Cyclic Loading Experiment for Characterizing Foam Viscoelastic Behavior
Several open-cell flexible foams, including aged polyurethane foams, were mechanically characterized over a temperature range of −40 to 20 °C. Quasi-static compression was performed to obtain the stress-strain behavior of the foams. The stress-strain rela
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Cyclic Loading Experiment for Characterizing Foam Viscoelastic Behavior Wei-Yang Lu, Matthew Neidigk, and Nicholas Wyatt Abstract Several open-cell flexible foams, including aged polyurethane foams, were mechanically characterized over a temperature range of 40 to 20 C. Quasi-static compression was performed to obtain the stress-strain behavior of the foams. The stress-strain relation is nonlinear, but typically there is a small range of linear behavior initially. Compressive cyclic loading at different amplitudes and frequencies of interest (20–60 Hz) were applied to measure foam’s hysteresis properties, i.e. stiffness and energy dissipation. The cyclic characterization includes foams with different amount of pre-strains, some are beyond the initial linear range as occurred in many applications. Keywords Flexible foam • Viscoelasticity • Nonlinear viscoelasticity • Pre-strain • Soft materials
16.1
Introduction
Flexible foams are used in packaging applications for shock and vibration isolation during shipping and transportation. A common scenario is that the foam component deforms slowly to support the weight of the object when packing. During the transporting environment, the package will be subjected to dynamic load, for example from bumpiness of the road. The loading is vibratory in nature and its amplitude is typically low. The loading profile is known or can be measured. Some critical engineering questions about the packaging are: what are the resonance frequencies and how much force is transmitted through the foam to the object? The foam’s behavior is known to be very complex. Modeling and analyzing the problem using non-linear viscoelastic analysis is very challenging. To circumvent this difficulty, the foam can be considered to be undergoing a small amplitude vibration superposed onto a larger compression [1]. The analysis can be simplified by approximating the foam behavior to be linear over the small amplitude of strain of interest; therefore, only linear analysis is needed to analyze this small amplitude vibration problem. The approximated linear viscoelastic properties of the pre-stressed foam, however, need to be characterized experimentally before a validated flexible foam model is developed. This paper presents the experiments performed to obtain linear viscoelastic parameters of several flexible foams at various frequencies, temperatures, and pre-strains.
16.2
Foam’s Nonlinear Viscoelastic Behavior
Figure 16.1 shows typical compressive stress-strain behaviors of a flexible open-cell foam TF5070-10 [2], which is 10 pcf (pound per cubic foot, lb/ft3), at different strain rates 0.1, 100, and 200 s1. The constant strain rate stress-strain curve is generally nonlinear. The loading portion shows typical elastomeric foam behavior with three regimes of initial linear elasticity, elastic-buckling plateau, and densification [3]. Young’s modulus E (the initial slope of the curve) and the buckling stress (the boundary separates the linear elastic regime from that of elastic buckling) are clearly rate
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