The gap test is a new type of fracture test developed in 2020, in which the end supports of a notched beam are installed with gaps that close only after the elasto-plastic pads next to notch introduce a desired constant crack-parallel compression ${\sigma }_{xx}$ (also called the T-stress). The test uses the size effect method to identify how such a compression alters the material fracture energy, $G_f$, and the characteristic size $c_f$ of the fracture process zone (FPZ). In 2020, experiments showed that a moderate ${\sigma }_{xx}$ doubled the $G_f$ of a quasibrittle material (concrete) and a high ${\sigma }_{xx}$ reduced its $G_f$ to almost zero. A preliminary study by Nguyen et al. (2021) showed that the gap test can be extended to plastic-hardening polycrystalline metals. A generalized scaling law with an intermediate asymptote for large-scale yielding in small structures was derived, and limited tests of aluminum alloy showed its applicability. In this study, geometrically scaled gap tests of notched three-point bend fracture specimens of aluminum are conducted at three different levels of ${\sigma }_{xx}$. An extended structural strength scaling law that captures the transition from the micrometer-scale FPZ through millimeter-scale yielding zone (YZ) to large-scale structures which follow linear elastic fracture mechanics (LEFM) is derived and then applied to analyze the effect of ${\sigma }_{xx}$. Presented here are the gap tests of aluminum alloy, in which three different levels of ${\sigma }_{xx}$ are applied to scaled notched four-point-bend beams of depths $D$ = 12, 24, 48 and 96 mm. Using an extended size effect law for plastic-hardening metals, it is found that, at crack-parallel stress ${\sigma }_{xx}\approx -40\text{\%}$ of the yield strength, the critical J-integral value gets roughly quadrupled, not only because of the well-known enlargement of the hardening YZ whose width is of millimeter scale, but also because of the increase of the FPZ width of micrometer scale. These results can be reproduced neither by line crack models, including the LEFM, cohesive crack and phase-field models, nor by peridynamic and various nonlocal models that ignore the tensorial nature of the material stress at the crack tip. The crack band models, being able to represent an FPZ of finite width and a YZ whose size evolves depending on ${\sigma }_{xx}$, can capture the effect of crack-parallel stresses provided that a realistic 3D tensorial damage constitutive model is used. Here, Bai–Wierzbicki’s model is shown to capture the ${\sigma }_{xx}$ effect on the $G_f$ and $J_{cr}$ qualitatively.

间隙测试是 2020 年开发的一种新型断裂测试，其中带缺口梁的端部支撑安装有间隙，只有在缺口旁边的弹塑性垫引入所需的恒定裂缝平行压缩后，间隙才会闭合${\sigma }_{XX}$（也称为 T 应力）。该测试使用尺寸效应方法来确定这种压缩如何改变材料断裂能，$G_F$, 以及特征尺寸$C_F$断裂过程区 (FPZ)。2020 年，实验表明适度${\sigma }_{XX}$加倍$G_F$准脆性材料（混凝土）和高${\sigma }_{XX}$减少其$G_F$几乎为零。Nguyen 等人的初步研究。(2021) 表明间隙试验可以扩展到塑性硬化多晶金属。导出了用于小结构中大尺度屈服的具有中间渐近线的广义标度律，并通过铝合金的有限试验证明了其适用性。在这项研究中，铝的缺口三点弯曲断裂试样的几何比例间隙试验在三个不同的水平上进行。${\sigma }_{XX}$. 推导了一种扩展的结构强度比例定律，该定律捕获从微米级 FPZ 通过毫米级屈服区 (YZ) 到遵循线性弹性断裂力学 (LEFM) 的大型结构的过渡，然后应用于分析${\sigma }_{XX}$. 这里介绍的是铝合金的间隙测试，其中三个不同级别的${\sigma }_{XX}$应用于按比例缩放的缺口四点弯曲梁的深度$丁$= 12、24、48 和 96 毫米。使用塑性硬化金属的扩展尺寸效应定律，发现在裂纹平行应力下${\sigma }_{XX}\approx -40\text{\%}$屈服强度的临界 J 积分值大约翻了四倍，这不仅是因为众所周知的毫米级宽度的硬化 YZ 的扩大，而且是因为 FPZ 宽度的微米级的增加。这些结果既不能通过线裂纹模型（包括 LEFM、内聚裂纹和相场模型）重现，也不能通过近场动力学模型和各种忽略裂纹尖端材料应力张量性质的非局部模型重现。裂纹带模型，能够表示有限宽度的 FPZ 和大小随时间变化的 YZ${\sigma }_{XX}$, 如果使用逼真的 3D 张量损伤本构模型，则可以捕获平行裂纹应力的影响。在这里，Bai–Wierzbicki 的模型显示捕捉${\sigma }_{XX}$影响$G_F$和${杰}_{Cr}$定性地。