Fatigue Crack Initiation Mechanism Research Articles

Microstructural influences on the fatigue crack initiation and propagation mechanisms in hypo-eutectic Al-Si cast alloys

Al-Si cast alloys are accompanied by different defects and microstructural heterogeneities like porosity, varying secondary dendrite arm spacings (SDAS) and multiple appearances of the eutectic. These issues are well known but technically difficult to prevent for the industry. To improve the light weight performance of cast aluminum alloys the present work deals with influences on crack initiation and propagation mechanisms of such heterogeneities in the high-cycle-fatigue (HCF) and very-high-cycle-fatigue (VHCF) regime for the two hypo-eutectic cast aluminum alloys AlSi8Cu3 (in-series sand castings) and AlSi7Mg0.3 (laboratory gravity die castings). Furthermore, the occurrence of facet-regions and their role in early and late state of fatigue damage evolution is discussed by showing results from modified Kitagawa-Takahashi analysis and ex-situ computed tomography investigation during intermitted HCF experiments. In the early HCF regime crack initiation starts at multiple pores. Due to coalescence of these defects, shear-stress-controlled crack propagation along slip bands leads to the occurrence of facets. In the VHCF regime, porosity provides strong scattering of the number of cycles to failure. A reduction in SDAS increases the fatigue strength significantly. However, in absence of porosity fatigue crack initiation and propagation is controlled by shear stresses on facet-like slip-planes. Additionally, the barrier role against dislocation movement of eutectic regions can be shown by means of these specimens.

Microscopic characterization on low cycle fatigue behavior at room temperature of Zircaloy-4 alloy with recrystallized microstructure

Low cycle fatigue (LCF) tests, with the total strain amplitude (ΔƐt/2) in the range of ± (0.5–3.0%), were employed at room temperature on Zircaloy-4 (Zr-4) alloy with recrystallized microstructure, aiming to investigate its fatigue life, fatigue crack initiation and propagation mechanisms. Results indicated that the fatigue behavior of Zr-4 alloy followed Coffin-Mason relation with fatigue ductility exponent of 0.2426. Fractographic investigation of all mechanically tested specimens revealed that the fatigue specimen surface is the preferential crack initiation place. SEM observation on the crack propagation regions of fatigue specimens showed that a large number of fatigue striations distributed in the crack propagation regions. Further detailed investigation revealed that micro-crack growth rates can be quantitatively evaluated in terms of strain amplitude.

Four-point bending fatigue behavior of an equimolar BCC HfNbTaTiZr high-entropy alloy: Macroscopic and microscopic viewpoints

High-entropy alloys (HEAs) have attracted a great deal of interest and high expectations not only from the scientific community, but also from the industrial point of view, mainly because of a remarkable combination of promising properties. However, the characterization of HEAs under fatigue loading is still very scarce, especially in the case of body centred cubic (BCC) HEAs. As an attempt to fill this gap, we report here the first study on the fatigue behavior of a refractory BCC HEA in four-point bending loading conditions. Equimolar HfNbTaTiZr HEA was subjected to load-control fatigue tests in air and at room temperature to assess its fatigue properties in the high-cycle regime. Experimental results show that the maximum stress level reached within one fatigue cycle exceeds the uniaxial yield stress of the material. This particularity is observed within the whole range of fatigue life investigated, i.e., from approximately 104 to 107 cycles, showing no macroscopic plastic deflection of the beam in the process. A fatigue endurance of approximately 430 MPa (at 107 cycles) is found, for a stress ratio of approximately 0.1. A fatigue crack initiation mechanism occurs at the specimen surface through an intergranular phenomenon and propagates in a transgranular crystallographic manner. The fatigued samples underlined the low local lattice rotation, implying a restrained cyclic strain hardening. Finally, the fatigue study of the HfNbTaTiZr HEA highlights a fatigue endurance over an ultimate tensile strength ratio of 0.43, which is significantly larger than the actual conventional structural metallic materials under similar conditions.

The effect of tensile pre-straining on fatigue crack initiation mechanisms and mechanical behavior of AA7050 friction stir welds

This work examines the plastic strain gradients developed by pre-straining AA7050 friction stir welds (FSW), the damage to intermetallic particles caused by this strain, and the effect of these fractured particles on the subsequent fatigue performance of the weld. After pre-straining FSW samples to 1% and 3%, the local plastic strain distribution was measured along the crown (top) and root (bottom) of the weld, and the maximum local plastic strain was found to be 3.34 and 1.83 times higher than the average plastic strain. Metallography conducted on pre-strained welds showed cracked intermetallic particles in both the 1% and 3% levels which may have influenced fatigue crack initiation. Pre-straining was found to decrease the fatigue life at high cycles for the 1% pre-strain; however the 3% pre-strain level showed essentially the same fatigue life as the 0% pre-strained welds. Fractography showed that increasing pre-strain was correlated with increasing prevalence of Fe-rich particles at the fatigue crack initiation sites. When corrected for maximum local strain, the fatigue results for non-pre-strained AA7050 FSWs were shown to match the base material, AA7050-T7451.

Very High Cycle Fatigue of a Cast Aluminum Alloy: Size Effect and Crack Initiation

Fatigue properties of an Al-Si-Cu cast alloy were investigated with life up to very high cycle fatigue regime. Particular attention was given to specimen size effect and fatigue crack initiation mechanism. Uniaxial fatigue tests with constant amplitudes were carried out by using ultrasonic fatigue testing machine operating at 20 kHz, compared to the results obtained by using conventional hydraulic fatigue machine at 35 Hz. In order to evaluate the size effect, two sets of specimens with different dimensions were used. The results show that the fatigue strength of the studied alloy decreases with the increase in specimen size. Scanning electron microscopy observation of fracture surfaces revealed that most fatigue cracks initiated from microstructural defects such as porosity located on specimen surface or in subsurface. Weibull statistical analysis of fatigue data accounting for size effect was performed to predict the fatigue life of the Al-Si-Cu alloy in the very high cycle regime. Good agreement was found between the fatigue life prediction and the experimental data.

Variation of the Corrosion Behavior Prior to Crack Initiation of E690 Steel Fatigued in Simulated Seawater with Various Cyclic Stress Levels

The electrochemical process and corrosion behavior prior to crack initiation of E690 steel fatigued in simulated seawater are investigated under different cyclic stress levels by methods of electrochemical impedance spectroscopy, cyclic current response, electrochemical noise and morphology observation. The results show that the electrochemical behavior changes with the peak stress of cyclic loading and the corrosion behavior transforms as the peak stress increases over the proof strength. The charge transfer resistance (Rct) decreases with elevated peak stress. Continuous decreasing Rct with cycling number under peak stress of 1.02 σp0.2 indicates that continuous fresh metal induced by work-softening is exposed to the seawater, which is also proved by cyclic current response induced by cyclic stress and contact angle test. As a result, the corrosion type of E690 steel changes from general corrosion to pitting corrosion with the peak stress close to and above the proof strength. That induces the corrosion fatigue crack initiation mechanism to change correspondingly.

Low cycle fatigue test and enhanced lifetime estimation of high-strength steel S550 under different strain ratios

This study investigates the low-cycle fatigue (LCF) behavior of the high-strength steel S550 (commonly used in ship and floating structures) under different strain amplitudes with different strain ratios at a room temperature. The test results characterize the cyclic stress-strain relationship, the mean stress relaxation behavior and the cyclic plasticity parameters of S550 steels. The scanning electron microscopy (SEM) examinations on the failure surface reveal the fatigue crack initiation and growth mechanism. Based on the experimental results, this study presents two enhanced approaches to estimate the low-cycle fatigue life of S550 steels. The energy-based approach modifies the original Smith-Watson-Topper model using the applied energy calculated in the first cycle to enhance the accuracy and facilitate engineering implementations. The damage mechanics-based approach calibrates the material parameters from the measured total fatigue life by combining the fatigue crack initiation model and the damage growth model. The computed fatigue life using the calibrated material parameters demonstrates a close agreement with the measured fatigue life in the experiment.

Micromechanical characterisation of Indian rail steel

Railway transport dynamics cause rails in general to experience a time varying rolling contact load to add shear strain to that from traction and normal strain from the axle loads. These loading conditions together with the frictional heat at the contact interface alters the microstructure in the near-surface region of the rails and wheels. This paper presents the hardness and microstructural study of used Indian rail steel to expose the influence of load parameter active at the interface. Samples are prepared from the rail section taken from Dhanbad railway division. The hardness variation along the rail head cross-section and metallurgical variations of the test sample are presented. Microstructural examinations reveal existence of micro cracks near to the rail surface favouring formation of wear debris. Also, the presence of the pro-eutectoid ferrite and cementite phase at the grain boundaries of the pearlitic rail material is observed. The fatigue cracks are found to initiate across the grain boundaries containing the pro-eutectoid ferrite phase. This study will help to develop a better understanding of rolling contact fatigue (RCF) crack initiation and wear debris formation mechanisms on the rails. Thus, these results can be used to improve rail maintenance planning and risk assessments.

Modelling of corrosion fatigue crack initiation on martensitic stainless steel in high cycle fatigue regime

This paper presents an analytical model for assessing the corrosion fatigue crack initiation life on a martensitic stainless steel X12CrNiMoV12-3 in high cycle fatigue regime (between 105 and 107 cycles). Based on in-situ electrochemical measurements during corrosion fatigue tests in NaCl aqueous solution, the corrosion fatigue crack initiation mechanism was identified. Two main stages were investigated: (i) the fracture of the passive film by slip bands and (ii) the free dissolution of the metal developing fatigue crack initiation from a critical corrosion defect. The depassivation stress threshold corresponds to the median fatigue strength at 107 cycles for fatigue corrosion tests. For an applied stress range less than this threshold, the depassivation phenomenon was not observed at 107 cycles and no crack initiation occurred. The proposed model takes into account the depassivation process induced by the slip bands emergence at the specimen surface and the corrosion rate under cyclic loading. The experimental results are compared to the proposed model taking into account mechanical and electrochemical material parameters.

Study of Relation between Shot Peening Parameters and Fatigue Fracture Surface Character of an AW 7075 Aluminium Alloy

Shot peening is a well-known surface treatment method used for fatigue life improvement of cyclically loaded structural components. Since three main variables are considered in the peening process (peening intensity, coverage and peening media type), there is no direct way to choose the best combination of treatment parameters for the best performance, thus it has to be based on experience and laboratory tests. When shot peening is performed with inadequate parameters, or the peening process is not stable in time (decrease of the peening pressure, deterioration of the peening media and so on), it can result in significant degradation of the treated component fatigue properties, what is commonly called as the “overpeening” effect. When a premature fatigue fracture occurs in operation, the fracture surface analysis is usually the most important method of revealing the damage mechanism. This work is aimed at the study of the relation between the shot peening parameters and the fatigue fracture surface character on an AW 7075 aluminium alloy with an objective of identifying marks of overpeening and investigating the fatigue crack initiation mechanism. After performing the tests, it was observed that shot peening with optimized parameters creates a surface layer that is able to change the mechanism of the fatigue crack propagation and improve fatigue strength. On the other hand, using extensive peening parameters decrease the fatigue strength due to the creation of surface cracks and surface layer delamination.

Fractographic analysis of fatigue crack initiation and propagation in CP titanium with a bimodal harmonic structure

Commercially pure (CP) titanium with a bimodal harmonic structure, which is defined as coarse grained regions surrounded by a network of fine grains, was fabricated by consolidating mechanically milled powders to improve both strength and ductility. In order to determine the fatigue properties of bimodal harmonic structured CP titanium, four-point bending fatigue tests were conducted at a stress ratio of 0.1 under ambient conditions. The fracture surfaces were analyzed by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) to identify the mechanisms of small fatigue crack initiation and propagation on the basis of fractography and crystallography. The fatigue crack initiation resistance was increased compared to sintered compacts fabricated from as-received initial powders, which was attributed to grain refinement during mechanical milling. Transgranular fracture occurred from coarse grains with a basal plane perpendicular to the loading direction. The fatigue crack path in harmonic structured CP titanium was not influenced by the microstructure.

Effect of initial hardness on the thermal fatigue behavior of AISI H13 steel by experimental sand numerical investigations

AbstractBased on the Uddeholm thermal fatigue test, the mechanism of thermal fatigue crack initiation and propagation and the influence of initial hardness on the thermal fatigue behavior of AISI H13 steel were investigated. Furthermore, an electromagnetic‐thermo‐mechanical coupled finite element model was established to analyze the temperature evolution and stress accumulation in specimen during thermal cycles. The experimental results demonstrate that, after 3000 thermal cycles, the surface hardness of specimen markedly decreases, and the lath martensite structure seems to completely evolve into a mixture of ferrite and irregularly sphere‐like and bar‐like M23C6 and M6C carbides. According to the numerical results, the effective stress of specimen will increase slightly after every thermal cycle. It presents a distinct stress accumulation phenomenon with increasing number of thermal cycles. Especially at the corner of specimen, it is more significant. The thermal fatigue test results also prove that it is a major site where the initiation and propagation of thermal fatigue primary cracks occur. Both the numerical and experimental results suggest that specimen with initial hardness of 46HRC has the slowest stress accumulation rate, the lowest thermal fatigue damage factor, and the longest thermal fatigue life.

Influence of friction stir processing on the room temperature fatigue cracking mechanisms of A356 aluminum alloy

Failure by fatigue is a common problem associated with cast aluminum alloys due to defects like shrinkage porosities, non-metallic inclusions, etc. Friction stir processing (FSP) has recently emerged as an effective technique for local modification of microstructure. This study investigates the fatigue crack initiation and growth mechanisms in cast and FSPed A356 aluminum alloy. Two sets of parameters were used to friction stir the cast alloy resulting in the complete modification of the cast microstructure to a wrought microstructure. Both the FSPed microstructures exhibited severe abnormal grain growth (AGG) after heat treatment leading to a multimodal grain size distribution – the grain sizes ranging from a few microns to a few millimeters. One of the FSP conditions displayed an excellent improvement in fatigue life by an order of magnitude, while the other condition displayed an unexpectedly large scatter in fatigue lives. Detailed study of the fractured fatigue specimens by electron back scattered diffraction (EBSD) revealed that both, fatigue crack initiation and propagation, were intimately tied to the grain size as well as the grain misorientations in the microstructure.

Mechanisms of fatigue crack initiation and propagation in 6005A CMT welded joint

Detailed microstructures of (base metal) BM, (welding zone) WZ, and (heat affected zone) HAZ, fracture appearance, and surface fatigue damage were analyzed by optical microscopy (OM), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The phases distribution which had serious effects in fatigue crack initiation in the welding joint was analyzed by X-ray diffraction (XRD). The fatigue test of the polished samples has been performed at stress ratio of 0.1. The grains in the WZ, refined by stirring action of the arc, hindered crack propagation. All samples fractured in the WZ due to micropores, which was the primary crack initiation model in the weld joint. Microcracks, with no propagation, initiated around secondary phases due to interaction between dislocations and phases. Z-shaped extension, which is typical in the stable crack-propagation stage, existed on the surface due to tearing-stress action. Plastic distortion caused by fatigue-crack closure, was found near the crack tip.

Fatigue crack initiation of magnesium alloys under elastic stress amplitudes: A review

The most advantageous property of magnesium (Mg) alloys is their density, which is lower compared with traditional metallic materials. Mg alloys, considered the lightest metallic structural material among others, have great potential for applications as secondary load components in the transportation and aerospace industries. The fatigue evaluation of Mg alloys under elastic stress amplitudes is very important in ensuring their service safety and reliability. Given their hexagonal close packed structure, the fatigue crack initiation of Mg and its alloys is closely related to the deformation mechanisms of twinning and basal slips. However, for Mg alloys with shrinkage porosities and inclusions, fatigue cracks will preferentially initiate at these defects, remarkably reducing the fatigue lifetime. In this paper, some fundamental aspects about the fatigue crack initiation mechanisms of Mg alloys are reviewed, including the 3 followings: 1) Fatigue crack initiation of as-cast Mg alloys, 2) influence of microstructure on fatigue crack initiation of wrought Mg alloys, and 3) the effect of heat treatment on fatigue initiation mechanisms. Moreover, some unresolved issues and future target on the fatigue crack initiation mechanism of Mg alloys are also described.

The effect of friction stir processing and post-aging treatment on fatigue behavior of Ca-added flame-resistant magnesium alloy

Friction stir processing (FSP) was applied to the microstructural modification of the extruded flame-resistant magnesium (Mg) alloy, AMX602. The as-received material exhibited microstructure, in which intermetallic compounds (IMCs) were inhomogeneously dispersed in the matrix. FSP broke up some large IMCs and resulted in homogeneous distribution of IMCs and fine grains of the matrix. The micro hardness of the FSPed material was lower than that of the as-received one because the dislocation density and hardening precipitates decreased due to the heat input during FSP. The hardness of the FSPed material increased by the post-aging treatment. However, the fatigue strengths of the FSPed and post-aged specimens were lower than those of the as-received ones. The lower fatigue strengths of the FSPed specimens were attributed to the decrease of hardness by FSP. EBSD analyses revealed that strong texture was developed by FSP. The fatigue cracks of the post-aged specimens initiated at the locations with strong texture, in which basal slip planes had an angle about 45° to the fatigue loading direction. The lower fatigue strengths in the post-aged specimens were attributed to the texture-induced fatigue crack initiation mechanism.

Effect of stress ratio on very high cycle fatigue properties of Ti-10V-2Fe-3Al alloy with duplex microstructure

In fatigue critical applications, Ti-10V-2Fe-3Al alloy components are expected to endure cyclic loading with cycles above 109. To assess their operating safety, S–N relations of Ti-10V-2Fe-3Al alloy in very high cycle fatigue (VHCF) regime are of concern and have been investigated in this work. Fatigue behavior including S–N curves and crack initiation mechanisms is reported. Two transitions of fatigue crack initiation mechanism, from internal crack initiation to surface crack initiation and from αp cleavage to αs/β decohesion, occur when the stress ratio (R) and stress level are reduced. Fatigue limits exist at Nf=6×107 cycles for all stress ratios except for 0.5. In the VHCF regime two kinds of internal crack initiation mechanisms exist, i.e., coalescence of cluster of αp facets and αs/β decohesion. Their mutual competition depends on the stress ratio and can be interpreted in terms of different stress character required for promotion on different internal crack initiation mechanism. Small crack propagation is discussed to be life controlling process under the stress ratio range from −0.5 to 0.1 during VHCF regime while under the stress ratio 0.5 VHCF, life almost refers to the life required for crack initiation.

Corrosion fatigue crack initiation and initial propagation mechanism of E690 steel in simulated seawater

In the present paper, the corrosion fatigue crack initiation and initial propagation mechanism of E690 steel in simulated seawater were studied by stress-controlled fatigue tests and a series of subsequent characterizations on the fracture surface, microstructure and secondary cracks. Results show that the corrosion fatigue crack initiation and initial propagation mechanism evolves with elevated peak stress level in simulated seawater. When peak stress is far below the proof stress, cracks preferentially initiate at the parent austenite grain boundaries (PAGBs) with 68.4% probability and at the ferrite lath boundaries (FLBs) with 31.6% probability. Meanwhile, the cracks also preferentially propagate along the PAGBs and FLBs. Upon the peak stress close to or above the proof stress, cracks turn to initiate from the emerging corrosion pits and propagate without zigzag detour but by splitting the ferrite laths which transversely block its propagation way.

Microstructure characterization and mechanism of fatigue crack initiation near pores for 6005A CMT welded joint

The fatigue test of 6005A cold metal transfer welded joint were carried out at room temperature and crack initiation near pores was systematically revealed. The fatigue strength of welded joint at 5 × 106 cycles based on S-N curve can be estimated as half of yield strength, which can be predicted reliably in engineering application. The fatigue test showed welded zone is the weakest zone due to the existence of micropores defects in the surface of the joint, even if there is softening zone in HAZ which is shown in hardness curve. Thinking about the existence of micropores, stress concentration will be produced near the pore even in smaller cyclic stress, which will drop down the crack initiation threshold, and the cupped grain boundary inside the pore may aggravate this condition. The pore in the equiaxed crystal microstructure would produce microcrack more easily than in columnar crystal structure. Moreover, there will be cracks with short-haul near the pore in the sample which isn't failed. These cracks won't impact the fatigue life of sample. But the microcrack occur in each of pores will converge into a macrocrack, when more than two pores gather in one region, then the fatigue property of the joint will be affected, seriously. Therefore, pores should be avoided in the process welding aluminum alloy as much as possible, especially the gathering of multiple pores.

Ex and in situ investigations on the role of persistent slip bands and grain boundaries in fatigue crack initiation

Abstract