Incorporation of Obstacle Hardening into Local Approach to Cleavage Fracture to Predict Temperature Effects in the Ductile to Brittle Transition Regime.

Maria S Yankova,Andrey P Jivkov,Rajesh Patel

doi:10.3390/ma14051224

Maria S Yankova, Andrey P Jivkov + Show 1 more

Open Access

https://doi.org/10.3390/ma14051224

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Journal: Materials	Publication Date: Mar 5, 2021
Citations: 2	License type: CC BY 4.0

Affiliation: University of Manchester, National Nuclear Laboratory

Abstract

Ductile-to-brittle-transition refers to observable change in fracture mode with decreasing temperature—from slow ductile crack growth to rapid cleavage. It is exhibited by body-centred cubic metals and presents a challenge for integrity assessment of structural components made of such metals. Local approaches to cleavage fracture, based on Weibull stress as a cleavage crack-driving force, have been shown to predict fracture toughness at very low temperatures. However, they are ineffective in the transition regime without the recalibration of Weibull stress parameters, which requires further testing and thus diminishes their predictive capability. We propose new Weibull stress formulation with thinning function based on obstacle hardening model, which modifies the number of cleavage-initiating features with temperature. Our model is implemented as a post-processor of finite element analysis results. It is applied to analyses of standard compact tension specimens of typical reactor pressure vessel steel, for which deformation and fracture toughness properties in the transition regime are available. It is shown that the new Weibull stress is independent of temperature, and of Weibull shape parameter, within the experimental error. It accurately predicts the fracture toughness at any temperature in the transition regime without relying upon empirical fits for the first time.

Highlights

ductile-to-brittle transition (DBT) presents a challenge for a number of industries, where structural components are made of bcc metals, with a very important example being ferritic steels
At which to 10 fracture toughness measurements are available, we perform at leastat8least to 108fracture toughness measurements are available, we perform a finite aelement finite element (FE) analysis each load increment, the elements within (FE) analysis and forand eachfor load increment, we find we the find elements within fracture fracture zonebased (FPZ),onbased on two conditions for the maximum stress process process zone (FPZ), two conditions for the maximum principalprincipal stress and the and the equivalent plastic strain, typically applied in the local approaches to fracture, as equivalent plastic strain, typically applied in the local approaches to fracture, as follows: p p p follows: ε, where ε
It represents the effect of easier dislocation motion through the lattice at higher temperatures due to a reduced lattice resistance, and an increase in the plastic flow

Summary

Introduction

Metallic materials with body-centred cubic (bcc) lattices exhibit a unique behaviour with decreasing temperature—their fracture toughness decreases rapidly as the mode of fracture shifts from slow ductile crack growth, typically by void growth and coalescence, to fast brittle fracture, typically by transgranular cleavage. This behaviour is referred to as the ductile-to-brittle transition (DBT). Stress-critical applications of ferritic steels include reactor pressure vessels (RPV) in light water reactors and pressurized equipment in hydrocarbon processing These applications require reliable assessments of the fracture toughness in the DBT regime. Such assessments are challenging because the fracture mechanisms encompass multiple length scales—from atomic to component—and remain an active area of research

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