Direction Of Crack Initiation Research Articles

Modelling of microscopic crack initiation behaviour of fretting fatigue based on X-ray micro-computed tomography scan

Fretting fatigue is a particularly destructive form of fatigue failure that has drawn significant attention from researchers. This paper proposes a numerical modelling method based on tomography scan to investigate the impact of inclusions on fretting fatigue crack initiation behaviour. The results reveal that inclusions can affect the crack initiation direction and reduce the initiation lifetime. The difference between the properties of inclusions and matrix leads to local mechanical response mismatch, resulting in local stress, strain, and damage concentration. The mismatch in strain and damage follows a gradient distribution along the thickness as the distribution in these mechanical responses. These could explain why particles closer to the surface are more likely to cause damage. Additionally, the aggregation distribution of inclusions results in merging and connecting local regions with high stress, strain, and damage concentration, ultimately promoting fracture behaviour. Finally, the numerical models are validated with experimental data from fretting fatigue tests. Compared to the homogeneous model, the numerical model that considers inclusions from topography data can more accurately analyse the material damage and provide a more comprehensive understanding of fretting fatigue behaviour.

The effect of dilatational and distortional strain energy density on crack initiation

AbstractTo investigate the distinct influences of dilatational strain energy density (Dil‐SED) and distortional strain energy density (Dis‐SED) on crack initiation, quasi‐static loading tests were performed on centrally cracked polymethyl methacrylate samples, employing digital image correlation equipment. Our findings unveiled that the Dil‐SED promotes crack initiation, while the Dis‐SED inhibits it. Additionally, it was found that the direction obtained by the ratio of these two strain energy densities was closer to the crack initiation direction than the direction associated with the maximum Dil‐SED and minimum Dis‐SED values. Furthermore, experimental and finite element method (FEM) results indicated that the distribution of these two strain energy densities remains relatively insensitive to the core region's radius . By understanding Dil‐SED and Dis‐SED magnitudes and their impacts on crack initiation, material failure can be predicted and prevented.

Influences of inclined crack defects on vibration characteristics of cylindrical roller bearings

Crack initiation and propagation directions on the raceway surface of cylindrical roller bearings (CRBs) can generate different forms of excitation patterns and affect the vibration level of CRBs. For inclined crack defects on the raceway surface, the direction of a roller passing through the cracked raceway determines the order and amplitude of time-varying displacement (TVD) excitation and time-varying stiffness (TVS) excitation. In this study, two different excitation modes are proposed to simulate the interactions between the roller and the raceway. Then, the excitation modes are integrated into the dynamic model of CRB and used to study the effects of crack inclination and propagation direction on the vibration characteristics of CRBs. The results show that the bearing vibration acceleration caused by excitation mode I is greater than that caused by excitation mode II. The distribution direction of the inclined crack defect on the CRB raceway has a great influence on its vibration level. It can provide useful guidance for early bearing fault detection and diagnosis.

Static axial tension or compression stress effects on shear cracks initiating under cyclic torsion loads in multiaxial fatigue tests

This work proposes models that can better fit and explain apparently contradictory mean stress effects observed when superimposing axial tension or compression loads on cyclic torsion fatigue tests. It introduces new shear-based multiaxial fatigue damage models, modifying the classic Fatemi-Socie equation to consider the peak deviatoric stress normal to the initiating shear crack as the actual driving force for the peak or mean effects of the normal stresses. This deviatoric hypothesis can quantify the odd detrimental effects of static axial compression observed on axial shear cracks tested under cyclic torsion. Additionally, applying the peak deviatoric correction term only to the elastic component of the shear strain range allows an even better description of the increasing mean normal stress effects in high-cycle fatigue, eliminating the need for a variable normal stress sensitivity coefficient in the multiaxial fatigue damage parameter. To evaluate the proposed shear-based models, their life and crack initiation direction predictions are compared for cyclic torsion experiments both with and without superimposed static axial stresses for steels and aluminum alloys, including both literature and original data. The experiments show that axial compression can be detrimental to fatigue crack initiation lives, depending on the cracking mode and on the presence of bifurcations, which effects could be explained from the proposed deviatoric damage parameters.

Shaping characteristics of excavation contours in sequential controlled fracture blasting of rock-anchored beams in Shuangjiangkou underground powerhouse

Influences of high in-situ stress generally need to be considered when excavating deep underground caverns. The dynamic fracture behaviors of rocks under blast loads were investigated by using the rock-anchored beam excavation in underground powerhouses of Shuangjiangkou Hydropower Station in Sichuan Province, China as the engineering background. To solve the problems of the poor blasting breakage effect of rocks and the difficulty in protecting surrounding rocks during excavation, mechanical properties of granite under static and dynamic loads were investigated and the sequential controlled fracture blasting (SCFB) method was adopted during in-situ tests. Based on the Riedel-Hiermaier-Thoma constitutive model and the strength criterion, software LS-DYNA was employed to simulate the dynamic propagation of blasting-induced cracks. The contour shaping effect obtained via numerical simulation is generally consistent with the test results. The results show that SCFB can to some extent control the direction of crack initiation and rock fracture behavior of the blasthole wall cracks and the spacing of successive bursting holes is about 10 times the diameter of the blastholes when the cracks between the blastholes are shaped the best effect. Moreover, the magnitude and direction of principal in-situ stress can both affect the propagation path and length of blasting-induced cracks. The results of the research on the excavation and construction of deeply buried underground caverns have a certain reference value.

A new criterion based on the ratio of strain energy density for mixed-mode crack initiation prediction in ductile materials

In consideration of the different mechanisms of microscopic separation governed by dilation and microscopic shearing controlled by distortion on crack initiation, a new criterion based on the ratio of two strain energy densities (RSED) is proposed by differentiating the dilatational and distortional parts in strain energy density. The influence of the weights of two strain energy densities under different stress states on fracture resistance of crack initiation is also considered in the RSED criterion. The size and shape of the core region do not affect the direction and fracture resistance of crack initiation predicted by the RSED criterion. Moreover, the direction predicted by the RSED criterion is theoretically demonstrated to be identical to the orientations predicted by the maximum stress triaxiality (MST) criterion and the T criterion on crack initiation. The crack initiation directions which are obtained by the experiments for different materials and predicted by various criteria are found to be mostly coincident near the mode-I crack, and so are the fracture resistances. In the vicinity of the mode-II crack, the direction of crack initiation predicted by the RSED criterion shows a superior consistency for the experimental results of the ductile materials reported by Theocaris et al. (1982) and Kong et al. (1995) over the three classical criteria, such as the maximum tangential stress (MTS), the maximum energy release rate (MERR) and minimum strain energy density (MSED). The RSED criterion also has the potential to amend the issue of the underestimation for KIIC/KIC in some typical materials, e.g., the face-centered-cubic (f.c.c) metals. Therefore, the RSED criterion prefers to predict the direction and fracture resistance of crack initiation in ductile materials.

Study on Fatigue Cracking of Diaphragm's Arc Opening of OSD in Steel Bridges by Using Biaxial Stress Method.

Changes in loading position have a significant impact on the stress field of each vulnerable area of an orthotropic steel deck (OSD). The arc opening area of the diaphragm and the connecting area between the U-rib and the diaphragm under the moving load are prone to fatigue cracking. By comparing the stress responses under different methods, the hot spot stress (HSS) method is used as the main stress extraction method in fatigue performance evaluation. The control stress of fatigue cracking was analyzed by comparing the direction of the principal stress field with the crack direction in this experiment. According to the stress amplitude deviation under the biaxial stress state, a set of methods for evaluating the effects of in-plane biaxial fatigue was developed. An improved luffing fatigue assessment S-N curve was applied to analyze the fatigue life of the diaphragm's arc opening area. The results show that when the moving load is exactly above the connection of the deck and the web of the U-rib on one side, it is in the most unfavorable position in the transverse direction, and the diaphragm is mainly under the in-plane stress state. The longitudinal range of the stress influence line of the arc opening is approximately twice the diaphragm spacing. Two to three stress cycles are caused by one fatigue load. Fatigue crack control stress is the principal stress tangential to the arc opening's edge in this area. The normal direction of the principal stress in the model test is roughly consistent with the crack initiation direction. The variation in the stress amplitude deviation in this area is caused by changes in the action position of the moving load. When the moving load is at a certain distance from the involved diaphragm, it is reduced to zero, implying that the in-plane fatigue effect is the greatest in this area.

Fretting fatigue crack propagation under out-of-phase loading conditions using extended maximum tangential stress criterion

In this paper, a numerical crack growth analysis is conducted by using the Maximum Tangential Stress (MTS) criterion and its extension, which considers the contact stress between the crack surfaces. Moreover, fretting fatigue is a multiaxial non-proportional loading problem, and most components are subjected to asynchronous loads. Therefore, this paper mostly focuses on the influence of loading phase difference (Φ) on the crack propagation behavior. Meanwhile, for the fretting fatigue problem, as there is a relationship between the crack initiation and propagation, the effect of initiation characteristics, including crack initiation position and direction is studied under different loading phase differences using Linear Elastic Fracture Mechanics (LEFM) theory. It is observed that the predicted crack path and lifetime by using the extended MTS criterion and Paris’ law is in good agreement with the experimental results in the literature. It is found that the phase difference has a strong effect on the crack propagation behavior. Furthermore, we found that the crack initiation position has a greater influence on crack propagation behavior than the crack initiation direction.

Initiation and propagation of a single internal 3D crack in brittle material under dynamic loads

Different defects are widely distributed inside natural rock masses, and the development of these defects poses a potentially severe threat to the entire structural stability and safety. Most studies at present are concentrated on the propagation of two-dimensional (2D) penetrated cracks under static loads. In this paper, a series of tests were carried out on cubic glass specimens containing a closed pre-crack using a split Hopkinson pressure bar (SHPB) system, aiming to investigate the internal three-dimensional (3D) crack growth under dynamic compression. The results indicate that the dynamic compressive strength of the cubic specimen is influenced by the inclination direction of the 3D pre-crack. Theoretical analysis of the circumferential stress field at the pre-crack tip reveals the mechanism behind dominant crack initiation and favorable direction, which are also affected by non-singular T stress along and perpendicular to the pre-crack plane, as well as friction between crack planes. Through the post-mortem examination, the mode III fracture characteristics, crack bifurcation and Wallner lines are observed in fracture morphology. The terminal length of wrapped wing cracks subjected to dynamic loads is sensitive to pre-crack inclination angle, while it is approximately 1–1.5 times the pre-crack radius under static compression.

Experiments and discrete element simulations of crack initiation angle of mixed-mode I/II in PMMA material

Complex crack configuration causing mixed-mode I/II usually occurs in various situations in engineering, and it is difficult to accurately predict the direction of crack initiation by theory. In fact, there is a great difference between the finite-plate theoretical prediction and the experimental results. To investigate the effects of crack configuration on crack initiation angle, new brittle polymethyl methacrylate (PMMA) specimens were designed. Multiple sets of fracture experiments of mixed-mode I/II were performed systematically to obtain the crack initiation angle of PMMA specimens. The variation of the crack initiation angle with the size ratio was obtained experimentally, which indicated that the crack initiation angle increases with the increase of the size ratio. As a supplement to the physical experiments, corresponding numerical simulations were carried out based on the Discrete Element Method. Firstly, the material parameters of the numerical simulation were calibrated by reproducing the failure process of the sample in the experiment. Then, the numerical modeling and calculation were carried out for the samples that are inconvenient to process in the experiment. Combining the results of both experiment and numerical simulation, a prediction formula for the crack initiation angle was obtained. Furthermore, the effects of symmetry factor and preexisting crack length on the tensile capacity and the crack initiation angle were investigated by simulation systematically. By adjusting the crack configuration parameters, the direction of crack initiation can be artificially controlled, which is of great importance for ensuring engineering safety.

Examining the effect of crack initiation angle on fracture behavior of orthotropic materials under mixed-mode I/II loading

In this research, the maximum strain energy release rate concept is adapted for predicting the fracture response of orthotropic materials under mixed-mode I / II loading with an arbitrary precrack-fiber angle. Adaptive maximum strain energy release rate criterion (AMSER) is derived using the crack tip stress field of orthotropic materials with a precise look on the crack initiation direction. In precrack along the fibers, the crack initiation angle is assumed to be in the direction of maximum strain energy release rate (SER). Several experimental observations on a macroscopic scale prove that in the arbitrary angle between the precrack and fiber case, precrack kinks and propagates along the fibers; thus in the derivation of AMSER, crack initiation angle is considered as a superposition of the angle between precrack and fibers and the predicted angle based on the SER criterion (θC). The main purpose of this paper is to investigate the effect of crack initiation angle on the predicted fracture behavior. Due to the arbitrary angle between precrack and fibers, it is impossible to establish pure mode I conditions and derive pure mode I stress intensity factor. Thus, the equivalent fracture toughness concept in pure mode I is presented. AMSER criterion is validated by comparing fracture envelope curves with available experimental data in mixed-mode I / II loading. It can be concluded that accurate prediction of the crack initiation angle based on SER criterion causes the fracture behavior of orthotropic materials to be extracted with high accuracy. The results prove that the hypothesis of precisely predicting crack initiation angle based on the superposition of kink and θC angles complies with the nature of fracture of orthotropic materials.

Early crack orientation prediction methods under fretting fatigue loading including wear effects

This paper investigates two different methods to predict crack initiation direction in fretting problems subjected to partial slip conditions. One of them is based on the Critical Direction Method (CDM) combined with the Smith Watson and Topper (SWT) fatigue damage parameter. The other consists in simulating early crack propagation using a Linear Elastic Fracture Mechanics (LEFM) approach associated with the SWT parameter. In addition, this paper also investigates the influence of considering wear in the crack initiation direction estimates. For validation of the methods, experimental data from fretting tests conducted on an AA7050- T7451 alloy were used. Results have shown that the CDM-based model provides very accurate estimates, whereas the LEFM-based approach failed to properly reproduce experimental observations. Besides, it was also verified that the inclusion of wear has little effect on the estimates despite the method considered.

Early crack orientation prediction methods under fretting fatigue loading

This paper investigates two different methods to predict crack initiation direction in fretting problems subjected to partial slip conditions. One of them is based on the Critical Direction Method (CDM) combined with the Smith Watson and Topper (SWT) fatigue parameter. The other consists in simulating early crack propagation through a Linear Elastic Fracture Mechanics (LEFM) approach associated with the SWT parameter. For validation of the methods, experimental data from fretting tests conducted on an AA7050-T7451 alloy were used. Results have shown that the CDM-based model provides very accurate estimates, whereas the LEFM-based approach failed to properly reproduce experimental observations.

Experimental investigation on hydraulic fracturing in cement mortar with tensile stress

A novel device is proposed to test hydraulic fracturing in specimen with tensile stress. The device is composed of splitting apparatus and sealing apparatus. The tensile stress in specimen is realized via splitting load. It is confirmed that the sealing apparatus has a less effect on peak splitting load and the Young’s modulus of specimen. Some hydraulic fracturing experiments of cement mortar specimens with tensile stress are conducted. Critical water pressure of hydraulic fracturing will decrease with increase of the tensile stress. Hydraulic fracturing mechanism of cement mortar with tensile stress is revealed based on the Biot’s theory and fracture process zone. Applied high water pressure on initial crack surface, water pressure in water flow zone, and drag force along initial crack surface and water flow zone have a promotional effect on fracture propagation. The T stress in the test and pore water pressure caused by tensile strain near crack tip will restrain fracture propagation. A critical water pressure model (CWP) is proposed based on general maximum tangential stress criterion, and it is verified that the CWP model is rational and accurate. The T stress and pore water pressure gradient along polar angle direction have an effect on direction of crack initiation.

On the Topology Update of the Numerical Manifold Method for Multiple Crack Propagation

Crack initiation and propagation are the two key issues of concern in the geotechnical engineering. In this study, the numerical manifold method (NMM) is applied to simulate crack propagation and the topology update of the NMM for multiple crack propagation is studied. The crack-tip asymptotic interpolation function is incorporated into the NMM to increase the accuracy of the crack-tip stress field. In addition, the Mohr-Coulomb criterion with tensile cut off is adopted to be the crack propagation criterion to judge the direction of crack initiation and propagation. Then a crack tip searching method is developed to automatically update the position of the crack tips. The inapplicability of the original loop search method in the NMM is also illustrated and a novel loop search method based on manifold elements is developed for physical loop updating. Moreover, methods for the manifold element updating and physical cover updating are provided. Based on the above study, the developed numerical method is capable to simulate multiple crack propagation. At last, typical rock rupture problems are numerically simulated to manifest the effectiveness of the developed numerical method.

Analysis of the crack propagation rules and regional damage characteristics of rock specimens

To study the evolution mechanism of cracks in rocks with multiple defects, rock-like samples with multiple defects, such as strip-shaped through-going cracks and cavity groups, are used, and the crack propagation law and changes in AE (acoustic emission) and strain of cavity groups under different inclination angles are studied. According to the test results, an increase in the cavity group inclination angle can facilitate the initial damage degree of the rock and weaken the crack initiation stress; the initial crack initiation direction is approximately 90°, and the extension angle is approximately 75~90° from the strip-shaped through-going cracks; thus, the relationship between crack development and cavity group initiation strengthens. The specific performance is as follows: when the initiation angle is 30°, the cracks between the cavities in the cavity group develop relatively independently along the parallel direction of the external load; when the angle is 75°, the cracks between the cavities in the cavity group can interpenetrate, and slip can occur along the inclination of the cavity group under the action of the shear mechanism rupture. With the increase in the inclination angle of the cavity group, the AE energy fluctuation frequency at the peak stress increases, and the stress drop is obvious. The larger the cavity group inclination angle is, the more obvious the energy accumulation and the more severe the rock damage; when the cavity group angle is 30° or 75°, the peak strain of the local area below the strip-shaped through-going fracture plane is approximately three times that when the cavity group angle is 45° and 60°, indicating that cracks are easily generated in the local area monitored by the strain gauge at this angle, and the further development of the cracks weakens the strength of the rock, thereby increasing the probability of major engineering quality damage. The research results will have important reference value for hazard prevention in underground engineering projects through rock with natural and artificial defects, including tunnels and air-raid shelters.

Multiaxial fatigue life prediction method of notched specimens considering stress gradient effect

AbstractThis paper investigates the effect of stress gradient on the fatigue life of U‐notched specimens under multiaxial proportional loading by studying the shear stress gradient and normal stress gradient on the crack initiation plane separately. First, based on the energy method and critical plane method, an energy‐critical plane method for determining the direction of crack initiation is proposed. Second, with the help of the concept of stress field intensity, the shear stress gradient and normal stress gradient on the critical plane are studied. Finally, a multiaxial fatigue life prediction model is established by combining the equivalent stress field intensity, composed of the shear stress field intensity and normal stress field intensity, with the Manson–Coffin equation. Theoretical analysis is verified by the experiments of Q345 steel notched specimens, and the results show that the predicted accuracy of the proposed method is not conservative and is superior to the local stress–strain method.

In-situ observation and finite element analysis of fretting fatigue crack propagation behavior in 1045 steel

In this paper fretting fatigue crack behavior in 1045 steel is studied by in-situ observation and finite element analysis. in-situ fretting fatigue experiments are conducted to capture real-time fretting fatigue crack formation and propagation process. The fretting fatigue tests under different load conditions are carried out, then the lifetime and fracture surface are obtained. The crack propagation rates under different loading conditions are measured by in-situ observations. With in-situ observation, crack initiation location and direction are analyzed. Finite element model is used to calculate J-integral which then is applied to fitting with experimental crack growth rate, and establishing crack growth rate model. From fitted S-N curve, it turns out that smaller load ratio leads to higher lifetime. Crack initiates slightly below the point equivalent to line contact of the contact surface in different test conditions, and crack direction shows no obvious relationship with load parameters. The established crack growth rate model well agrees with the test results.

Influence of Bedding Planes on Mode I and Mixed-Mode (I–II) Dynamic Fracture Toughness of Coal: Analysis of Experiments

To determine the influence of bedding planes on pure mode I and mixed-mode I–II dynamic fracture toughness (DFT) and crack propagation characteristics of coal, a modified split Hopkinson pressure bar (SHPB) system is used to test notched semi-circular bend (NSCB) specimens. The DFT is calculated by the finite element code Abaqus. Two strain gauges are used to measure the crack propagation velocity and the crack propagation path is recorded by a high-speed digital camera. The results show that bedding planes have significant influences on the effective DFT and crack propagation characteristics of coal. As the bedding-plane angles increase (meaning the geometric positional relationship of the loading direction and the bedding-plane direction transforms from perpendicular to parallel), the effective DFT and peak force decrease. The bedding planes affect the crack initiation direction, and the crack propagation path is jointly determined by the direction of maximum principal stress and the bedding planes. For pure mode I, the crack propagation velocities rise with the increase of bedding-plane angles. However, for mixed-mode I–II, the largest velocity is at the bedding-plane angle of 45°. Moreover, the largest loading velocities at the linear stage of the loading history all occur at 45° and decrease rapidly thereafter.

Fretting fatigue and shot peening: a multiaxial fatigue criterion including residual stress relaxation

Fretting fatigue damage is a typical material damage arising in mechanical components. In such a damage condition, surface cracks can initiate at the surface contact zone, and under a fluctuating loading they can grow up to the complete failure of the component, resulting in a catastrophic failure.In order to prevent/mitigate fretting fatigue damage, shot peening is widely used in mechanical components. The improvement due to shot peening is related to different factors, but the main one is the compressive residual stress field produced by such a treatment in a region near the contact surface. Unfortunately, both the uncertainty in measurements and stability (relaxation) of residual stress field, make shot peening effects difficult to be included in models aimed to the estimation of fatigue life. Due to this lack in the literature, especially relatively to the fretting fatigue field, in the present paper a new fretting fatigue model, that is able to consider shot peening residual stress and its relaxation, is presented. A 3D elastic-plastic finite element model, which incorporates the shot peening residual stress and its relaxations, is employed. The results obtained, in terms of stress field, are used as input data of a multiaxial stress-based criterion to estimate both fatigue lifetime and crack initiation direction.