The increased demand for improved high-temperature materials in structural applications has driven researchers to examine a variety of light weight, high melting point alloys such as NiAl. This intermetallic compound has the desirable properties of high specific strength and oxidation resistance, although the strain to failure and toughness at room temperature are prohibitively low. Currently a combined experimental and numerical investigation is being conducted in order to characterize and clarify the failure behavior of NiAl. Experiments have revealed that NiAl specimens loaded in tension or 3-pt bending display little or no plastic deformation prior to failure. However, when a purely elastic analysis is applied to a notched 3-pt bending specimen, the calculated σ max at the instance of catastrophic failure can be as much as an order of magnitude higher than that observed in tensile or pure bending failure. Experimental observations suggest that this great discrepancy is a result of plastic, or pseudo-plastic deformation at the notch tip prior to failure. Utilizing experimental yielding and hardening data, FEM calculations were performed in order to quantify the stress and strain distributions around the notch tip. Results show that plastic deformation reduces the maximum stress at the notch tip at failure from the value calculated elastically. With the stress and strain distributions known, a general fracture criterion is discussed. This criterion is applied in the analysis in order to compare predicted values to experimental data for other specimen geometries.
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