Study on COD Bend Test as an Engineering Brittle Fracture Initiation Characteristics Test

Abstract

The brittle fracture initiation characteristics have successfully been investigated in terms of COD concept in these few years. The COD concept seems to be promising as one of the possible useful fracture criteria to assess the fracture behavior of steel structures. In order to clarify the significance of defects in structures and select the proper materials, it is very important for the fabricators to be able to obtain the critical COD value from small sized and simple test. The present authors have investigated the effectiveness and utility of COD bend test as one of the fundamental material tests. The bend test specimen can be made smaller and can fracture at lower load, so that it could be very useful testing method from the industrial point of view.
But one of the troubles associated with the measurement of critical COD on a bend specimen is that the notch tip COD is not so easily obtained for bend test as for tension test. So far a COD value is estimated from the clip gauge reading using “rotational factor”, on the assumption that a notched bend specimen rotates about a sort of “pivot”, the location of which in the specimen is dependent of applied load.
In this paper the method of determining COD value at notch tip in a bend specimen is disscussed on the basis of the results of experiment and calculation using the Dugdale-Barenblatt crack model in bending stress field and Finite Element Method. It is revealed that the critical COD value can be obtained only from fracture load of bend specimen.
The COD bend test were carried out with various specimen size and notch acuity for the purpose of clarifying the characteristics of critical COD value. It is confirmed that not only the critical COD value obtained from COD bend test compares well with that obtained from tension test but also it is independent of specimen geometry to a certain extent.
Finally the tentative method of COD bend test is proposed as a fundamental material test.