Session: 03-01-01 Fracture Assessment and Control

Paper Number: 78305

78305 - Study on Correcting Method for Predicting Brittle Fracture of Surface Cracked Plates in Mixed-Mode Loading 

This study aims to establish a fracture prediction method that considers the effect of crack opening modes to accurately predict brittle fracture occurring at the beam-to-column joints of building structures. This study consider the effect of mode differences on fracture initiation using finite element analysis of edge cracked plates.

The brittle fractures occurred in the 1994 Northridge and 1995 Kobe earthquakes. The post-earthquake reports indicate that the brittle fracture was initiated at ductile cracks starting from geometrical singularity points. The singularities were where the high triaxial stress constraint and strain concentration occurs, such as the bottom of cope halls or weld defects. Improvements have been made to the joints to reduce the strain concentration. However, the danger of weld defects cannot be eliminated in welded joints. The technology to accurately assess the risk of brittle fracture arising from weld defects is essential for steel structures.

Engineering methods based on fracture mechanics have been used to evaluate the brittle fracture of structural members. However, the fracture mechanics approach cannot apply directly to building structures. One of the reasons for this is the difference in plastic constraints between cracks as weld defects and cracks in test specimens. For example, a single edge notched bend (SENB) specimen used to determine the fracture toughness values has a notch half the thickness of the specimen, which is much deeper than the defects of the real structures. Therefore, these deep notch tips are subjected to plastic constraints much more strongly than actual defect tips. In the local approach study, the Weibull stress was proposed by Beremin as a new fracture evaluation parameter to consider the effect of plastic constraint. The critical value based on the Weibull stress is expected to be independent of specimen size, crack length and loading modes, and it has been the subject of numerous experiments. We have also investigated the Weibull stress applicability to building structures and have shown that the Weibull stress prediction can improve the accuracy of brittle fracture prediction.

Although the Weibull stress prediction shows highly accurate under Mode I loading only, the accuracy is reduced under the mixed mode loading. To establish a unified brittle fracture prediction method even under large yielding and mixed modes, correcting the Weibull stress considering the mixed modes effects is necessary. As a preliminary correction step, we proposed a quantitative parameter mixed mode ratio to indicate the rate of Mode I and Mode II. The mixed-mode ratio defines from stress intensity factors. Therefore, this parameter cannot be applied after the crack has grown significantly, but it is a valid parameter in the practical range. We have shown that it is possible to quantify different modes by using mixed-mode ratios, as long as the ductile crack growth is not excessive.

To investigate the effects of different crack opening modes on the initiating of the brittle fracture, we adopted a composite mode experiment using a crack plate of the same size as the SENB specimens. This is to prevent influences other than the mode from entering as much as possible. In this study, edge cracked plates with various modes are reproduced by finite element analysis and compared with the mixed-mode ratio. Then, we considered the relationship between the occurrence of brittle fracture and the crack opening modes.

Presenting Author: Takuya Akahoshi Sojo University

Authors:

Takuya Akahoshi Sojo University
Koji Azuma Sojo University
Tsutomu Iwashita National Institute of Technology, Ariake College