Adhesion toughness of soft materials measured in peel and lap shear

Peel and lap shear are commonly used tests for soft materials. Our previous comparison of peel and lap shear for measuring bulk toughness leads to a fundamental question: do peel and lap shear measure identical adhesion toughness? Here we compare peel and lap shear for measuring adhesion toughness and use the adhesion between polyacrylamide hydrogel and polyacrylic acid hydrogel as a model system. To prepare samples, we introduce a precut crack to the adhesion interface of two adhered hydrogels and sandwich the adhered hydrogels between two stiff layers. In both types of tests, we pull one stiff layer at a constant velocity and record the force-displacement curve. In peel, as the displacement increases, the force increases to a peak and subsequently drops to a plateau, the crack propagates steadily in the sample, and the adhesion toughness is determined by the plateau force. In lap shear, as the displacement increases, the force increases to a peak and subsequently drops to zero, the crack propagates unstably in the sample, and the adhesion toughness is determined by the peak force. Our experiments show that the average values of adhesion toughness measured in peel and lap shear are nearly identical. We also prepare hydrogels of different values of pH and measure the adhesion toughness using peel and lap shear. When pH is high, the adhesion is brittle, the crack propagates along the adhesion interface, and adhesive fracture takes place. When pH is low, the adhesion is tough, the crack kinks into the hydrogels, and cohesive fracture takes place. In either case, the average values of adhesion toughness measured in peel and lap shear are comparable. We further measure the adhesion toughness of hydrogels of different crosslink densities, and peel and lap shear give nearly identical adhesion toughness for all the crosslink densities. These results guide the use of peel and lap shear for characterization of adhesion of soft materials.