WHY DIRECT TENSION SPECIMENS FLEX AND BREAK AT MIDLENGTH

In a direct tension test, unnotched specimens of quasibrittle materials that exhibit post-peak strain softening do not deform symmetrically. The equilibrium path bifurcates and the thermodynamic criterion of stable path requires the specimen to flex to one side, even if the geometry is perfect and if the straightening effect of the moment of the axial force about the centroid of the deflected cross section is taken into account. In absence of rotational restraints at the ends, the specimen fails right after the peak load. But if the attachments to the loading machine exert a sufficient restraint against rotation, the flexing to the side is retarded and failure occurs at midlength. The phenomenon (which is similar to the recently discovered behavior of notched tensile fracture specimens) is illustrated using a simple model in which the specimen consists of two bars of unequal length, joined by a strain-softening link.