Effect of Temperature and Strain Rate on the Brittleness of China Sandstone

To quantitatively study the influence of temperature and strain rate on the brittleness of sandstone, the mechanical parameters of sandstone under different temperatures and strain rates are collected from the previous literature, and two empirical equations for calculating rock brittleness are used to quantitatively calculate and evaluate the brittleness of sandstone. The results show that both BI1 and BI2 can characterize the brittleness of sandstone, but the applicable conditions are different. The BI1 method is more accurate in calculating the variation in the sandstone brittleness with a strain rate, while the BI2 method is more accurate in calculating its variation with temperature. The brittleness of sandstone increases with the increase in the strain rate, especially when the strain rate exceeds 100 s-1. Under low-temperature conditions, the strength and brittleness of rocks increase due to the strengthening of ice. Under the condition of high temperature, the thermal damage to sandstone is intensified after 400°C, and the quartz phase changes after 600°C, which leads to the increase in microcrack density and the decrease in brittleness of sandstone. The conditions of low temperature and high strain rate are beneficial to the enhancement of sandstone brittleness.

Interaction of Microcracks and Tissue Compositional Heterogeneity in Determining Fracture Resistance of Human Cortical Bone

Recent studies demonstrated an association between atypical femoral fracture (AFF) and long-term bisphosphonate (BP) use for osteoporosis treatment. Due to BP treatment, bone undergoes alterations including increased microcrack density and reduced tissue compositional heterogeneity. However, the effect of these changes on the fracture response of bone is not well understood. As a result, the goal of the current study is to evaluate the individual and combined effects of microcracks and tissue compositional heterogeneity on fracture resistance of cortical bone using finite element modeling (FEM) of compact tension (CT) specimen tests with varying microcrack density, location, and clustering, and material heterogeneity in three different bone samples. The simulation results showed that an increase in microcrack density improved the fracture resistance irrespective of the local material property heterogeneity and microcrack distribution. A reduction in material property heterogeneity adversely affected the fracture resistance in models both with and without microcracks. When the combined changes in microcrack density and tissue material property heterogeneity representing BP treatment were evaluated, the models corresponding to BP-treated bone demonstrated reduced fracture resistance. The simulation results also showed that although microcrack location and clustering, and microstructure significantly influenced fracture resistance, the trends observed on the effect of microcrack density and tissue material property heterogeneity did not change. In summary, these results provide new information on the interaction of microcracks, tissue material property heterogeneity, and fracture resistance and may improve the understanding of the influence of mechanical changes due to prolonged BP use on the fracture behavior of cortical bone.

Determining Cyclic Durability of Piezoceramic Structures Using Probabilistic-Structural Approach

The present paper addresses the new probabilistic-structural approach to determining the service life of piezoceramic structural members under multiple static or cyclic loading. The cyclical service life of structures is related to the intensity with which a microcrack density increases with the cycles of loading. To illustrate the approach the problem of durability of piezoelectric transformer plate for idle mode is considered. Two estimation methods of the durability of this type transformer are proposed.

Understanding Effect of Grain Boundaries in the Fracture Behavior of Polycrystalline Tungsten under Mode-I Loading

Polycrystalline tungsten is considered as an important material in aerospace, automobile, and energy industries due to its excellent thermal and mechanical properties. While grain boundaries (GBs) are perceived to play a major role in polycrystalline tungsten failure resistance, experimental data are scarce on explicit contribution of GBs to tungsten failure resistance. The present work focuses on understanding the effect of GB property variation on fracture resistance of polycrystalline tungsten. The cohesive finite element method is used for the simulation of crack propagation in polycrystalline tungsten microstructures. The results show a significant effect of GB property variation on change of crack propagation patterns during tungsten fracture. A variation of 10% in GB fracture energy resulted in distinctly different crack patterns with different primary crack propagation direction and the microcrack density. Based on the observed microstructural fracture attributes, a relation between cohesive energy dissipation and microcrack density in polycrystalline tungsten microstructures is proposed.