Fatigue Crack Initiation Sites Research Articles

Fatigue Crack Growth Behaviour of an Out-of –Plane Gusset Welded Joints under Biaxial Tensile Loadings with Different Phases

Most of in-service welded built-up structures, which contain many welded joints as fatigue crack initiation sites, are subjected to many types of loading and these loadings have different axial components with different phases. However, the structural integrities are evaluated according to design codes based on theoretical and experimental investigations under a uniaxial loading condition. Most of these codes are based on the S-N curves approach. On the other hand, authors proposed the numerical simulation method of fatigue crack propagation histories of a cracked plate subjected biaxial loadings with phase difference of each loading component. Fracture mechanics approach is applied to establish our method. In this study, fatigue crack growth behaviour of an out-of-plane gusset welded joint under biaxial loading with two different phase conditions were investigated. The phase difference effect for fatigue crack shape evolution under biaxial loading was confirmed by measured ones. Besides, comparisons of measured crack evolution with the numerical simulation results were performed to validate of our fatigue crack growth simulation for welded joints.

Numerical simulation of fatigue initiation life for notched specimens with gradient surface layer

Based on Tanaka Mura's fatigue model and introduced the elastic strain energy release of the crack initiation, a new fatigue model is proposed for fatigue initiation life under complex loading. Then, with the gradient characteristics of the hardness and residual stress, this new model is used to assess the fatigue life distribution and the crack initiation site for the notched components with a gradient surface layer. The results of numerical simulation indicate that surface hardening treatment increases the fatigue life. And the gradient layer thickness changes the crack initiation site. If the gradient layer thickness is smaller than a critical value, fatigue crack will initiate at the interface of surface and matrix, otherwise at the surface or subsurface. The increase of hardness ratio will lead to an increase of the critical thickness value. Excessive residual compress stress reduces fatigue initiation life. For notched samples with the same stress concentration factor, after the same hardening treatment, the fatigue life and crack initiation site would change with the notch size of the sample.

Multiaxial fatigue resistance of shot peened high-strength aluminum alloys

This paper is aimed at investigating multiaxial fatigue of shot peened Al-7075-T6 alloy. Plain axi-symmetric specimens were subjected to combined in-phase tension and torsion loading, under nominal load ratio R=0.05 and biaxiality ratio λ=τa/σa=2. The results from multi-axial tests are discussed together with those obtained under pure tension and pure torsion loading. Fatigue crack initiation sites have been investigated through scanning electron microscopy fractography and the role of surface roughness on fatigue resistance has been analyzed. The initial and the stabilized residual stress profiles were used to discuss the improvement in the fatigue response in the hypothesis of crack initiation and early crack propagation as fatigue controlling parameters. For this purpose, several multiaxial fatigue criteria were used to account for the residual stress field. The Crossland and the Findley fatigue criteria incorporating stabilized residual stresses as mean stresses were found to predict the fatigue strength with a relative error lower than 15% within the entire fatigue life interval, for all the material variants and the loading conditions considered.

Welding Residual Stresses in Tubular Joints

In spite of an increased awareness of welding residual stress threat to structural integrity, the extent of its influence on fatigue especially under multiaxial loading is still unclear and is a matter of debate. One important reason for this lack of clarities is that the determination of the initial welding residual stress field in welded structures even at the fatigue crack initiation sites is difficult and requires complementary instruments. Since the fatigue crack initiation in sound welds almost always occurs on the surface, the determination of surface residual stresses could increase the awareness of the extent of their threat to the structural safety. In this paper the development of residual stresses in different TIG-welded tubular specimens out of S355J2H and S690QL steel is studied and compared. The mechanisms of the development of residual stresses based on heat input and cooling rate are discussed. The welding parameters and thus heat inputs are varied and the mechanisms leading to different residual stress states are investigated. X-ray method was used for residual stress state characterization.

Grain refinement and fatigue strengthening mechanisms in as-extruded Mg–6Zn–0.5Zr and Mg–10Gd–3Y–0.5Zr magnesium alloys by shot peening

This paper studies the fatigue properties and fracture behavior of as-extruded Mg–6Zn–0.5Zr and Mg–10Gd–3Y–0.5Zr alloys before and after shot peening. Compared to Mg–6Zn–0.5Zr alloy, Mg–10Gd–3Y–0.5Zr alloy shows higher optimal Almen intensity, and possesses a broader process window. The stress-controlled rotating bending fatigue property improvement for Mg–10Gd–3Y–0.5Zr alloy by shot peening is significantly superior to that of Mg–6Zn–0.5Zr alloy. With the increase in peening (Almen) intensity, the fatigue crack nucleation site of Mg–6Zn–0.5Zr alloy under stress control shifted from the surface to subsurface, and then back to the surface again. Meanwhile, a significantly higher number of fatigue crack initiation sites can be seen as a consequence of overpeening. However, the fatigue cracks of the peened Mg–10Gd–3Y–0.5Zr alloy initiated subsurface at all Almen intensities, showing unchanged crack initiation location with the increase in Almen intensity. The observed phenomenon is related to differences between the two alloys both in the deformation mechanisms during shot peening and the residual stress relaxation mechanisms during subsequent fatigue process. Namely, at present test conditions such as Almen intensity range and high cycle fatigue stress, in the Mg–6Zn–0.5Zr alloy twinning dominates deformation during shot peening and detwinning during fatigue. Comparatively, dislocation slip dominates deformation in both shot peening and fatigue process in the Mg–10Gd–3Y–0.5Zr alloy.

Influence of Mean Stress and Stress Variation on Fatigue Strength of Stainless Cast Steel SCS5 for Hydraulic Turbine Runner

In hydraulic turbine runners, accidents due to cyclic loading often occur. In this study, in order to clarify the fatigue properties and fracture mechanism of stainless cast steel SCS5 cut out from the hydraulic turbine runner vane for about 22 years, fatigue tests under constant amplitude with positive mean stress and variable amplitude loading were carried out by using tension-compression specimens. The results are summarized as follows : (1)The S-N curves of aged SCS5 used in this study have wide scatter. (2) From SEM observation of the fracture surface, fatigue crack initiation site were found to be the casting defect which consists of dendritic phase and shrinkage cavity. (3) It is possible to evaluate the fatigue life of aged SCS5 by using the defect size √area and the initial stress intensity factor range calculated by casting defect size. The value of the threshold stress intensity factor range obtained in this study were 3.8 MPa·m1/2. (4) Under repeated two-step loading pattern, a fatigue crack was found to grow when the stress intensity factor range was lower than the threshold stress intensity factor range of under constant amplitude loading.

General relation between tensile strength and fatigue strength of metallic materials

With the development of high-strength materials, the existing fatigue strength formulae cannot satisfactorily describe the relation between fatigue strength σw and tensile strength σb of metallic materials with a wide range of strength. For a simple but more precise prediction, the tensile and fatigue properties of SAE 4340 steel with the tensile strengths ranging from 1290MPa to 2130MPa obtained in virtue of different tempering temperatures were studied in this paper. Based on the experimental results of SAE 4340 steel and numerous other data available (conventional and newly developed materials), through introducing a sensitive factor of defects P, a new universal fatigue strength formula σw=σb(C−Pσb) was established for the first time. Combining the variation tendency of fatigue crack initiation sites and the competition of defects, the fatigue damage mechanisms associated with different tensile strengths and cracking sites are explained well. The decrease in the fatigue strength at high-strength level can be explained by fracture mechanics and attributed to the transition of fatigue cracking sites from surface to the inner inclusions, resulting in the maximum fatigue strength σwmax at an appropriate tensile strength level. Therefore, the universal fatigue strength formula cannot only explain why many metallic materials with excessively high strength do not display high fatigue strength, but also provide a new clue for designing the materials or eliminating the processing defects of the materials.

Influence of plasma and cold spray deposited Ti Layers on high-cycle fatigue properties of Ti6Al4V substrates

This paper presents a summary of the research on the influence of plasma and cold spray deposition of bio-grade Ti powder on the high-cycle fatigue properties of Ti6Al4V substrates.Four sets of flat specimens (as-received, grit-blasted, plasma and cold sprayed) were prepared and subjected to cantilever–beam cyclic bend loading with a constant deflection of the free end.It was found that the grit-blasting procedure significantly increased the fatigue lives of the specimens compared to the as-received set (1.81× increase). The deposition of the coatings onto grit-blasted specimens led to deterioration of fatigue properties. The average fatigue lives of the plasma sprayed and cold sprayed samples reached 1.16× and 0.91× of the as-received specimens, respectively.In order to understand the positive effect of the grit-blasting procedure and the adverse effect of the coatings deposition on fatigue lives, chemical analyses, fractographic analysis, microstructural investigations as well as layers elastic moduli estimation were carried out. It was found that the cold spray deposition retained the composition of the powder feedstock while a complete transformation of Ti into nitrides and oxides was detected in the plasma deposited layers. Owing to the different coating build-up principles and their respective different porosity levels, the moduli of the coatings were found to vary (7.2GPa for plasma sprayed and 36.7GPa for cold sprayed layers). The fatigue crack initiation sites and propagation directions in the plasma and cold sprayed specimens were found to differ substantially. Based on the obtained results, two explanations of the fatigue loading results are suggested in the paper.

Rotating bending fatigue properties of two case hardening steels after nitriding treatment

Fatigue properties of two Al-containing steels have been investigated by rotating bending fatigue tests. Results show that the fatigue limits (the fatigue strength at 107cycles) were improved remarkably by plasma nitriding due to the high hardness of 1000Hv and compressive stress of 400MPa in the nitrided layer. Scanning electron microscopic observations show that after nitriding the fatigue crack initiation sites moved from the surface flaws or near-surface matrix into the AlN inclusions at around the case-core interface. Degassing treatment can increase the fatigue limit because it prevented fatigue crack initiation at AlN inclusions due to the reduced [N] contents and refined inclusion size.

Fatigue properties of cast aluminium joints by FSW and MIG welding

Fatigue properties of cast aluminium welded joints by friction stir welding (FSW) and MIG welding were investigated, comparing with that of the base plate. Fatigue crack propagation tests for the da/dN − ΔK relation and bending fatigue tests for the S–N relation were carried out. Fatigue cracks in both FSW and MIG specimens were accelerated, when the fatigue crack tip reached the stir zone or the weld metal. This behaviour was discussed based on the crack closure induced by the crack surface roughness and the residual stress. In the S–N properties, the influence of specimen surface finishing on fatigue life was also examined. Fatigue lives of the FSW and MIG specimens in the ‘as weld’ condition were in the range of the largely scattered base plate fatigue lives, in spite of the different fatigue crack initiation sites in each specimen such as the porosity in the base plate, the tool mark bottom in the FSW and the weld toe in the MIG. The FSW specimens with the polished surface showed the particular improvement in fatigue strength for finite fatigue life.

Fatigue Life of Titanium Alloys Fabricated by Additive Layer Manufacturing Techniques for Dental Implants

Additive layer deposition techniques such as electron beam melting (EBM) and laser beam melting (LBM) have been utilized to fabricate rectangular plates of Ti-6Al-4V with extra low interstitial (ELI) contents. The layer-by-layer deposition techniques resulted in plates that have different surface finishes which can impact significantly on the fatigue life by providing potential sites for fatigue cracks to initiate. The fatigue life of Ti-6Al-4V ELI alloys fabricated by EBM and LBM deposition techniques was investigated by three-point testing of rectangular beams of as-fabricated and electro-discharge machined surfaces under stress-controlled conditions at 10 Hz until complete fracture. Fatigue life tests were also performed on rolled plates of Ti-6Al-4V ELI, regular Ti-6Al-4V, and CP Ti as controls. Fatigue surfaces were characterized by scanning electron microscopy to identify the crack initiation site in the various types of specimen surfaces. The fatigue life data were analyzed statistically using both analysis of variance techniques and the Kaplan-Meier survival analysis method with the Gehan-Breslow test. The results indicate that the LBM Ti-6Al-4V ELI material exhibits a longer fatigue life than the EBM counterpart and CP Ti, but a shorter fatigue life compared to rolled Ti-6Al-4V ELI. The difference in the fatigue life behavior may be largely attributed to the presence of rough surface features that act as fatigue crack initiation sites in the EBM material.

Demonstration of an in situ microscale fatigue testing technique on a titanium alloy

Under high cycle and very high cycle fatigue (HCF and VHCF) conditions, scatter in fatigue lifetimes is substantial; often 2–3 orders of magnitude. Characterization of fatigue crack initiation sites in laboratory scale fatigue specimens has led to the identification of characteristic initiation sites and microstructural arrangements. Despite these observations, in some cases, it is still unclear how apparently similar initiation sites exhibit such different total fatigue lifetimes. Differences in crack-initiation mechanisms can be further revealed if specific microstructural arrangements are isolated within a micro-specimen. Towards this end, an in situ microscale tension testing technique was adapted to enable microscale fatigue testing on tensile dog-bone specimens. Microscale tensile fatigue specimens with approximate gage diameters of 20μm were prepared with a focused ion beam (FIB) microscope. Initial tensile experiments were conducted to characterize the mechanical behavior of microscale specimens for this microstructure. The microscale tensile specimens were observed to exhibit reduced yield and flow stresses in comparison to bulk tensile specimens. However, in this feasibility demonstration of in situ microscale fatigue testing, microscale specimens exhibit enhanced fatigue properties relative to conventional specimens, which may result from the differences in strain rate and, potentially, the method of test control (load vs. displacement) between these two testing methods. Both tensile and fatigue specimens have been characterized with electron backscatter diffraction to identify the neighborhood and the specific slip systems that were activated under local deformation conditions within the microstructure. Both basal and prism slip were observed, although prism slip was more prevalent. Results of the tension and fatigue experiments are discussed in the context of the growing body of literature on microscale testing, and avenues for future work are highlighted.

Characterization of Cut-Edges for Improved Automotive Chassis and Suspension Fatigue CAE Life Predictions

The durability of safety-critical automotive vehicle steel structures has been observed to be influenced significantly by the condition of the component’s cut-edge properties. The importance of the mechanical cut-edge properties on structural durability has been observed due to surface imperfections generated during the cutting process. These then become the preferred initiation sites for fatigue cracks due to the increased intensity of stress applied to a structure. The current work outlines the development of an improved finite element (FE)-based life prediction method, based on strain-life fatigue data of S355MC steel cut-edges. The resulting best method of predicting fatigue lives of cut-edges using the Coffin–Manson method has been determined as providing the best means for predicting the durability of high strength steel components. This process has been validated using a bespoke laboratory test component representative of those in automotive chassis and suspension assemblies. The outcome of this work is that it allows increased reliance on FE life predictions rather than extensive physical laboratory testing.

Fatigue properties in high strength bulk metallic glasses

Fatigue tests were carried out on ultra-high strength Co- and Fe-based bulk metallic glasses (BMGs), and the fatigue results were investigated together with the ones which had already been obtained from the fatigue tests on the super-high strength Ni-based BMG and the high strength Ti- and Cu-based BMGs. The test alloy rods with a diameter of 2 mm were prepared in [(Co0.6Fe0.4)0.75B0.2Si0.05]96Nb4, (Fe0.5Co0.5)72B20Si4Nb4, Ti41.5Zr2.5Hf5Cu42.5 Ni7.5Si1, Ni60Zr20Nb15Al5 and Cu60Zr30Ti10 at% systems by the copper mold casting method. The test specimens were machined to hourglass shape type with a minimum diameter of 0.9 mm. The specimens were tested at a stress ratio of 0.1 and a frequency of 10 Hz. The fatigue limits in the Co- and Fe-based BMGs exceeded 2 GPa, and those in the Ni- and Ti-based BMG surpassed about 1.5 GPa. Fatigue ratios in all BMGs were found to be about 0.5. Exceptionally, only the Ti-based BMG showed the high ratio, which may indicate the possibility that the BMG does not cause the fatigue phenomenon. In front of the fatigue crack initiation sites, fracture surfaces showed a radial ridges and valleys morphology as well as a striation like pattern. Unstable fracture surfaces consisted of facets with vein patterns. As the facet and vein pattern sizes increase, the strength and fatigue limit decreased, whereas the fracture toughness increased. Fracture surfaces in front of the fatigue cracks in strong BMGs showed the clear elastic wave traces.

Correlation between Scatter in Fatigue Life and Fatigue Crack Initiation Sites in Cast Aluminum Alloys

Correlation between Scatter in Fatigue Life and Fatigue Crack Initiation Sites in Cast Aluminum Alloys

Fatigue Strength of Weathering Steel

Fatigue behaviour of Atmofix 52 steel (comparable to COR-TEN â steel) exposed to atmospheric corrosion for 20 years was investigated. S-N curves for load symmetrical cycling and cycling with stress ratio R = 0 were determined on specimens detracted from a failed transmission tower. The data were compared with those on material without a rust layer. The fracture surfaces and, in particular, the sites of fatigue crack initiation were analyzed. Substantial decrease of fatigue life and fatigue limit due to corrosion exposition was found. Based on observation of surface layer with corrosion products and on fractographic analysis of failed specimens conclusions on fatigue damage mechanism were drawn. No grain boundary corrosion, which can be responsible for fatigue crack initiation, was observed. Initiation of fatigue cracks was related to surface roughness and took place exclusively on corrosion dimples. DOI: http://dx.doi.org/10.5755/j01.ms.18.1.1335

Effect of Inhomogeneity of Zr-Based Bulk Metallic Glass Plate on Fatigue Strength under Torsion

Fatigue tests of Zr-based bulk metallic glass plates were conducted under cyclic-torsion, and the results were compared with those obtained under cyclic-bending and axial-loading. The fatigue strength of axial-loading was lower than that under cyclic-bending because the fatigue strength depends on the cooling rate in the casting process of the material, which is different in the thickness direction. The fatigue strength of plane-bending reflects the strength at the specimen surface while that of axial-loading is determined by the weakest strength in the thickness direction. For cyclic torsion of specimens with rectangular cross-section, the fatigue crack initiation site must be determined by the ratio of specimen width to specimen thickness, and fatigue strength depends on the ratio. Therefore, the criteria for fatigue limit under combined stress for metallic glass should be constructed considering the crack initiation site.

Relaxation of welding residual stresses – Part I: under quasi-static loading

The influence of welding residual stress on fatigue performance depends strongly on how stable these stresses are during cyclic loading. If the tensile residual stresses at the fatigue crack initiation sites relax, their influence on service life decreases and if they stay stable their influence on life assessments should be taken into consideration. The relaxation of residual stresses under cyclic loading is most considerable during the very first load cycle which could be considered as relaxation under quasi–static loading. It was shown here that the von Mises failure criterion describes the relaxation mechanisms in welded S355J2G3 and S690QL specimens under quasi–static tensile and compressive loading. By increasing the tensile load a continuous relaxation of residual stresses occurs. Under compression the relaxation sets in with delay which is an evidence of stability of tensile residual stresses under compression. The residual stresses were determined by means of X–ray diffraction.

Casting defects and high temperature fatigue life of IN 713LC superalloy

High-cycle high temperature fatigue life of a superalloy IN 713LC in as cast state and after hot isostatic pressing was experimentally determined for symmetrical cycling and cycling with tensile mean stress of 300MPa. Fatigue tests were conducted at 800°C in laboratory air. It has been found that the hot isostatic pressing improves the fatigue life. Large casting defects are sites of fatigue crack initiation in both states of the alloy. The hot isostatic pressing reduces the size of casting defects, however the broad scatter band of the lifetime data remains. Determination of casting defects size by optical microscopy on metallographic sections and an analysis of the size distribution by extreme value statistics indicates two types of defects: (i) small isolated defects and (ii) defect clusters consisting of complicated interconnected shrinkages in the three-dimensional space. The size distribution of both types of defects follows the extreme value statistics. This enables to estimate the maximum size of a defect likely to occur in a defined volume. The predicted maximum defect size in a volume of a fatigue specimen reasonable corresponds to the size of defect observed on the fracture surface of failed specimen.

Optimising plasma cut-edge properties for improving the durability of bridge structures

The qualities of plasma cut-edges generated on S355 structural steel were investigated to understand their effect on durability. These edges are left exposed in often critical areas after final assembly of bridge and load bearing applications, in which the damaging characteristic defects formed during the cutting process can act as initiation sites for fatigue cracks. Cuts were generated using oxygen and air at a range of commercial traverse cutting speeds that are used to fabricate structural components. A comparison of topographical properties and microstructural observations confirmed that the changes in the cut-edge surface and internal properties were significantly altered by the plasma gas type. Through the characterisation of cut-edges, optimum process parameters were determined that minimise the level of cut-edge microstructural and surface damage. An improvement in understanding the mechanisms that form irregularities on plasma cut-edge surfaces has been developed, so that damage inducing characteristic properties can be minimised. This has resulted in establishing the influence of characteristic properties on resulting fatigue life, as a means to generate plasma cut-edges that are near to optimum. This was tested using a representative top-hat beam component, in order to validate these results and act as guidance towards the fabrication of structural steel which have optimum cut-edge durability.