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.