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 σ_xx, σ_zz, and σ_xz on the material primary fracture energy G_f and on the characteristic size of the fracture process zone (FPZ) c_f. For quasibrittle materials, previous tests showed that σ_xx could either enhance or reduce G_f to almost zero. In this paper, we adapt the gap tests to study the effect of σ_xx 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 σ_xx. Our findings reveal that a crack-parallel stress σ_xx = 40% of the yield strength will roughly double the fracture energy and triple the size of the YZ.