An improved test fixture for biaxial-tension strength testing of ceramics featuring uniform pressure loading of disks was developed and qualified. Biaxial data were obtained for an alumina ceramic, along with comparable uniaxial data from three-and four-point flexure tests. Weibull statistics provided a good description of the size effect on data from the two uniaxial tests, but underestimated the effect of stress biaxiality. The biaxial strengthening observed in the alumina ceramic is consistent with that observed previously in a glass-ceramic.

The direct-current electric potential method is receiving increasing attention for monitoring crack extension in J-resistance curve testing. Among its advantages over the unloading-compliance method are: (1) no time-consuming unloadings are required, (2) a continuous record of crack extension versus displacement can be obtained, and (3) the method can be used at higher strain rates where unloading compliance cannot be used. Despite the advantages of the direct-current electric potential method, questions persist regarding its ability to monitor large amounts of crack growth in highly ductile materials where large displacements and large amounts of plastic strain occur. This paper presents details of an experiment conducted on a 3T planform-size compact specimen of 25.4 mm thickness to assess the ability of the direct-current electric potential method to accurately measure crack extension in a highly ductile material. The material selected was Type 304 austenitic stainless steel. It was found that the Johnson expression, often used to calculate crack extension from direct-current electric potential data, significantly underestimated the actual amount of crack extension. However, a simple modification of the Johnson expression resulted in excellent agreement between calculated and measured crack extensions.

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Fracture toughness tests were conducted at room temperature and at – 196°C on a martensitic stainless steel that had been quenched and tempered to a hardness of Rockwell C 61-63. The fracture toughness specimens were chevron-notch, short-bar specimens for which B was 12.7 mm and W was 25.4 mm. Tests were conducted in accordance with ASTM Test Method for Plane-Strain (Chevron Notch) Fracture Toughness of Metallic Materials (E 1304-89). Five tests were conducted at each temperature, and each test produced a valid KIv result. Test results were unusually reproducible for plane-strain fracture toughness tests; the standard deviation for room temperature tests was approximately ±5% and for -196°C tests was approximately ±2% of the mean value of KIv. For comparison with the chevron-notch test results, three standard KIc tests were conducted on fatigue-precracked compact (tension) specimens at room temperature. The tests were conducted in accordance with ASTM Test Method for Plane-Strain Fracture Toughness of Metallic Materials (E 399). As was the case for the KIv tests, the KIc tests produced very reproducible results: the standard deviation for three tests was approximately ±2% of the mean value. However, it was found that the mean KIv values for this high-hardness steel were approximately 18% greater than the mean KIc values. That result confirms the warning included in ASTM E 1304 that KIc values may be larger than KIc values in some materials.