The Dependence of Strength-Controlling Fracture Energy on the Flaw-Size to Grain-Size Ratio

The transition from single‐crystal to polycrystalline fracture energies was studied as a function of the flaw‐size to grain‐size ratio by two methods. The primary method was calculating fracture energies from observed flaw sizes found at fracture origins in strength‐test specimens. Some measurements were also made by varying the number of grains across the web in the applied‐moment DCB test. Both methods agreed and generally snowed the transition to polycrystalline fracture energies being completed at flaw‐size to grain‐size ratios of ∼1 to ∼ 6 for the cubic materials studied. It is estimated that cracks less than ∼½ to ¼ of the grain size cannot be arrested at grain boundaries and that single‐crystal fracture energies can be applied below this limit. The grain‐size range over which this fracture‐energy transition occurs was shown to be a function of extrinsic factors, such as texture, as well as intrinsic factors, such as the number and multiplicity of low‐energy single‐crystal fracture planes.