Residual Stress Gradient Research Articles

Microstructure and residual stress gradient evolution mechanism of electronic copper strip for lead frame

The miniaturization of integrated circuits has led to a shift in lead frame fabrication methods from physical stamping to chemical etching. However, this process is highly sensitive to residual stress. We investigated the gradient distribution of residual stress, microstructure, and macroscopic texture in electronic copper foils using a layer-by-layer peeling method, and proposed a novel strategy for evaluating residual stress levels. Our findings reveal that the rolling-induced residual stress exhibits a gradient distribution along the thickness direction. Specifically, from the surface to the central layer, the stress transitions gradually from tensile to compressive, with the stress type primarily influenced by metal flow during rolling. Additionally, we observed gradient variations in the S and Cube texture. Tensile stress favors the formation of Cube-texture, while compressive stress promotes S-texture development. Notably, Cube-texture can lead to strain localization during rolling, directly impacting the local residual stress levels and distribution. Furthermore, quantitative analysis of dislocation density using peak profile methods revealed a consistent trend between {111} plane dislocation density and lateral residual stress. Further investigation highlighted that dislocations and precipitates associated with {111} planes in face-centered cubic metals significantly influence residual stress, with their combined effects determining the overall stress magnitude in relation to texture, dislocations, and precipitates.

Effect of compound surface modification of shot peening and DLC coating on surface integrity and fatigue properties of gear steel 16Cr3NiWMoVNbE

We conducted a study on the surface compound modification of shot peening and pure carbon DLC coating to simultaneously meet the requirements of wear resistance and fatigue resistance of spline structure. The effects of surface compound modification were investigated on the surface morphology, residual stress profile, microstructure, and nano-indentation hardness of 16Cr3NiWMovNbE gear steel, and conducted a comparative study on fatigue performance. The results show that the surface compound modification inherits the surface morphology and compressive residual stress gradient of shot peening, while the surface residual stress is slightly smaller than that of shot peening. In addition, surface compound modification still reflects the characteristics of high hardness and high fracture resistance of DLC coatings. Under the bending load based on spline tooth root, compared to the original specimen, the fatigue life after shot peening, pure carbon DLC coating, and surface compound modification is increased by 3.68, 2.35, and 3.36 respectively. Although the compound modified surface still maintains the shot peening morphology with a increasing surface roughness and stress concentration coefficient, the 100 μm-depth compressive residual stress profile and the subgrain refinement layer introduced, as well as the hard surface layer with good load-bearing capacity, have played the role of fatigue strengthening.

Gradient residual stress evolution and its influence on fatigue life under combined carburising heat treatment and ultrasonic surface rolling process

This study combined carburising heat treatment (CHT) with the ultrasonic surface rolling process (USRP) to improve the rotary bending fatigue properties of 18CrNiMo7-6 alloy steel. Based on the experimentally obtained fatigue limit (σ-1), the evolution of gradient compressive residual stress (CRS) of CHT- and CHT + USRP-treated specimens was analysed below the fatigue limit stress level. Subsequently, the evolution mechanism of gradient CRS of the two reinforced specimens was discussed and analysed from the opposing effects of austenite transformation and dislocation density change on the evolution of CRS. The results demonstrated that the selective bending fatigue performance of the alloy was evidently improved after the introduction of CRS; however, the change in fatigue limit was not evident. The durability under other loads (such as cyclic tension/compression and torsion loads) needs further study. The gradient CRS evolution of CHT specimens was mainly caused by the large residual austenite transformation and small dislocation density change. Therefore, the volume expansion caused by residual austenite transformation gradually increased the gradient compression residual stress of CHT specimens with an increased number of cycles. Contrarily, the degree residual stress evolution of CHT + USRP specimens was more affected by the dislocation density, and the gradient compression residual stress gradually decreased with an increased number of cycles. Finally, the residual stress evolution rate of the two reinforced specimens was divided into two stages: fast and slow.

Backside metallization affects residual stress and bending strength of the recast layer in laser-diced Si

Thin Silicon dies separated by laser dicing form a thin layer via redeposition of ablated silicon known as recast layer. This work analyzed the influence of the recast layer microstructure and nanoscale residual stress gradients on the bending strength of bare and metalized silicon dies <100 μm. Scanning and transmission electron microscopy revealed an intricate microstructure of ablated silicon and elements of the wafer backside metallization within the recast layer. Refined silicon grains decorated by nanoscopic metallic precipitates at their grain boundaries were observed. Cross-sectional synchrotron X-ray nanodiffraction revealed that the altered microstructure increased the tensile residual stress from 200 to 295 MPa for bare and metalized dies, respectively. Additionally, the metalized die exhibited gradients in residual stress and grain size between the die front- and backside. Despite their similar frontside bending strengths of ∼340 MPa, observed in 3-point bending experiments, a considerable strengthening of the backside from 425 up to 957 MPa was measured for bare and metalized die, respectively. The origins of the tensile residual stress and the influence of the backside metallization on the die bending strength are discussed.

A study of stress, composition and grain interaction gradients in energy-dispersive X-ray stress analysis on materials with cubic symmetry.

The influence of various combinations of residual stress, composition and grain interaction gradients in polycrystalline materials with cubic symmetry on energy-dispersive X-ray stress analysis is theoretically investigated. For the evaluation of the simulated sin2ψ distributions, two different strategies are compared with regard to their suitability for separating the individual gradients. It is shown that the separation of depth gradients of the strain-free lattice parameter a 0(z) from residual stress gradients σ(z) is only possible if the data analysis is carried out in section planes parallel to the surface. The impact of a surface layer z* that is characterized by a direction-dependent grain interaction model in contrast to the volume of the material is quantified by comparing a ferritic and an austenitic steel, which feature different elastic anisotropy. It is shown to be of minor influence on the resulting residual stress depth profiles if the data evaluation is restricted to reflections hkl with orientation factors Γ hkl close to the model-independent orientation Γ*. Finally, a method is proposed that allows the thickness of the anisotropic surface layer z* to be estimated on the basis of an optimization procedure.

Optimization of tensile properties and anisotropy in a cryogenically treated laser additively manufactured high entropy alloy

Repetitive deep cryogenic soaking treatment (DCT) of laser metal deposition (LMD) processed CrMnFeCoNi high entropy alloy (HEA) significantly enhances its strength without compromising ductility. This is attributed to the compressive stress induced nanotwin formation, which in turn facilitates twin induced plasticity. In this work, a parametric study on the effect of the residual stress profile and the DCT cycles on the tensile properties of the HEA, along the build and scan directions is conducted. Towards this end, builds fabricated with 5 different laser powers, 1100, 1400, 1700, 2000 and 2300 W, are examined and the ones with highest and lowest residual stress gradient are considered for further DCT treatments. Results indicate that the build fabricated with 1400 W laser power, which has the highest gradient in initial residual stresses, exhibits a greater enhancement in dislocation and twin density with increasing number of DCT treatments. Compared to its as-built state the peak yield and tensile strength of the HEA (along the scanning direction) increases to 592 ± 13 and 778 ± 15 MPa, without significant decrease in its ductility after 12 DCT cycles. However, the enhancement in the dislocation, twin density and therefore, the strength, is minimal after it is treated to 15 DCT cycles. Anisotropy in both strength and ductility, which is of the order of 20-25 %, is also observed in the DCT treated builds along the build and scan directions. These observations were rationalized on the basis of dislocation and twin evolution and distribution during DCT and deformation of the build when deformed in different directions. Implications of these results in the context of employing DCT for strengthening LMD fabricated HEA components are discussed.

The effect of fine particle peening on aviation gear performance, part II: Rolling contact fatigue

AbstractFine particle peening (FPP) technology offers an effective method to achieve outstanding surface integrity and fatigue performance in aviation gears. However, the lack of sufficient research on the correlation between process parameters, surface integrity parameters, and fatigue life undermines the full potential of FPP in strengthening aviation gears. Therefore, we have developed an elastic–plastic contact fatigue life model to predict the fatigue life of AISI 9310 gear steel after different strengthening processes. This model can consider surface integrity parameters such as surface micro‐topography, residual stress gradient, and hardness gradient. To validate the model, we conducted rolling contact fatigue tests and compared the results with the predictions from our model. The influence of process parameters on the rolling contact fatigue of AISI 9310 gear steel was further investigated. A process window for FPP was established, and an optimization method was proposed in order to provide guidance for selecting process parameters for FPP on high‐performance aviation gears.

Strains and Stresses in Multilayered Materials Determined Using High-Energy X-ray Diffraction

This work explores the advantages and disadvantages of a methodology using high-energy X-ray diffraction to determine residual stresses in multilayer structures produced by atmospheric plasma spraying. These structures comprise a titanium alloy substrate (Ti64), a bonding layer (Ni-Al), and an abrasive coating (Al2O3). This study focuses on analyzing the residual stress gradients within these layers. The presented method is used to determine stresses across the entire thickness of multilayer structures. Experiments were carried out using a high-energy rectangular beam, operating in transmission mode, on the cross-section of the sample. The results indicate variable stresses throughout the depth of the sample, particularly near the layer interfaces. The semi-automatic methodology presented here enables us to follow stress evolution within the different layers, providing indications of the load transfer between them and at their interfaces. The sin2ψ method was used to analyze the diffraction data and to determine the stresses in each phase along the sample depth. However, interpreting results near the interfaces is complex due to the geometric and chemical effects. We present a discussion of the main advantages and disadvantages of the methodology for this kind of industrial sample.

Composite effects of residual stress and hardness gradient on contact fatigue performance of rough tooth surfaces and optimization design of detailed characteristic parameters

Residual stress and hardness gradient significantly affect the contact fatigue failure process of rough tooth surfaces, and the meticulous design of their characteristic parameters can help improve the fatigue resistance of gears. In this paper, an efficient computational method for rough tooth surface contact is established based on the reconstruction modeling of asperities and mixed lubrication analysis. By comprehensively considering the effects of residual stress and hardness gradient, the Dangvan multiaxial fatigue criterion is adopted to predict and analyze the contact fatigue life of the gear surface. The correlation between the surface residual stress, maximum residual stress depth, surface hardness, and effective hardening layer depth, among other characteristic parameters, and the contact fatigue performance of tooth surfaces is established. The results show that: (1) Increasing the magnitude of surface residual compressive stress and surface hardness can significantly reduce the risk of fatigue failure in the near-surface layer of the gear. To achieve the same fatigue performance, there is a linear negative correlation between the design values of the two parameters. (2) Affected by the double stress peaks in the rough tooth surface stress field, the maximum residual stress depth significantly affects the depth at which the highest failure risk occurs. With changes in roughness, the optimal design value of the maximum residual compressive stress depth changes from about 0.5 times the Hertzian contact half-width depth in the subsurface layer (Sa < 0.2 μm) to about 15 μm in the near-surface layer (Sa > 0.4 μm). (3) Considering both process cost and gear surface fatigue resistance, the design threshold for the effective hardening layer depth should be greater than the Hertzian contact half-width. This paper establishes a composite correlation between the meticulous characteristic parameters of residual stress, hardness gradient, and the contact fatigue performance of tooth surfaces, providing theoretical support for optimal design of tooth surface anti-fatigue performance.

Experimental and numerical insights into fatigue crack propagation of railway component with gradient residual stress

As a representative structure with an acute radial gradient in terms of residual stress (RS) and grains, the induction-hardened railway S38C axle exhibits excellent mechanical properties and fatigue resistance. To investigate the effect of gradient features on fatigue crack growth (FCG) behaviors, we designed three-point bending (TPB) tests with different gradient features, including the negative-gradient, positive-gradient, and matrix specimens. Finite element modeling was introduced to simulate the FCG of the TPB specimens in terms of the cyclic J-integral (ΔJ) and equivalent plastic strain at the growing crack tip. The results reveal that the FCG rates in both negative- and positive-gradient structures decrease and a more pronounced tendency of decline is achieved in the positive-gradient ones. The substantial compressive residual stress (CRS) within the negative-gradient structure directly influences the crack tip area, while the CRS in the positive-gradient structure indirectly mitigates the crack driving force by impeding the overall structural deformation. Coarser grains aid in enhancing energy absorption along the zigzag crack path, thereby diminishing the crack driving force. ΔJ and the equivalent plastic zone surrounding the crack tip indicate significant reductions in the presence of CRS.

Gradient residual stress and fatigue life prediction of induction hardened carbon steel S38C axles: Experiment and simulation

Gradient distribution of triaxial residual stresses to a depth of several millimeters is retained in middle carbon steel S38C axles after high-frequency induction hardening, which has become a critical concern for fatigue structural integrity. To address this, the axial, hoop, and radial gradient residual strains inside the axles were measured for the first time by advanced neutron diffraction. The SIGINI Fortran subroutine was then adopted to reconstruct the global initial residual stress field from the measured data. Experimental and simulation results show that residual stresses of about −520 MPa (axial), −710 MPa (hoop), and −40 MPa (radial) residual stress were retained below the axle surface. Subsequently, the fatigue crack propagation behavior of S38C axles was numerically investigated in the framework of fracture mechanics. The calculated results clearly show that the compressive residual stresses at a depth of 0–3 mm from the axle surface lead to a low crack growth driving force, and that fatigue cracks do not propagate as long as the crack depth is less than 3.7 mm for hollow S38C axles. These results further indicate that the maximum defect size allowed in routine inspections is acceptable from a safety and economic point of view. Accurate measurement and characterization of the global gradient residual stress field through experiments and simulations can provide an important reference for optimizing the mileage intervals of nondestructive testing (NDT) of surface defects in these surface-strengthened railway axles.

Fabrication of cemented carbides with bidirectional gradient structure via graphene deposition to simultaneously enhance surface hardness and toughness

The adoption of a continuous gradient in Co content stands as the foremost strategy for attaining heightened surface hardness and improved core toughness in cemented carbides. Nevertheless, the reduction in surface toughness stemming from a decreased Co content often precipitates premature surface failure of components. In this paper, WC-11Co powders was utilized as the precursor materials for fabricating a bidirectional gradient cemented carbide using the combined approach of low-temperature presintering and ultrasonic treatment techniques. The resultant cemented carbide exhibited a distinct gradient distribution, characterized by a forward gradient of graphene content, a reverse gradient of Co content, a forward gradient of WC grain size, and a forward gradient of residual compressive stress. The hardening and toughening mechanisms of the fabricated bidirectional gradient cemented carbides were comprehensively characterized and quantitatively discussed. The results provide compelling evidence that the gradient distribution of Co content plays a crucial role in attaining an impressive surface hardness of 1820 kgf/mm2. By utilizing the crack bridging mechanism of graphene, which efficiently exploits its strain transfer capabilities across a semi-coherent interface, in conjunction with the toughening effects of crack deflection and residual compressive stress, an exceptional surface fracture toughness of 17.62 MPa m1/2 is achieved. Additionally, the gradient layer exhibited a higher critical load for cracking. The present findings presents a novel strategy for the fabrication of high-performance ceramic-based composites.

Microstructure and residual stress distribution of electron beam-welded joints of a 50 mm-thick TA15 titanium alloy plate

Abstract Vacuum electron beam welding was applied to a 50 mm-thick TA15 titanium alloy plate. Microstructure observation and microhardness testing were performed. Meanwhile, the residual stress on the surface at different distances from the weld centre was measured via the hole drilling method. A finite element model for electron beam welding of the TA15 titanium alloy plate was constructed on commercial finite element software. Experimental results showed that the microstructure of the weld zone consisted of a basket martensitic phase. The highest microhardness in the weld zone was 409 HV, and the microhardness of the base metal was the lowest. The maximum gradient change area was located at the HAZ near the base metal. The peak value of longitudinal residual stress from the simulation was 880 MPa, which was in the weld metal. The sharp change area of longitudinal residual stress gradient was located in the base metal near HAZ.

Laser shock-induced gradient twinning microstructure in AZ31B alloy

Magnesium alloys are highly promising in biodegradable materials for medical applications. However, their applications are limited by weaknesses in mechanical strength and corrosion resistance. In recent years, advancements in a novel laser method, Laser Shock Peening (LSP), have been explored. Researchers suggest that LSP can induce surface compressive stresses and induce changes in residual stress and grain size gradients, thereby enhancing the material's fatigue resistance, corrosion resistance, and resistance to stress corrosion cracking. However, there is ongoing debate about whether LSP treatment can, in effect, create a gradient microstructure. In this study, we focused on the AZ31B alloy, notable for its dense hexagonal close-packed (HCP) crystal structure. This material is particularly interesting because, due to its low stacking fault energy, it is more prone to twinning deformation than to dislocation slip when it undergoes plastic deformation. To analyze the changes wrought by LSP, we employed optical microscopy (OM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) for examining the microstructures both before and after LSP treatment. Our findings revealed that LSP treatment refined the grain size, leading to a gradient distribution. In addition, we observed a direct correlation between the volume fraction of twinning and grain size. In contrast, the geometrically necessary dislocation (GND) density exhibited an inverse relationship. By measuring hardness and residual stress in the region affected by LSP, we determined that LSP could introduce a layer of plastic deformation up to a depth of 800–1000 μm. We also established a link between the gradient microstructure and hardness and provided a thorough discussion on the types of twinning and the mechanisms behind the induction of gradient microstructure on the surface.

Effect of laser shock peening on fretting wear behaviour of AISI 304 stainless alloy

Unlike many other surface treatment techniques, such as shot peening (SP), low plasticity burnishing (LPB), ultrasonic surface rolling process (USRP), ultrasonic nanocrystal surface modification (UNSM), laser shock peening (LSP) is a non-contact process. This means that LSP achieves surface modification without direct physical contact, thereby eliminating the risk of media contamination and undesired wear on the peening equipment. As a surface hardening technology, LSP tends to produce a hardened layer with a high gradient of residual stress (RS) in the near-surface, significantly enhancing wear resistance. The well-known wear theories applied for numerical prediction, whether Archard model or dissipated frictional-energy model, contain a multiplier for contact pressure and relative slip amplitude p·ds or shear stress and relative slip amplitude q·ds, respectively. These multipliers are sensitive to the mechanical properties near the surface. Additionally, the wear profile is governed not only by the wear effect but also by the deformation of materials. Therefore, to accurately evaluate the wear performance for surface-hardened materials, the influence of RS is hard to be overlooked. In this work, the finite element method (FEM) is utilised to construct a 3D numerical model, which ignores and considers RS, for both as-received and LSP-treated samples. Two distinct RS distributions induced by different scanning strategies (one- and five-time), are introduced in the finite element (FE) model. The numerical simulation results are then compared with the experimental data to verify the accuracy of the fretting wear model built in this study. On this basis, the impact of different RS distributions on fretting wear characteristics, including wear profile and volume, is extensively investigated. The findings reveal that higher amplitude RS can reduce the degree of fretting wear to a greater extent, highlighting the beneficial role of RS in enhancing wear resistance. Furthermore, the numerical models incorporating RS demonstrate superior predictive accuracy compared to those that omit RS. This naturally underscores the importance of integrating RS in numerical simulations of LSP and other surface-hardened materials.

Tensile properties and microstructural evolution of 17–4 PH stainless steel fabricated by laser hybrid additive manufacturing technology

Wire-based laser directed energy deposition (LDED) technology is an attractive and efficient additive manufacturing technology, which shows great potential in the process of manufacturing high-value engineering components. However, the thermal effect will lead to a large amount of residual tensile stress in the components, and the large crystal structures after solidification seriously affects the tensile performance. The pursuit of high mechanical properties in additive manufacturing components have prompted researchers to seek new strengthening process to produce high-strength components. Therefore, in this paper, laser shock peening (LSP) composite wire-based LDED were used for the additive manufacturing of 17–4 PH stainless steel under the specified experimental conditions, and the effects of LSP on microstructure evolution, microhardness, and tensile properties of wire-based LDED components were systematically studied. Results showed that uniform grains with an average size of 2.31 μm were generated on the top surface of specimen due to the ultra-high plastic strain induced by LSP shock wave. The size of hierarchical martensitic units shows varying degrees of reduction after LSP treatment, which is shown by the width decrease of packet, block and lath. Dynamic recrystallization caused by rearrangement and annihilation of high-density dislocations promoted the grain refinement. Meanwhile, the residual stress was completely converted from tensile state into compressive state with a maximum value of 425 MPa, and the microhardness was also enhanced to a peak value of 432 HV. In addition, we observed that the high-density dislocation environment created by LSP treatment promoted the further precipitation, and the aggregation phenomenon of precipitates was observed in the dense dislocation area. Furthermore, the tensile strength and elongation of LSP treated specimen were improved significantly, which was attributed to the combined effects of grain refinement, introduction of dislocation structures, precipitation strengthening and gradient residual compressive stress. In summary, laser hybrid additive manufacturing technology provides a new idea and method for the production of high-performance precipitation hardening stainless steel components.

Determination of gradient residual stress for elastoplastic materials by nanoindentation

Residual stress is an important manufacture factor affecting the working life of components and structures. In this paper, a dimensionless method is proposed to determine the residual stress profile by using the nanoindentation technique. An exponential function is formulated in a generalized form to describe the residual stress profile in the elastoplastic substrate material. A finite element (FE) model is created to simulate the entire process including the loading stage with the indenter penetrating into the substrate up to the maximum depth and also the subsequent unloading stage. For elastoplastic substrate materials following the power-law constitutive model, a wide range of FE predictions are made by considering various gradient residual stresses of interest. The results show that the applied load–penetration depth curve is highly consistent with the variation of gradient residual stress. Based on FE simulations, dimensionless functions considering the residual stress is derived for both the applied load and the total work done. Thereafter, a reverse algorithm is proposed to elegantly predict the gradient residual stress in elastoplastic materials. With several work examples, the proposed reverse algorithm is confirmed to be excellent for determining residual stresses by comparing the existing experimental results. More importantly, the proposed reverse algorithm can effectively determine the type and trend of gradient residual stresses, which helps to guide the optimization of manufacturing process for critical components and structures.

Modeling of the fatigue behavior of functionally graded materials: Study of the residual stresses induced by surface severe plastic deformation

AbstractA new finite elements approach has been used to model the evolution of residual stresses during cyclic loading in functionally graded 316 L stainless steel, obtained by surface severe plastic deformation. The affected layer modeling was divided in two stages: the compressive residual stress state initialization and the update of the mechanical properties characteristic of the gradient microstructure determined from local characterization. The capacity of the model to simulate both the strain‐ and stress‐controlled fatigue behavior was evaluated, and a good consistency between the numerical and experimental results was obtained. Numerical residual stress fields were compared to the experimental measurements, and a very good agreement was observed. A mean error of approximately 65 MPa over all the compared data was measured. Especially, the relaxed and inverted residual stress gradient after the different testing conditions were nicely retrieved, indicating the capabilities of the newly proposed model.

Residual stress analysis in industrial parts: a comprehensive comparison of XRD methods

A recently emerged XRD-based cosα residual stress measurement method, which utilizes imaging plate detectors, has attracted special attention from both academia and industry. There are uncertainties about to which extent the method could be used and about the accuracy of the measurements when analyzing industrial components. This work investigates the accuracy of the method by targeting four common types of material structures for the XRD experiments: preferred orientation of the microstructure (texture effect), coarse grain microstructure (coarse grain effect), a combination of both, and materials with steep lateral or in-depth residual stress gradients. The analysis was carried out by the conventionally used sin2ψ and the newly developed cosα methods on ferritic and austenitic steels, aluminum alloys, and SiSiC ceramics. The results indicate that both methods are reliable in most cases. However, cosα method has higher uncertainties and is more sensitive to the initial microstructure of the material.

Micro-mechanical behavior of lamellar structured Ti6Al4V alloy upon compression via experimental and crystal plasticity study

The micro-mechanical behavior of lamellar structured Ti6Al4V alloy to a small compressional strain of 4.5% was studied. Strain and stress intends to localize along grain boundaries (GBs), triple points (TPs) as well as grain interiors (GIs). The maximum strain and stress is found at some GB-TPs with soft and grain clustering mostly owing to their strong strain incompatibility. Their heterogeneous distribution is highly related to individual crystallographic orientation, their size, shape and grain-grain interactions. The local dislocation behavior forms ledges along with Geometrically necessary dislocations (GNDs) to accomodate strain incompatibility. EBSD and crystal plasticity (CP) simulation disclosed some long range meso-bands (about 45° to loading direction) which crosse multi-grains and some mini-bands within grain interiors (GIs) reflecting a concentrated strain behavior and ductile nature of the Ti6Al4V material. Prismatic slip dominates Ti6Al4V lattice (mainly from surface dislocation activation) determined by its low critical resolved shear stress (CRSS) which is away from some findings of literature. Dislocation is found commenced from some high angle GBs (HAGBs) and extened into GI which can pass through GBs of both low (LA) and high angles given their slip plane coherency. The substructures (α-lamellae) separated by LAGBs within α-colonies, have no significant effect to dislocation movement. Fatigue failure can be inferred from the early stage micro-mechanical behavior through accumulated plastic strain and strong strain incompatability. Fractography reaveals fatigue failure from surface, sub-surface, shearing, cleavage and intergranular modes are presumed highly related to surface defects, compressional residual stress gradient, easy prismatic slip and soft and hard GBs. Multiple crack origins from such combinations are deemed for reduced fatigue life of shot peened samples regardless of the surface compressional residual stress being generated.