Generalized fracture strength as a function of stress rate for ceramics and glasses exhibiting environmentally induced slow crack growth. Crack growth theory indicates that the slope of the curve changes over the life of the component.
Long description of figure 1.
Fracture stress as a function of stress rate for alumina used in a microwave window. The data were generated in accordance with the American Society for Testing and Materials standard test method C1368-97, and at slower rates than required. The ultra-slow-rate data agree with the theory that the slope should change at short failure times. Measurements made with the fast rates recommended in the standard result in a factor of 3 error in the estimated life-prediction parameter n.
Long description of figure 2.
One of the factors limiting structural ceramics in long-term applications is stress corrosion, or slow crack growth. For example, airborne water can cause degradation and failure of ceramic components subjected to static loads. Thus, the design of structural components from advanced ceramics requires the measurement of the life-prediction parameters for the particular ceramic material and environment of interest. Recently, the American Society for Testing and Materials standard C1368-97 (ref. 1) allowed life-prediction parameters to be measured with such fast stress rates and correspondingly short experimental failure times that significant errors in the life-prediction parameters occurred (ref. 2). As a result, components could not be accurately designed for a long time-to-failure.
One application where a long time-to-failure is expected is microwave windows that are used in the production of nuclear materials (ref. 3). Recent ultraslow and standard-rate testing of a window material for Los Alamos National Laboratory demonstrated the existence of multiple regions of the slow crack growth curve, in agreement with the theory shown in the top figure. The tests indicated that the short failure times allowed by the standard resulted in a factor of 3 error in the life parameter n, as shown in the bottom figure. In addition, we determined that by increasing the concentration of the corrosive media, the diffusion-related phenomenon that made the parameters inaccurate could be avoided, thereby allowing faster, time-saving rates to be employed in some situations.
The results have been used to revise the American Society for Testing and Materials standard C1368-01 (ref. 4) and thereby provide engineers with better data for designing industrial components with long failure times.
Glenn contact: Dr. Jonathan Salem, 216-433-3313, Jonathan.A.Salem@nasa.gov
Author: Dr. Jonathan A. Salem
Headquarters program office: OAT
Programs/Projects: HOTPC, Space Act
Special recognition: American Society for Testing and Materials Award of Merit
Last updated: June 25, 2003
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