Huilan Ren

Compressive Behaviors of Alumina: Experiment and Modeling

Mechanical behaviors and failure characteristics of alumina are investigated by plate impact experiments under shock waves. Stress histories are measured by using in-material manganin gauges. A velocity interferometer system for reflectors (VISAR) is used to obtain velocity profile of the particle free surface, which consists of an elastic wave followed immediately by a dispersive inelastic wave. The nonlinear dynamic responses are analyzed. A constitutive model for the alumina subjected to biaxial compressive loading is developed. The model takes non-interacting sliding micro-cracks into account. The stress-strain curves predicted by the model are in good agreement with experimental results. Dynamic fracture behaviors of alumina target under impact loading are investigated numerically. Analyzing the fracture mechanism and damage process of the alumina target shows that the nucleation and growth of various cracks play a dominant role in the fracture behavior of the alumina target.

A quantitative acoustic emission study on fracture processes in ceramics based on wavelet packet decomposition

We base a quantitative acoustic emission (AE) study on fracture processes in alumina ceramics on wavelet packet decomposition and AE source location. According to the frequency characteristics, as well as energy and ringdown counts of AE, the fracture process is divided into four stages: crack closure, nucleation, development, and critical failure. Each of the AE signals is decomposed by a 2-level wavelet package decomposition into four different (from-low-to-high) frequency bands (AA2, AD2, DA2, and DD2). The energy eigenvalues P0, P1, P2, and P3 corresponding to these four frequency bands are calculated. By analyzing changes in P0 and P3 in the four stages, we determine the inverse relationship between AE frequency and the crack source size during ceramic fracture. AE signals with regard to crack nucleation can be expressed when P0 is less than 5 and P3 more than 60; whereas AE signals with regard to dangerous crack propagation can be expressed when more than 92% of P0 is greater than 4, and more than 95% of P3 is less than 45. Geiger location algorithm is used to locate AE sources and cracks in the sample. The results of this location algorithm are consistent with the positions of fractures in the sample when observed under a scanning electronic microscope; thus the locations of fractures located with Geigers method can reflect the fracture process. The stage division by location results is in a good agreement with the division based on AE frequency characteristics. We find that both wavelet package decomposition and Geigers AE source locations are suitable for the identification of the evolutionary process of cracks in alumina ceramics.

Impact-initiated behavior and reaction mechanism of W/Zr composites with SHPB setup

The dynamic compressive behavior of a hot pressed tungsten/zirconium (W/Zr) composite with a mass proportion of 34:64 (W:Zr) was experimentally investigated using a split Hopkinson pressure bar and a high-speed camera. The W/Zr composite has high strength but some brittle characteristics; when subjected to a strong enough impact loading, the sample is crushed, rapidly releasing high amounts of energy as a result. This impact-initiated reaction depends on the loading conditions, where a higher loading strain rate resulting a smaller fragment size. The Zr phase is involved in the reaction as the active component of the composite, and these fragments can be divided into small, medium, and large fragments with their reactions labeled as “fire ball,” “spark,” and “no react” respectively. A simple model is constructed to analyze the heat generated during plastic deformation based on yield stress, crack speed and the thermal properties of the brittle material. Our proposed model’s prediction of temperature increase at initiation may reach several hundred degrees Celsius.