Assessment of Ductile Fracture Model in Pipe Fracture Scenario

Abstract

QinetiQ has a major and long standing interest in the dynamic deformation and fracture response of materials and has been very active in developing constitutive and fracture models. These models have been validated across a number of different tests, triaxial stress states and strain rates [1]. The models have also been used on a range of Defence and commercial applications where the material characterisation has been controlled by QinetiQ. However, they have not been exercised extensively on industrial applications using available data in the literature.The constitutive models are based on interrupted tensile testing to separate the strain hardening, strain rate and thermal softening and for body centred cubic (BCC) steels give rise to the modified Armstrong-Zerilli model [1]. The ductile fracture model is based on a path dependent approach linking the damage to the stress-state resulting in the Goldthorpe Path Dependent Fracture (PDF) model [2]. Essentially the Goldthorpe PDF model gives a single value of critical damage (Void Fracture Number – VFN) applicable to all stress states and strain rates.This paper describes the application of the QinetiQ constitutive and fracture models in the simulation of an industrial application and assesses their ability to capture the main deformation and fracture mechanisms. The application was an anchor dropping onto a pipe both dynamically and statically to determine whether the pipe is fractured [3]. The DYNA3D hydrocode was used to simulate the dynamic impact and the subsequent deformation and fracture of the pipe.The material constants for the model were estimated for the actual materials by using the testing published in the literature [3] and comparing them with ‘similar’ materials within the QinetiQ materials database. Given there is some subjectivity in this process, the likeliness of failure was investigated, particularly regarding the VFN relating to the fracture process.The results are discussed with respect to the degree of deformation and fracture within the pipe and the sensitivity of these features to the input conditions. The results are also discussed in the context of whether the predictive accuracy of the simulations is such that they could ultimately be used to design a pipe that is more resistant to the impact loading, for example by increasing the yield strength through processing.