Charles Joseph Aloisio

A predictive model for impact response of viscoelastic polymers in drop tests

We show for the first time that a classical Hookean viscoelastic constitutive law for rubbery materials can predict the impact forces and deflections measured with a commercial drop tester when a mass, or “tup” with a flat impacting surface is dropped onto a flat pad of commercial impact-absorbing rubber. The viscoelastic properties of the rubber, namely the relaxation times and strengths, are obtained by a standard rheological linear-oscillatory test, and the equation of momentum transfer is then solved, using these measured parameters, assuming a uniaxial deflection of the pad during the impact. Good agreement between measured and predicted forces and deflections is obtained for a series of various drop heights, tup masses, impact areas, and pad thicknesses, as long as the deflection of the pad relative to its thickness is small or modest (<50% or so), and as long as the area of the pad is less than or equal to that of the tup. When the pad area is greater than the tup, forces are higher than predicted, unless an empirical factor is introduced to account for the nonuniaxial stretching of the ring of material that extends outside of the impact area. These results imply that the impact-absorbing properties of a rubbery polymeric material can be assessed by simply examining the materials linear viscoelastic spectrum.

THERMAL ANALYSIS CHARACTERIZATION OF A COMMERCIAL THERMOPLASTIC/THERMOSET ADHESIVE

DSC, TGA and DMA thermal analysis techniques are used to characterize a complex adhesive blend. The chemical and thermomechanical property development shown to follow a two-stage process. Beneficial synergy between these analysis tools is demonstrated in this study.

Quantitative Prediction of Impact Forces In Elastomers

We measure the impact forces and deflections resulting from drop tests of a mass with a flat impact surface onto flat pads of various elastomeric materials, and show that the forces can be predicted quantitatively with no adjustable parameters by using a theory whose only inputs are the linear viscoelastic characteristics of the material, measured in small-amplitude oscillatory deformations. The theory, which models the elastomer as a nonlinear neo-Hookean material, is accurate for several elastomeric solids including polyurethanes, polynorbornene, and poly-vinyl-chlorides (PVCs), over a wide range of impact velocities, masses, temperatures and pad thicknesses. Some steps are taken to extend the model to surfaces which are not flat. The application in mind is the rational design of elastomeric components in impact-tolerant portable electronic equipment.