Estimation of the Fracture Toughness of Structural Steels by Means of the CTOD Evaluation on Notched Small Punch Specimens

Abstract

By means of the experimental analysis of two structural steels, one with a ductile behaviour and the other one with brittle behaviour, this paper compares the results obtained by means of small punch test (SPT) performed on notched samples with those obtained in standard fracture toughness tests, in order to obtain a relationship between them and to analyse the suitability of the SPT for estimating the fracture toughness. With the aim of analysing the evolution of the defect tip opening displacement (δSPT) during the test, loading was interrupted at different levels of punch displacement (d) and these specimens were analysed under SEM.In the case of the ductile steel, δSPT at the onset of crack growth was larger than the value of CTOD obtained in conventional fracture tests, but this could be expected due to the lower constraint condition of the SPT. By the other hand, the brittle steel exhibits ductile fracture micromechanisms in the SPT, so fracture mechanisms have changed from the standard to the small punch test, due to the different load conditions. Relationship between d and δSPT seems to be material independent.