Adhesion of soft materials is commonly characterized by strength and toughness. Their applicability, however, is often questionable even for widely used test methods. Here we study lap shear using a combination of experiment, theory, and computation. Lap shear has long been used to measure adhesion strength of soft materials, but our experiments show that the measured adhesion strength is constant only when the sample is thin and short enough. When the sample thickness exceeds the fractoadhesive length, the corner of the sample concentrates stress, and the adhesion strength decreases as the sample thickness increases. When the sample length exceeds the shear lag length, the backing layer deforms appreciably, much of the sample does not carry shear stress, and the adhesion strength decreases as the sample length increases. Furthermore, we derive an analytical expression for energy release rate for a crack long compared to the sample thickness. For samples thicker than the fractoadhesive length, we show that the adhesion toughness is constant, independent of the sample length. These findings will guide the development of the methods to characterize strength and toughness of adhesion.

软材料的粘合通常以强度和韧性为特征。然而，即使对于广泛使用的测试方法，它们的适用性也经常受到质疑。在这里，我们结合实验，理论和计算来研究搭接剪切。长期以来一直使用弯曲剪切来测量软材料的粘合强度，但是我们的实验表明，只有当样品薄且足够短时，所测量的粘合强度才是恒定的。当样品厚度超过分形粘合长度时，样品的角会集中应力，并且粘合强度会随着样品厚度的增加而降低。当样品长度超过剪切滞后长度时，背衬层会明显变形，许多样品不承受剪切应力，并且粘合强度随样品长度的增加而降低。此外，我们得出了与样品厚度相比较长的裂纹能量释放率的解析表达式。对于厚度大于分形粘合剂长度的样品，我们显示出粘合韧性是恒定的，与样品长度无关。这些发现将指导表征粘合强度和韧性的方法的发展。