EFFECTS OF TORSIONAL BUCKLING ON THE CLEAVAGE FAILURE OF LOW-ALLOY STEEL TENSION PIPE SPECIMENS

The local approach criterion of fracture mechanics, initially develope d by Beremin for brittle cleavage fracture, is applied here to A508 cl ass 3 low-alloy ferritic steel. This criterion, based on the maximum p rincipal stress and Weibull statistics, has previously been verified i n the case of uniaxial tests. In this study, it is extended to multiax ial loading tests, that can lead to more significant levels of plastic strain, and thus permit a study of the effect of plastic strain on cl eavage fracture. Uniaxial tests on axisymmetric notched tensile bars ( AE2-6) were used to determine Beremin's model parameters m and sigma(u ). The cleavage fracture behaviour, described by these parameters, was then verified by multiaxial tension-torsion tests carried out on thin tubular specimens. Numerical simulations of the tension-torsion tests , by the finite element method, were also performed, taking into accou nt the nonlinear geometrical effects and the specimen plastic buckling . The buckling critical loads were calculated and used to ascertain wh ether fracture was associated with the instability phenomenon. Beremin 's model is shown to correctly describe experimental data which are no t affected by buckling.