Gap Test Consequences for Fracture Mechanics of Quasibrittle Materials and Plastic-Hardening Metals

In a new type of fracture experimental setup, which was developed in 2020 and usually referred to as gap test, we use the size effect method to identify the interaction between crack-parallel compressive stresses σxx, σzz, and σxz on the material primary fracture energy Gf and on the characteristic size of the fracture process zone (FPZ) cf. For quasibrittle materials, previous tests showed that σxx could either enhance or reduce Gf to almost zero. In this paper, we adapt the gap tests to study the effect of σxx on fracture of plastic-hardening metals. First, the scaling law of structural strength for this type of material is re-examined and found to be different, showing a transition from the micrometer-scale FPZ through millimeter-scale yielding zone (YZ) to small-scale yielding fracture of large structures. This transition has been formulated by means of asymptotic matching approximations. Next, the gap tests are performed on aluminum alloy using scaled notched three-point-bend beams of depths D = 12–96 mm at three different levels of σxx. Our findings reveal that a crack-parallel stress σxx = 40% of the yield strength will roughly double the fracture energy and triple the size of the YZ.