Fatigue Crack Propagation Direction Research Articles

Measuring fatigue crack growth using microscale specimens: Si-modified Inconel 939 alloy processed by laser powder bed fusion additive manufacturing

Quantitative assessment of fatigue crack growth (FCG) in a Si-modified Inconel 939 alloy, processed by laser powder bed fusion additive manufacturing (L-PBF AM), was conducted through cyclic bending of pre-notched cantilevers with a characteristic cross-sectional dimension of 25 μm × 25 μm. To the best of our knowledge, this work is the first to show that cyclic bending of such pre-notched microscale cantilevers can sense both the threshold regime and the Paris-law regime of FCG quantitatively, despite the small specimen size. The deliberate fabrication of such microscale test specimens enables control of the direction of fatigue crack propagation with respect to a given microstructure, while avoiding the inclusion of volumetric defects within the test volume. Significant differences in FCG behavior were observed when the direction of crack propagation was switched from being parallel to the AM build direction to being perpendicular to it. The present work demonstrates the efficacy of FCG assessment using microscale specimens, which can enhance our understanding of the influence of a given microstructure on crack growth behavior.

Investigation of fatigue strength of root of lap fillet welds of thin sheet and improvement method

This study investigates the dominant factors of fatigue strength at the root of thin-sheet lap fillet arc welds, which are frequently used in automobile chassis components. A method to improve the root fatigue strength was investigated. The direction of fatigue crack propagation at the root coincided with the direction of maximum circumferential stress range. The circumferential stress was reduced by increasing the distance from the toe to the root (penetration width). Fatigue strength originating from the root was improved in welded joints where the penetration width was increased by adjusting the welding aim point and heat input.

Fatigue crack propagation direction under different loading conditions using MTS and MSS criteria

Fatigue Crack Growth plays a major role on structural integrity. When mixed mode fatigue crack propagation is introduced by complex geometry or loading conditions, numerical methods are essential for fatigue life determination. This paper explores the differences between two crack propagation models. Using an automatic fatigue crack growth algorithm and the finite element method, crack propagation on CTS and four-point bending specimens was simulated. Both specimens allow for pure mode I, mixed mode and pure mode II loading conditions. According to the maximum tangential stress criteria, fatigue crack propagation occurs along the direction where tangential stress is maximized. Comparing crack propagation results with literature, it was possible to conclude that this criterion can be used under pure mode I or mixed mode loading conditions. The maximum tangential stress criterion was also compared to the maximum shear stress criterion, where crack propagation occurs along the direction that maximizes shear stress. Crack propagation directions between these criteria were compared and it was possible to conclude that the maximum shear stress criterion should be used for pure mode II loading conditions. Overall, this methodology allows for fatigue crack growth simulations under complex loading conditions.

The influence of building direction on the fatigue crack propagation behavior of Ti6Al4V alloy produced by selective laser melting

Additive Manufacturing (AM) which combines with computer aided design, materials processing and forming technologies can manufacture near net shape components. In recent years, many studies have revealed the effects of AM parameters on metal material microstructures and mechanical properties. Few studies focus on the fatigue performance of AM materials. Therefore, in the present study, the Ti6Al4V samples produced by Selective Laser Melting (SLM) were used for in-situ fatigue tests. The effects of SLM building directions (0°, 45°, 90°) on sample fatigue properties were investigated. The in-situ scanning electron microscope (SEM) observation technology was applied to observe the fatigue crack initiation and propagation behavior. An obvious worse crack propagation resistance of the 90° samples can be identified. The fracture mechanism of SLM samples was also revealed. The results indicated that the fatigue small cracks always extend between paralleled α laths. The fatigue crack propagation direction can also deflect an angle accompanied with α colonies boundaries. Inter-granular fracture was predominant. For long cracks, the inhibition effect of microstructures can be ignored. Fatigue cracks can penetrate α colonies directly. Intra-granular fracture can be observed. At last, a simplified fatigue life evaluation of AM aero-engine blades was conducted. The fatigue life difference of blades resulted from different building directions was proved. The evaluation method of complex AM component building directions was provided. The best fatigue damage tolerance performance can be obtained.

An experimental approach for estimating the effect of heat affected zone (HAZ) microstructural gradient on fatigue crack growth rate in aluminum alloy FSW

The microstructural gradient of the heat affected zone (HAZ) in aluminum alloy FSW has been simulated experimentally by subjecting the parent alloy to a controlled aging process. The purpose of the simulation is to estimate the effect of the HAZ microstructural gradient on fatigue crack propagation rate by excluding the effect of weld residual stresses. The microstructure in the gradient material produced by aging was evaluated against the actual FSW heat affected zone by comparing microstructural features and microhardness variation. Fatigue crack growth tests were conducted on the gradient material to evaluate its influence on crack propagation rate. The results showed that the gradient has a negative influence on fatigue crack propagation rate, accelerating crack growth with regard to the conditions at onset of the gradient. The impact on crack growth rate is dependent on the slope of the gradient with regard to the direction of fatigue crack propagation. The results of the study suggest that independent from the effect of weld residual stresses the micostructural influence in fatigue crack growth analysis of welded joints should not be neglected.

Microstructure, tensile and fatigue properties of ultrasonic spot welded aluminum to galvanized high-strength-low-alloy and low-carbon steel sheets

The microstructure evolution, tensile lap shear strength and fatigue properties of dissimilar ultrasonic spot welded (USWed) joints of aluminum to two commercial steel sheets at different welding energies were investigated. The main intermetallics at the weld interface were θ (FeAl3) in both joints along with eutectic Al-Zn in Al-to-galvanized high-strength-low-alloy (HSLA) steel joints and Fe3Al in Al-to-ASTM A36 steel joints. The welding strengths of both joints were higher than those of other dissimilar joints reported in the literature. With increasing welding energy, the maximum tensile lap shear strength increased in the Al-to-galvanized HSLA steel joints, while the lap shear strength increased up to a peak value and then decreased in the Al-to-ASTM A36 steel joints. Both the average peak welding strength and fracture energy of the Al-to-galvanized HSLA steel joints were higher than those of the Al-to-ASTM A36 steel joints. The fatigue lives of both welded joints were in agreement with or somewhat longer than other Al-to-steel USWed joints in the literature. The fatigue fracture mode changed with increasing cyclic loads in both welded joints. Fatigue crack growth was mainly characterized by the formation of fatigue striations perpendicular to the fatigue crack propagation direction.

Micro-morphology of Fatigue Crack Initiation and Propagation Behavior in High Strength Aluminum Alloy

The microscopic morphology of fatigue crack initiation and propagation behavior was investigated for B93пч high strength aluminum using optical microscopy, scanning electron microscopy and transmission electron microscopy. The results show that, the fatigue micro-crack generally initiates on the surface from broken coarse particle, interface between particle and matrix and grain boundary. The micro-crack initiation has the characteristics of randomness and diversity and the main crack is generally formed on the surface of the specimen. Fatigue crack propagation path may deviate from the direction with the max loading and appear in the form of deflection, fork, convergences and bridge connectivity. This can be attributed to the influence of microstructural characteristics, which also affect the fatigue crack propagation direction and growth rate.

Low cycle fatigue behavior in a medium-carbon carbide-free bainitic steel

In the paper, different morphologies of bainite were obtained through isothermal quenching at 320°C and 395°C in a medium-carbon carbide-free bainitic steel. The cyclic deformation mechanism was explored by using low cycle fatigue testing. The volume fraction of retained austenite was measured by X-ray diffraction and the space partitioning of the solute atoms was constructed by three-dimensional atom probe. Results showed that the fatigue life at 320°C was always higher than that at 395°C under low and high total strain amplitude. The cyclic softening at the early fatigue stage increased the plastic strain of the sample which was responsible for the reduction of the fatigue life at 395°C. Strain-induced retained austenite to martensite contributed to initial cyclic hardening, but almost having no effect on the subsequent cyclic stable/softening behaviors. The finer bainitic ferrite sheaves obtained at 320°C changed the small fatigue crack propagation direction and delayed the crack propagation rate, which was beneficial for the fatigue properties. In addition, the substitutional atoms did not redistribute between the retained austenite and bainitic ferrite before and after cyclic deformation.

Visual Inspection on Failure Surface of Constant Force Spring (CFS) Fitted in a Counterweight Balancing Mechanism

A failure characteristic of a fractured constant force spring (CFS) or flat spiral spring fitted in a counterweight balancing mechanism is investigated via series of visual experimentation. Macroscopic examination reveals several beach marks that shows direction of fatigue crack propagation has indicated that the CFS fracture had initiated and propagated due to fatigue from an inner surface origin. Macro cracks resulted from stress concentration were also visible on grain boundaries. The crack was initiated at the center of the CFS which later propagated in the direction perpendicular to the applied cyclic load and finally fractured when it can no longer sustain the applied cyclic load. Inspection via Scanning Electron Microscopy (SEM) has indicatedsign of fatigue striations perpendicular to the fracture propagation which is a characteristic of fatigue failure mechanism. Examination of the fractured surface also pointed porosities that reflects points of crack initiation. Multiple crack initiation points identified shows that the fracture was a result of high stress or high stress concentration.

Fatigue damage behavior of a surface-mount electronic package under different cyclic applied loads

This paper studies and compares the effects of pull–pull and 3-point bending cyclic loadings on the mechanical fatigue damage behaviors of a solder joint in a surface-mount electronic package. The comparisons are based on experimental investigations using scanning electron microscopy (SEM) in-situ technology and nonlinear finite element modeling, respectively. The compared results indicate that there are different threshold levels of plastic strain for the initial damage of solder joints under two cyclic applied loads; meanwhile, fatigue crack initiation occurs at different locations, and the accumulation of equivalent plastic strain determines the trend and direction of fatigue crack propagation. In addition, simulation results of the fatigue damage process of solder joints considering a constitutive model of damage initiation criteria for ductile materials and damage evolution based on accumulating inelastic hysteresis energy are identical to the experimental results. The actual fatigue life of the solder joint is almost the same and demonstrates that the FE modeling used in this study can provide an accurate prediction of solder joint fatigue failure.

Crystallographic Orientation Dependence of Fatigue Crack Propagation Behavior in Rolled AZ31 Magnesium Alloy

This paper deals with fatigue crack propagation behavior of rolled AZ31B magnesium alloy. Two types of specimens with the loading axis parallel to rolling direction were machined; fatigue crack propagation direction was parallel to transverse direction (L-T specimen), and short transverse direction (L-S specimen). Fatigue crack propagation tests were performed with center cracked plate tension specimen with stress ratio R=0.1 and frequency of 10Hz at room temperature. Crack propagation rate of L-T specimen was approximately 10 times higher than that of L-S specimen. SEM-EBSD observations revealed that the c-axis direction is unfavorable for the fatigue crack propagation in textured polycrystalline magnesium alloy.

Prediction of Fatigue Crack Propagation Path from a Pre-Crack under Combined Torsional and Axial Loading

Fatigue tests of crack propagation from a pre-crack in thin-walled tubular specimens made of a medium-carbon steel were performed under cyclic torsion with and without superposed static or cyclic axial loading. The stress ratio of cyclic loading was-1. The experimental path of fatigue crack propagation from a pre-crack was compared with the predictions based on the maximum tangential stress criterion. The direction of fatigue crack propagation was perpendicular to the direction of the maximum of the range of the tangential stress, Δσ*θmax, near the crack tip determined from the stress intensity factor which was calculated by considering the contacts of crack faces at the minimum load. The stress intensity factor calculated from the actual crack path by using the body force method showed that the mode II stress intensity factor range quickly got close to zero after a small amount of crack extension. In other words, fatigue cracks propagate to satisfy the local symmetry of the cyclic components of crack-tip deformation.

Fatigue crack propagation from a precrack under combined torsional and axial loading

ABSTRACTFatigue tests of crack propagation from a precrack in thin‐walled tubular specimens made of a medium‐carbon steel were performed under cyclic torsion with and without superposed static or cyclic axial loading. The stress ratio of cyclic loading was −1. The fatigue crack‐propagation path from a precrack was compared with the predictions based on the maximum tangential stress criterion. The direction of fatigue crack propagation follows the direction of the maximum of the range of the tangential stress, Δσ*θ max, near the crack tip determined from the stress intensity factor (SIF) which was calculated by considering the contacts of crack faces at the minimum load. The SIF calculated from the actual crack path by using the body force method (BFM) showed that the mode II SIF range quickly approached zero after a small amount of crack extension. The crack‐propagation rate decreased first and then increased with crack extension. The initial dip of the crack‐propagation rate was caused by the development of crack closure with crack extension. As cracks extended a long distance, the propagation rate was faster than the uniaxial data. This acceleration of crack propagation was caused by excessive plasticity near the crack tip.

Research on the direction of fatigue crack propagation resulting from sharp concentrators of stress

On specimens of rail steel, experimental research on the propagation features of fatigue cracks of the mixed type is carried out. The direction of the growth of fatigue cracks at three kinds of loading was studied: sign-variable cross shear, and cross and longitudinal shear in the compressing stress field. One general feature of fatigue crack development for all researched specimens is established: the growth of crack direction coincides with a trajectory of the principal stresses, minimal on the module.

Fatigue behaviour of a box‐welded joint under biaxial cyclic loading: effects of biaxial load range ratio and cyclic compressive loads in the lateral direction

ABSTRACT The biaxial fatigue of a steel plate (JIS SM400B) having a box‐welded (wrap‐around) joint was experimentally studied. Special concerns were focused on the effects of the biaxial load range ratio and compressive cyclic loading in the lateral direction. The direction of fatigue crack propagation under biaxial cyclic tensile loading, which has a phase difference of π, changed according to the biaxial load range ratio, Rxy = ΔPx/ΔPy. When Rxy was less than 0.56, fatigue cracks propagated along the toe of the weld in the x‐direction because the principal tensile stress range Δσy at that location exceeded the orthogonal value Δσx at the box‐weld toe. The fatigue lives in biaxial tests related well to the data from uniaxial tests when invoking the Δσ5 criterion. However, the location and direction of Δσ5 should be chosen according to the Rxy value and the failure crack direction. An increase in Δσ5, as induced by the Poisson's ratio effect from either the out‐of‐phase tensile loading or the in‐phase compressive loading in the y‐direction, leads to an increase in fatigue damage (decrease in fatigue resistance or specifically a faster crack propagation rate), and this effect can be successfully estimated from uniaxial fatigue test data.

OS11W0458 Fatigue crack propagation from a pre-crack under combined torsional and axial loading

The fatigue crack propagation behavior from a tesile pre-crack under cyclic torsion combined with static or cyclic axial loading was predicted on the basis of the maximum tangential stress criterion. The prediction was compared with the experimental results obtained for a medium carbon steel. The direction of fatigue crack propagation follows the direction of the maximum of the total range of the tangential stress, Δσ_<θmax> near the crack tip and then gradually changes to the direction perpendicular to the maximum of the total range of the principal nominal stress. The mode II stress intensity factor range ΔK_<II> quickly gets close to zero after small amount of crack extension. The propagation rate was faster than the uniaxial data as cracks extended a long distance.

Propagation and Arrest of Fatigue Cracks from a Precrack under Cyclic Torsional Loading in Medium-Carbon Steel.

Fatigue tests of crack propagation from a pre crack in thin walled tubular specimens made of a medium carbon steel were performed under cyclic torsion with and without superposed static tension. The condition of the transition from shear to tensile mode and the propagation behavior during this transition were investigated from the viewpoint of fracture mechanics. The direction of fatigue crack propagation follows the direction of the maximum of the total range of the tangential stress, Δσθ max near the crack tip and then gradually changes to the direction perpendicular to the maximum of the total range of the principal nominal stress. The propagation rate is faster than the uniaxial data when compared at the onset of crack extension and at high stress intensity factors. The former acceleration is caused by crack closure and the latter acceleration is due to the non singular T stress. The negative nonsingular stress induces excessive plasticity ahead of the fatigue crack, and then accelerates the fatigue crack.

Particle size dependence of fatigue crack propagation in SiC particulate-reinforced aluminium alloy composites

Fatigue crack propagation (FCP) and fracture mechanisms have been studied for two orientations in powder metallurgy 2024 aluminium alloy matrix composites reinforced with three different sizes of silicon carbide particles. Particular attention has been paid to make a better understanding for the mechanistic role of particle size. The FCP rates of the composites decreased with increasing particle size regardless of orientation and were slightly faster in the FCP direction parallel to the extrusion direction. After allowing for crack closure, the differences in FCP rate among the composites and between two orientations were significantly diminished, but the composites showed lower FCP rates than the corresponding unreinforced alloy. Fracture surface roughness was found to be more remarkable with increasing particle size and in the FCP direction perpendicular to the extrusion direction. Taking into account the difference in the modulus of elasticity in addition to crack closure, the differences in FCP rate between the unreinforced alloy and the composites were almost eliminated.

マッシュシーム溶接継手の疲労き裂進展特性

Fatigue crack propagation (FCP) tests have been conducted using CT specimens cut from mash seam welded joints coupled with two different steels, cold rolled low carbon steel and high tensile strength steel. FCP rate for two different directions, paralled and normal to the weld, were determined. In both directions, the FCP rates for the welded joints were considerably lower than those for the unwelded base steel regardless of the coupling of steel and slightly decreased FCP rates were seen within the weld zone in the FCP direction normal to the weld. After allowing for crack closure, the welded joints showed almost the same FCP behaviour as the base steel, indicating that the higher apparent FCP resistance were mainly due to compressive residual stress resulting from the welding. The differences in FCP behaviour between mash seam and laser welded joints are discussed on the basis of material property changes.

Detecting gear tooth fatigue cracks in advance of complete fracture

AbstractSome results from using vibration‐based methods to detect gear tooth fatigue cracks are presented. An experimental test rig was used to fail a number of spur‐gear specimens through bending fatigue. The gear tooth fatigue crack in each test was initiated through a small notch in the fillet area of a tooth on the gear. The primary purpose of these tests was to verify analytical predictions of fatigue crack propagation direction and rate as a function of gear rim thickness. The vibration signal from a total of three tests was monitored and recorded for gear fault detection research. The damage consisted of complete rim fracture on the two thin rim gears and single tooth fracture on the standard full rim test gear. Vibration‐based fault detection methods were applied to the vibration signal both on‐line and after the tests were completed. The objectives of this were to identify methods capable of detecting the fatigue crack, and determine how far in advance of total failure positive detection was given. Results showed that the fault detection methods failed to respond to the fatigue crack prior to complete rim fracture in the thin rim gear tests. In the standard full rim gear test all of the methods responded to the fatigue crack in advance of tooth fracture; however, only three of the methods responded to the fatigue crack in the early stages of crack propagation.