Synergism of Toughening Mechanisms in Whisker-Reinforced Ceramic-Matrix Composites

The improved fracture resistance of whisker-reinforced ceramic-matrix composites involves more than one energy-absorbing mechanism. The possible mechanisms are reviewed and a micromechanical model evaluating the relative contributions to the overall toughness is presented. The mechanisms involve microcracking, load transfer, bridging, and crack deflection. The synergism of these mechanisms is examined using an energy release rate balance equation. The basic assumption of the proposed model is that the load transfer between the matrix and the whiskers is due to Coulomb friction. The model has been applied to an Al₂O₃/SiC whisker composite and shows reasonable agreement with reported experimental results. The role of the thermal residual stresses is also examined in light of the frictional load transfer assumption.     

Elastic wave emission from damage processes

The theory of elastic wave emission (i.e., acoustic emission; AE) from damage processes such as slip and microcracking is discussed. Analogous developments in the literature on earth-quake seismology and dynamic dislocation theory are noted and utilized. A general representation of the displacement field of an AE event is given in terms of the double-couple response to a distribution of moment density tensor in the source region. Results are specialized to a point source model and to a general far-field analysis of outgoing elastic waves, and conditions for validity of such representations and their low-frequency specializations are noted. Emitted wave fields are compared for tensile opening and slip events, and procedures which might enable the approximate determination of the size or area increase of tensile microcracks are discussed.     

混凝土损伤断裂过程研究

本文运用损伤力学理论，从微裂缝损伤演化发展的角度研究凝混凝土裂缝的形成与扩展机理，提出相应的理论判据，并用算例和模型试验数据作了验证。     

Failure Strains in Micromachined Piezoelectric Membranes

Abstract:  The strain at failure was investigated for micromachined lead zirconate titinate (PZT) piezoelectric membranes. The effect of microcracking in the PZT films was overshadowed by the support layers present in the devices. Bare Si membranes achieved the highest strain at failure, 0.49% with the pressure applied to the back side of the membranes compared to Si/SiO₂ and composite PZT membrane structures. Variation in the volume of PZT that was strained and the side length of the composite membranes did not decrease the strain at failure. In addition, the materials utilised in the composite structure for the bottom electrode material and PZT chemistry were investigated and the strain at failure levels was effectively the same for each material system. Composite structures with significantly different levels of effective residual stress failed at applied strains of up to 0.2%. This paper details each of the tests that were performed and the results obtained.     

Stress-Biased Anisotropic Microcracking in Zirconia Polycrystals

Anisotropic elastic response of microcracked bodies has been studied using transformation-toughened Mg-PSZ and Ce-TZP as model materials. These zirconias have been previously deformed in triaxial compression to effect various extents of phase transformation. Microcracks were found to align with the compression axis and their normalized density increases linearly with the extent of transformation plasticity. The measured elastic constants are anisotropic and well described by an elasticity theory. An additional anomalous elastic anisotropy was also found in Ce-TZP due to a transformation texture of both the remaining tetragonal phase and the newly formed monoclinic phase. The terminal crack density ω= Na ³, where N is the number of cracks per unit volume and a is the crack radius, reaches 0.05 in Mg-PSZ and 0.1 in Ce-TZP at the completion of transformation plasticity.     

Residual Stresses in a Wo-Phase Microcracking Ceramic

Residual stresses in a SiC-TiB₂ particulate composite before and after stressing have been measured using X-ray diffraction. Tensile residual stresses in the TiB₂ drop by 60% after bending stresses of 250 MPa are applied. Likewise, the compressive residual stresses in the Sic phase decrease accordingly. Such behavior is consistent with stress-induced microcracking.     

Thermoplastic Sizing of Carbon Fibres in High Temperature Polyimide Composites

Thermoplastic sizing of various carbon fibres has been evaluated as a means of controlling the microcracking resistance, transflexural strength (TFS), and thermo-oxidative stability (TOS) of PMR-15 matrix composites. Four different fibre/thermoplastic size combinations were selected for this study, based on their higher TOS than appropriate controls. Data are presented for the transverse microcrack (TVM) density/inch, and the subsequent delamination of [0,90]_4s laminates induced by thermal cycling (-196 to 350°C for 20 cycles). Results for composite TFS, conditioned TFS (after heating in air at 350°C), and TOS (350°C and 316°C) are also reported. One fibre/thermoplastic size combination gave both good microcracking resistance, and significantly improved TOS, in composite when compared with current commercially-available material.     

Atomic Force and Ultrasonic Force Microscopic Investigation of Laser-Treated Ceramic Head Sliders

The surface of laser-treated ceramic hard disk drive head sliders has been imaged with the atomic force microscope (AFM) and ultrasonic force microscope (UFM). The surface topography image from the AFM is compared with the elasticity image generated by the UFM on the same region. Images of the surface structure changes along with microcracking in the laser-treated regions are presented. The possible reasons for the development of microcracking and the enhanced contrast that the UFM provides of the microcracks and the material microstructure changes in the laser-treated region are discussed.     

Thermoplastic Sizing of Carbon Fibres in High Temperature Polyimide Composites

Thermoplastic sizing of various carbon fibres has been evaluated as a means of controlling the microcracking resistance, transflexural strength (TFS), and thermo-oxidative stability (TOS) of PMR-15 matrix composites. Four different fibre/thermoplastic size combinations were selected for this study, based on their higher TOS than appropriate controls. Data are presented for the transverse microcrack (TVM) density/inch, and the subsequent delamination of [0,90]_4s laminates induced by thermal cycling (-196 to 350°C for 20 cycles). Results for composite TFS, conditioned TFS (after heating in air at 350°C), and TOS (350°C and 316°C) are also reported. One fibre/thermoplastic size combination gave both good microcracking resistance, and significantly improved TOS, in composite when compared with current commercially-available material.