On the influence of specimen size and geometry on the fracture toughness of tungsten heavy metal alloys

We previously showed that commercial tungsten (W)-based heavy metal alloy (WHA) composites containing 90 to 97 wt% W, reinforced with 3 to 10 wt% of a NiFe ductile phase (DP), have room temperature (RT) maximum load elastic-plastic fracture toughness (KJm) values from 69 to 110 MPa√m. This is far higher than for monolithic RT W toughness (≈ 8 ± 4 MPa√m). In all cases, fracture took place by stable crack growth. However, these results were based on very small (16/3.3/1.65 mm) pre-cracked bend bar tests, which naturally raises the question of size effects on the measured KJm. While they are best thought of as being composites, here we experimentally assess size and geometry effects on the KJm of WHA, in the broad context of validity criteria that have been developed for both cleavage and ductile tearing, R-curve fracture in steels. Specifically, we examine the relation between KJm and a dimensionless elastic-plastic zone small scale yielding deformation limit criterion, M ≥ σobo/Jm. Here σo is the flow stress, Jm is the measured J at maximum load, and bo is the initial unbroken ligament dimension. Tests on specimens from 3x to 8x larger than the original small (1x) bend bars show only weak or no size effect up to 95W with KJm ≈ 82 ± 5 MPa√m. However, 3x 97W alloy fractured elastically at KIm ≈ 38 ± 4 MPa√m. The corresponding average for the 1x specimens was KJm 100 ± 15 MPa√m for the 90–95W and 69 ± 12 MPa√m for the 97W alloys. The approximately constant KJm for 3–8x specimen sizes suggests that the M required for constraint-related size independence is ≈ 200. However, DP fraction also plays a role, and there are additional effects of compact tension versus bend bar specimen geometries.