Abstract
In fracture toughness testing on beryllium and aluminum-beryllium materials, the load-displacement response exhibits a significant amount of nonlinearity that requires special, nontraditional methods for analysis. This work is directed toward determining what causes this nonlinearity, focusing on plasticity and subcritical crack extension. Plasticity was indirectly assessed by varying the size of the fracture toughness specimen, whereas crack advance was monitored with unloading compliance during the fracture toughness tests. Neither of these influences appears to be the primary contributor to the nonlinearity. Rather, the range of loading employed during precracking influences the load-displacement response of the material. The central, more linear portion of the load-displacement response appears bounded at the low end by the crack opening load and at the high end by the maximum load applied during the terminal stage of precracking. This region of the load-displacement curve represents an alternative to the standard 20 % to 80 % maximum load analysis region. Although using this region for analysis offers the advantage of representing the portion of the curve where the crack is fully open and behaving in an elastic manner, it will likely increase the number of invalid tests as a result of violation of the Pmax/PQ < 1.1 criterion.
