When a soft material (gel, elastomer, or biological tissue) is compressed beyond a critical strain, its smooth surface forms creases. During this process, a material particle near a crease undergoes a path of load and unload. If the material is elastic, the load and unload do not dissipate energy, and the crease sets in at a critical strain about 0.35. If the material is inelastic, however, the load and unload dissipate energy, and the crease is expected to set in at a higher critical strain. Here we study the effect of inelasticity on the onset of creases using alginate-polyacrylamide hydrogels. Such a hydrogel consists of two interpenetrating polymer networks: a polyacrylamide network of covalent crosslinks, and an alginate network of ionic crosslinks. The former is stretchy and elastic, while the latter unzips at a small stretch and causes inelasticity. We prepare alginate-polyacrylamide hydrogels with various degrees of inelasticity by varying the concentration of alginate. Our experiment confirms that larger degree of inelasticity leads to larger critical strain for crease. This study shows an example that the chemistry of materials affects the mechanics of creases.

当软质材料（凝胶，弹性体或生物组织）受压超过临界应变时，其光滑表面会形成折痕。在此过程中，折痕附近的材料颗粒会经历加载和卸载的路径。如果材料是弹性的，则加载和卸载不会耗散能量，折痕的临界应变约为0.35。但是，如果材料是无弹性的，则加载和卸载会耗散能量，并且折痕预计会以更高的临界应变出现。在这里，我们使用藻酸盐-聚丙烯酰胺水凝胶研究了无弹性对折痕开始的影响。这种水凝胶由两个互穿的聚合物网络组成：共价交联的聚丙烯酰胺网络和离子交联的藻酸盐网络。前者是有弹性的，而后者一小段拉开并导致无弹性。我们通过改变藻酸盐的浓度来制备具有不同程度的非弹性的藻酸盐-聚丙烯酰胺水凝胶。我们的实验证实，更大程度的无弹性会导致更大的折皱临界应变。这项研究显示了一个示例，表明材料的化学性质会影响折痕的力学。