Small-Specimen Brittle-Fracture Toughness Testing

Abstract

Accurate estimates of valid plane-strain fracture toughness, KIc, for low-alloy steels in the ductile-to-brittle transition temperature range may be made by using JIc-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, σf*, 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 JIc is represented by J1∕J2=(B2∕B1)1∕m where m is the Weibull modulus, Bi is the specimen thickness for specimen i, and Ji is the mean JIc for several specimens. Experimental evidence supporting the theory is shown.