A theoretical model for predicting crack tip opening displacement (CTOD) in small scale yielding situation has been developed by combining the elastic solution of Muskhelishvili with Irwin's idea of notional crack. The model is used to calculate the CTOD from the displacement at the mouth of the crack, without the use of rotational factor, r. An attempt has also been made to relate the rotational factor, r with the yield strength of the material.
The above model has been used to compute CTOD values in a low alloy steel with yield strength ranging from 400-2100 MPa and the results have been compared with Wells' as well as Xiao's CTOD values. The significance of the factor m' in the K-CTOD relationship has been investigated in the light of the results observed in the present investigation.
IN LEFM, Irwin's stress intensity factor, K[l] provides the necessary fracture parameter the critical value of which in plane strain opening mode is the fracture toughness (K& of the material. The K,= approach is, however, applicable in small scale yielding regime only. Wells proposed that when a significant amount of plasticity occurs at the tip of the crack, the fracture process at the crack tip is controlled by at~i~ent of a critical strain adjacent to the crack tip which can be measured by crack tip opening displa~ment (CTOD), 6. The critical value of the crack tip opening displacement in plane strain opening mode (6,,), then, represents the fracture toughness of materials including materials of high crack resistance. Compatibility between K and CTOD approaches in small scale yielding situation has been shown by Irwin[3], Egan[4] etc.
A direct measurement of CTOD is difficult and various methods have been proposed and used in the literature to obtain the CTOD values. Well@] has reported in BS DD (19) the method to obtain CTOD from the clip gauge measurements taken at the mouth of a notch. The analysis is based on the assumption of an apparent-axis of rotation in a bend specimen. The location of the apparent axis of rotation from the tip of the crack is at r(wa), where I is the rotational factor and (w -a), the ligament length. The value of rotational factor, r, is a function of deformation. The values of CTOD have also been inferred from the notch-root contraction, angle of bend measurements, double notch technique, high speed photo~aphy etc. .
Robinson and Tetelman [7] measured directly the CTOD by infiltrating crack tip with hardening silicone rubber and found a relationship between clip gauge and true tip (infiltration) COD, The rotational factor, r, was found to be independent of the material properties and varying from zero in fully elastic to close to 0.5 in fully plastic situation. More recently, Xiao and Huang[8] studied '