Study of estimation of ductile-brittle transition temperature using U-notched small punch test specimens

•The notch depths of U-notched small punch specimens have a significant effect on fracture and stress triaxiality.•Lower shelf and higher transition part are strongly related to the constraints of specimens.•Notch depth-to-thickness ratio a/t = 0.3 can be regarded as an optimum selection to design U-notched small punch specimen. Various notched small punch test (SPT) specimens have been designed to determine ductile–brittle transition temperature (DBTT) of materials, but the use of notched specimens is still lacking an in-depth understanding. This paper aims to study the effects of notch depths on SPT results. U-notched specimens made from 3Cr1MoV were tested at different temperatures. It is found that the specimen with a notch of depth-to-thickness ratio a/t = 0.3 generates a clear lower shelf and higher ductile-brittle transition area as compared to the specimens with notches of a/t = 0 and 0.5. Finite element simulations reveal that the lower shelf and higher transition part are related to the geometric constraint of specimens. Fracture behaviors of notched specimens were also thoroughly studied using simulations. It suggests that a/t = 0.3 can be regarded as a reference depth for U-notched specimens. This finding is confirmed with metallic alloys 2.25Cr1MoV, A106 and 16MnR. The present work gives a new insight into the design of notched SPT specimens for estimating DBTT.