Several ASTM standards on the fracture of glued and welded joints need attention because they do not consider the Griffith energy criterion of cracking which was proposed a century ago. It is almost as if Griffith never existed because the ASTM definition of failure is the stress criterion postulated by Galileo in 1638 in which stress at failure (i.e. strength = force/area) is defined as the determinant of fracture. Irene Martinez Villegas (Villegas, Rans 2021 Phil. Trans. R. Soc. A 376, 20200296. (doi:10.1098/rsta.2020.0296)) shows in this volume that attempts to use ASTM D5868 to standardize welded composite (carbon fibre reinforced polymer, CFRP) lap joints reveal major problems. First, the test is a low angle bend–peel test; not shear. Second, the energy required to break the joint is not emphasized so that joints may have high strength properties but also low toughness; third, the fracture force is not proportional to the lap joint area so the concept of strength independent of sample size is false; fourth, as the CFRP panels are made thicker, the strength rises at constant overlap area so the strength can be any value you want; fifth, the strength of larger joints goes down; this is the size effect noted in many bend-cracking tests, much as Galileo suggested for bent beam fracture in his famous book ‘the larger the machine, the greater its weakness'. The purpose of this paper is to demonstrate that poor ASTM ‘shear strength’ standards should be replaced by a definition of welded lap joint performance based on Griffith's energy conservation argument in which fracture surface energy is the main parameter resisting failure. The foundation of this Griffith-style lap joint analysis for long cracks goes back to 1975 but has been largely ignored until now because it does not fit the Griffith equation for cracked sheets, has no ‘stress intensity factor’, and travels at constant speed, not accelerating like the standard Griffith tension crack. This study of tensile delamination shows that a long lap crack is not driven by stress near the crack but by changes in stored elastic energy in the stretched strips remote from the crack tip, while strain energy release rate is negative. It would be more appropriate to call this lap failure a tensile delamination crack.

This article is part of a discussion meeting issue ‘A cracking approach to inventing new tough materials: fracture stranger than friction’.

一些关于胶合和焊接接头断裂的 ASTM 标准需要注意，因为它们没有考虑一个世纪前提出的格里菲斯开裂能量标准。几乎就像格里菲斯从未存在过一样，因为 ASTM 对失效的定义是伽利略在 1638 年提出的应力标准，其中失效时的应力（即强度 = 力/面积）被定义为断裂的决定因素。Irene Martinez Villegas (Villegas, Rans 2021 Phil. Trans. R. Soc. A376, 20200296. (doi:10.1098/rsta.2020.0296)) 在本卷中表明，尝试使用 ASTM D5868 来标准化焊接复合材料（碳纤维增强聚合物，CFRP）搭接接头揭示了主要问题。首先，该测试是低角度弯曲-剥离测试；不剪切。其次，没有强调破坏接头所需的能量，因此接头可能具有高强度特性但韧性较低；第三，断裂力与搭接面积不成正比，因此强度与样本量无关的概念是错误的；第四，随着碳纤维板做得更厚，强度在恒定的重叠区域上升，所以强度可以是你想要的任何值；第五，较大关节的强度下降；这是许多弯曲开裂测试中注意到的尺寸效应，正如伽利略在他的著名著作《机器越大，其弱点越大》中所建议的弯曲梁断裂。这篇论文的目的是表明，根据格里菲斯的能量守恒论点，断裂表面能是抵抗失效的主要参数，应该用焊接搭接接头性能的定义代替糟糕的 ASTM“剪切强度”标准。这种用于长裂纹的格里菲斯式搭接接头分析的基础可以追溯到 1975 年，但直到现在基本上都被忽略了，因为它不适合裂纹板的格里菲斯方程，没有“应力强度因子”，并且以恒定速度行进，不像标准的格里菲斯张力裂纹那样加速。这项拉伸分层研究表明，长搭接裂纹不是由裂纹附近的应力驱动，而是由远离裂纹尖端的拉伸条中存储的弹性能的变化驱动，而应变能释放速率为负值。将这种搭接失效称为拉伸分层裂纹更为合适。

这篇文章是讨论会问题“发明新的坚韧材料的破解方法：断裂比摩擦更奇怪”的一部分。