Tunable buckling strength of magnetically active elastomeric shells

It has long been recognized that the buckling of shell structures is highly sensitive to material or geometric imperfections, leading to observed critical loads that are significantly lower than classic predictions. In this class of problems, the knockdown factor is defined as the ratio between the experimentally measured critical load and the classic theoretical prediction. This knockdown is typically regarded as an intrinsic property of the structure since the type and distribution of defects are encoded into the shell during fabrication. Here, we demonstrate the ability to actively tune the knockdown factor of pressurized spherical shells. We fabricate our spherical shells with a magneto-rheological elastomer (MRE) using a coating technique. The shells are first magnetized and then loaded by pressure under a uniform magnetic field. We find that, by adjusting the strength and polarity of the field, the knockdown factor of the magnetically active shells can be increased or decreased up to a maximum change of 30%. As such, we can externally tune their intrinsic buckling strength, on-demand. An axisymmetric shell model is used to rationalize the experimental results on how the magnetic field interacts with the buckling of imperfect shells.