Research was initiated to measure the characteristics and properties of materials to support a valid simulation for general design purposes. Cushioning systems are used for personal protection in sports and transportation and for the safe transport of fragile materials. Design costs can be reduced by replacing prototype construction with simulation procedures.
Materials impact characteristics are in response to transient dynamic loading involving a sequence of structural processes during the period of contact. In most applications the engineering system responds in an overdamped configuration to eliminate oscillations. The cushioning component, however, is usually a viscoelastic, low-density substance with distinctive characteristics and properties. Cushioning is defined as the redirecting of motion under a controlled level of deceleration. The damping function is separate, being a dissipation of kinetic energy to terminate motion.
A variety of foamed plastics, including polypropylene, polystyrene, and polyurethane formulations were tested in the form of homogeneous blocks with dimensions of 3.8 by 3.8 by 2.5 cm (1.5 by 1.5 by 1.0 in.). Flat-surface impact was employed with drop heights ranging to 122 cm (48 in.). Voltage signals from an accelerometer in the impactor and a force cell below the specimen stage were recorded using a digital storage oscilloscope. These data were processed separately for analysis.
Specimen characterization also included measurement of density and stress relaxation and static testing at rates allowing effective specimen compliance. The objective of the experimental measurements was to determine the density, damping, and structural stiffness to provide the needed information for the deformation process in the governing differential equation Mü + C˙u + Ku = R where ü,
Simulation was performed by finite element (FE) structural analysis developed in prior research. The FE program provided transient, dynamic analysis using nonlinear structural stiffness and damping. The FE model included the corresponding size, shape, and properties of the impactor, specimen, and stage in an appropriate manner. Simulation results were matched with measured responses to establish the validity of the simulation procedure.
Additional experimental drop tests showed pronounced differences between the characteristics of different materials. These observations and specific test procedures allowed conclusions to be drawn relative to the nature of the mechanical and structural responses at different stages of cushioning. These include separate effects of inertial stiffness, dynamic stiffness (due to damping), and structural stiffness occurring when motion is arrested.