Experimental Study on Dynamic Tensile Failure of Sandstone Specimens with Different Water Contents

Understanding the effect of water saturation on dynamic failure of rocks is of great importance to tunnel excavation at water-rich coal mines and prevention of rock bursts by water injection. Dynamic Brazilian disc tests are performed to study mechanical behaviour of sandstones in this paper. The results indicate that water saturation significantly weakens the dynamic tensile strength of sandstones and increases the specimen strain at which the specimen fails. The damage degree of sandstones reduces gradually with increasing water contents. Failure of the sandstone specimen includes the crack initiation at the center of the specimen, macroscopic crack propagation, and stretch of the macroscopic crack through the specimen. In addition, parallel macroscopic crack propagation is found in the specimen with a low water content. From the observation of fracture sections, microstructures are compact in the specimen with high water contents. This is due to the swell of the kaolinite in the specimen after water saturation. The failure mechanism of microstructures is typical brittle failure in the specimen with a high water content, whereas ductile fracture is found in the specimen with a low water content. Different failure processes of microstructures lead to the differences between mechanical properties and macroscopic failure characteristics of the specimens with various water contents.

MODELS OF PHYSICALLY SHORT AND MACROSCOPIC CRACK DEVELOPMENT AND THEIR APPLICATION FOR TIN BASED BABBIT

The stages of fatigue crack development are analyzed. The well-known models of physically short cracks and generalized models describing some stages are shown. A perspectivity of a statistical generalized model offered is shown. The parameters obtained of a cyclic crack resistance of tin-based babbit are presented.

Study on Failure Process and Permeation Evolution of Single-Cracked Rock

To evaluate the mechanical properties and permeation evolution of cracked rock mass, failure evolution tests were designed by RFPA software for single-cracked rock mass with (i) different inclination angles under uniaxial compression and (ii) different confining pressures and pore pressures under triaxial compression. The results show the following: (1) Angle of the crack significantly affects the crack propagation mode and slightly affects the bearing capacity of rock. During the crack propagation, the peak of permeation is delayed at the peak of stress. The stress-strain curve shows a different behavior in the postcritical part of the curve, especially in the case of 45°, where a smooth postcritical curve was clearly observed instead of an abrupt decrease in the stress of other two cases. (2) When the confining pressure is constant, the trend is almost the same when varying pore pressures, and with the increase in pore pressure, crack propagation is accelerated. At a low confining pressure, the crack is extended vertically to the upper and lower ends of the specimen, forming a longitudinal macroscopic crack. At a high confining pressure, the crack gradually extends to the left and right boundaries of the specimen, forming a transverse macroscopic crack. (3) The rate of crack initiation and destruction first decreases and then increases with the increase in confining pressure when pore pressure is constant.

ADAPTIVE QUASICONTINUUM SIMULATION OF ELASTIC-BRITTLE DISORDERED LATTICES

The quasicontinuum (QC) method is a computational technique that can efficiently handle atomistic lattices by combining continuum and atomistic approaches. In this work, the QC method is combined with an adaptive algorithm, to obtain correct predictions of crack trajectories in failure simulations. Numerical simulations of crack propagation in elastic-brittle disordered lattices are performed for a two-dimensional example. The obtained results are compared with the fully resolved particle model. It is shown that the adaptive QC simulation provides a significant reduction of the computational demand. At the same time, the macroscopic crack trajectories and the shape of the force-displacement diagram are very well captured.

Fractographic analysis of fatigue crack growth in lightweight integral stiffened panels

PurposeThe purpose of this paper is to complement available macroscopic fatigue crack growth measurements in flat stiffened panels with scanning electron microscopy (SEM) measurements of striation spacing.Design/methodology/approachThe paper's approach is fatigue testing of two‐stiffener flat panels manufactured using three different processes, with a central initial crack perpendicular to the stiffeners and load, in order to identify striation spacing during crack growth up to final fracture, using SEM.FindingsAn increase of striation spacing as cracks grow was quantified. Although when cracks approach the stiffeners the stress intensity factor decreases, there is no clear decrease of striation spacing in that region. Striation spacing is roughly similar to macroscopic crack‐propagation rate da/dN measured in the panels testing. This observation is no longer valid once the stiffeners are reached; this stage is characterized by fast acceleration of the cracking process until final complete rupture is verified, and macroscopic crack growth measurements are made difficult because of the “T” geometry in that region.Originality/valueA complete picture of the striation spacing during the fatigue crack growth up to final fracture of a two‐stiffener flat panel is provided for three different manufacturing processes: high‐speed machining, laser beam welding and friction stir welding.

Analysis of macroscopic crack branching patterns in chemically strengthened glass

Residual stress profiles were introduced in sodium aluminosilicate glass disks using an ion-exchange process. They were fractured in two loading conditions: indentation and biaxial flexure. The fractal dimension of the macroscopic crack branching pattern called the crack branching coefficient (CBC), as well as the number of fragments (NOF) were used to quantify the crack patterns. The fracture surfaces were analyzed to determine the stresses responsible for the crack branching patterns. The total strain energy in the body was calculated. The CBC was a good measure of the NOF. They are directly related to the tensile strain energy due to the residual stress profile for fractures due to indentation loading. However, in general for materials with residual stresses, CBC (or NOF) is not related to the strength or the stress at fracture, or even to the total stored tensile strain energy. Instead, the CBC appears to be related, in a complex manner, to the distribution of stresses in the body. Therefore, in general, the characterization of the CBC of fractured materials cannot be used to ascertain the prior stress distribution.

A Theoretically-Based Statistical Model of Transition Toughness

A program was undertaken to develop a fully predictive model of the scatter in toughness across a wide range of transition temperatures based on a physical understanding of deformation and fracture behavior. The temperature dependence of the proposed model is taken from previous work in which the local mechanisms of cleavage fracture were used to define the plastic work to fracture. The local to global stress transference is achieved by a dislocation-mechanics based examination of the interaction between the globally applied stresses, a macroscopic crack and a nearby accumulation of dislocations blocked by a second phase particle, i.e. slip band, whose position relative to the macroscopic crack tip is variable. The scatter of toughness values at each temperature is captured through variation of this macro-crack / micro-crack geometry, and of the particle size. Once the local stress field is determined using the dislocation-based transference equations, an energy balance criterion for fracture is applied that incorporates the temperature-dependent fracture work term and the local stresses determined from the transference equations. This paper summarizes this multiscale fracture model, which serves as a foundation for more detailed descriptions of the mathematics of the quantitative model, its temperature dependence and scatter characteristics and coding efforts. These latter topics will be addressed in greater detail in subsequent papers.