The Ductile-Brittle Fracture Transition: A Comparison of Macro and Microscopic Observation on Compact Tension Specimens

Rapid development of failure analysis has brought increased attention to the concept of fracture toughness in recent years. The existing criteria for valid plane strain fracture toughness testing based on a macroscopic view have been shown to be conservative in some cases, and for other cases to be of questionable validity.

Compact tension specimens of variable thickness were fabricated from annealed 01 tool steel between 0.125 inch and 1.00 inch. Load-COD data and gross plastic flow measurements are compared to scanning electron microscopy (SEM) fracture surface analysis in order to correlate macroscopic observations of the ductile-brittle transition on a macroscopic scale with microscopic data permit the validity of macroscopic criteria for plane strain fracture toughness to be examined. Criteria examined include criteria based on continuum mechanics, shape of the load-crack opening displacement curve, plan view plastic zone size (PZS) and percent slant fracture.

Data indicate, for example, that PZS criteria need not be met in order to obtain totally brittle fracture on a microscopic scale, so that this criterion is conservative. Alternatively plane strain fracture toughness does decrease as the percent flat fracture approaches 100 percent, but SEM data indicate that 100 percent macroscopic flat fracture does not correspond to 100 percent microscopic brittle fracture at the minimum thickness required to obtain 100 percent macroscopic flat fracture. This implies that a larger thickness is required than is indicated by macroscopic appearance and that larger thickness must be utilized to obtain a valid plane strain fracture toughness.