Shear-mode Crack Growth Research Articles

Fatigue Crack Growth under Non-Proportional Mixed Mode Loading in Rail and Wheel Steel Part 2: Sequential Mode I and Mode III Loading

Rolling contact fatigue cracks in rail and wheel undergo non-proportional mixed mode I/II/III loading. Fatigue tests were performed to determine the coplanar and branch crack growth rates on these materials. Sequential and overlapping mode I and III loading cycles were applied to single cracks in round bar specimens. Experiments in which this is done have been rarely performed. The fracture surface observations and the finite element analysis results suggested that the growth of long (does not branch but grown stably and straight) coplanar cracks was driven mainly by mode III loading. The cracks tended to branch when increasing the material strength and/or the degree of overlap between the mode I and III loading cycles. The equivalent stress intensity factor range that can consider the crack face contact and successfully regressed the crack growth rate data is proposed for the branch crack. Based on the results obtained in this study, the mechanism of long coplanar shear-mode crack growth turned out to be the same regardless of whether the main driving force is in-plane shear or out-of-plane shear.

Evaluation of crack growth direction criteria on mixed-mode fatigue crack growth experiments

The predictive capabilities of different fatigue crack growth direction criteria in finite element simulations are investigated. Crack growth direction criteria based on stress intensity factors, energetic measures and kinematic (displacement) measures are evaluated for mixed-mode fatigue crack growth experiments in the literature. More specifically, the development of the experimentally found crack path is simulated numerically, and the directions from the different criteria are compared to the known crack path along the development of the fracture. The simulated experiments featured proportional and non-proportional loading. Of the evaluated criteria, those based on energetic and displacement measures are able to accurately capture the tensile-mode fatigue crack growth direction for all examined experiments. Furthermore, the displacement-based criterion is able to capture the direction of shear-mode crack growth as well as the transition from shear- to tensile-mode growth and the subsequent tensile-mode growth. Modeling the cyclic elastic–plastic material response does not improve the accuracy of the predicted directions for the considered cases.

Quantitative analysis of intrinsic mode III fatigue thresholds in bcc metals

Intrinsic (effective) threshold is a material characteristic important for both engineering and science connected to fatigue crack propagation processes. This paper presents the first attempt to give a quantitative interpretation of the mode III effective threshold. The approach of a local growth mode previously applied to the remote mode II cracks, which led to a prediction of values of the mode II effective threshold in metallic materials, is applied to remote mode III cracks in materials with coplanar shear-mode crack growth (bcc metals). Local mode II stress intensity factors of a zig-zag shaped crack front loaded in the remote mode III were determined by numerical modelling for various angles of in-plane crack front asperities. The results were compared with experimentally measured effective thresholds and geometry of real crack fronts. It was found that for relatively small angles of the in-plane precrack asperities (23° for the ARMCO iron and 25° for niobium) the remote mode III crack propagation can be realized purely by the local mode II growth mechanism. Experimentally measured average angles of asperities on real precrack fronts (25° for the ARMCO iron and 26° for niobium) were very close to those obtained theoretically. This represents a quantitative confirmation of a dominance of the local mode II mechanism previously deduced only qualitatively by observation of fractographical patterns.

Mixed mode II and III fatigue crack growth in a rail steel

Rolling contact fatigue cracks in rails undergo non-proportional mixed-mode I + II + III, in variable proportions along their front. In order to determine the crack growth thresholds and kinetics in mixed-mode II/III, asymmetric four point bending tests are run on a rail steel with different angles between the crack front and the shearing load, so as to vary the mode mixity ratio. For sufficiently high loading ranges, these tests give rise to coplanar shear-mode crack growth. The effective stress intensity factors (SIFs) are derived by an inverse method from the measured crack face relative displacements. It appears to be 10–70% lower than the nominal SIFs and to allow a reasonable correlation of the measured crack growth rates. The local application, ahead of the crack front, of shear-driven or tension-driven fatigue damage models – after 3D elastic-plastic computations of local stress and strain ranges – allows a successful prediction of crack fronts paths and growth rates.

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.

Friction and roughness induced closure effects on shear-mode crack growth and branching mechanisms

The understanding of shear-mode crack growth mechanisms and crack branching phenomena is of great interest for a variety of practical engineering situations. Despite this fact, relatively little research is available regarding these topics. Of the studies that have been performed, few provide a means of quantifying such effects and most consider crack growth starting from a precrack. The current study is aimed at trying to fill some of the research voids in these areas by investigating the effects of microcrack coalescence, loading level, and superimposed normal stresses on the shear-mode crack behavior of naturally initiated fatigue cracks. Based on the experimental results and subsequent analyses, it was determined that microcrack networks and coalescence have little to no effect on the experimentally observed crack paths, regardless of the applied loading level. Instead, the preferred crack growth mode was shown to have a dependence on the applied shear stress magnitude and stress normal to the crack plane, indicating a significant role of fiction and roughness induced crack closure in the crack growth process. A simple model was then proposed to quantify these effects based on the idea that crack face interaction reduces the effective shear-mode driving force by allowing a portion of the nominally applied loading to be transferred through a crack. Crack growth predictions based on the model were compared to experimental results with respect to both crack branching and crack growth rate. The model was found to agree well, both qualitatively and quantitatively, with experimentally observed trends for a variety of multiaxial loading conditions.

Assessment of mixed mode crack propagation of crane runway girders subjected to cyclic loading

Crane runway girders are subjected to travelling wheel loads resulting in mixed mode crack tip loading. The paper addresses the assessment of crack propagation of crane runway girders being travelled over by wheel loads. Fractographic investigations of crack surfaces show shear mode crack growth and scabs due to friction between the crack flanks. Crack growth rates for steels subjected to tensile mode crack growth are evaluated. Stress intensity factors of crane runway girders under cyclic loading are obtained by numerical investigations. Numbers of cycles of crack growth are calculated by use of the shear stress intensity factor Kτ.

Crack paths in smooth and precracked specimens subjected to multiaxial cyclic stressing

The understanding of shear-mode crack growth mechanisms and crack branching phenomena is of great interest for a variety of practical engineering situations. Despite this fact, relatively little research is available regarding these topics. Of the studies that have been performed, few provide a means of quantifying such effects and most consider crack growth starting from a precrack. The current study is aimed at trying to fill some of the research voids in these areas by investigating the effects of microcrack coalescence, loading level, and superimposed normal stresses on the mode II crack behavior of naturally initiated fatigue cracks. Based on the experimental results and subsequent analyses, it was determined that microcrack networks and coalescence have little to no effect on the experimentally observed crack paths regardless of the applied loading level. Instead, the preferred crack growth mode is shown to have a dependence on the applied shear stress magnitude and stress normal to the crack plane, indicating a significant role of fiction and roughness induced crack closure effects in the crack growth process. A simple model is then proposed to quantify these effects based on the idea that crack face interaction reduces the effective mode II SIF by allowing a portion of the nominally applied loading to be transferred through a crack. The model agrees qualitatively with the experimentally observed trends for pure torsion loading and predicts crack branching lengths within a factor of 2 for all loadings considered.

Experiments under pure shear and rolling contact fatigue conditions: Competition between tensile and shear mode crack growth

In the present paper, the mechanism of shear crack growth under both pure torsion and mixed mode loadings, simulating rolling contact fatigue testing conditions, has been investigated for a bearing steel and the role of the superimposed compressive stress in subsurface RCF has been clarified both numerically and experimentally. In particular a previous data set of fatigue tests on micro-notched specimens subjected to torsion and out-of-phase loads with |σmin|/τmax≅3.5 (LP1) has been complemented with the new tests onto micro-notched specimens loads with |σmin|/τmax≅0.7 (LP2) and a test under pure compression. The same tests have been also simulated numerically with a non-linear FE analysis of crack advance. The numerical analyses have been conducted with the aim of demonstrating that the compressive stress fully suppresses the tendency to tensile mode growth as the crack extends.Eventually, the competition between tensile and shear mode growth during a fatigue cycle has been investigated theoretically in terms of local branch SIFs. In particular, the conditions for the branch crack growth have been examined on the basis of the effective SIFs: the crack tip shielding effects due to the crack surface interference (both the mode I contribution caused by the asperity mismatch and the shear attenuation produced by the frictional stresses) have been quantified by employing a model for crack sliding interaction under pure mode III and mixed mode I+III loadings.

Effects of Crack Size and Crack Face Interference on Shear Fatigue Crack Growth in a Bearing Steel

The near-threshold fatigue behavior of small, semi-elliptical surface cracks in a bearing steel was investigated under cyclic shear-mode loading in ambient air. Fully-reversed cyclic torsion was combined with a static axial compressive stress to obtain a stable shear-mode crack growth in the longitudinal direction of cylindrical specimens. Shear stress amplitude was gradually decreased with an increase in crack length and the crack finally became non-propagating. Abrasive wear on the crack faces was inferred by debris and also by changes in microstructure in the wake of crack tip. Further, it was found that these effects resulted in a significant decrease in the crack growth rate. In this study, we shed light on the important role of the crack size and crack face interference on the crack growth behavior.

Shear mode threshold for a small fatigue crack in a bearing steel

The fatigue behaviour of small, semi-elliptical surface cracks in a bearing steel was investigated under cyclic shear-mode loading in ambient air. Fully reversed torsion was combined with a static axial compressive stress to obtain a stable shear-mode crack growth in the longitudinal direction of cylindrical specimens. Non-propagating cracks less than 1 mm in size were obtained (i) by decreasing the stress amplitude in tests using notched specimens and (ii) by using smooth specimens in constant stress amplitude tests. The threshold stress intensity factor ranges, ΔKIIth and ΔKIIIth, were estimated from the shape and dimensions of non-propagating cracks. Wear on the crack faces was inferred by debris and also by changes in microstructure in the wake of crack tip. These effects resulted in a significant increase in the threshold value. The threshold value decreased with a decrease in crack size. No significant difference was observed between the values of ΔKIIth and ΔKIIIth.

The influence of biaxial strain ratio and strain range on crack growth mode and crack shape

Axial-torsion tests were used to determine and model the combinations of strain range and crack length at which the crack growth changed from a shear to a tensile mode. Biaxial tests and confocal scanning laser microscopy (CSLM) were used to observe crack shape evolution at various strain ratios. Constant and variable amplitude loading of notched specimens and CSLM were used to determine the dependence of crack shape development on strain range. The results showed that, except for very high strain levels, at strain ratios causing shear mode crack growth multiple cracks initiated and grew until they were semicircular and then linked up to cause failure. For tensile mode crack growth at low strains a single crack grew into and maintained a semicircular shape. At high strain ranges multiple cracks formed and linked up to maintain an elongated crack shape.

Modelling and full-scale trials to investigate fluid pressurisation of rolling contact fatigue cracks

Fluid penetration of cracks has been regarded as an important mechanism of crack growth for inclined surface breaking cracks under contact loading since the 1930s. However, there are only limited cases in which it is realistic to assume fluid is sealed in and pressurised by wheels crossing cracks of complex three-dimensional morphology in rolling contact fatigue affected rail. To investigate, modelling to predict crack growth rates was conducted following experimental verification that fluid penetration occurs for a full size rail-wheel contact. A fluid filled crack beneath a three-dimensional elliptic contact was modelled through development of an existing two-dimensional model. Stress intensity factor results from this model were in good agreement with available literature values, indicating very high growth rates for cracks containing pressurised fluid. At larger crack sizes, for which it was assumed the contact could not seal fluid inside the crack, a shear mode crack growth model was applied. This predicted much lower crack growth rates, of around ten times the rail wear rate, indicating a crack growing sufficiently fast to “keep ahead” of wear, but not unrealistically fast. Further work to correlate the predictions of the fluid pressurisation model with field measurements was suggested.

A material and environment-dependent criterion for the prediction of fatigue crack paths in metallic structures

Crack growth under mode II cyclic loading was investigated in maraging steel, ferritic–pearlitic steel and TA6V. When Δ K II exceeds a threshold value, cracks do not bifurcate but grow in mode II over a distance which increases with Δ K II. Shear mode crack growth was much more extensive in maraging steel than in TA6V and ferritic–pearlitic steel. This result is discussed in relation with the cyclic behaviour of the materials and the importance of friction along the crack faces. The maximum growth rate criterion is shown to be suitable for the prediction of crack paths when shear mode crack growth is likely to occur.

純アルミニウムの疲労き裂伝ぱ特性に関して―混合モードき裂伝ぱの観察と評価―

The mechanism of fatigue crack propagation in pure aluminum was investigated. Two types of fatigue crack propagation tests were carried out. One of them was a cyclic torsion test with static tension using a pre-cracked specimen. In this test, the effect of static tension on shear mode crack propagation by cyclic torsion was examined. The other was a push-pull fatigue test of plate type specimens in which a slit was cut in the center section of the test section. The slit direction cut in specimen was perpendicular or a 45 degree inclined direction to the specimen axis. In the former, it was found that the application of some static tension was necessary to continue the shear mode crack growth by cyclic torsion. In the latter, crack propagated by mode I or by a mixed mode, which combined mode I and shear mode, depended on the testing conditions. In these cases, the crack propagation rate da/dN was evaluated with a modified effective stress intensity factor range, which was independent of the crack propagation mode. In the pure aluminum case, the shear stress strongly affected crack propagation rate. This was thought to be related to the crystal structure.

Shear mode fatigue crack growth in 1050 aluminium

ABSTRACTThe shear mode crack growth mechanism in 1050 aluminium was investigated using pre‐cracked specimens. A small blind hole was drilled in the centre section of the specimens in order to predetermine the crack initiation position, and a push–pull fatigue test was used to make a pre‐crack. Crack propagation tests were carried out using both push–pull and cyclic torsion with a static axial load. With push–pull testing, the main crack grew by a mixed mode. It is thus apparent that shear deformation affects the fatigue crack growth in pure aluminium. In tests using cyclic torsion, the fatigue crack grew by a shear mode. The micro‐cracks initiated perpendicular and parallel to the main crack's growth direction during the cyclic torsion tests. However, the growth direction of the main crack was not changed by the coalescence of the main crack and the micro‐cracks. Shear mode crack growth tends to occur in aluminium. The crack growth behaviour is related to a material's slip systems. The number of slip planes in aluminium is smaller than that of steel and the friction stress during edge dislocation motion of aluminium is lower than many other materials. Correlation between the crack propagation rate and the stress intensity factor range was almost the same in both push–pull and cyclic torsion with tension in this study.

Mechanism of Shear Mode Fatigue Crack Growth in Pure Aluminium

The mechanism of shear mode fatigue crack growth in pure aluminum was investigated using pre-cracked specimen. The crack propagation tests were carried out by the cyclic torsion with a static axial load and the push-pull, respectively. In the case of the cyclic torsion, the fatigue crack grew by shear mode. The micro-cracks initiated perpendicular and parallel to the direction of main crack growth during the cyclic torsion. However, the interactions were not observed between the main crack and the micro-cracks, which directed perpendicular to the direction of main crack growth. The behavior of micro-cracks on pure aluminum was different from that of 4340 steel in which the pure shear mode crack growth hard to occurred. The crack growth behavior is related to the slip systems of the materials. The number of slip plane and the friction resistance between molecules of aluminum are smaller than those of steel. These and the loading conditions are related to the mechanism of the shear mode crack growth in aluminum. Under condition in the prenent study, the relation between the crack propagation rate and the stress intensity factor range was almost the same in the cases of the push-pull and the cyclic torsion with tension. However, the effects of friction between the crack surfaces on the crack propagation in a shear mode were not exactly evaluated.

Crack growth mechanism in precracked torsional fatigue specimens

The mechanism of crack growth of precracked materials was investigated under cyclic torsion loading with and without axial static stress using 4340 steel. During the crack growth, branching of the crack was observed. The length of the crack between the first branching points was dependent on loading conditions. This length was longer when the applied shear amplitude and the static axial stress level were higher. When the crack tips were opened by the tension loads, the crack had the tendency to grow in a shear mode during cyclic torsion. It was found that friction of the crack surfaces prevented shear mode crack growth. Furthermore, at higher stresses the initiation of new microcracks was observed in front of the main crack and their density was dependent on loading conditions. This helped the crack grow in a shear mode before and after branching.

予き裂材における疲労き裂の分岐と伝ぱ挙動について

The growth behavior of crack was investigated under cyclic torsion with and without axial static stress using pre-cracked steel. After shear mode crack growth, branching of the crack was observed on both sides of the crack. The length between the branching points was dependent on loading conditions. This length was longer when the applied shear amplitude and the static axial stress level were higher. The crack propagation curves have turning points. Before the turning point, shear mode crack growth was observed and the crack propagation rate had good correlation with the branching behavior. It is found that the crack opening and the initiation of micro cracks help the shear mode crack growth After the branching of crack, loading method was changed from cyclic torsion to push-pull. When applied normal stress was higher, the crack grew under the condition of maximum shear stress criterion. On the other hand, when applied stress was lower, the crack grew under the condition of maximum principal stress criterion. Those behaviors are also related to slip band density and crack opening.

K-0625 予き裂材の低サイクル疲労試験におけるき裂伝ぱ挙動(G03-7 疲労・き裂)(G03 材料力学部門一般講演)

The growth behavior of crack was investigated under cyclic torsion with and without axial static stress using precracked steel. After shear mode crack growth, branching of the crack was observed on both sides of the crack. The length between the first branching points was dependent on loading conditions. This length was longer when the applied shear amplitude and the static axial stress level were higher. The crack propagation behavior had good correlation with the branching behavior. It is found that the crack opening and the initiation of micro cracks help the shear mode crack growth After the branching of crack, loading method was changed from cyclic torsion to push-pull. When applied normal stress was higher, the crack grew under the condition of maximum shear stress criterion. On the other hand, when applied stress was lower, the crack grew under the condition of maximum principal stress criterion. Those behaviors are also related to slip band density and crack opening.