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.

Fatigue Crack Growth under Non-Proportional Mixed Mode Loading in Rail and Wheel Steel Part 1: Sequential Mode I and Mode II Loading

Fatigue tests were performed to estimate the coplanar and branch crack growth rates on rail and wheel steel under non-proportional mixed mode I/II loading cycles simulating the load on rolling contact fatigue cracks; sequential and overlapping mode I and II loadings were applied to single cracks in the specimens. Long coplanar cracks were produced under certain loading conditions. The fracture surfaces observed by scanning electron microscopy and the finite element analysis results suggested that the growth was driven mainly by in-plane shear mode (i.e., mode II) loading. Crack branching likely occurred when the degree of overlap between these mode cycles increased, indicating that such degree enhancement leads to a relative increase of the maximum tangential stress range, based on an elasto–plastic stress field along the branch direction, compared to the maximum shear stress. Moreover, the crack growth rate decreased when the material strength increased because this made the crack tip displacements smaller. The branch crack growth rates could not be represented by a single crack growth law since the plastic zone size ahead of the crack tip increased with the shear part of the loading due to the T-stress, resulting in higher growth rates.

Fracture Property of Y-Shaped Cracks of Brittle Materials under Compression

In order to investigate the properties of Y-shaped cracks of brittle materials under compression, compression tests by using square cement mortar specimens with Y-shaped crack were conducted. A true triaxial loading device was applied in the tests, and the major principle stresses or the critical stresses were measured. The results show that as the branch angleθbetween the branch crack and the stem crack is 75°, the cracked specimen has the lowest strength. In order to explain the test results, numerical models of Y-shaped cracks by using ABAQUS code were established, and the J-integral method was applied in calculating crack tip stress intensity factor (SIF). The results show that when the branch angleθincreases, the SIFKIof the branch crack increases from negative to positive and the absolute valueKIIof the branch crack first increases, and asθis 50°, it is the maximum, and then it decreases. Finally, in order to further investigate the stress distribution around Y-shaped cracks, photoelastic tests were conducted, and the test results generally agree with the compressive test results.

Fracture Splitting Mechanism of Jointed Rock Mass under Compressive-Shear Stress and Grouted Pressure

The cracking and propagation regulations of compressive-shear rock cracks and the evolution regulations of the stress intensity factor at the branch crack tip under the grouting pressure were discussed in this article. The fracture failure mechanism of rock bridge in the compressive-shear stress state under the effect of grouting pressure was also studied and a fracture failure criterion was put forward. The fracture failure criterion was applied to a example and some useful conclusions were drawn. The grouted pressure and the lateral stress are the key factors which remarkably effect the stress intensity factor KI at the branch crack tip when the certain axial stress and friction coefficient is known. The larger grouting pressure and the smaller value of the lateral stress result in the larger the initial stress intensity factor KI at the branch crack cusp.Under the certain stress and friction coefficient, the higher grouted pressure and lateral tension stress together is easier to lead to crack transfixation. A new way was offered which studied the fracture transfixation of fractured rock mass in the interaction of grouting pressure field and stress field.

Study on the Difference of the Propagation Characteristics between Open Flaws and close Flaws in Brittle Materials under Compressive Loading

The second development of ABAQUS is implemented to simulate the initiation, propagation processes of flaws in brittle materials under compressive loading (in the paper ‘flaw’ means ‘the initial crack’, and ‘crack’ means ‘the branch crack’), by which the propagation paths and the corresponding stress intensity factors of the branch crack can be calculated. Further more the experiment is carried out to verify the validity of the above numerical method. By the numerical method, the propagation processes of open flaws and close flaws are simulated, and the comparative analysis of propagation characteristics between the open flaw and the close flaw is carried out. The results show the obvious difference in the propagation characteristics between open flaws and close flaws with the same initial flaw length and angle. Firstly, compared with the close flaw, the branch crack of the open flaw grows along a more obviously curvilinear path, and the propagation path gradually approaches to a line, which passes through the middle point of the open flaw and parallel to the maximum principal stress. Secondly in the early stage of the crack propagation, the stress intensity factors of the branch crack of the open flaw are greater than of the close flaw, but with the further propagation of the branch cracks, the stress intensity factors of the branch crack of the open flaw will be less than of the close flaw. Additionally, according to the close flaw, with the decrease of the friction coefficients, the curve characteristics of the crack propagation paths become more obvious. Therefore, it is noteworthy that the wing crack of the close flaw can be regard as the straight line if the friction coefficient of the flaw surface is very small. The above differences of the propagation characteristics between the open flaws and the close flaw show that the two flaws should be distinguished strictly in the fracture analysis.

Fracture Analysis on the Interface Crack of Concrete Gravity Dam

Some numerical parameters-sensitivity analysis has been conducted to evaluate the stability and propagation of the interface cracks at heel of concrete gravity dam. In this paper, utilizing the software ANSYS to simulate the stress and displacement fields of the tip of the interface cracks between concrete gravity dam and foundation, the stress intensity factor (SIF) of the interface crack is analyzed using facture mechanics. Three impacting factors have been discussed, such as the crack length, the angle of crack, and the water height. Critical length and loads of interface crack propagation are obtained using composite fracture criteria. The results indicate that the coarse interface retards the propagation of interface crack, and redounds to stability of gravity dam. It is found that the interface crack often propagates alone the interface between dam and foundation, simultaneously the branch crack kinks to foundation at the specific condition.