Transient Electrically Driven Stiffness-Changing Materials from Liquid Metal Polymer Composites.

Stiffness-changing materials (SCMs) have received lots of interests due to their reversible transition between their soft and rigid states for modern applications. However, the irreversible stiffness transition, slow response, and sustained external stimuli strictly hinder the broad utilizations of SCMs. Here, this work reports electrically driven SCMs based on supercooled liquid metals (LMs). A small voltage (5 V) can successfully initiate the stable and reversible stiffness change of the SCMs in electrolyte solution. Surprisingly, the LM-based SCMs (LM-SCMs) exhibited a significant change in 1000 times difference of moduli (65 kPa versus 79 MPa). Moreover, such a stiffness transition of the LM-SCM was ultrarapidly completed in a few seconds (<30 s). Importantly, after transient stimulation of LM nucleation, the rigidity of the LM-SCM could be maintained when the external stimulus (voltage) was removed, highly different from previously reported SCMs that require sustained energy to maintain their mechanical states. Based on the unique features of LM-SCMs, advanced robotics like smart valves and mechanical paws in seawater were successfully fabricated.

Experimentation and Modeling of the Tension Behavior of Polycarbonate at High Strain Rates.

A comprehensive understanding of the mechanical behavior of polycarbonate (PC) under high-rate loadings is essential for better design of PC products. In this work, the mechanical behavior of PC is studied during tensile loading at high strain rates, using a split Hopkinson tension bar (SHTB). A modified experimental technique based on the SHTB is proposed to perform the tension testing on PC at rates exceeding 1000 s-1. The effect of strain rates on the tension stress⁻strain law of PC is investigated over a wide range of strain rates (0.0005⁻4500 s-1). Based on the experiments, a physically based constitutive model is developed to describe the strain rate dependent tensile stress⁻strain law. The high rate tensile deformation mechanics of PC are further studied via finite element simulations using the LSDYNA code together with the developed constitutive model.

Processing-Induced Inhomogeneity of Yield Stress in Polycarbonate Product and Its Influence on the Impact Behavior.

In this study, an integrated methodology for impact analysis of polycarbonate (PC) product is proposed which incorporates the processing-induced inhomogeneity of yield stress. A previously developed model is extended to predict the inhomogeneous yield stress distribution along the specimen by using the thermal history experienced during injection molding. A strain rate-dependent elastic-plastic model combining the processing-induced yield stress is applied to model the mechanical behavior of PC. Finite element simulation for notched Izod impact test is then conducted to analyze the impact behaviors of PC specimens with different thermal histories. Numerical results of the fracture energies are compared with experimental measurements.