Small-Specimen Brittle-Fracture Toughness Testing

Accurate estimates of valid plane-strain fracture toughness, K_Ic, for low-alloy steels in the ductile-to-brittle transition temperature range may be made by using J_Ic-valid specimens and accounting for a size effect evident for cleavage fracture. The size effect is explained using a “weakest link” theory that predicts variance in test results using constant-size specimens and decreasing average results when the specimen size is increased. Cleavage fracture occurs when the maximum tensile stress near the crack tip equals or exceeds the cleavage stress, σ_f*, over a microstructurally significant distance, the process-zone diameter. Variation in specimen toughness arises due to variations in the microstructure within the process zone; the weakest feature in the process zone causes catastrophic cleavage fracture of the specimen.

Our work indicates that the size effect on J_Ic is represented by $J1∕J2=(B2∕B1)1∕m$ where m is the Weibull modulus, B_i is the specimen thickness for specimen i, and J_i is the mean J_Ic for several specimens. Experimental evidence supporting the theory is shown.