Surface Flaw Effect on LCF Life of a Ni-based Superalloy under Shot Peen Conditions ISABE-2015-20239 Ross Miller Honeywell Aerospace Phoenix, AZ Abstract Residual stresses are known to influence the fatigue life of components. Shot peening is a post-machining process used to increase fatigue resistance by inducing cold work and compressive residual stresses on the surface and subsurface of the material. Due to the increased compressive residual stresses shot peened specimens have a higher average total low-cycle fatigue (LCF) life than as-machined surfaces, however it is ambiguous as to the effect on low LCF life events. Failures associated with low fatigue life are typically associated with machining anomalies or near surface material defects that cause a severe debit to fatigue life. The intent of this experiment was to evaluate the effect of small surface flaws, introduced prior to shot peen, on total LCF life. shot at the surface while controlling the velocity, distance, size of shot, and time of exposure. Shot peen creates areas of overlapping plastic deformation leaving a dimpled layer that increases cold work (10%-20%) to a depth of 0.001 and compressive residual stresses to a depth of 0.005. Figure 1 shows a typical residual stress profile of a peened surface using X-ray diffraction techniques with peak residual stresses being subsurface [1]. Introduction Typical machining methods used for creating features in aerospace hardware have the possibility for inducing anomalies that may act as small flaws reducing the total fatigue life of the component. Special consideration is taken in high stress applications to control the machining parameters in order to minimize the variation in the machined surface. It is typical in these critical pieces of hardware to use component specimens that mimic the geometry, stress and operating temperature to test under low-cycle fatigue (LCF) conditions in order to characterize the fatigue life. For certain applications, it may be of interest to increase the fatigue life or reduce the risk of a component failure by increasing near surface compressive residual stresses by using postmachining surface treatments. One of the most common and economical methods for increasing fatigue resistance is shot peening. Shot peening is completed by spraying high velocity glass or steel Figure 1 - Representative residual stresses of a shot peened surface measured using XRD techniques. However, it has been shown that thermal exposures relax residual stresses induced by post-machining surface treatments and is exceptionally true with shot peened specimens [1]. This is believed to occur due to the increased cold work that is also added to the surface during the shot peen process. In order to fully understand long term effects of small flaws on the notched specimens, thermal relaxation must therefore be taken into consideration. When characterizing LCF life, total life is a combination of cycles to crack initiation and crack propagation cycles. For a given stress state propagation cycles are consistent for as-machined and shot peened surfaces, because the compressive 1
residual stresses induced during shot peening are near surface and do not affect the bulk of the component. The benefit of shot peen comes from increasing cycles to crack initiation as LCF mean stress is decreased due to compressive residual stresses. It is the intent of this work to combine the effects of machining-like flaws underneath the shot peened surface in order to characterize the flaw size that may create a debit to total LCF life. Experiment DP-718 is a delta processed version of alloy 718 (more commonly referred to as Inconel 718). This is a Ni-based superalloy used in a variety of aerospace applications including seals, turbines and compressors. The variation of the heat treatment causes the delta phase to become spherical in shape (see figure 2), different from the typical Widmastatten delta that is apparent along the grain boundaries of alloy 718. This allows for increased fatigue life and better tensile properties and refines the grain structure to minimize the variation in the mechanical properties [2]. (approximately 25 hours) then the second step was a cyclic triangular waveform, f=0.333hz, T=1225 F, R- ratio = 0.05, and σ max = 75% of yield strength for 30,000 cycles. After 30,000 cycles the frequency was increased to 3.33Hz until failure. The purpose of the low stress, 90s hold time portion of the fatigue is to represent long term engine usage since thermal exposure at elevated temperatures is known to relax surface residual stresses. This is accentuated in heavily cold worked materials such as those with shot peened surfaces [1][3]. Since cold work is highest at the surface, the residual stresses have completely relaxed after thermal exposure. Less relaxation occurs as cold work decreases internally leading to a peak residual stress at a similar location as the original shot peen condition. As shown in figure 3 both static-no stress and cyclic-stress thermal exposures decrease the residual stresses, but cyclic-stress conditions were applied to the specimens in order to better represent practical engine conditions. The specimen geometry used is a double edge notch as shown in figure 4a, notches were machined using low stress grind, and polished to limit fatigue life variation caused by surface roughness. Figure 3 - Effect of shot peening and thermal exposure on machined specimens. Figure 2 - Typical Kalling s etched microstructure of DP718. The fatigue testing was separated into two steps. The first step was cyclic trapezoidal waveform with 90s hold time at peak stress, T = 1225 F, R-ratio = 0.05, and σ max = 33% of yield strength. The trapezoidal waveform was run for 1000 cycles Figure 4 - Initial specimen geometry (a) followed by the assumed skim cut required to remove the precrack (b). Representative EDM starter flaw (c) and anticipated small flaw geometry (d). 2
Specimens were tested in multiple conditions in order to better represent different machining characteristics, as shown in table 1. In order to create the notch defect, an EDM flaw was placed in one of the notches. Specimens were pre-cracked at a nominal peak stress of 50% of yield strength, R- ratio = 0.05 until 2c was approximately 50% larger in length with an assumption that a/c ratio was 1. Using submerged wire-edm the notches were skim cut to remove the starter notch and leave a flaw on the order of 0.003 0.005 while maintaining the original notch radius, see figure 4(d). The recast layer left by the EDM surface was polished off leaving a surface roughness similar to the as-machined condition to maintain consistency. Then the specimen notches were shot peened using consistent parameters covering any remaining flaw. Striations were used to determine the crack propagation cycles by measuring the density of the striations in relation to the distance from the initiation location. By plotting striation density vs crack depth and integrating over the total propagation length (0.015 to overload), crack propagation cycles can be estimated [4]. Using asmachined specimens it was determined that crack propagation cycles were approximately 5,000 cycles of total fatigue life. For striation analysis it was assumed that the initial flaw size was 0.015 in depth with an a/c ratio = 1. Since is it anticipated that subsurface crack propagation rate is not affected by surface residual stresses, the asmachined striation analysis will be used for shot peened specimens. Test Condition as-machined, 5 100% 100% large precrack large precrack, 6 100% 100% shot peen Small precrack, 12 75% 25% shot peen Table 1 - Specimen testing per each condition and breakdown of failure mode. Results & Discussion # of specimens tested % surface initiation % initiation due to added flaw as-machined 6 0% NA The first evaluation was to determine the fatigue life of the as-machined condition and to understand the failure mechanisms associated with the current component specimens. Average total fatigue life for the as-machined specimens was greater than 150,000 cycles. Initiation site documentation, using a scanning electron microscope (SEM), showed that all of the asmachined specimens had subsurface primary initiations being either a crystallographic facet or Ti/Nb carbides. Average depth of the subsurface initiation location was 0.001. This depth correlates to the maximum compressive residual stress depth (figure 3). The fracture path showed the majority of the crack propagation to be transgranular in nature allowing for striation analysis to be completed on the specimens. Figure 5 - lognormal probability plot of total cycles to failure for all tested specimens. Specimens tested with a large flaw contained the EDM notch plus the precrack but did not have any portion of the notch or precrack removed nor were these specimens shot peened. These specimens had extremely low fatigue lives, 25% of the propagation life, as the crack began to propagate immediately and at an accelerated crack growth rate due to the large size of the initial flaw and subsequently a high stress intensity factor. During fracture surface analysis, it was determined that these specimens showed an all intergranular crack propagation path correlating to the high stress intensity. A subset of the large flaw specimens were shot peened to determine if there was any benefit or variation in fatigue life. As shown in figure 5, there is an increase in total fatigue life 2x the as-machined, large flaw specimens. However, it is due to a decrease in crack propagation rate from the near 3
surface compressive residual stresses from a decrease in the mean stress from surface residual stresses. Propagation still occurred immediately and was 60% of propagation life compared to the asmachined specimens. These flaws are larger than what typically would occur due to machining defects in controlled aerospace applications categorizing them as beyond a worst case scenario and show no benefit from shot peening. Figure 6 shows a typical representation that compares the original EDM notch surface with the remaining flaw for failures lower than the propagation life. The large flaw size had an average depth of 0.007 which is about half the minimum predicted flaw size to immediately start crack propagation. The flaw size depth ranged from 0.003 to 0.012 with little variation in the total cycles to failure all of which were less than the calculated propagation cycles. The final set of specimens had their flaws decreased in size to better represent a machining flaw. These flaws averaged 0.002 in depth. This set of specimens is broken into two groups, one in which the induced flaw remained after the skim cut removal and the other in which no induced flaw remained. The group which had a flaw remaining can further be separated as represented in both figures 5 and 7. Two specimens still contained large enough flaws to initiate crack propagation immediately and were less than 50% of the propagation cycles, however there were two specimens that 40% of the total life was represented by crack propagation cycles. These two specimens had an initial flaw depth just slightly larger than the peak residual stress. The final two specimens had <2% of total life represented by crack propagation which is on order of the as-machined specimens and shot peened specimens that contained no remaining induced flaw. These final two specimens showed no debit due to the initial flaw size. Specimens not containing the induced flaw showed an average fatigue life of 3x the as-machined specimens and had multiple initiations both surface and subsurface. Surface initiations were assumed to have arrested due to the compressive residual stresses as their initial depth never exceeded the maximum compressive residual stress. Since the surface residual stresses were mostly relaxed it is likely that initiation occurred early since the initiation sites were heavily oxidized. Subsurface initiations were similar to those of the as-machined being either Ti/Nb carbides or faceted in nature. Figure 6 - Overall macro photo representing a large flaw fracture surface. (b) Represents a close-up SEM photo of the precrack surface transitioning into the LCF crack face (separation occurs at the dashed line). (c) An SEM image showing intergranular failure that occurs during cyclic LCF of large flaw initiations 4
Conclusions Shot peened specimens do show a benefit in both average and minimum fatigue life when compared to as-machined specimens. In situations where the flaw is large enough, crack propagation occurs immediately with a high crack tip stress, inducing intergranular crack propagation. Since the specimens underwent a thermal exposure prior to fatigue testing, surface residual stresses are near due to the decrease in mean applied stress from the compressive residual stresses near to the crack tip. This is shown in figure 7 by the vertical shift in cycles to failure of the large flaw, shot peen specimens over the as-machined, large flaw specimens even though the initial flaw depths are similar. However, any remaining residual stresses are not significant enough to delay crack initiation since total fatigue life is still less than the 5000 propagation cycles. Figure 7 - Primary initiation depth vs. cycles to failure for all specimens. For subsurface initiations the measure depth is the length of the flaw perpendicular to the notch surface. zero which is more representative of long term engine exposure conditions. Upon viewing figure 7, a flaw can be considered small when it is less than the depth of the peak compressive residual stress, otherwise it will be considered large. This also correlates with the as-machined specimens, since the peak residual stress is similar in location to the depth of primary initiation. However, the notches in this study were machined in order to keep residual stresses near zero. In typical controlled machining tensile residual stresses can be induced at the surface that may accelerate crack initiation. When a large flaw occurs the crack propagation rate is initially reduced under shot peen conditions. This is As the flaw depth decreases, approaching the peak residual stresses, it is more prone to resisting propagation. Historically, most considerations for initial flaw size is 0.015 in depth based on nondestructive testing s minimum threshold for crack detection [4], and although conservative, is a representative point for calculating crack propagation. Secondary surface initiation sites also correlate to the maximum compressive residual stresses as they stall once that depth is reached. This further validates that cracks that are smaller than the peak compressive residual stress are suppressed. Since most machining defects relate to pulled out and 5
dragged material, or surface scratches, shot peen does appear to alleviate any negative effects these features may induce. As long as the initial flaw is smaller in depth than peak compressive residual stresses no debit in total fatigue life appears. As the peak depth is approached there appears to be some debit that may occur, however further investigation will need to take place in order to better characterize what effect that debit may be. Future evaluations may include better characterization of the flaw size and shape on the fatigue life as well as determining the effect of varying peak residual stress depth as a function of stress amplitude and mean stress. References 1. Prevey, P.S.; Hornbach, D.J; Mason, P.W. Thermal Residual Stress Relaxation and distortion in surface enhanced gas turbine engine components. Proceedings of the 17 th Heat Treating Society Conference and Exposition and the 1 st International Induction Heat Treating Symposium. 1997. 3-12. 2. Ruiz, Carlos; Obabueki, Abel; Gillespie, Kathy. Evaluation of the Microstructure and Mechanical Properties of Delta Processed Alloy 718. Superalloys 1992. 1992. 33 42. 3. Cammett, J.T.; Prevey, P.S.; Jayaraman, N. The Effect of Shot Peening Coverage on Residual Stress, Cold Work, and Fatigue in a Nickel-Base Superalloy. Proceedings of ICSP 9. 2005. Paper 261. 4. Cherolis, Nicholas E. Fatigue in the Aerospace Industry: Striations. Journal of Failure Analysis and Prevention. 2008. 8:255-258. 6