Strength and toughness of adhesion of soft materials measured in lap shear

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.