Mechanical instability, such as buckling of films, plates and shells, has emerged as a viable means of functional shape morphing, and has attracted much attention. Despite the extensive studies, examples with elastoplastic double-curved shells are rare, primarily because of the difficulties in fabrication and the complexities involved in analyzing their underlying high nonlinearities in materials, geometry, and contact. Here we use ping pong balls as specimens to study the post-buckling behavior of thin elastoplastic shells compressed by circular and polygonal indenters via compression tests and finite element simulations. We show that, for circular indenters, the nucleation and evolution of the buckling patterns, i.e., s-cones, are strongly affected by the indenter size and the yield stress of the shell. We also show that plastically deformed s-cones can be formed via the compression of a polygonal indenter of proper size with respect to the shell. Our findings may shed new light on rationalizing this highly nonlinear process, and may provide design guidelines that circumvent the need for creating inhomogeneous materials or geometrical imperfections.

机械不稳定性，例如薄膜，板和壳的屈曲，已经成为功能性形状变形的可行方法，并引起了广泛关注。尽管进行了广泛的研究，但具有弹塑性双曲线壳的示例仍然很少见，这主要是由于制造困难以及分析它们在材料，几何形状和接触方面潜在的高度非线性所涉及的复杂性。在这里，我们以乒乓球为样本，通过压缩测试和有限元模拟研究了圆形和多边形压头压缩的弹塑性薄壳的屈曲后行为。我们表明，对于圆形压头，屈曲模式（即s-圆锥）的形核和演化受压头大小和壳的屈服应力的强烈影响。我们还表明，可以通过相对于壳体压缩适当大小的多边形压头来形成塑性变形的s-圆锥。我们的发现可能为合理化这种高度非线性的过程提供新的思路，并可能提供设计准则，从而避免了创建不均匀材料或几何缺陷的需求。