Smoothening creases on surfaces of strain-stiffening materials

When an elastic block (e.g., an elastomer or a soft tissue) is compressed to a critical strain, the smooth surface of the block forms creases, namely, localized regions of self-contact. Here we show how this instability behaves if the solid stiffens steeply. For a solid that stiffens steeply at large strains, as the compression increases, the surface is initially smooth, then forms creases, and finally becomes smooth again. For a solid that stiffens steeply at small strains, creases will never form and the surface remains smooth for all levels of compression. We also obtain the critical conditions for the onset of wrinkles. When the surface does become unstable, we find that creases always set in at a lower compression than wrinkles. Our findings may shed light in developing crease-resistant materials.