and it is well understood that geometric features such as notches or holes are typical fatigue crack initiation points because they act as stress concentrations. However, very little information has been published regarding the residual stress induced by LSP in different geometries. One such study that has been published correlates the effect of LSP on the fatigue performance of open hole 2024-T3 aluminum samples [11]. The study showed that LSP was effective in reducing fatigue crack growth from notched open holes in 2024-T3 aluminum samples with two types of LSP patterns around the hole. However, that work contained only cursory data on the residual stress induced by LSP and did not address crack formation. This paper presents the results of recent experiments designed to study the relationship between the laser shock peening pattern and the fatigue performance of open hole samples. Multiple laser
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S.D. Cuellar et al. / International Journal of Fatigue 44 (2012) 8–13
shock peening patterns were studied using residual stress measurement and fatigue testing. The first objective of this study is to determine relationship between various LSP patterns and the residual stress distribution in the vicinity of the hole in open hole fatigue test samples. The second objective is to determine the fatigue performance of open hole samples for the same LSP patterns.
(a)
2. Methods
(b)
Fig. 2. Description of (a) S1 pattern and (b) S2 pattern.
2.1. Sample geometry and material properties the laser to run at a high repetition rate without damaging the optics. The LSP parameters used for this study were an irradiance of 10 GW/cm2 and a pulse duration of 18 ns, which had previously been used as an effective treatment for Ti–6Al–4V. The spot size was nominally 3 3 mm (somewhat larger than the grain size of the material) but the LSP pattern (number of spots, spot location, and number of layers) varied from sample to sample. The four different LSP patterns used for this study are outlined in Table 1. The LSP treatment was applied to both sides of the sample in an alternating fashion (i.e. peen a group of spots on one side, flip the specimen over, peen a group of spots on the other side). The first two patterns (S1 and S2) have rectangular shape with S1 covering an area roughly 4.5 mm by 3.0 mm near the high stress region adjacent to the hole (Fig. 2a) and S2 covering an area roughly 4.5 mm by 6.0 mm (Fig. 2b). The other two patterns studied were circular, consisting of a single ring of spots around the circumference of the hole (pattern R1 Fig. 3) or multiple concentric rings of spots around the hole (pattern R4). The radius of each ring of spots was defined as the distance from the center of the hole to the center of the peened ring. Pattern R1 consisted of 16 spots and had a 4.7 mm radius. Pattern R4 consisted of four concentric rings of spots with various radii, applied in a specific order: the first ring had a radius of 4.7 mm (16 spots), the second ring a radius of 7.7 mm (64 spots), the third a radius of 6.2 mm (32 spots), and a final fourth ring was identical to the first.
All of the samples used in this study were typical open hole samples with a thickness of 3.81 mm, a width of 38.0 mm, and a hole diameter of 6.35 mm (Fig. 1). These dimensions provided an elastic stress concentration factor of Kt = 3.1, based on gross section stress. Samples used for fatigue testing had a length of 230 mm and samples used for residual stress measurement had a length of 76 mm. The samples were fabricated from forged, beta-solution-treated and over-aged (BSTOA) Ti–6Al–4V having extra-low interstitials (ELI). The BSTOA heat treatment provides a transformed beta microstructure, which provides improved damage tolerance [12], and has a large and variable grain structure with grain sizes on the order of 1–2 mm. Elastic material properties for this material are taken to be E = 115 GPa and m = 0.33. Tensile yield and ultimate strengths are taken to be 793 MPa and 883 MPa, respectively. Further information on this material and condition, including microstructure and nominal fatigue properties (crack nucleation and crack growth), is available in earlier work (and material used here was drawn from the same supply chain) [12]. 2.2. Laser shock peening The LSP treatment for this study was applied at Lawrence Livermore National Laboratory (LLNL). The laser system employed at LLNL is capable of generating up to 20 J of energy at a repetition rate of 6 Hz [13]. The laser system at LLNL also includes a stimulated Brillouin scattering (SBS) phase conjugation device, which provides uniform wavefront control. The SBS ensures that a uniform energy distribution exists within the laser spot, which allows
2.3. Residual stress measurement Residual stresses were measured in samples treated with each of the four LSP patterns using the contour method [14]. The contour method measurements resulted in maps of the residual stress acting normal to the plane shown in Fig. 4. The basic principle behind the contour method is that a part containing residual stress, when cut in half on a flat plane, will exhibit a non-flat cut surface due to stress release perpendicular to the cut surface (z-direction, Fig. 4b). The cut surface deformation, if known, can provide the released residual stress on the cut surface. The cut is typically made with a wire electric discharge machine (EDM) as it provides a good surface finish and does not introduce significant levels of residual stress. The deformations on the cut surface are measured with a high precision inspection device and the inverse of the measured deformations are applied to a finite element (FE)
w = 38mm
L = 230mm r = 3.175mm
t = 3.81mm Fig. 1. Open hole specimen geometry and nomenclature.
Table 1 Summary of the four laser shock peening patterns studied. Pattern
Spots
Pattern size (mm)
S1
Pattern Geometry
4
4.5 3.0 mm
S2
8
4.5 6.0 mm
R1
16
r = 4.7 mm
R4
128
r1 = 4.7 mm, r2 = 7.7 mm, r3 = 6.2 mm, r4 = 4.7 mm
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S.D. Cuellar et al. / International Journal of Fatigue 44 (2012) 8–13
16
2
14
4
12
6 10
1
15
+
13
5
11
8
Layer 1
16 1 2 15 3 4 14 13 5 6 12 11 7 10 9 8
3
=
7
9
Layer 2
Full pattern
Fig. 3. Description of circular pattern R1.
3. Results w
3.1. Residual stress measurements y
L
x
L/2
z
w
(a)
(b)
Fig. 4. (a) Location of contour method measurement plane and (b) geometry of sample after cutting required for contour method measurement.
model of the cut part as a set of displacement boundary conditions [6,14] to determined residual stress existing before the cut. For these experiments, the samples were cut using a submerged wire EDM with 0.25 mm diameter brass wire and finish cut settings to minimize the roughness along the cut surface. Prior to cutting, the samples were securely clamped to a large backing plate to minimize movement of the samples during cutting [14]. After the samples were cut, they were measured using a scanning laser profilometer. Data points were taken at 0.02 mm intervals in the x-direction and 0.2 mm intervals in the in the ydirection (Fig. 4b) on both cut surfaces. An FE model was made representing the geometry of half of the original sample prior to any deformation (Fig. 4b). The model was made by creating the outline of the xz-plane of half the sample (Fig. 4b) and extruding the plane 3.8 mm in the y-direction. The mesh density was increased in the vicinity of the hole to provide for better resolution in the area of most significance. The measured cut surfaces from each half of a given sample were individually fit to a smooth Fourier surface to eliminate surface roughness effects [6]. Each smoothed surface was then interpolated at the node locations of the FE model and averaged to remove anti-symmetric effects from shear stress or cut path deviation [6,14]. Finally the inverse of the averaged smoothed surface deformations were applied to the FE model as displacement boundary conditions on the cut surface of the model (z-direction displacements on the xy cut plane). Linear elastic stress analysis was performed using commercial FE software, which gave a resulting stress map normal to the cut surface, which is the estimate of residual stress before cutting.
2.4. Fatigue testing Load controlled fatigue testing was performed on samples treated with each of the four LSP patterns to determine the degree of correlation between the measured residual stress and fatigue performance. All samples were tested in tension–tension (axial) fatigue at a frequency of 8 Hz, a stress ratio of R = 0.1, and a maximum gross stress between 230 MPa and 480 MPa (specimen dependent). Failure in the fatigue tests was defined as complete separation. As machined (AM) samples were also fatigue tested for comparison with the LSP results.
Maps of residual stress over the xy-plane for each of the LSP patterns and one AM sample are shown in Fig. 5. These residual stress maps show that there are significant differences in the residual stress generated by each of the LSP patterns. The two rectangular patterns (S1 Fig. 5a and S2 Fig. 5b) generate compressive residual stress adjacent to the hole, near the surfaces of the specimens (y = 0, y = 3.81 mm); however, both rectangular patterns resulted in an area of tensile residual stress adjacent to the hole at the mid thickness of the sample. The residual stress map for pattern R1 (Fig. 5c) shows compressive residual stress through the entire thickness along the edge of the hole (Fig. 5c). The sample treated with pattern R4 resulted in significantly greater levels of compressive residual stress than the other LSP patterns (Fig. 5d). The sample showed complete through thickness, high magnitude compressive residual stress near the hole, and compressive stress extending 5 mm away from the hole edge at the mid thickness (Fig. 5d). Line plots of residual stress versus position were created to allow for quantitative comparison of the various LSP patterns. Two sets of line plots were created for each pattern. One set near the hole edge (x = 15 mm) for residual stress versus thickness (Fig. 6) and the other at the mid thickness (y = 1.95 mm) for residual stress versus width (Fig. 7). The line plots essentially reinforce the key observations of the previous paragraph. First, patterns S1 and S2 result in tensile residual stress at the hole edge near the
(a) S1
(b) S2
(c) R1
(d) R4
(e) AM
Fig. 5. Maps of residual stress measured using the contour method for patterns (a) S1, (b) S2, (c) R1, (d) R4, and (e) AM.
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S.D. Cuellar et al. / International Journal of Fatigue 44 (2012) 8–13
200
200
0
Residual Stress (MPa)
Residual Stress (MPa)
R4 R1
-200
-400 S1 S2 As Machined
-600
-800
0
0.5
1
1.5
2
2.5
3
3.5
0
-200
-400
-600
-800
4
0
0.5
1
1.5
2
2.5
y (mm)
y (mm)
(a)
(b)
3
3.5
4
Fig. 6. Line plots of residual stress along the line x = 15 mm, z = 0 mm for (a) patterns S1, S2, and AM and (b) patterns R1 and R4.
Maximum applied gross stress (MPa)
mid thickness. Second, pattern R1 has no tensile residual stress along the hole edge. Third, pattern R4 has significantly more compressive residual stress in the vicinity of the hole than any of the other LSP patterns. 3.2. Fatigue testing The results of all of the fatigue tests are presented in Fig. 8. The rectangular LSP patterns (S1 and S2) resulted in fatigue lives somewhat less than the AM samples tested at the same stress level, which is consistent with the tensile residual stress found adjacent to the hole near the sample mid-thickness (Fig. 5). Only one sample for each of patterns S1 and S2 was fatigue tested because little benefit was expected from these patterns. The R1 pattern showed a slight improvement over AM at the lower stress levels (below 350 MPa) and similar performance compared to AM at the higher stress levels (above 350 MPa). Pattern R4 fatigue results were clearly superior to any other results (Fig. 8), showing an improvement over AM samples at every stress level and approaching 10X over AM samples at the lower stress levels. 4. Discussion
200
200
-100 -200 S1 S2 AM
-300 -400 -500
350 300 R4
250 S1
200
10
4
S2
10
R1
5
10
R4 R1
100 0 -100 -200 -300 -400
0
5
10
15
20
25
30
35
40
6
distribution though the entire thickness of the sample at the area of interest. The contour method was probably the best method to provide the needed residual stress information. Other common residual stress measurement techniques used for the measurement of LSP residual stress include X-ray diffraction (XRD) and slitting (crack compliance method). However, both of these methods are 300
0
400
Fig. 8. Fatigue life verses applied stress for all samples tested.
300
100
450
N (cycles)
Residual Stress (MPa)
Residual Stress (MPa)
This study required a residual stress measurement method that would provide an accurate description of the residual stress
As Machined S1 S2 R1 R4
500
-500
0
5
10
15
20
x (mm)
x (mm)
(a)
(b)
25
30
35
40
Fig. 7. Line plots of residual stress along the line y = 1.95 mm, z = 0 mm for (a) patterns S1, S2, and AM and (b) patterns R1 and R4.
S.D. Cuellar et al. / International Journal of Fatigue 44 (2012) 8–13
only capable of producing 1D measurements of residual stress versus depth as compared with the 2D capability of the contour method. Additionally, XRD results from large grained materials are oftentimes difficult to interpret [6,15]. Another residual stress measurement technique that is capable of generating 2D residual stress maps is neutron diffraction. However, neutron diffraction measurements require significantly more time and effort than contour method measurements and neutron diffraction would be even more difficult given the large grain structure of this material. Measurements taken with the contour method have been shown to compare well with neutron diffraction measurements. Thus, the contour method was deemed to be a suitable residual stress measurement technique for the requirements of this study. Earlier work has shown that increasing the number of layers will introduce more and deeper compressive residual stress into the workpiece [1,6], supporting the results of this study. Pattern R4 had the most compressive residual stress and had the most layers of peening, while pattern R1 had the second most compressive residual stress, with the second most layers of LSP. Patterns S1 and S2, with only one layer of peening, produced inferior results based on the high tensile stress area at the mid-thickness of the samples and the least amount of volume of material under compressive stress. It is noteworthy that the circular peening patterns covering the entire circumference of the hole produce more favorable residual stress distributions than rectangular peening patterns applied in the area of high applied stress. This shows that it is important to select an LSP pattern that covers the entire circumference of a hole. Although it was not studied, a large rectangular pattern covering the entire circumference of the hole would probably exhibit results similar to the circular patterns. One may likewise expect that treatment of an entire filet radius would provide a better result than peening only the high stress area at the filet toe. 4.1. Fatigue testing The redistribution of tensile stress away from the hole is a key feature that helps provide fatigue life enhancement of the open hole samples. The open hole geometry results in a stress concentration at the edge of the hole and therefore compressive residual stress provides a fatigue benefit. Tensile residual stress arises from equilibrium, but, for the circular patterns here, the tension exists where applied stress is of sufficiently low magnitude. A similar situation would exist in a surface treated filet, where tensile residual stress would arise below the treated surface, and remote from a near-surface stress concentration. The fatigue results for the various LSP patterns qualitatively correlate with the residual stress measurements. Pattern R4, which resulted in the largest area of compressive residual stress, also had the best fatigue life performance. Pattern R1, which resulted in compressive residual stress around the entire circumference of the hole, resulted in the second best fatigue performance (of the four LSP patterns studied). The samples treated with patterns S1 and S2, which resulted in tensile residual stress at mid thickness on the circumference of the hole, performed slightly worse than the AM samples tested at the same stress level. This would suggest that the proximity of tensile residual stress to the edge of the hole plays a significant role in fatigue performance. The development of a given laser shock peening application currently involves iteration for parameters and peened area, requiring a significant empirical burden (e.g. fatigue testing and residual stress measurement) to take advantage of the technology. Further adoption of the technology would be enabled by predictive models for the spatial distribution of residual stress given a treatment specification, and for fatigue life given predicted or measured
Maximum applied gross stress (MPa)
12
As Machined S1 S2 R1 R4 AM fit S1 or S2 prediction R1 prediction R4 prediction
500 450 400 350 300 250 200 10
4
10
5
10
6
N (cycles) Fig. 9. Predicted and actual fatigue life verses applied stress for all samples.
residual stress. With the present results at hand, it is therefore worthwhile to assess the degree to which the present LSP fatigue results could be predicted from the AM fatigue results and the measured residual stresses. Assuming fatigue life is the result of the linear combination of residual and applied stress, an analysis was conducted by fitting a base material curve and using a common mean stress correction scheme. The base material curve consisted of a power-law fit to the AM open hole fatigue results, with the fit relating local applied stress, rapp to fatigue life, N. Local applied stress was determined by multiplying applied gross stress, S, by Kt = 3.1. The SWT method [16] was used for mean stress correction, where AM data have a stress ratio of 0.1 and LSP data have a stress ratio found from the sum of residual stress and applied mean stress, which varies for each combination of residual stress and applied stress. A single value of residual stress was determined for each peening pattern as a representative, most positive residual stress from the data in Figs. 6 and 7. The assumed values were +120 MPa for both S1 and S2, 100 MPa for R1, and 400 MPa for R4. Resulting predictions are shown in Fig. 9. Although the predictions somewhat under-predict the effects of residual stress at low applied stress levels and somewhat over-predict the effects at high applied stress levels, the results of this simple model are in reasonable agreement with the fatigue data. We attribute the beneficial effects of LSP found here to compressive residual stress, and note that these benefits arise despite potential for negative effects from cold working. Earlier unpublished work, using metallographically prepared sections into the depth of simple block-shaped samples treated uniformly on one surface with LSP, showed intragranular slip bands at and within a few mm below the peened surface. Follow-up work [17] investigated in detail surface deformation attending LSP of this material using electron backscatter diffraction, scanning electron microscopy, and atomic force microscopy. Slip steps were noted in grains having their c axis nearly normal to the sample surface, and such steps were noted to terminate near grain boundaries and triple points. Localized lattice rotations in grains with slip steps were concentrated near the steps, with almost no orientation variations in between the slip steps, and suggested potential for stress concentration at slip steps, which could negatively affect fatigue performance. It would be useful to investigate the influence of these small scale features on crack initiation behavior in further work.
5. Summary The results here clearly show that the LSP pattern can have a significant effect on the performance of LSP specimens and that
S.D. Cuellar et al. / International Journal of Fatigue 44 (2012) 8–13
care should be taken to design an effective LSP pattern for the specifics of the problem at hand. When improperly applied, laser shock peening can have a detrimental effect on material performance. This was demonstrated by the poor fatigue performance of open hole coupons having LSP applied only near areas of high applied stress (patterns S1 and S2), which exhibited residual tensile stress near the mid-thickness, adjacent to the hole. On the other hand, LSP patterns having rings of LSP spots around the circumference of the open hole produced through-thickness compression in the vicinity of high applied stress, which had a beneficial effect on fatigue performance. This study also demonstrates that the contour method is a useful tool to correlate LSP induced residual stress with fatigue performance, as there is a clear correlation between the measured residual stress and the fatigue results. Residual stress plays a major role in fatigue performance, and in this work, residual stress measurements enabled reasonable predictions of the fatigue performance of LSP-treated samples given the performance of as-machined samples. Acknowledgements Funding for this work was provided by Boeing Integrated Defense Systems (IDS). The authors benefited from helpful advice provided by Boeing engineers Jim Pillers, Bob Frantz, and Jeff Bunch. Laser shock peening was performed at Lawrence Livermore National Laboratory (LLNL), and the authors are grateful for the support of Lloyd Hackel, formerly program leader for Laser Science and Technology at LLNL, and the efforts of Laurie Lane. The LLNL laser shock peening capability was co-developed with Metal Improvement Company (MIC), and Fritz Harris of MIC deserves credit for his many efforts in establishing and maintaining that facility. This work was carried out under the auspices of the US Department of Energy by the University of California, LLNL under Contract No. W-7405-Eng-48.
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References [1] Fabbro R et al. Physics and applications of laser-shock processing. J Laser Appl 1998;10(6):265–79. [2] Peyre P et al. Corrosion reactivity of laser-peened steel surfaces. J Mater Eng Perform 2000;9(6):656–62. [3] Shepard MJ, Smith PR, Amer MS. Introduction of compressive residual stress in Ti–6Al–4V simulated airfoils via laser shock processing. J Mater Eng Perform 2001;10(6):670–8. [4] Montross CS, Florea V, Swain MV. The influence of coatings on subsurface mechanical properties of laser peened 2011-T3 aluminum. J Mater Sci 2001;36:1801–7. [5] Devaux D et al. Generation of shock waves by laser-induced plasma in confined geometry. J Appl Phys 1993;74(4):2268–73. [6] DeWald AT et al. Assessment of tensile residual stress mitigation in alloy 22 welds due to laser peening. J Eng Mater Technol 2004;126(4):465–73. [7] Rodopoulos CA et al. Effect of controlled shot peening and laser shock peening on the fatigue performance of 2024-T351 aluminum alloy. J Mater Eng Perform 2003;12(4):414–9. [8] Rankin JE, Hill MR, Hackel LA. The effects of process variations on residual stress in laser peened 7049 T73 aluminum alloy. Mater Sci Eng A – Struct Mater Prop Microstruct Process 2003;349(1–2):279–91. [9] Clauer AH, Walters CT, Ford SC. The effects of laser shock processing on the fatigue properties of 2024-T3 aluminum. Lasers Mater Process 1983:7–22. [10] Tan Y et al. Laser shock peening on fatigue crack growth behavior of aluminum alloy. Fatigue Fract Eng Mater Struct 2004;27(8):649–56. [11] Clauer HA, et al. Laser shock processing for treating fastener holes in aging aircraft. In: Atluri SN, et al., editor. Durability of Metal Aircraft Structures, Atlanta: Atlanta Technology Publications; p. 350–361. [12] Cotton JD, Clark LP, Phelps HR. Titanium alloys on the F-22 fighter airframe. Adv Mater Process 2002;160(5):25–8. [13] Dane CB et al. High power laser for peening of metals enabling production technology. Mater Manuf Process 2000;15(1):81–96. [14] Prime MB. Cross-sectional mapping of residual stresses by measuring the surface contour after a cut. J Eng Mater Technol – Trans ASME 2001;123(2):162–8. [15] Society for experimental mechanics. Handbook of measurement of residual stresses. In: Lu J, Lilburn GA, editors. Fairmont Press; 1996. [16] Dowling NE. Mechanical behavior of materials: engineering methods for deformation, fracture, and fatigue. In: 2nd ed., Upper Saddle River, NJ: Prentice Hall. xviii; 1999. p. 830. [17] El-Dasher BS et al. Surface deformation behavior of beta solution treated and overaged Ti–6Al–4V during laser shock processing. J Appl Phys 2006;99:103506.
Contents lists available at SciVerse ScienceDirect
International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue
Residual stress and fatigue life in laser shock peened open hole samples Servando D. Cuellar a, Michael R. Hill a,⇑, Adrian T. DeWald a,b, Jon E. Rankin a a b
Department of Mechanical and Aerospace Engineering, University of California, One Shields Avenue, Davis, CA 95616, United States Laser Science and Technology, Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94550, United States
a r t i c l e
i n f o
Article history: Received 22 April 2012 Received in revised form 15 May 2012 Accepted 8 June 2012 Available online 16 June 2012 Keywords: Residual stress Laser shock peening Fatigue Contour method
a b s t r a c t This study investigates the effects of various laser shock peening patterns on the residual stress distribution and fatigue performance of beta-solution-treated and over-aged (BSTOA) Ti–6Al–4V open hole fatigue samples. The residual stress produced by various laser shock peening patterns was measured using the contour method. Additional samples were laser peened with similar patterns and fatigue tested to establish a correlation between the residual stress distribution and fatigue performance. A description of each of the patterns is presented along with the corresponding residual stress measurements and fatigue testing results. The results clearly show that the laser shock peening pattern can have a large impact on the residual stress produced. A laser shock peening pattern with multiple concentric rings of spots around the circumference of the hole produced the most favorable residual stress distribution in the vicinity of the hole and also resulted in the best fatigue performance. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Laser shock peening (LSP) is an emerging tool for enhancing the mechanical performance of metallic materials. The LSP process is capable of introducing deep subsurface compressive residual stress [1], which is beneficial in extending the fatigue life of metallic components, preventing stress corrosion cracking [2], and reducing fatigue crack growth rates [3]. Laser shock peening is currently being applied to fan and compressor blades of commercial and military aircraft engines to increase fatigue performance and to protect against failures due to foreign object damage (FOD). The compressive residual stress produced by the LSP process is the result of plastic deformation caused by a laser induced shock wave. Prior to LSP treatment, the workpiece surface is covered with an ablative layer (e.g. paint or tape) [4]. Next, a laminar flow of water (or sometimes glass) [5], called the inertial tamping layer, is directed to the surface of the ablative layer. Then a laser pulse (having duration between a few and several nanoseconds) is fired at the surface of the part. The laser beam passes through the transparent inertial tamping layer and is absorbed by the ablative layer, forming a high-pressure plasma at the interface between the ablative and tamping layers. The expansion of the plasma is confined by the inertial tamping layer, which allows high pressure to build on the surface of the workpiece. The pressure on the surface causes a shock wave to travel through the material and if the pressure is high enough, the shock wave induces plastic deformation in the workpiece. The plastically deformed material is left in a state of ⇑ Corresponding author. Tel.: +1 530 754 6178; fax: +1 530 752 4158. E-mail address: [email protected] (M.R. Hill). 0142-1123/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijfatigue.2012.06.011
compressive residual stress as a result of geometric compatibility with nearby unprocessed material and stress equilibrium. Previous studies published in the literature have shown that laser irradiance, pulse duration, and number of successive treatment layers are the parameters that most strongly affect the depth and magnitude of the compressive stress imposed by LSP [6]. There has recently been an increase in LSP research demonstrating the influence of these parameters for specific metals [6–9]. Such studies show it is possible to tailor the distribution of residual stress produced by laser shock peening (such as deep compressive residual stress or high magnitude near surface compressive residual stress) to meet the needs of the specific application. One LSP parameter that has not been extensively documented in the literature is the LSP pattern or coverage area. It is known that the residual stress field due to LSP is extremely sensitive to geometric features [10] and it is well understood that geometric features such as notches or holes are typical fatigue crack initiation points because they act as stress concentrations. However, very little information has been published regarding the residual stress induced by LSP in different geometries. One such study that has been published correlates the effect of LSP on the fatigue performance of open hole 2024-T3 aluminum samples [11]. The study showed that LSP was effective in reducing fatigue crack growth from notched open holes in 2024-T3 aluminum samples with two types of LSP patterns around the hole. However, that work contained only cursory data on the residual stress induced by LSP and did not address crack formation. This paper presents the results of recent experiments designed to study the relationship between the laser shock peening pattern and the fatigue performance of open hole samples. Multiple laser
9
S.D. Cuellar et al. / International Journal of Fatigue 44 (2012) 8–13
shock peening patterns were studied using residual stress measurement and fatigue testing. The first objective of this study is to determine relationship between various LSP patterns and the residual stress distribution in the vicinity of the hole in open hole fatigue test samples. The second objective is to determine the fatigue performance of open hole samples for the same LSP patterns.
(a)
2. Methods
(b)
Fig. 2. Description of (a) S1 pattern and (b) S2 pattern.
2.1. Sample geometry and material properties the laser to run at a high repetition rate without damaging the optics. The LSP parameters used for this study were an irradiance of 10 GW/cm2 and a pulse duration of 18 ns, which had previously been used as an effective treatment for Ti–6Al–4V. The spot size was nominally 3 3 mm (somewhat larger than the grain size of the material) but the LSP pattern (number of spots, spot location, and number of layers) varied from sample to sample. The four different LSP patterns used for this study are outlined in Table 1. The LSP treatment was applied to both sides of the sample in an alternating fashion (i.e. peen a group of spots on one side, flip the specimen over, peen a group of spots on the other side). The first two patterns (S1 and S2) have rectangular shape with S1 covering an area roughly 4.5 mm by 3.0 mm near the high stress region adjacent to the hole (Fig. 2a) and S2 covering an area roughly 4.5 mm by 6.0 mm (Fig. 2b). The other two patterns studied were circular, consisting of a single ring of spots around the circumference of the hole (pattern R1 Fig. 3) or multiple concentric rings of spots around the hole (pattern R4). The radius of each ring of spots was defined as the distance from the center of the hole to the center of the peened ring. Pattern R1 consisted of 16 spots and had a 4.7 mm radius. Pattern R4 consisted of four concentric rings of spots with various radii, applied in a specific order: the first ring had a radius of 4.7 mm (16 spots), the second ring a radius of 7.7 mm (64 spots), the third a radius of 6.2 mm (32 spots), and a final fourth ring was identical to the first.
All of the samples used in this study were typical open hole samples with a thickness of 3.81 mm, a width of 38.0 mm, and a hole diameter of 6.35 mm (Fig. 1). These dimensions provided an elastic stress concentration factor of Kt = 3.1, based on gross section stress. Samples used for fatigue testing had a length of 230 mm and samples used for residual stress measurement had a length of 76 mm. The samples were fabricated from forged, beta-solution-treated and over-aged (BSTOA) Ti–6Al–4V having extra-low interstitials (ELI). The BSTOA heat treatment provides a transformed beta microstructure, which provides improved damage tolerance [12], and has a large and variable grain structure with grain sizes on the order of 1–2 mm. Elastic material properties for this material are taken to be E = 115 GPa and m = 0.33. Tensile yield and ultimate strengths are taken to be 793 MPa and 883 MPa, respectively. Further information on this material and condition, including microstructure and nominal fatigue properties (crack nucleation and crack growth), is available in earlier work (and material used here was drawn from the same supply chain) [12]. 2.2. Laser shock peening The LSP treatment for this study was applied at Lawrence Livermore National Laboratory (LLNL). The laser system employed at LLNL is capable of generating up to 20 J of energy at a repetition rate of 6 Hz [13]. The laser system at LLNL also includes a stimulated Brillouin scattering (SBS) phase conjugation device, which provides uniform wavefront control. The SBS ensures that a uniform energy distribution exists within the laser spot, which allows
2.3. Residual stress measurement Residual stresses were measured in samples treated with each of the four LSP patterns using the contour method [14]. The contour method measurements resulted in maps of the residual stress acting normal to the plane shown in Fig. 4. The basic principle behind the contour method is that a part containing residual stress, when cut in half on a flat plane, will exhibit a non-flat cut surface due to stress release perpendicular to the cut surface (z-direction, Fig. 4b). The cut surface deformation, if known, can provide the released residual stress on the cut surface. The cut is typically made with a wire electric discharge machine (EDM) as it provides a good surface finish and does not introduce significant levels of residual stress. The deformations on the cut surface are measured with a high precision inspection device and the inverse of the measured deformations are applied to a finite element (FE)
w = 38mm
L = 230mm r = 3.175mm
t = 3.81mm Fig. 1. Open hole specimen geometry and nomenclature.
Table 1 Summary of the four laser shock peening patterns studied. Pattern
Spots
Pattern size (mm)
S1
Pattern Geometry
4
4.5 3.0 mm
S2
8
4.5 6.0 mm
R1
16
r = 4.7 mm
R4
128
r1 = 4.7 mm, r2 = 7.7 mm, r3 = 6.2 mm, r4 = 4.7 mm
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S.D. Cuellar et al. / International Journal of Fatigue 44 (2012) 8–13
16
2
14
4
12
6 10
1
15
+
13
5
11
8
Layer 1
16 1 2 15 3 4 14 13 5 6 12 11 7 10 9 8
3
=
7
9
Layer 2
Full pattern
Fig. 3. Description of circular pattern R1.
3. Results w
3.1. Residual stress measurements y
L
x
L/2
z
w
(a)
(b)
Fig. 4. (a) Location of contour method measurement plane and (b) geometry of sample after cutting required for contour method measurement.
model of the cut part as a set of displacement boundary conditions [6,14] to determined residual stress existing before the cut. For these experiments, the samples were cut using a submerged wire EDM with 0.25 mm diameter brass wire and finish cut settings to minimize the roughness along the cut surface. Prior to cutting, the samples were securely clamped to a large backing plate to minimize movement of the samples during cutting [14]. After the samples were cut, they were measured using a scanning laser profilometer. Data points were taken at 0.02 mm intervals in the x-direction and 0.2 mm intervals in the in the ydirection (Fig. 4b) on both cut surfaces. An FE model was made representing the geometry of half of the original sample prior to any deformation (Fig. 4b). The model was made by creating the outline of the xz-plane of half the sample (Fig. 4b) and extruding the plane 3.8 mm in the y-direction. The mesh density was increased in the vicinity of the hole to provide for better resolution in the area of most significance. The measured cut surfaces from each half of a given sample were individually fit to a smooth Fourier surface to eliminate surface roughness effects [6]. Each smoothed surface was then interpolated at the node locations of the FE model and averaged to remove anti-symmetric effects from shear stress or cut path deviation [6,14]. Finally the inverse of the averaged smoothed surface deformations were applied to the FE model as displacement boundary conditions on the cut surface of the model (z-direction displacements on the xy cut plane). Linear elastic stress analysis was performed using commercial FE software, which gave a resulting stress map normal to the cut surface, which is the estimate of residual stress before cutting.
2.4. Fatigue testing Load controlled fatigue testing was performed on samples treated with each of the four LSP patterns to determine the degree of correlation between the measured residual stress and fatigue performance. All samples were tested in tension–tension (axial) fatigue at a frequency of 8 Hz, a stress ratio of R = 0.1, and a maximum gross stress between 230 MPa and 480 MPa (specimen dependent). Failure in the fatigue tests was defined as complete separation. As machined (AM) samples were also fatigue tested for comparison with the LSP results.
Maps of residual stress over the xy-plane for each of the LSP patterns and one AM sample are shown in Fig. 5. These residual stress maps show that there are significant differences in the residual stress generated by each of the LSP patterns. The two rectangular patterns (S1 Fig. 5a and S2 Fig. 5b) generate compressive residual stress adjacent to the hole, near the surfaces of the specimens (y = 0, y = 3.81 mm); however, both rectangular patterns resulted in an area of tensile residual stress adjacent to the hole at the mid thickness of the sample. The residual stress map for pattern R1 (Fig. 5c) shows compressive residual stress through the entire thickness along the edge of the hole (Fig. 5c). The sample treated with pattern R4 resulted in significantly greater levels of compressive residual stress than the other LSP patterns (Fig. 5d). The sample showed complete through thickness, high magnitude compressive residual stress near the hole, and compressive stress extending 5 mm away from the hole edge at the mid thickness (Fig. 5d). Line plots of residual stress versus position were created to allow for quantitative comparison of the various LSP patterns. Two sets of line plots were created for each pattern. One set near the hole edge (x = 15 mm) for residual stress versus thickness (Fig. 6) and the other at the mid thickness (y = 1.95 mm) for residual stress versus width (Fig. 7). The line plots essentially reinforce the key observations of the previous paragraph. First, patterns S1 and S2 result in tensile residual stress at the hole edge near the
(a) S1
(b) S2
(c) R1
(d) R4
(e) AM
Fig. 5. Maps of residual stress measured using the contour method for patterns (a) S1, (b) S2, (c) R1, (d) R4, and (e) AM.
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S.D. Cuellar et al. / International Journal of Fatigue 44 (2012) 8–13
200
200
0
Residual Stress (MPa)
Residual Stress (MPa)
R4 R1
-200
-400 S1 S2 As Machined
-600
-800
0
0.5
1
1.5
2
2.5
3
3.5
0
-200
-400
-600
-800
4
0
0.5
1
1.5
2
2.5
y (mm)
y (mm)
(a)
(b)
3
3.5
4
Fig. 6. Line plots of residual stress along the line x = 15 mm, z = 0 mm for (a) patterns S1, S2, and AM and (b) patterns R1 and R4.
Maximum applied gross stress (MPa)
mid thickness. Second, pattern R1 has no tensile residual stress along the hole edge. Third, pattern R4 has significantly more compressive residual stress in the vicinity of the hole than any of the other LSP patterns. 3.2. Fatigue testing The results of all of the fatigue tests are presented in Fig. 8. The rectangular LSP patterns (S1 and S2) resulted in fatigue lives somewhat less than the AM samples tested at the same stress level, which is consistent with the tensile residual stress found adjacent to the hole near the sample mid-thickness (Fig. 5). Only one sample for each of patterns S1 and S2 was fatigue tested because little benefit was expected from these patterns. The R1 pattern showed a slight improvement over AM at the lower stress levels (below 350 MPa) and similar performance compared to AM at the higher stress levels (above 350 MPa). Pattern R4 fatigue results were clearly superior to any other results (Fig. 8), showing an improvement over AM samples at every stress level and approaching 10X over AM samples at the lower stress levels. 4. Discussion
200
200
-100 -200 S1 S2 AM
-300 -400 -500
350 300 R4
250 S1
200
10
4
S2
10
R1
5
10
R4 R1
100 0 -100 -200 -300 -400
0
5
10
15
20
25
30
35
40
6
distribution though the entire thickness of the sample at the area of interest. The contour method was probably the best method to provide the needed residual stress information. Other common residual stress measurement techniques used for the measurement of LSP residual stress include X-ray diffraction (XRD) and slitting (crack compliance method). However, both of these methods are 300
0
400
Fig. 8. Fatigue life verses applied stress for all samples tested.
300
100
450
N (cycles)
Residual Stress (MPa)
Residual Stress (MPa)
This study required a residual stress measurement method that would provide an accurate description of the residual stress
As Machined S1 S2 R1 R4
500
-500
0
5
10
15
20
x (mm)
x (mm)
(a)
(b)
25
30
35
40
Fig. 7. Line plots of residual stress along the line y = 1.95 mm, z = 0 mm for (a) patterns S1, S2, and AM and (b) patterns R1 and R4.
S.D. Cuellar et al. / International Journal of Fatigue 44 (2012) 8–13
only capable of producing 1D measurements of residual stress versus depth as compared with the 2D capability of the contour method. Additionally, XRD results from large grained materials are oftentimes difficult to interpret [6,15]. Another residual stress measurement technique that is capable of generating 2D residual stress maps is neutron diffraction. However, neutron diffraction measurements require significantly more time and effort than contour method measurements and neutron diffraction would be even more difficult given the large grain structure of this material. Measurements taken with the contour method have been shown to compare well with neutron diffraction measurements. Thus, the contour method was deemed to be a suitable residual stress measurement technique for the requirements of this study. Earlier work has shown that increasing the number of layers will introduce more and deeper compressive residual stress into the workpiece [1,6], supporting the results of this study. Pattern R4 had the most compressive residual stress and had the most layers of peening, while pattern R1 had the second most compressive residual stress, with the second most layers of LSP. Patterns S1 and S2, with only one layer of peening, produced inferior results based on the high tensile stress area at the mid-thickness of the samples and the least amount of volume of material under compressive stress. It is noteworthy that the circular peening patterns covering the entire circumference of the hole produce more favorable residual stress distributions than rectangular peening patterns applied in the area of high applied stress. This shows that it is important to select an LSP pattern that covers the entire circumference of a hole. Although it was not studied, a large rectangular pattern covering the entire circumference of the hole would probably exhibit results similar to the circular patterns. One may likewise expect that treatment of an entire filet radius would provide a better result than peening only the high stress area at the filet toe. 4.1. Fatigue testing The redistribution of tensile stress away from the hole is a key feature that helps provide fatigue life enhancement of the open hole samples. The open hole geometry results in a stress concentration at the edge of the hole and therefore compressive residual stress provides a fatigue benefit. Tensile residual stress arises from equilibrium, but, for the circular patterns here, the tension exists where applied stress is of sufficiently low magnitude. A similar situation would exist in a surface treated filet, where tensile residual stress would arise below the treated surface, and remote from a near-surface stress concentration. The fatigue results for the various LSP patterns qualitatively correlate with the residual stress measurements. Pattern R4, which resulted in the largest area of compressive residual stress, also had the best fatigue life performance. Pattern R1, which resulted in compressive residual stress around the entire circumference of the hole, resulted in the second best fatigue performance (of the four LSP patterns studied). The samples treated with patterns S1 and S2, which resulted in tensile residual stress at mid thickness on the circumference of the hole, performed slightly worse than the AM samples tested at the same stress level. This would suggest that the proximity of tensile residual stress to the edge of the hole plays a significant role in fatigue performance. The development of a given laser shock peening application currently involves iteration for parameters and peened area, requiring a significant empirical burden (e.g. fatigue testing and residual stress measurement) to take advantage of the technology. Further adoption of the technology would be enabled by predictive models for the spatial distribution of residual stress given a treatment specification, and for fatigue life given predicted or measured
Maximum applied gross stress (MPa)
12
As Machined S1 S2 R1 R4 AM fit S1 or S2 prediction R1 prediction R4 prediction
500 450 400 350 300 250 200 10
4
10
5
10
6
N (cycles) Fig. 9. Predicted and actual fatigue life verses applied stress for all samples.
residual stress. With the present results at hand, it is therefore worthwhile to assess the degree to which the present LSP fatigue results could be predicted from the AM fatigue results and the measured residual stresses. Assuming fatigue life is the result of the linear combination of residual and applied stress, an analysis was conducted by fitting a base material curve and using a common mean stress correction scheme. The base material curve consisted of a power-law fit to the AM open hole fatigue results, with the fit relating local applied stress, rapp to fatigue life, N. Local applied stress was determined by multiplying applied gross stress, S, by Kt = 3.1. The SWT method [16] was used for mean stress correction, where AM data have a stress ratio of 0.1 and LSP data have a stress ratio found from the sum of residual stress and applied mean stress, which varies for each combination of residual stress and applied stress. A single value of residual stress was determined for each peening pattern as a representative, most positive residual stress from the data in Figs. 6 and 7. The assumed values were +120 MPa for both S1 and S2, 100 MPa for R1, and 400 MPa for R4. Resulting predictions are shown in Fig. 9. Although the predictions somewhat under-predict the effects of residual stress at low applied stress levels and somewhat over-predict the effects at high applied stress levels, the results of this simple model are in reasonable agreement with the fatigue data. We attribute the beneficial effects of LSP found here to compressive residual stress, and note that these benefits arise despite potential for negative effects from cold working. Earlier unpublished work, using metallographically prepared sections into the depth of simple block-shaped samples treated uniformly on one surface with LSP, showed intragranular slip bands at and within a few mm below the peened surface. Follow-up work [17] investigated in detail surface deformation attending LSP of this material using electron backscatter diffraction, scanning electron microscopy, and atomic force microscopy. Slip steps were noted in grains having their c axis nearly normal to the sample surface, and such steps were noted to terminate near grain boundaries and triple points. Localized lattice rotations in grains with slip steps were concentrated near the steps, with almost no orientation variations in between the slip steps, and suggested potential for stress concentration at slip steps, which could negatively affect fatigue performance. It would be useful to investigate the influence of these small scale features on crack initiation behavior in further work.
5. Summary The results here clearly show that the LSP pattern can have a significant effect on the performance of LSP specimens and that
S.D. Cuellar et al. / International Journal of Fatigue 44 (2012) 8–13
care should be taken to design an effective LSP pattern for the specifics of the problem at hand. When improperly applied, laser shock peening can have a detrimental effect on material performance. This was demonstrated by the poor fatigue performance of open hole coupons having LSP applied only near areas of high applied stress (patterns S1 and S2), which exhibited residual tensile stress near the mid-thickness, adjacent to the hole. On the other hand, LSP patterns having rings of LSP spots around the circumference of the open hole produced through-thickness compression in the vicinity of high applied stress, which had a beneficial effect on fatigue performance. This study also demonstrates that the contour method is a useful tool to correlate LSP induced residual stress with fatigue performance, as there is a clear correlation between the measured residual stress and the fatigue results. Residual stress plays a major role in fatigue performance, and in this work, residual stress measurements enabled reasonable predictions of the fatigue performance of LSP-treated samples given the performance of as-machined samples. Acknowledgements Funding for this work was provided by Boeing Integrated Defense Systems (IDS). The authors benefited from helpful advice provided by Boeing engineers Jim Pillers, Bob Frantz, and Jeff Bunch. Laser shock peening was performed at Lawrence Livermore National Laboratory (LLNL), and the authors are grateful for the support of Lloyd Hackel, formerly program leader for Laser Science and Technology at LLNL, and the efforts of Laurie Lane. The LLNL laser shock peening capability was co-developed with Metal Improvement Company (MIC), and Fritz Harris of MIC deserves credit for his many efforts in establishing and maintaining that facility. This work was carried out under the auspices of the US Department of Energy by the University of California, LLNL under Contract No. W-7405-Eng-48.
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