On Fracture Toughness of Isotopic and Anisotropic Magnetoactive Elastomer Under an External Magnetic Field

The emerging studies in the field of soft robotics include design, modeling, and manufacturing of soft active materials that can control soft bodies. Magneto-mechanical coupling has been recently explored as a mechanism for soft robotic control, for example in elastomeric materials that incorporate magnetic particles, typically known as magnetoactive elastomers (MAEs). Magneto-mechanical properties of MAEs such as tunable damping, vibration control, and soft structure deformation under magnetic fields have been extensively studied. Further, the magneto-mechanical coupling can be amplified by curing the MAEs in a magnetic field, producing a highly anisotropic chain-like microstructure. However, fracture toughness mechanisms in MAEs, and their dependence on anisotropy and an applied magnetic field, have not yet been explored. Cracks or any kind of damage can lead to impaired function and control of soft robotic materials. While the dependence of volume fraction and orientation of reinforcing particles on the damage response of soft materials has been studied, the fracture behavior of MAEs, which contain micron-sized reinforcing particles, is currently unknown. In this work, we experimentally study the fracture toughness and toughening mechanisms of MAEs and their dependence on particle volume fraction and chain anisotropy. To fabricate the MAEs, micron-sized carbonyl iron particles (CIPs) were suspended, and then cured, in a polydimethylsiloxane (PDMS - Sylgard 184) matrix. Magnetic fields were applied in different directions while curing to introduce anisotropy in the particle chains. We quantified the effect of particle volume fraction and chain orientation on fracture toughness through a unified experimental method with and without magnetic fields. Various volume fractions and chain orientations of CIPs were used to explore the dependence of the damage mechanisms on CIP volume fraction and chain orientation without a magnetic field. The results show that increasing particle volume fraction increases the fracture toughness. Further, fracture toughness is enhanced when particles are aligned with the direction of loading. To elucidate the dependence of fracture toughness on the magneto-mechanical coupling, similar experiments were repeated under an applied magnetic field. Finally, SEM micrographs were used to explain the micromechanics behind the variations in fracture toughness. This revealed that the mechanism is driven by inducing and controlling crack pinning, crack deflection, and interface debonding of particles. These experiments can provide a basis to understand fracture of MAEs that could enable improved damage mechanisms in soft robots controlled by magneto-mechanical coupling.