Shear-mode Fatigue Crack Growth Research Articles

Criteria for determining the fatigue limit of Ni-based superalloy 718, considering the competition between opening- and shear-mode fatigue crack-growth

The mechanism that determines the fatigue limit associated with small-crack arrest in Ni-based superalloy 718 remains under debate. In order to comprehensively assess the alloy’s fatigue limit in relation to the growth threshold of a small fatigue crack, (i) a ΔKI-decreasing fatigue crack-growth test, (ii) torsional fatigue tests and (iii) tension-compression fatigue tests were conducted under fully-reversed loading conditions. In addition, elastic, crack-tip stress fields were analyzed for cracks under various loading conditions. Based on the results, a novel strategy for evaluating the alloy’s fatigue limit is proposed after consideration of two crack-arresting modes (i.e., opening and shear modes).

In-situ observation of microstructurally small fatigue crack initiation and growth behaviors of additively-manufactured alloy 718

To reveal the effects of microstructural features on fatigue property, microscopic fatigue crack initiation and growth behavior were successively observed for rolled plate and additively-manufactured nickel base 718 alloy. Three types of additive manufacturing (AM) methods: powder bed Electron Beam Melting (EBM), Selective Laser Melting (SLM), and wire-fed Electron Beam Additive Manufacturing (EBAM) were adopted for specimen preparation to create various microstructures for comparison. The results showed that the fatigue lives of the EBM and EBAM specimens, which have a robust (001) texture, were shorter than those of the rolled plate and SLM specimens. This fatigue life difference could not be explained by the defects, because the fatigue cracks in the part of EBAM specimen were initiated along a slip plane. On the other hand, acceleration of the early stage fatigue crack growth rate (FCGR) was found in the EBM and EBAM specimens. Cross-sectional electron back scatter diffraction (EBSD) observations also showed that early stage cracks initiated and grew along the {111} slip plane; but in the EBAM specimens, the cracks continuously propagated straight to an adjacent grain due to their (001) texture. It thus appears that the low-angle grain boundaries which tend to be generated as a result of the (001) texture do not act as barriers to the penetration of dislocations; instead, they accelerate the shear mode fatigue crack growth rate in the early stages and decrease fatigue life.

Microstructurally small fatigue crack initiation behavior of fine and coarse grain simulated heat-affected zone microstructures in low carbon steel

Successive observation of fatigue crack initiation process was conducted on six types of thermally simulated HAZ microstructures to clarify the fundamental reason of fatigue life difference between coarse grain (CG) and fine grain (FG) heat-affected zone (HAZ) microstructures. As a result, regardless of microstructure, fatigue cracks initiated and propagated along shear stress plane when the crack is smaller than prior austenite grain size. In addition, this shear mode fatigue crack growth of CGHAZ were much faster than that of FGHAZ and categorized as microstructurally small fatigue cracks. Although prior austenite grain size of crack initiation life of the FG and CGHAZ were comparable, the remaining fatigue life after the grain size crack initiation of the CGHAZ become shorter than that of the FGHAZ as a function of grain size difference. As a result, total fatigue life of CGHAZ becomes shorter than that of FGHAZ. Therefore, relatively larger part of CGHAZ's fatigue life is consumed by shear mode microstructurally small fatigue crack growth process.

Fracture and fatigue of the surface layer of layered media due to adhesive sliding contact

Layer cracking in a layered medium consisting of an elastic-perfectly plastic half-space (substrate) and a relatively stiff, elastic surface layer in adhesive sliding contact with a rigid asperity was analyzed with the finite element method using linear elastic fracture mechanics. The directions of tensile and shear mode fracture were determined from the maximum tensile and shear stress intensity factors (SIFs), respectively, whereas the directions of tensile and shear mode fatigue crack growth were determined from the maximum range of tensile and shear SIFs, respectively. The effects of asperity position, interfacial adhesion (Maugis parameter), crack depth, asperity interaction distance, layer thickness, and material properties of the elastic layer and the elastic-plastic substrate on fracture and fatigue crack growth in the layer are interpreted in the context of simulation results. It is shown that the propensity for fracture and fatigue in the layer is enhanced with increasing asperity interaction distance and Maugis parameter and decreasing crack depth and layer thickness. The numerical results of this study reveal that, for similar tensile and shear fracture toughness, layer fracture in adhesive sliding contact predominantly occurs by the out-of-plane tensile mode of fracture, whereas fatigue crack growth in the layer is a manifestation of the in-plane shear and out-of-plane tensile modes of fatigue crack growth.

Crack size dependency of shear-mode fatigue threshold in bearing steel subjected to continuous hydrogen charging

Premature delamination failure, characterized by the white structure flaking (WSF) or the white etching crack (WEC), often occurs in rolling element bearings and it deteriorates the durability of bearing substantially. It is known that this failure is caused by shear-mode (Mode II and Mode III) crack growth in conjunction with evolution and invasion of hydrogen into material during operation. To ensure the structural integrity associated with rolling element bearing, it is important to clarify the effect of hydrogen on the shear-mode fatigue crack growth behavior near the threshold level.In our previous study, the effect of hydrogen on the shear-mode fatigue crack growth behavior in a bearing steel of JIS SUJ2 was examined near the threshold level. Consequently, it was shown that the threshold stress intensity factor (SIF) range for shear-mode fatigue crack growth decreased significantly by action of hydrogen. However, the investigation was made only for a crack with a surface length of about 900 mm. To thoroughly understand the critical condition for delamination failure, it is important to investigate the crack size dependency of the threshold level for a shear-mode small fatigue crack in the presence of hydrogen. In the present study, correspondingly, the threshold SIF ranges for a shear-mode crack with different length were additionally measured in the same material by using a novel technique that enables continuous charging of hydrogen in a specimen during long-term fatigue test. Then, a clear reduction in crack growth rate and a crack size dependency of the threshold SIF range were observed under the environmental condition of continuous hydrogen charging.

Micro-scale frictional behavior of a bearing steel (JIS SUJ2) in cyclic sliding motion

Application of fracture mechanics has recently been emphasized to establish the mechanics-based evaluation of the rolling contact fatigue strength for designing mechanical elements such as bearing, rail/wheel, gear, etc. Flaking that is a typical rolling contact failure mode is intimately related to the shear-mode (modes II and III) fatigue crack growth, which is significantly affected by the interaction of opposing crack faces. Understanding the tribological properties on the crack faces being in the reciprocating sliding contact is therefore essential for the material design as well as the mechanical design of many components associated with rolling contact fatigue. The present study carried out a ring-on-ring test by making use of a combined axial-torsional fatigue testing machine to investigate the micro-scale frictional phenomena under the non-lubricated condition. The material investigated was a heat-treated high-carbon-chromium bearing steel (JIS SUJ2) with a Vickers hardness of 753. The tests were conducted up to 104 cycles under the various conditions of mean contact pressures of 10-100 MPa, relative displacement amplitudes of about 10-100 μm, sinusoidal frequencies of 0.1-10Hz, and initial surface roughness values, Ra, of 0.2-1.7. The time variations of the tangential force, contact load and the relative displacement between contact surfaces were periodically monitored at a predetermined number of cycles during testing.

Characteristics of shear-mode fatigue crack growth behaviors in roll materials

The shear-mode fatigue crack behavior of work roll materials for hot strip mills was evaluated using cyclic torsion with superimposed static compression. Most of the deep cracks in rolls have grown under shear-mode during rolling, however there has been no practical methods to evaluate the fatigue behavior of roll materials in the presence of shear-mode crack, which has been a big obstacle for development of the work roll materials, especially the advanced materials like the high speed steel type roll materials. The present study shows that a high speed steel material has preferable torsional fatigue behavior than an enhanced ICDP (Indefinite Chill Double Poured cast iron). However, in actual rolling, the enhanced ICDPs generally show better crack resistance than high speed steel roll type materials. Accordingly, further research is necessary utilizing the testing method.

A new testing method for investigating the shear-mode fatigue crack growth behavior in hydrogen environment

Ball bearing is widely used in a variety of machines including the transportation equipments of the automobiles and airplanes. Flaking failure is a common problem for ball bearing and it is caused by shear-mode fatigue crack growth under cyclic shear stress. Further, it is known that the premature flaking is attributed to the combined effect of hydrogen penetration into the material and cyclic shear stress during the operation. Therefore, in order to ensure the integrity of ball bearing, it is necessary to clarify the effect of hydrogen on shear-mode fatigue crack growth behavior, in particular, the threshold behavior. The evaluation of the shear-mode crack growth behavior is not easy because mode I crack branching occurs easily. Our previous studies revealed that it is required to apply static compression in the direction of specimen axis to attain a stable shear-mode fatigue crack growth. In addition, successive hydrogen supply to the specimen is essential for the evaluation of hydrogen effect on the fatigue threshold because hydrogen emits from the specimen during the fatigue test. In other words, the hydrogen-precharging method, commonly used for the research on hydrogen embrittlement, is not appropriate for the evaluation of fatigue threshold. In this study, to solve these problems, we have developed a novel, easy-to-use experimental method to evaluate the threshold behavior of shear-mode fatigue crack in the presence of hydrogen. The fundamental principle of the method is introduced in this paper.

Effect of initial contact surface condition on the friction and wear properties of bearing steel in cyclic reciprocating sliding contact

Delamination failure is one of the most important engineering problems. This failure can frequently be detrimental to rolling contact machine elements such as bearings, gear wheels, etc. This phenomenon, called rolling contact fatigue, has a close relationship not only with opening-mode but also with shear-mode fatigue crack growth. The crack face interference is known to significantly affect the shear-mode fatigue crack propagation and its threshold behavior. Quantitative investigation on friction and wear at fatigue crack faces in the material is essentially impossible. Previously, thus, a novel ring-on-ring test by making use of fatigue testing machine was proposed to simulate a cyclic reciprocating sliding contact of crack surfaces. However, this test procedure had some problems. For instance, in order to achieve the uniform contact at the start of test, the rubbing of specimens must be conducted in advance. By this treatment, the specimen surfaces were already damaged before the test. In this study, an improvement of experimental method was made to perform the test using the damage-free specimens. The friction and wear properties for heat-treated high carbon-chromium bearing steel were investigated with this new method and the results were compared to the results obtained by using the initially damaged specimens.

Microscopic observation of shear-mode fatigue crack growth behavior under the condition of continuous hydrogen-charging

Rolling bearings facilitate the smooth motion of moving mechanical transmission devices substantially reducing the energy loss. However, flaking failure caused in bearings by rolling contact fatigue can crucially deteriorate the integrity of an overall mechanical system composed using bearings. It is understood nowadays that this phenomenon is intimately associated with shear-mode (Mode II and III) fatigue crack growth caused by cyclic shear stress in the presence of large compression. Further, a problem has become of importance that the premature flaking in ball bearings could be caused by the assistance of the penetration of hydrogen into material during the operation. In this study, torsional fatigue test of a bearing steel (JIS SUJ2) was performed by using a newly-developed method of continuous hydrogen-charging. Based on the microscopic observation, it was found that continuous hydrogen-charging had an influence on the shear-mode fatigue crack growth behavior in the high-cycle fatigue regime and reduced the threshold level.

Fatigue crack growth of aluminium alloy 7075-T651 under proportional and non-proportional mixed mode I and II loads

This study aims to experimentally investigate fatigue growth behaviour in AA7075-T651 under proportional and non-proportional mixed mode I and II loads. Fatigue tests were performed under cyclic tension and torsion using a thin-walled tubular specimen with a key-hole style crack starter. After the generation of a single-side mode I pre-crack, varied forms of mixed mode loads were applied, which in most cases led to coplanar growth for a short distance followed by a long and stable crack path deviation. It was found that under most of the non-proportional mixed mode load cases, the direction of the deviated crack path could not be reasonably predicted using the commonly accepted maximum tangential stress criterion. Meanwhile, in some cases, the crack path directions could be approximately predicted using the maximum shear stress criterion. It was also confirmed for the first time that a long, stable and non-coplanar shear mode fatigue crack growth could be produced in AA7075-T651 under non-proportional mixed mode I and II loads. The implications of transient co-planar as well as stable shear-mode fatigue crack growth were discussed, revealing the needs of future investigations. ©2016 Commonwealth of Australia

The influence of static crack-opening stress on the threshold level for shear-mode fatigue crack growth in bearing steels

An investigation was conducted into the influence of statically-applied, mode I, crack-opening load on the threshold condition for propagation of a shear-mode fatigue crack in a bearing steel. Torsional fatigue test was carried out at an R of −1 using a hollow cylindrical specimen into which a semi-elliptical, small slit was axially introduced. A static axial compressive stress was simultaneously applied to suppress crack branching. A coplanar, shear-mode, non-propagating fatigue crack emanating from the slit was attained by appropriate control of shear stress amplitude. Internal pressure was then applied to generate a hoop stress as a static crack-opening stress, σθ static. Consequently, the threshold shear-mode stress intensity factor range, ΔKτth, was significantly decreased with increase of the static mode I stress intensity factor. To further understand the contribution of σθ static to the reduction in ΔKτth, microstructural observations for the cross-sections of a non-propagating crack were conducted using a scanning electron microscope in conjunction with the electron backscatter diffraction analysis. The results revealed that the excess loading of σθ static accounts for the change in the crack-path, resulting in a further reduction in ΔKτth.

Fatigue crack growth of aluminium alloy 7075-T651 under non-proportional mixed mode I and II loads

This study aims to investigate fatigue growth behaviour in AA7075-T651 under non-proportional mixed mode I and II loads. Fatigue tests were performed under cyclic tension and torsion using a thin-walled tubular specimen with a key-hole style crack starter. After the generation of a single-side mode I pre-crack, varied forms of mixed mode loads were applied, which in most cases led to a short distance coplanar growth followed by a long and stable crack path deviation. It was found that under most of the non-proportional mixed mode load cases, the direction of the deviated crack path could not be reasonably predicted using the commonly accepted maximum tangential stress criterion. Meanwhile, in some cases, the crack path directions could be approximately predicted using the maximum shear stress criterion. It was also confirmed for the first time that a long, stable and noncoplanar shear mode fatigue crack growth could be produced in AA7075- T651 under non-proportional mixed mode I and II loads.

A New Fatigue Testing Machine for Investigating the Behavior of Small Shear-Mode Fatigue Cracks

The investigation of the behavior of small shear-mode fatigue cracks in the high-cycle fatigue regime is essential to understand the mechanism of rolling-contact fatigue failures, such as flaking in bearings and shelling in rails, from the fracture mechanics point of view. The stable growth of a shear-mode fatigue crack was achieved by applying static compression to a specimen in a cyclic torsion fatigue test. This loading condition is usually obtained by a combined tension-torsion testing machine with a servo-hydraulic control system. In this study, a new testing machine was developed and found to be superior to the servo-hydraulic testing machine in terms of price, operation/maintenance costs, operating speed, and installation volume. For substantiation and demonstration purposes, a shear-mode fatigue crack growth test with a bearing steel was also carried out using both the new and the conventional servo-hydraulic testing machines. The experiments revealed that under the same loading conditions, nonpropagating shear-mode cracks of similar size and geometry could be obtained by the respective testing machines. Thus, it was concluded that the new testing machine has equivalent capabilities to the servo-hydraulic testing machine in performing shear-mode fatigue crack growth tests.

Progress in understanding of intrinsic resistance to shear-mode fatigue crack growth in metallic materials

Recently made experiments allowed measurement of effective data (free of crack tip shielding) and enabled a better understanding of intrinsic behaviour of shear-mode cracks. Influence of crystal structure and microstructure was distinguished. Experimental results for niobium showed that growth micromechanisms and effective thresholds (ΔKIIeff,th≈1.0MPam1/2, ΔKIIIeff,th≈1.4MPam1/2) of modes II and III cracks were different, which was also observed in other metals. Validity of the simple formula for mode II effective threshold for single-phase metals based on the dislocation emission model was confirmed. The cracks propagated in a coplanar manner along the maximum shear plane, which is a characteristic behaviour for bcc metals. Model of propagation of remote mode III loaded cracks by local mode II advances was confirmed.

軸受鋼の繰返し往復すべり接触における摩擦挙動と表面損傷

Delamination failure is one of the most important engineering issues for rolling contact machine elements. It is known that this failure is intimately related to shear-mode (mode II and mode III) fatigue crack growth. Generally, the shear-mode fatigue crack growth behavior and its threshold condition can be greatly influenced by the crack face interference. Therefore, understanding of the mechanism of friction and surface damage of crack faces is of importance and a new testing method that can examine appropriately these properties is required. In this study, the influences of the cyclic reciprocating sliding contact with micro-scale relative motion on the frictional behavior and the surface damage of a bearing steel were studied under dry condition. As a novel friction and wear testing method, a cyclic ring-on-ring test was conducted by making use of a hydraulic-controlled combined axial and torsional fatigue testing machine. The coefficient of kinetic friction was ranged from 0.4 to 1.0, the average value being about 0.75.

Effects of microstructure and crystallography on crack path and intrinsic resistance to shear-mode fatigue crack growth

ABSTRACT. The paper focuses on the effective resistance and the near-threshold growth mechanisms in the ferritic-pearlitic and the pure pearlitic steel. The influence of microstructure on the shear-mode fatigue crack growth is divided here into two factors: the crystal lattice type and the presence of different phases. Experiments were done on ferritic-pearlitic steel and pearlitic steel using three different specimens, for which the effective mode II and mode III threshold values were measured and fracture surfaces were reconstructed in three dimensions using stereophotogrammetry in scanning electron microscope. The ferritic-pearlitic and pearlitic steels showed a much different behaviour of modes II and III cracks than that of the ARMCO iron. Both the deflection angle and the mode II threshold were much higher and comparable to the austenitic steel. Mechanism of shear-mode crack behaviour in the ARMCO iron, titanium and nickel were described by the model of emission of dislocations from the crack tip under a dominant mode II loading. In other tested materials the cracks propagated under a dominance of the local mode I. In the ferritic-pearlitic and pearlitic steels, the reason for such behaviour was the presence of the secondary-phase particles (cementite lamellas), unlike in the previously austenitic steel, where the fcc structure and the low stacking fault energy were the main factors. A criterion for mode I deflection from the mode II crack-tip loading, which uses values of the effective mode I and mode II thresholds, was in agreement with fractographical observations.

Description of Fatigue Crack Growth under Modes II, III and II+III in Terms of J-integral

Mode II and mode III fatigue crack growth rates and the related intrinsic stress intensity factor range thresholds ΔKIIeff,th and ΔKIIIeff,th were for a long time expected to be equal. However, there are many theoretical and experimental indications of a slower crack growth rate and a higher threshold for mode III than that for mode II for various metallic materials. In this article, a description of shear-mode fatigue crack growth for specimens made of ARMCO iron in terms of a diagram da/dN vs. ΔJ is presented and compared with that based on ΔK. For higher applied loads the description in terms of ΔK is no more valid which turned out from a precise assessment of plastic zone size for a wide range of shear-mode loading levels in these specimens. This numerical elasto-plastic analysis revealed that the small-scale yielding conditions were only fulfilled in the near-threshold region and, for higher loading levels, the ΔJ approach should be utilized. The measured curves plotted for loading modes II, III and II+III showed higher effective mode II threshold than that of mode III but approximately equal crack growth rates for both modes II and III. The mode II cracks in metals usually propagate faster than the mode III ones because the “factory-roof” morphology created under mode III loading produces a high friction, thus reducing the effective crack driving force significantly. The unusual behavior of ARMCO iron can be explained here by a coplanar crack propagation without forming the factory roof morphology.

Investigation for Small Shear-Mode Fatigue Cracks in Bearing Steels

Flaking and spalling caused by rolling contact fatigue associate with a small crack, and a special testing method and machine are required to study the small fatigue crack behavior under shear mode loading. It was found by authors that the behaviors of small shear-mode fatigue cracks from the inclusions and the artificial defects could be successfully observed by applying the fully-reversed torsion coupled with static axial compressive stress. However, the servo-hydraulic fatigue testing machine is quite expensive for purchase and maintenance, and large installation space is necessary for the hydraulic and cooling systems. Moreover, the presence of axial compression significantly lowers the frequency of torsional loading, which consequently results in low testing speed. In this study, a cost-effective, space-saving and high-speed fatigue testing method was newly proposed, and the shear-mode fatigue crack growth tests were carried out by using the developed machine. Based on the obtained experimental data, the potential of the new testing machine is discussed.

DESIGNING OF A TESTING MACHINE FOR SHEAR-MODE FATIGUE CRACK GROWTH

As recognized, flaking-type failure is one of the serious problems for railroad tracks and bearings. In essence, flaking-type failure is closely related to the growth of the shear-mode (Mode-II and Mode-III) fatigue crack. In our research group, it is demonstrated that a shear-mode fatigue crack can be reproduced for cylindrical specimens by applying the cyclic torsion in the presence of the static axial compressive stress. However, a biaxial servo-hydraulic fatigue testing machine is quite expensive to purchase and costly to maintain. The low testing speed (about 10Hz) of the testing machine further aggravates the situation. As a result, study on shear-mode fatigue crack growth is still in the nascent stage. To overcome the difficulties mentioned above, in this research activity, we developed a high-performance and cost-effective testing machine to reproduce the shear-mode fatigue crack growth by improving the available resonance-type torsion fatigue testing machine. The primary advantage of using the resonance-type torsion fatigue testing machine is cost-efficiency. In addition, the testing speed effectively can be improved, in comparison with that of a biaxial servo-hydraulic fatigue testing machine. By utilizing the newly-designed testing machine, we have demonstrated that we can successfully reproduce the shear-mode fatigue crack.