Initiation and propagation behaviour of fatigue cracks in hard-shot peened Type 316L steel in high cycle fatigue

Observations of fatigue crack growth behaviour were made during rotating‐bend testing of hard‐shot peened Type 316L steel. From the results of these observations, the crack that developed in the axial direction was observed and the mechanism of the fatigue crack properties was clarified as follows: (1) Small circumferential surface fatigue cracks were detected at 60% of the fatigue lifetime. These cracks propagated very slowly in both the circumferential and radial directions. (2) When a radial crack reached a depth of between 150 and 350 μm, axial fatigue cracks were formed. (3) In the next stage, either the radial or the axial fatigue cracks continued propagating, or an inwards growing radial crack formed from the axial crack. (4) In the final stage, the circumferential surface crack began to grow rapidly and resulted in fracture. (5) The fracture type of hard‐shot peened Type 316L is a particular type of surface fracture.     

Influence of overloading on fatigue durability and stability of residual stresses in shot peened normalized steel

The present investigation deals with the effect of overloads on the stress relaxation and fatigue life time of shot-peened near-pearlitic steels. Single cycles with a total strain amplitude of 0.6% with start in tension or compression were exerted either at the very beginning or after 1000 cycles and superimposed on the constant total strain amplitude test at 0.3%. The results were compared with the constant amplitude test data. It was shown that such overloading can reduce the fatigue life time by 25-60% of the life time obtained in constant amplitude tests. Maximum amount of reduction was obtained in overloading histories started with straining in tension. The stress amplitudes and corresponding mean stress development after overloading were also recorded and used to interpret the effects of overloads on the fatigue life times. In addition the residual stress relaxation throughout the whole lifetime was followed.     

Effect on fatigue performance of shot peened components: An analysis using DOE technique

Shot peening is widely used to improve the fatigue properties of components and structures. Residual stresses, surface roughness and work hardening are the main effects induced in the superficial layer from shot peening, which depend on the correct choice of the peening parameters. Compressive stresses are beneficial in increasing resistance to fatigue failures, corrosion fatigue, stress corrosion cracking, hydrogen assisted cracking, fretting, galling and erosion. In this paper design of experiment (DOE) technique was used in carrying out test, using an air blast type shot peening machine. This investigation examines the effect of process parameters such as pressure, shot size, nozzle distance and the exposure time on the fatigue performance of AISI 1045 and 316L material. After going through confirmation test the analysis reveals the right combination of the parameters for better process control. An ANOVA was carried out to identify significant peening parameters. Expressions correlating fatigue life and the process parameters for both materials were developed, which are useful in predicting fatigue life. This technique could prove beneficial in industries for reduction of performance variation and cost and to increase productivity.     

Effect of shot peening on fatigue performance of ductile iron castings

Abstract

Ductile iron is a commonly used structural material. However the unsatisfactory fatigue performance has limited its application for some dynamic loads. Shot peening is a mechanical surface modification process to extend the fatigue life of materials. Results of the influence of the shot peening treatment on ductile iron castings with as-cast surface and machined surface are presented. The results showed that shot peening ductile iron castings could double the fatigue life for an as-cast surface and quadruple the fatigue life for a machined surface. It is believed that shot peening affects fatigue life through the retardation of crack nucleation and growth as a result of the introduction of work hardening, the existence of compressive stresses on the surface layer, and the removal of the surface irregularities of the ductile iron castings.     

Relaxation of residual stress in shot peened Udimet 720Li under high temperature isothermal fatigue

The extent of residual stress relaxation in turbine disc material Udimet 720Li was measured using laboratory X-rays with the sin²ψ technique, for fatigue samples as a function of temperature and number of fatigue cycles for strain controlled loading to 1.2%. Results showed that extensive relaxation occurs upon the initial fatigue cycle. The maximum compressive residual stress (RS) parallel to the loading direction is found to decrease by 50% for all testing temperatures. The extent of relaxation upon further cycling increased with temperature. In the plastically deformed near surface region, the diffraction peak width decreased with increasing testing temperature and number of fatigue cycles (and exposure time), indicating that the relaxation of cold work is controlled by both thermal and mechanical processes.     

Effect of loading type on fatigue properties of high strength bearing steel in very high cycle regime

Very high cycle fatigue tests under axial loading at frequencies of 95 Hz and 20 kHz were performed to clarify the effect of loading type on fatigue properties of a high strength bearing steel in combination with experimental result of this steel under rotating bending. As a result, this steel represents the single P-S-N (probabilistic-stress-life) curve characteristics for surface-induced fracture and interior inclusion-induced fracture, just like that under rotating bending. However, fatigue strength is lower, where the run-out stress at 10⁹ cycles is evaluated to be 588 MPa, less than that under rotating bending with about 858 MPa. Occurrence probability of larger and deeper inclusion-induced fracture is much higher than that under rotating bending. Furthermore, the formation process of fine granular area (FGA) is independent of the type and frequency of loading, which is very slow and is explained as the crack nucleation process under the special dislocation mechanism. The stress intensity factor range at the front of FGA, ΔK_FGA, is approximately regarded as the threshold value controlling the stable propagation of interior crack. For the control volume of specimen under axial loading, the estimated value of fatigue limit by FGA is similar to experimental run-out stress value at 10⁹ cycles, but that by inclusion is larger. However, the corresponding estimated results under rotating bending are all conservative.     

Notable size effects on very high cycle fatigue properties of high-strength steel

Very high cycle fatigue (VHCF) properties were compared between two types of specimens: enlarged specimens and our standard specimens. Fatigue tests were conducted by ultrasonic fatigue testing; the material used was commercial spring steel. All tests ended in internal fracture, with large-size effects observed, i.e., the enlarged specimens showed lower VHCF strength than the standard specimens. Most of the internal fracture origins were oxide-type inclusions that were larger in the enlarged specimens than in the standard specimens, indicating the size effect to be caused by the difference in oxide-type inclusion sizes at the origins of internal fractures. The large-size effect strongly urges the use of large specimens when conducting VHCF tests on high-strength steel. Moreover, the large-size effect implies that fatigue strength cannot in this case be determined using the conventional S-N curve approach, since the S-N curve depends on the specimen size. The evaluation of the VHCF strength thus needs two steps: an estimation of the maximal inclusion size, followed by an estimation of the VHCF strength based on the maximal inclusion size.     

Very high cycle fatigue properties of bearing steel under axial loading condition

It has been observed to appear a step-wise or duplex S–N curve under the test of rotary bending fatigue using high strength steel. This behavior was caused by the transition of fracture mode from surface-induced fracture to subsurface inclusion-induced fracture. The aim of this study is to clarify the S–N characteristic under an axial loading fatigue in the very high cycle fatigue regime. In order to investigate the mean-stress effects, fatigue tests were carried out in air at room temperature under three applied stress ratios of ?1, 0 and 0.5 using a hour-glass shaped specimen of high carbon–chromium bearing steel, JIS SUJ2. From the results, three types of fracture mode were observed on the fracture surface, such as surface-induced fracture, subsurface inclusion-induced fracture without granular bright facet (GBF) area and that with GBF area around an inclusion. Fatigue lifetime for transition in the fracture mode depended on the applied stress ratio. Shape of the S–N curve was a smooth and continuous under three testing conditions in spite of the occurrence on the three types of fracture. Detail discussion for fatigue fracture behavior was made through the observation of fracture surface and from point in view of the fracture mechanics. In addition, an effect of residual stress in the specimen surface layer on the transition of fracture mode was discussed and compared with the experimental results.     

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.     

High- and very high-cycle plain fatigue resistance of shot peened high-strength aluminum alloys: The role of surface morphology

The present paper is aimed at investigating the effect of shot peening on the high and very-high cycle plain fatigue resistance of the Al-7075-T651 alloy. Pulsating bending fatigue tests (R = 0.05) were carried out on smooth samples exploring fatigue lives comprised between 10⁵ and 10⁸ cycles. Three peening treatments were considered to explore different initial residual stress profiles and surface microstructural conditions. An extensive analysis of the residual stress field was carried out by measuring with the X-ray diffraction (XRD) technique the residual stress profile before and at the end of the fatigue tests. Fatigue crack initiation sites were investigated through scanning electron microscopy (SEM) fractography. The surface morphology modifications induced by shot peening were evaluated using an optical profilometer. The influence of surface finishing on the fatigue resistance was quantified by eliminating the surface roughness in some peened specimens through a tribofinishing treatment. The capability of shot peening to hinder the initiation and to retard the subsequent propagation of surface cracks is discussed on the basis of a model combining a multiaxial fatigue criterion and a fracture mechanics approach.     

The influence of partial surface shot peening on fatigue crack growth behaviour of a high‐strength ferritic steel

The effects of partial surface shot peening on the fatigue crack growth behaviour of a ferritic steel have been experimentally investigated in this paper. Dog‐bone specimens fabricated from Optim700QL were tested under tension‐tension fatigue loads. Three distinct extents of partial shot peening, with respect to the crack tip and specimen symmetry line, were tested. The fatigue crack growth results from these experiments have been compared with those obtained from the same specimen geometry but with no peening. The results show that the residual stress fields formed ahead of the initial notch tip due to the partial peening process play a significant role in the fatigue crack growth behaviour of the material and effectively result in accelerated crack propagation at the midwidth of the specimens. It has been shown in this study that partial peening can lead to a fatigue crack growth rate around twice as fast as that of the unpeened specimen.     

Effect of shot peening on fatigue limit of surface flawed samples

The effects of shot peening on the fatigue limit of specimens having a semicircular notch of varied surface length, 2a , are investigated. In the case of un-peened specimens, the fatigue limit of specimens having a notch of a = 0.05 mm was equal to that of the un-notched specimens. However, the fatigue limit of a = 0.3 mm was 46% smaller than that of the un-notched specimens. On the contrary, in the case of peened specimens, the fatigue limit of a = 0.2 mm was equal to that of the un-notched specimens and furthermore, that of a = 0.3 mm was only 5% smaller than that of the un-notched specimens. Multiple non-propagating cracks were observed in peened specimens after fatigue testing. The stress intensity factor of the maximum non-propagating crack size corresponded to that of a = 0.2 mm notch. These results indicate that shot peening increases fatigue limit and decreases the likelihood that a surface flaw will result in failure.     

Effect of severe shot peening on ultra-high-cycle fatigue of a low-alloy steel

It is well known that shot peening is able to increase the fatigue strength and endurance of metal parts, especially with a steep stress gradient due to a notch. This positive effect is mainly put into relation with the ability of this treatment to induce a compressive residual stress state in the surface layer of material and to cause surface work hardening. Recently the application of severe shot peening (shot peening performed with severe treatment parameters) showed the ability to obtain more a remarkable improvement of the high cycle fatigue strength of steels. In this paper severe shot peening is applied to the steel 50CrMo4 and its effect in the ultra-high cycle fatigue regime is investigated. Roughness, microhardness, X-ray diffraction residual stress analysis and crystallite size measurement as well as scanning electron microscopy (SEM) observations were used for characterizing the severely deformed layer. Tension–compression high frequency fatigue tests were carried out to evaluate the effect of the applied treatment on fatigue life in the ultra-high cycle region. Fracture surface analysis by using SEM was performed with aim to investigate the mechanism of fatigue crack initiation and propagation. Results show an unexpected significant fatigue strength increase in the ultra-high cycle region after SSP surface treatment and are discussed in the light of the residual stress profile and crystallite size.     

Effect of internal hydrogen on very high cycle fatigue of precipitation-strengthened steel SUH660

The fatigue life of SUH660 steel is dominated by crack initiation in the region of very high cycle fatigue owing to the new crack initiation behavior near the tip of temporarily arrested crack. The effect of internal hydrogen on very high cycle fatigue life is investigated focused on crack initiation life via fatigue and Vickers hardness tests. Hydrogen inhibits cracks initiation, and accelerates the increase in crack initiation lives with decreasing stress in low and medium hardness zones. Hydrogen increases the hardness in low and medium hardness zones. Hydrogen extends new crack initiation lives and causes longer very high cycle fatigue life.     

Effect of ultrasonic nanocrystal surface modification on the characteristics of AISI 310 stainless steel up to very high cycle fatigue

Effects of ultrasonic nanocrystal surface modification (UNSM) on the very high cycle fatigue response of AISI 310 stainless steel have been investigated. The higher impact force used in UNSM treatment showed a higher fatigue life improvement. The fatigue life improvement was higher in crack initiation from the surface of specimens. The subsurface crack initiation depth in the alloy increased with increase in the fatigue failure cycles. It was concluded that UNSM treatment can increase the life of the alloy significantly up to very high cycle fatigue.     

Resistance spot weldability and high cycle fatigue behaviour of martensitic (M190) steel sheet

Resistance spot welding characteristics of martensitic sheet steel (M190) was investigated using a peel test, microhardness test, tensile shear test and fatigue test. Tensile shear test provides better spot weld quality than conventional peel test and hardness is not a good indicator of the susceptibility to interfacial fracture. Unlike DP 600 steel, the maximum load carrying capability is affected by the mode of fracture. At high load low cycle range, weld parameters have a significant difference in the S–N curves. But, almost similar fatigue behaviour of the spot welds is noted at low load high cycle range. However, when applied load was converted to stress intensity factor, the difference in fatigue behaviour between welds and even DP 780 steel diminished. Furthermore, a transition in fracture mode, that is, interfacial and plug and hole type at about 50% of yield load were observed.
[* Note: Correction made on 16 Aug 2010 after first publication online on 28 June 2010. The authors' affiliations were corrected. Under Results and Discussion, in reference to the HAZ hazardness in the ‘Micro hardness profile’ section, Figure 2 was changed to Figure 3. In reference to the welding parameters under ‘Tensile properties’ section, note that Figure 4 represents 7/200 and Figure 5 represents 5/300. In reference to the low cycles behaviour of S‐N curves in the ‘Fatigue’ section, Figure 5 was changed to Figure 6.]     

Evaluating surface deformation and near surface strain hardening resulting from shot peening a tempered martensitic steel and application to low cycle fatigue

The plastic deformation resulting from shot peening treatments applied to the ferritic heat resistant steel FV448 has been investigated. Two important effects have been quantified: surface roughness and strain hardening. 2D and 3D tactile and optical techniques for determining surface roughness amplitude parameters have been investigated; it was found that whilst R_a and S_a were consistent, S_z was generally higher than R_z due to the increased probability of finding the worst case surface feature. Three different methods for evaluating the plastic strain profile have been evaluated with a view to establishing the variation in yield strength near the surface of a shot peened component. Microhardness, X-ray diffraction (XRD) line broadening and electron backscatter diffraction (EBSD) local misorientation techniques were applied to both uniaxially deformed calibration samples of known plastic strain and samples shot peened at intensities varying from 4A to 18A to establish the variation in plastic strain and hence the variation in yield strength. The results from the three methods were compared; XRD and EBSD profiles were found to be the most similar with microhardness profiles extending much deeper into the sample. Changes in the measured plastic strain profile after exposure to low cycle fatigue and the correlation of these changes with the cyclic stress–strain behaviour of the material are also discussed with a view to assessing the importance of the dislocation profile in component life assessment procedures.     

Effect of shot peening on fatigue fracture for an as quenched martensitic steel

Abstract

A Mn–C steel with an as quenched martensite microstructure expected to be used as a high strength steel for automobile parts. However, the fatigue strength of the steel is relatively low in spite of its high tensile strength. Such low fatigue strength is attributed to a low elastic limit caused by mobile dislocations in the as quenched martensite structure. Shot peening improves the fatigue life markedly below 10⁷ cycles but has little effect at 10⁷ cycles. These results are explained by the fact that the initiation site of the fatigue crack changes from the surface of the as quenched specimen to the interior of the shot peened specimen.     

Low cycle fatigue and cyclic softening behaviour of martensitic cast steel

The paper presents the results of research on fatigue properties in a small number of cycles to failure of high-chromium martensitic GX12CrMoVNbN9-1 (GP91) cast steel designed for the casts working under the so-called supercritical parameters. The tests of fatigue life were carried out at three temperatures: room temperature, 550 °C and 600 °C, for five levels of controlled amplitude of total strain ɛ_ac: 0.25%; 0.30%; 0.35%; 0.50% and 0.60%. Cyclic softening was observed in the fatigue tests at all temperatures without a stabilization period. The fatigue lifetime curves at each temperature were obtained based on Basquin and Coffin–Manson equations. It has been shown that the influence of temperature on fatigue life depends on the level of strain and is the biggest for the lowest levels of strain covered in the research. The results of fatigue tests are connected with the quantitative changes in the microstructure of the cast steel. Quantitative tests were carried out using thin foils by means of TEM. Disappearance of martensitic laths, increase in the subgrains size, decrease in the dislocations density and coagulation of carbides were observed after low cycle fatigue. The scale of these changes in the microstructure is dependent on the temperature and level of strain.     

Effect of microstructure and low cycle fatigue deformation on tensile properties of P91 steel

Isothermal furnace heat treatments were carried out to simulate the microstructures of inter-critical, fine grain and coarse grain heat-affected zones of P91 steel weld joint at different soaking temperatures ranging from just above AC₁ (837 °C) to well above AC₃ (903 °C). Interrupted low cycle fatigue tests were performed on the specimens of P91 steel up to 5 %, 10 %, 30 %, and 50 % of the total fatigue life at the strain amplitude of ±0.6 %, strain rate of 0.003 s⁻¹ and temperatures of 550 °C and 600 °C. Subsequently, tensile tests were conducted on the interrupt tested specimens at the same strain rate and temperatures. Soaking at the inter-critical temperature region reduces / deteriorates the tensile and yield strengths of base metal compared to fine grain and coarse grain regions. The inter-critical heat-affected zone accounted higher damage contribution towards the overall tensile behavior of the actual P91 steel weld joint. Substructural coarsening during strain cycling at elevated temperatures attributes to the rapid reduction in the initial yield strength up to 10 % of fatigue life of P91 steel. A higher amount of plastic strain accumulation during low cycle fatigue deformation resulted in a decrease in fatigue life of the inter-critical heat-affected zone of P91 steel.