This paper reports the unique microstructure of polyurea foams that combines the advantages of open and closed cell polymeric foams, which were synthesized through a self-foaming process. The latter was the result of aggressive mechanical mixing of diamine curative, isocyanate, and deionized water at ambient conditions, which can be adjusted on-demand to produce variable density polyurea foam. The spherical, semi-closed microcellular structure has large perforations on the cell surface resulting from the concurrent expansion of neighboring cells and small holes at the bottom surface of the cells. This resulted in a partially perforated microcellular structure of polyurea foam. As a byproduct of the manufacturing process, polyurea microspheres nucleate and deposit on the inner cell walls of the foam, acting as a reinforcement. Since cell walls and the microspheres are made of polyurea, the resulting reinforcement effect overcomes the fundamental interfacial issue of different adjacent materials. The partially perforated, self-reinforced polyurea foam is compared to the performance of traditional counterparts in biomechanical impact scenarios. An analytical model was developed to explicate the stiffening effect associated with the reinforcing microspheres. The model results indicate that the reinforced microcell exhibited, on average, ~30% higher stiffness than its barren counterpart.

A systematic approach is presented to achieve the stable grasping of objects through a two-finger robotic hand, in which each finger cavity was filled with granular media. The compaction of the latter, controlled by vacuum pressure, was used to adjust the structural and contact stiffness of the finger. The grasping stability was studied under the concurrent effect of an external torque and applied vacuum pressure. Stable grasping was defined as the no slippage condition between the grasped object and the two fingers. Three control schemes were adopted and applied experimentally to ensure the effectiveness of the grasping process. The results showed that stable and unstable grasping regions exist for each combination of applied torque and vacuum pressure. The two-finger robotic hands can be further improved for applications that require high load-carrying capabilities.

Strain-mediated multiferroic composite structures are gaining scientific and technological attention because of the promise of low power consumption and greater flexibility in material and geometry choices. In this study, the direct magnetoelectric coupling coefficient (DME) of composite multiferroic cylinders, consisting of two mechanically bonded concentric cylinders, was analytically modeled under the influence of a radially emanating magnetic field. The analysis framework emphasized the effect of demagnetization on the overall performance. The demagnetization effect was thoroughly considered as a function of the imposed mechanical boundary conditions, the geometrical dimensions of the composite cylinder, and the introduction of a thin elastic layer at the interface between the inner piezomagnetic and outer piezoelectric cylinders. The results indicate that the demagnetization effect adversely impacted the DME coefficient. In a trial to compensate for the reduction in peak DME coefficient due to demagnetization, a non-dimensional geometrical analysis was carried out to identify the geometrical attributes corresponding to the maximum DME. It was observed that the peak DME coefficient was nearly unaffected by varying the inner radius of the composite cylinder, while it approached its maximum value when the thickness of the piezoelectric cylinder was almost 60% of the total thickness of the composite cylinder. The latter conclusion was true for all of the considered boundary conditions.