Methodical approach to the experimental acoustic-emission evaluation of the crack resistance of structural materials

We develop a general approach to the determination of the characteristics of crack resistance of structural materials by the method of acoustic emission. This approach includes the procedure of selection of acoustic signals from cracks and the proper choice of the most efficient parameters of acoustic signals, the operating frequency of the receiving circuit of an acoustic-emission device, and the location of an acoustic-emission transducer. It is shown that, in the process of propagation of a crack, acoustic radiation is directed and the frequency band of emitted acoustic signals becomes narrower in the stage of subcritical crack growth. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 5, pp. 17–30, September–October, 1997.     

Evaluation of statistical fracture toughness parameters on the basis of crack arrest experiment

The crack diffusion theory addresses the origins of the observed scatter, in brittle fracture, of macroscopic fracture parameters under seemingly identical test conditions. An essential question of experimental evaluation of the nonconventional fracture parameters introduced there appears to be buried in the formalism. In this paper we report a pilot crack arrest experiment, in which a methodology of evaluating some of the new fracture parameters is demonstrated. Experimental observations are found to not disprove the theory.     

Determination of the crack resistance of materials under impact loading

Differential equations in terms of the bending moment in a section of a prismatic specimen weakened by a crack during testing in accordance with a one- and three-point bending scheme are derived on the basis of analysis of the specimen's motion during deformation. The causes of the shifting of signals from sensors recording the force on the striker and supports are discussed. Solution of the equation yields values of the bending moment (stress intensity factor) over the specimen's entire loading interval. The results obtained agree well with experimental data. Translated from Problemy Prochnosti, No. 9, pp. 15–23, September, 1993.     

Simple method to measure the crack resistance of ceramic materials

A new and simple method to study the change in crack resistance during the process of crack growth in ceramic materials has been developed. The method is based on using the chevron-notched short-bar test which is generally accepted as a convenient method for measuring fracture toughness. The simple modification described here allows one to measure fracture toughness using the assumption that fracture toughness changes in the process of crack growth (presence of crack resistance curve, or R-curve). This method presents many advantages, especially the long stable crack growth under mode I fracture specimen and plane strain, small specimen size and no need of pre-cracking for measuring R-curve behaviour in ceramic materials.     

Critical stress diagrams for brittle materials with defects of the cusped void/crack type

An analysis of the strength of a brittle solid with defects of the hypocycloid void/crack type in plane stress state is presented. Based on the tenets of the theory of equilibrium cracks, critical stress diagrams for a brittle material with defects of this type are constructed and compared with published experimental data. It is shown that, using a model of a brittle material with defects of the cusped void/crack type, it is possible to describe laws governing the fracture of certain materials.     

Effect of material parameters on the erosion resistance of brittle materials

Erosion data are compared with two theories that have been suggested to explain the erosive behaviour of solids. A dimensional analysis is applied to the variables that are important to erosion, and a multivariate, linear regression analysis is used to fit the data to the dimensional analysis. The results of the linear regression analyses are compared with the two theories in order to evaluate the applicability of these theories to erosion. Although semi-quantitative agreement of the data with the theories is obtained, some discrepancies are apparent. In particular, the dependence of erosion rate on hardness and critical stress intensity factor is greater than predicted by either of the two theories. These discrepancies are attributed primarily to microstructural aspects of erosion that are not modelled by either of the theories.