For Better or For Worse: Self-tuning of the buckling strength of active bilayer shells

We study a new class of composite shells that can self-repair or self-aggravate existing imperfections, thereby tuning their buckling strength. Our bilayer polymeric shells, containing a precise geometric defect, are fabricated through a customizable coating technique. Upon curing, the diffusion of uncross-linked residual polymer chains across the two layers induces swelling, causing the natural curvature of the bilayer structure to evolve. This natural curvature can be made positive or negative depending on the order of the layers. Through precision experiments, we quantify the time-dependence of both the geometry and buckling strength of the shells. We find that the critical buckling pressure of the shells with negative natural curvature can increase with time, as long as the defect amplitude exceeds a critical value, hence, causing the shells to self-heal. This healing trend is reversed for defects with an amplitude below the threshold. By contrast, the shells with positive natural curvature always exhibit a self-destructive behavior. We combine the experiments with finite element simulations and a reduced analytical model to rationalize our results on how an evolving geometry and residual stresses can self-tune the buckling strength of bilayer shells, for better or for worse.