Simplified Prediction Method of Stress Intensity Factor in Mid-thick Plane in 3D Cracked Body and Its Difference from 2D Handbook Formula

Abstract

Abstract The authors have been working to improve the crack tip opening displacement (CTOD) evaluation standard as a fracture parameter for the cleavage-type brittle fracture critical condition of carbon steel. In 2016, WES1108, Standard Test Method for Crack-Tip Opening Displacement (CTOD) Fracture Toughness Measurement, which is the CTOD test standard in Japan, was revised, and subsequently, the CTOD calculation formula of ISO 15653:2010, Metallic Materials – Method of Test for the Determination of Quasistatic Fracture Toughness of Welds, was revised in 2018, focusing on the study of the formula that constitutes the basis of the standard. Similar to the original British Standards equation, the proposed CTOD calculation formula consists of a small-scale yield (SSY) term and a plastic term, but in the SSY term, the two-dimensional (2D) stress intensity factor (SIF) is used. The authors focused on the incorrectness of this treatment derived from the difference between 2D and three-dimensional (3D). Here, two accurate relationships have been explored through systematic finite element method analyses; one is the correlation of the 3D SIF values in the mid-thickness plane and 2D SIF, and the second is the relationship between the actual 3D SIF in mid-thickness and the CTOD in mid-thickness. The KI ratio (K3D/K2D) sharply coincides with the value 1.11, as inferred in several previous pieces of research in the idealistic infinite plate with a crack. However, if the ligament size is finite as in the actual fracture toughness test, the KI ratio drops significantly from 1.11, and in some cases, the KI ratio is less than 1. By unifying these findings, a new precise CTOD formula has been established. Lastly, the study inferred that the error of the calculation formula is improved, especially in small thickness regions, as compared with the current standard proposed in 2016 and 2018 by the authors.