Experimental results in fretting fatigue with shot and laser peened Al 7075-T651 specimens

This work focuses on determining the effect of shot and laser peening on fretting fatigue in the Al 7075-T651 alloy. These surface treatments generate a residual compressive stress field near the treated surface where contact under fretting fatigue produces high stress levels. The fretting fatigue resistance of shot and laser peened specimens was assessed in a series of tests involving measurements of the residual stress field, residual stress relaxation under the action of cyclic loads, the friction coefficient, surface roughness and material hardness. The obtained results are compared with those for untreated specimens. The tests show the beneficial effect of the compressive residual stresses and the improvement that surface roughness causes in fretting fatigue life, especially in shot peened specimens. Another important effect observed, is the partial residual stress relaxation produced during the fretting fatigue tests.     

Prediction of 3D small fatigue crack propagation in shot-peened specimens

Shot peening has been widely applied in industrial design to improve fatigue durability of high loaded machine components. The compressive residual stress induced by shot peening is in general assumed to be responsible for the improvement of material fatigue strength. In the present work a cyclic cohesive zone model is extended to analyze three-dimensional fatigue crack growth in shot-peened specimens. Fatigue crack growth behaviors in both unpeened and peened specimens are investigated using 3D finite element analysis. The parameters of the cohesive zone model have been identified in 2D unpeened specimens and are applied to predict peened specimens directly. The results indicate that shot peening strongly affects crack initiation time and crack profiles, but has little effect on crack propagation rate. It implies that the shot peening will hardly change Paris’ law used for the damage tolerant design.     

Influence of shot peening on fatigue durability of normalized steel subjected to variable amplitude loading

The influence of shot peening on the fatigue durability of normalized carbon steels subjected to variable amplitude loading has been investigated. The relaxation of residual stresses was recorded during the fatigue life time. Strain amplitude spectra were extracted from real spectra recorded from components in service. The results were compared with data achieved from constant amplitude testing. In both types of tests parallel studies were made on both peened and unpeened specimens. Shot peening leads to pronounced increase in life time, especially for smaller amplitudes. For both variable and constant amplitude loading shot peened specimens exhibit longer life provided the residual stresses during fatigue loading do not relax more than to about 60% of their initial value. To get an improvement in life time of at least a factor two for peened specimens, the stress amplitude in constant amplitude loading or the maximum stress amplitude in variable amplitude history must not be more than 20% larger than the magnitude of the initial residual stresses. This limit corresponds to 1.2 times the yield strength of the unaffected material.     

Effects of Shot‐Peening on Fretting Fatigue Crack Growth Behavior in Ti‐6Al‐4V

Abstract: The effects of shot‐peening on fretting fatigue crack growth behaviour in titanium alloy, Ti‐6A1‐4V were investigated. Three shot‐peening intensities: 4A, 7A and 10A were considered. The analysis involved the fracture mechanics and finite element sub‐modelling technique to estimate crack propagation lives. These computations were supplemented with the experimentally measured total fretting fatigue lives of laboratory specimens to assess the crack initiation lives. Shot‐peening has significant effect on the initiation/propagation phases of fretting fatigue cracks; however this effect depends upon the shot‐peening intensity. The ratio of crack initiation and total life increased while the ratio of the crack propagation and total life decreased with an increase of shot‐peening intensity. Effects of residual compressive stress from shot‐peening on the crack growth behaviour were also investigated. The fretting fatigue crack propagation component of the total life with relaxation increased in comparison to its counterpart without relaxation in each shot‐peened intensity case while the initiation component decreased. Improvement in the fretting fatigue life from the shot‐peening and also with an increase in the shot‐peening intensity appears to be not always due to increase in the crack initiation resistance from shot‐peened induced residual compressive stress.     

EFFECT OF SHOT PEENING ON FATIGUE BEHAVIOR IN AL-ALLOYS

The individual contributions of residual compressive stresses, high dislocation densities, and surface roughness that are induced by shot peening to the improvement of fatigue life of Al 2024 are assessed. Stress amplitudes to number of fatigue cycles (S-N) curves and microcrack propagation (da/dN-ΔK curves) are determined for both underaged (T3) and peak-aged (T6) conditions. The fatigue behavior of the electropolished (which is also the reference condition), shot peened, shot peened and stress relieved, shot peened and polished, and shot peened, stress relieved, and polished condition is evaluated in fatigue tests.     

Short‐crack thresholds and propagation in an AISI 4340 steel under the effect of SP residual stresses

The effect of residual stresses induced by shot‐peening in a high‐strength AISI 4340 steel has been studied with the purpose of deriving a consistent fatigue model incorporating the results of fatigue crack growth experiments in the threshold region for a broad range of load ratio (R‐ratio ranging from ?2.5 to 0.7), and the effect of short cracks by means of a modified El‐Haddad model. The proposed model, taking into account the effect of crack closure and being capable to assess the conditions for fatigue propagation of short cracks partially embedded in the shot‐peened surface layer, was validated against constant amplitude fatigue experiments conducted in the endurance strength region, ie, for fatigue lives up to 10⁷ cycles, with micronotched specimens in the presence of shot‐peening residual stresses. The proposed model was also validated by comparing the results of fatigue crack propagation simulations with fatigue crack growth experiments under variable amplitude loading, experimentally reproducing the combined effect of service fatigue loads and shot‐peening residual stresses.     

Eigenspannungsmessungen an unterschiedlich abgekohlten,kugelgestrahlten und torsionsschwellbeanspruchten Proben aus Federstahl

Residual Stress Measurements on Differently Decarburized, Shot Peened and Cyclic Torsional Loaded Specimens of a Spring Steel In addition to previous fatigue tests with the spring material 55 Cr 3, X-ray residual stress measurements were carried out on differently decarburized, shot peened and cyclic torsional loaded specimens. It was shown that longer duration of decarburization leads to higher residual tensile stress in the surface zone for not peened specimens. Shot peening, however, effects almost equal residual compressive stresses in the surface zone in spite of different starting conditions, with a slight decrease for specimens with a long duration of decarburization, which corresponds to the results of the fatigue tests. By cyclic torsional loading with mean stress the residual compressive stresses due to shot peening are diminished in the axial and circumferential directions. A main stress system is now to be found with an angle of 45° to axis, whereas the residual stresses in the s?₁-(tensile direction) retain their values, those in the s?₂-(compressive) direction are decreased distinctly.     

Increased fatigue strength of partially stabilised zirconia achieved by shot peening

The effects of shot peening (SP) on the fatigue strength of partially stabilised zirconia were studied. Smooth specimens and specimens containing a surface pre-crack with depths in the range 35–110?μm were subjected to SP. The cyclic fatigue tests were performed using a three-point bending setup. SP introduced compressive residual stresses on the specimens and improved their fatigue strengths. The shot peened specimens with pre-crack depths?≤?50?μm fractured outside the pre-crack area and exhibited considerably high fatigue limits, equivalent to those of the shot peened smooth specimens. Therefore, the pre-cracks with depths?≤?50?μm could be rendered harmless by SP, which was confirmed by the theoretical estimations based on fracture mechanics.     

Low cycle fatigue life prediction in shot‐peened components of different geometries—part I: residual stress relaxation

In this study, the residual stress relaxation behaviour occurring during low‐cycle fatigue in shot‐peened specimens with either a flat or a notched geometry has been studied. A representative low‐pressure steam turbine material, FV448, was used. The residual stress and strain hardening profiles caused by shot peening were measured experimentally and were then incorporated into a finite element model. By allowing for both effects of shot peening, the residual stress relaxation behaviour was successfully simulated using this model and correlated well with the experimental data. Although more modelling work may be required to simulate the interaction between shot peening effects and external loads in a range of notched geometries, the model predictions are consistent with the specimens tested in the current study. The novelty of this study lies in the development of such a modelling approach which can be used to effectively simulate the complex interaction between shot peening effects and external loads in notched regions. Compared with the un‐notched geometry, the notched geometry was found to be more effective in retaining the improvement in fatigue life resulting from shot peening, by restricting the compressive residual stress relaxation during fatigue loading.     

A stress approach model for predictions of fatigue life by shot peening of EN45A spring steel

A lot of research has been done to improve fatigue strength of materials by creating compressive residual stress field in their surface layers through shot peening. In this paper, fatigue strength of shot peened leaf springs has been calculated from laboratory samples. The axial fatigue strength of EN45A spring steel specimen is evaluated experimentally as a function of shot peening in the conditions used for full-scale leaf springs testing in industries. Optimum shot peening condition for specimen is found and S/N curves of the specimens are correlated with leaf springs curve. A mathematical model has been developed which predicts the fatigue life of leaf springs for a given stress at varying shot peening conditions. Predictions from this model are compared with experimental data. The estimation of fatigue life and relaxation of compressive residual stress field are discussed.     

Crack incubation in shot peened AA7050 and mechanism for fatigue enhancement

Shot peening is a dynamic cold‐working process involving the impingement of peening media onto a substrate surface. Shot peening is commonly used as a surface treatment technique within the aerospace industry during manufacturing to improve fatigue performance of structural components. The compressive residual stress induced during shot peening results in fatigue crack growth retardation, improving the performance of shot‐peened components. However, shot peening is a compromise between the benefit of inducing a compressive residual stress and causing detrimental surface damage. Because of the relatively soft nature of AA7050‐T7451, shot peening can result in cracking of the constituent precipitate particles, creating an initial damage state. The aim of this paper is to understand the balance and fundamentals of these competing phenomena through a comparative study throughout the fatigue lifecycle of baseline versus shot‐peened AA7050‐T7451. Microstructure and surface topology characterization and comparison of the baseline and shot‐peened AA7050‐T7451 has been performed using scanning electron microscopy, electron backscatter diffraction, energy dispersive spectroscopy, and optical profilometry techniques. A residual stress analysis through interrupted fatigue of the baseline and shot‐peened AA7050‐T7451 was completed using a combination of X‐ray diffraction and nanoindentation. The fatigue life performance of the baseline versus shot‐peened material has been evaluated, including crack initiation and propagation. Subsurface particles crack upon shot peening but did not incubate into the matrix during fatigue loading, presumably due to the compressive residual stress field. In the baseline samples, the particles were initially intact, but upon fatigue loading, crack nucleation was observed in the particles, and these cracks incubated into the matrix. In damage tolerant analysis, an initial defect size is needed for lifetime assessment, which is often difficult to determine, leading to overly conservative evaluations. This work provides a critical assessment of the mechanism for shot peening enhancement for fatigue performance and quantifies how incubation of a short crack is inhibited from an initially cracked particle into the matrix within a residual stress field.     

Fatigue assessment and failure analysis of shot‐peened leaf springs

The paper deals with the fatigue and failure analysis of serial shot‐peened leaf springs of heavy trucks emphasizing on the influence of thermal treatment and shot peening on fatigue life. Experimental stress–life curves are determined by investigating smooth specimens subjected to fully reversed rotating bending conditions. These test results are compared to corresponding ones determined from cyclic three‐point bend tests on shot‐peened serial leaf springs in order to reveal the influence of the applied thermal treatment and shot peening process on the fatigue life of the high‐strength steel used for leaf spring manufacturing, dependent on the load level. Microstructure, macro‐ and micro‐hardness analyses are performed to support the analyses and explain the effects resulting from the certain shot peening process on the surface properties of the high‐strength spring steel under investigation. The assessment of the fatigue results reveals nearly no life improvement due to the manufacturing, emphasizing the necessity for mutual adjustment of shot peening and thermal treatment parameters to take account for life improvement.     

The effect of residual stresses arising from laser shock peening on fatigue crack growth

Residual stresses have in the past been introduced to manipulate growth rates and shapes of cracks under cyclic loads. Previously, the effectiveness of shot peening in retarding the rate of fatigue crack growth was experimentally studied. It was shown that the compressive residual stresses arising from the shot peening process can affect the rate of crack growth. Laser shock peening can produce a deeper compressive stress field near the surface than shot peening. This advantage makes this technique desirable for the manipulation of crack growth rates. This paper describes an experimental program that was carried out to establish this effect in which steel specimens were partially laser peened and subsequently subjected to cyclic loading to grow fatigue cracks. The residual stress fields generated by the laser shock peening process were measured using the neutron diffraction technique. A state of compressive stress was found near the surface and tensile stresses were measured in the mid-thickness of the specimens. Growth rates of the cracks were observed to be more affected by the tensile core than by the compressive surface stresses.     

Influence of anodization on the fatigue strength of 7050-T7451 aluminium alloy

In recent years, with higher demand for improved quality and corrosion resistance, recovered substrates have been extensively used. Consequently residual stresses originated from these coatings reduce the fatigue strength of a component. Due to this negative influence occasioned by corrosion resistance protective coatings, an effective process like shot peening must be considered to improve the fatigue strength. The shot peening treatment pushes the crack sources beneath the surface in most of medium and high cycle cases due to the compressive residual stress field (CRSF) induced. The aim of this study was to evaluate the influence on the fatigue life of anodic films grown on 7050-T7451 aluminium alloy by sulphuric acid anodizing, chromic acid anodizing and hard anodizing. The influence on the rotating and reverse bending fatigue strength of anodic films grown on the aluminium alloy is to degrade the stress life fatigue performance of the base material. A consistent gain in fatigue life in relation to the base material was obtained through the shot peening process in coated specimens, associated to a residual stress field compressive near the surface, useful to avoid fatigue crack nucleation and delay or even stop crack propagation.     

激光喷丸强化对半圆孔件疲劳寿命的影响

为研究激光喷丸强化对7075-T6铝合金半圆孔件疲劳寿命的影响,对激光喷丸与未喷丸的试样进行了对比试验,利用X射线应力仪测定其表面残余应力,并对试样进行疲劳拉伸试验.用扫描电镜观察了两类试样疲劳断口的形貌,并采用数理统计方法对其疲劳寿命进行分析.研究表明:经激光喷丸处理区域,表面存在较大的残余压应力,幅值为310 MPa;未喷丸试样疲劳裂纹条带的宽度为0.7~0.8μm,而喷丸试样疲劳裂纹条带的宽度为0.3~0.4μm,说明喷丸试样裂纹扩展的速度比未喷丸试样慢很多;激光喷丸后半圆孔件的疲劳寿命比未喷丸的疲劳寿命提高了2.8~7.2倍.     

Influence of residual stresses from shot peening on fretting fatigue crack growth

One method to improve fretting fatigue life is to shot peen the contact surfaces. Experimental fretting life results from specimens in a Titanium alloy with and without shot peened surfaces were evaluated numerically. The residual stresses were measured at different depths below the fretting scar and compared to the corresponding residual stress profile of an unfretted surface. Thus, the amount of stress relaxation during fretting tests was estimated. Elastic–plastic finite element computations showed that stress relaxation was locally more significant than that captured in the measurements. Three different numerical fatigue crack growth models were compared. The best agreement between experimental and numerical fatigue lives for both peened and unpeened specimens was achieved with a parametric fatigue growth procedure that took into consideration the growth behaviour along the whole front of a semi‐elliptical surface crack. Furthermore, the improved fretting fatigue life from shot peening was explained by slower crack growth rates in the shallow surface layer with compressive residual stresses from shot peening. The successful life analyses hinged on three important issues: an accurate residual stress profile, a sufficiently small start crack and a valid crack growth model.     

Shot peening of plasma nitrided steel for fretting fatigue strength enhancement

Abstract

The potential of fretting fatigue strength enhancement by a duplex surface engineering process involving shot peening of plasma nitrided steel, termed duplex SP/PN, is demonstrated. Specimens of 709M40 steel were individually plasma nitrided, shot peened, or duplex SP/PN treated. Fretting fatigue properties of the surface engineered specimens were evaluated. Surface roughness, residual stress, and hardening effect following the various surface treatments were examined and compared. It has been found that the duplex treatment can significantly improve the fretting fatigue strength of the investigated low alloy steel. Under the present testing conditions, the duplex SP/PN treatment increased the fretting fatigue strength (at 10⁷ cycles) of 709M40 steel by more than 70% relative to the nitrided, 120% to the shot peened, and 500% to the untreated steel. The improvement has been explained in terms of the significantly increased surface hardness and compressive residual stress in the near surface region following the duplex SP/PN treatment. By analysing the stress distributions in a shot peened surface, the influence of surface roughness on fretting fatigue strength is also discussed.     

Surface modifications induced by shot peening and their effect on the plane bending fatigue strength of a Cr-Mo steel produced by powder metallurgy

The effect of shot peening on the plane bending fatigue strength of a 7.1 g/cm³ sintered Cr-Mo steel was investigated. Shot peening provides surface densification, strain hardening, compressive residual stresses up to −700 MPa, without impairing the dimensional and geometrical precision of specimens. Plane bending fatigue strength increases of 30%, irrespective to the different residual stress profiles obtained by changing the shot peening parameters. The improvement is mainly due to the surface densification and strain hardening.     

Fatigue behavior of X70 microalloyed steel after severe shot peening

The so called “severe plastic deformation” (SPD) processes are object of increasing interest due to their ability to obtain a nanostructured surface layer of material with supposed superior properties. Among these processes, severe shot peening (SSP) is very attractive from an industrial point of view, due to its versatility and wide applicability.In this study the X70 microalloyed steel is considered and treated by severe shot peening.Fatigue tests were performed to assess how SSP treatment affects the fatigue behavior of this steel.Roughness and X-ray diffraction residual stress measurements as well as microscopy observations have been carried out on the treated specimens. Rotating bending fatigue tests at room temperature on smooth and notched specimens were performed to evaluate the effect of the treatment on fatigue strength. Fracture surfaces have been then observed by scanning electron microscopy. The results were compared with the ones of a series of not peened specimens and another series of specimens shot peened with conventional parameters and interpreted by considering the characteristics of the nanostructured layer, the induced residual stresses and the surface work hardening. The comparison shows a very significant fatigue strength improvement for severely shot peened specimens, especially for the notched specimens.