Study on Establish a Brittle Fracture Prediction Considering Different Crack Opening Modes Using Mixed-Mode Ratio

Takuya Akahoshi,Tsutomu Iwashita,Toshiomi Itatani,Koji Azuma

doi:10.3390/applmech2040049

Abstract

In this study, we propose a method for predicting the occurrence of brittle fractures in the beam-to-column joints of steel structures, considering different crack opening modes. We conducted experiments on beam-to-diaphragm joint specimens with varying plastically constrained cracks to reproduce brittle fractures. The experiments’ results demonstrated the effectiveness of the toughness scale model and the Weibull stress approach. In addition, we propose the mixed-mode ratio, which is a quantitative index of the mode difference, and we applied it to the finite element models of the specimens. In this study, we evaluate the validity of the mixed-mode ratio and explore the differences in crack opening modes, as they pertain to the occurrence of brittle fractures.

Highlights

Brittle fractures in the welded joints of building structures were first observed in the devastating 1994 Northridge earthquake [1]
These fractures resulted from welding defects or cope holes in the webs on the bottom flange side of the welds [2]. Various experimental studies, such as those conducted by the Architectural Institute of Japan [3], proved that brittle fractures occur owing to ductile cracks propagation by singularities in the shapes around the bottoms of cope holes and end tabs. Several indices, such as J-value and crack tip opening displacement (CTOD) [4], which are used for the evaluation of brittle fractures, allow an overly safe assessment of building structures
This is because brittle fractures occur as a result of significant deformations of building structures, the stress–strain state at the crack tip constantly changes owing to ductile crack propagation, whereas the plastic constraint state at the crack tip of the material specimen used to obtain the fracture toughness significantly differs from that of the actual building structure

Summary

Introduction

Brittle fractures in the welded joints of building structures were first observed in the devastating 1994 Northridge earthquake [1]. Various experimental studies, such as those conducted by the Architectural Institute of Japan [3], proved that brittle fractures occur owing to ductile cracks propagation by singularities in the shapes around the bottoms of cope holes and end tabs. Several indices, such as J-value and crack tip opening displacement (CTOD) [4], which are used for the evaluation of brittle fractures, allow an overly safe assessment of building structures. Jc, which is the critical J value, was determined to be 152.46 N/mm through the single-edge notched bending (SENB) tests of the FTC and FSC specimens

Testing Procedures

Test Results

Weibull Stress Approach

Effect of the Crack Opening Modes

Mixed-Mode Ratio Methodology

Elastic Analysis of the Stress Intensity Factor