The relationship between crack tip opening displacement, local strain and specimen geometry

Abstract

A method is described whereby values of crack tip opening displacement (COD) can be measured at any section of a pre-cracked or slotted specimen. The relationship between COD and the degree of transverse strain at the notch root of a Charpy specimen has been investigated. A specimen will show some portion in plane strain up to a COD of about one twenty-fifth of the thickness. Transverse contraction results in smaller COD values, for a particular clip gauge displacement, at the outside than at the specimen midsection. An equation is presented relating COD to longitudinal notch root strain for Charpy specimens with root radius 0.002 in. COD and notch root strain are found to be not directly proportional. This finding is discussed in relation to the concept of a “miniature tensile specimen” at the notch root. The effect of specimen geometry on the value of COD at fracture initiation, (COD)c, has been investigated. Neither a change in specimen thickness nor a change from a fatigue pre-crack to a 0.002 in. root radius notch has been found to significantly affect (COD)c. The material used in this research was, for the most part, a low alloy pressure vessel steel, A533B.