Proof testing of ceramic materials--an analytical basis for failure prediction

International Journal of Fracture, Vol. 10, No. 3, September 1974 Noordhoff International Publishing-Leyden Printed in The Netherlands 379 Proof testing of ceramic materials--an analytical basis for failure prediction A. G. EVANS and S. M. WIEDERHORN Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234, U.S.A. (Received March 23, 1973; in revised form October 18, 1973) ABSTRACT An analysis is presented which permits the accurate prediction of component lifetimes after proof testing. The analysis applies to crack propagation controlled fracture but can be used as a conservative prediction when crack initiation is predominant. The analytical predictions are confirmed in a series of time-to-failure measurements. 1. Introduction In many ceramic systems of structural importance [1-4], slow crack growth precedes fast fracture and this leads to a time dependence of strength. The successful structural exploitation of these materials requires, therefore, a detailed understanding of the time dependent behavior so that accurate failure predictions can be made. The accuracy of failure prediction is very substantially enhanced by incorporating a component proof test prior to service. It is generally considered, therefore, that effective proof testing is an essential prerequisite for the successful structural application of ceramic materials. The primary objective of this paper is to present an analysis based on fundamental principles which enables proof test conditions to be accurately selected, thereby ensuring the "in-service" component lifetimes demanded by a particular application. The proof test analysis considers a rapid proof test (which does not lead to any significant slow crack growth in the unbroken components) and then a more practically realistic "slow" proof test (which may permit slow growth, and hence, lead to strength degradation in the unbroken components). The predictions of the analysis are then verified in a series of critical experiments. Finally, some general considerations of time dependent fracture in brittle materials are developed, which enable techniques for the rapid evaluation of the important crack propagation parameters to be established. 2. Fundamental aspects of time dependent failure Fracture involves two independent series processes, flaw initiation and flaw propagation. One of these processes is usually predominant, although it is important to recognize that sometimes both processes contribute in a significant way to failure. In most ceramic materials of structural importance, there are preexisting sharp cracks [5] so that flaw propagation is usually the predominant failure process. A crack propagation analysis will thus predict the time to failure for most ceramic materials (this will be a conservative underestimate when flaw initiation is also necessary). 2.1. Crack growth characteristics in ceramics It has been established by a number of investigators [1, 2, 6, 7] that for a given system (environment, temperature, material, etc.) there is a unique relation between the crack velocity v and the crack tip stress intensity factor, K r For example, in many ceramics the amount of water in the environment has a strong influence on crack propagation [1, 2]. Trimodal KI-v Int. Journ. of Fracture, 10 (1974) 379-392

Proof testing of ceramic materials--an analytical basis for failure prediction

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