Polyacrylamide hydrogels. III. Lap shear and peel

Lap shear and peel are common tests for soft materials. Their results, however, are rarely compared. Here we compare lap shear and peel as tests for measuring toughness. We prepare specimens for both tests by using stiff layers to sandwich a layer of a polyacrylamide hydrogel. We introduce a cut in the hydrogel by scissors, pull one stiff layer at constant velocity, and record the force. In lap shear, the force peaks and then drops to zero, the cut grows unstably through the entire hydrogel, and the peak force is used to determine toughness. In peel, the force peaks and then drops to a plateau, the cut grows in the hydrogel in steady state, and the plateau force is used to determine toughness. Our experimental data show that the average values of toughness determined by lap shear and peel are comparable. The peak forces in both tests scatter significantly, but the plateau force in peel scatters narrowly. Consequently, toughness determined by lap shear scatters more than toughness determined by peel. We hypothesize that the peak forces scatter mainly due to the statistical variation of the cuts made by scissors, and test the hypothesis using two additional sets of experiments. First, after a cut is made by scissors, we pre-peel the specimen to extend the cut somewhat, and then measure toughness by lap shear and peel. The peak force in lap shear scatters less, and the peak force in peel is removed. Second, we prepare cuts using spacers of various thicknesses, and find that the peak forces in both lap shear and peel vary with the thickness of the spacer. These findings clarify the use of lap shear and peel to characterize soft materials.