New formulation for fracture toughness characterization using four-point bend specimens

Abstract

The present work aims to present a new complete procedure to characterize fracture toughness in terms of stress intensity factor K or the J−integral for Mode I of loading using four-point bend specimens. Four-Point Bend (4PB) specimens are shown to be valid configurations for fracture toughness characterization tests with certain advantages when compared to traditional three-point bend geometries and single edge notch tension SE(T) specimens under clamped conditions. Fracture toughness is a key parameter in defining critical flaw sizes in a fitness-for-service (FFS) procedure based upon fracture mechanics principles. Accurate estimation of fracture toughness is therefore necessary to assess the integrity of critical engineering structures by following the failure assessment diagram concept. Extensive 3D finite element analyses were performed to obtain the analytical relationships necessary to calculate the fracture toughness parameters. New formulas are provided for the elastic compliance, stress intensity factor, and eta factor for calculating fracture toughness valid for four-point bend specimens. Using the distance between inner and outer roller supports as a normalization parameter, the stress intensity factor and the normalized elastic compliance equations are shown to be independent of the specimen supporting configuration. The eta factor based upon the CMOD is dependent on the distance between external and internal roller supports as well as a∕W. Experimental fracture tests were carried out to check the practicality of the methodology. J-R curves obtained by four-point bend specimens were compared to resistance curves derived from three-point SE(B) specimens and clamped SE(T) samples which are standardized specimens. The results show that the four-point bend specimens provide reliable fracture toughness values somewhat higher than three-point bend geometries.