Relating matrix stress to local stress on a hard microstructural inclusion for understanding cleavage fracture in high strength steel

Quanxin Jiang,V. M. Bertolo,V. A. Popovich,J. Sietsma,Carey L. Walters

doi:10.1007/s10704-021-00587-y

Abstract

Macroscale cleavage fracture toughness of high strength steels is strongly related to the fracture of hard microstructural inclusions. Therefore, an accurate determination of the local stress on these inclusions based on the matrix stress is necessary for the statistical modelling of macroscale cleavage fracture. This paper presents analytical equations to quantitatively estimate the stress of the microstructural inclusions from the far-field stress of the matrix. The analytical equations account for the inclusion shape, the inclusion orientation, the far-field stress state and matrix material properties. Finite element modelling of a representative volume element containing a hard inclusion shows that the equations provide an accurate representation of the local stress state. The equations are implemented into a multi-barrier model and compared with CTOD experiments with two different levels of constraint.

Highlights

Mechanical integrity assessment of steel structures frequently requires knowledge of their resistance to catastrophic failure by fast, unstable crack growth, expressed as fracture toughness
This paper aims to propose an analytical solution that can be used for the calculation of local stress on a hard inclusion based on the far-field stress on the matrix
In the above validations and applications, the analytical solution reflects the influence of several factors on the inclusion stress, which corresponds to the observations of hard inclusion behaviour in cleavage fracture that have been reported in literature

Summary

Introduction

Mechanical integrity assessment of steel structures frequently requires knowledge of their resistance to catastrophic failure by fast, unstable crack growth, expressed as fracture toughness. Toughness at lower temperatures and the transition temperature region are related to transgranular quasicleavage fracture, which will be called cleavage in this paper. The need for more accurate cleavage modelling is acute for a new generation of high- and very high-strength steels (yield strength of 500 to 1000 MPa) because they generally have lower toughness, and a lower safety margin. These classes of steels obtain their favorable properties through their complex, multiphase microstructures, which complicates microstructural modelling of cleavage-driven failure

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Conclusion