Fracture mechanics of brittle ceramics - 30 years of progress

The application of fracture mechanics to advanced technical ceramics was initiated nearly 30 years ago. It was heralded by an international conference at the Pennsylvania State University in 1973. Unbridled optimism prevailed as many believed that the coupling of fracture mechanics with ceramics would enable high temperature advanced ceramics to find extensive structural applications. Unfortunately, there were a number of barriers to the use of ceramics as structural materials, even with the application of fracture mechanics. The use of brittle ceramics in structural applications faced major challenges. These included: (1) the absence of a reliable data base of materials properties, (2) the lack of appropriate standards for the determination of the properties of these materials, (3) a lack of any rational, fracture mechanics based design methodology for brittle ceramics, and (4) the high costs of producing and utilizing structural parts from advanced ceramics. These were serious deficiencies that inhibited the achievement of successful applications of ceramics. Fracture mechanics tests provided a basis for the design methodology and also provided understanding of the basics of the fracture process in brittle ceramics. It informed the engineering community that ceramic fracture toughnesses were low, < 10 MPa root m and that there were serious problems with slow crack growth and fatigue. The presence of rising R-curve behavior was observed and related to the crack tip's following wake region. Cyclic fatigue was confirmed to exist and related to the presence of a rising R-curve. Today, the promise of structural ceramics remains mostly unfulfilled, except for a few niche applications that are nonetheless encouraging. In addition, there have been significant advances in the proper-ties and integrity of structural ceramics over the past three decades. Advanced structural ceramics are better than ever. This can be attributed to the use of fracture mechanics concepts to identify the microstructural processing defects and then improve the processing methodology to eliminate those defects. There have also been major advances in the development of standards applied to ceramics in which ASTM has played a major role. Unfortunately, we still have not learned to design with brittle ceramics and the cost of their utilization in structural applications remains prohibitively high.