Predicting plastic part life

Dynamic mechanical analysis helps designers build longer lasting products. Edited by Jean M. Hoffman DMA testing helps determine the effect of different curing conditions on thermoset polyester products. DMA demonstrates the degree of cure based on the stiffness and glass transition temperature of the samples. Optimum cure is important to insure that thermoset materials retain the required stiffness needed at elevated temperatures. A variable sinusoidal stress is applied to a sample and the resultant sinusoidal strain is measured. The relationships between complex modulus, storage modulus, and loss modulus are often shown as a right triangle. The hypotenuse is complex modulus. The tangent of the phase angle from a DMA test equals the ratio loss modulus/storage modulus. A DMA comparison of three polymers — epoxy, polyethylene terephthalate (PET), and polyethersulfone (PES) — shows all have similar deflection temperatures under load (DTUL). However, the mechanical response below and above the DTUL of each is different. This is important when the product will experience short-term temperature excursions. A short-term excursion above the DTUL for PES could result in a product failure because of the sharp drop in storage modulus above 218°C. Dynamic mechanical analysis (DMA) measures mechanical response of materials subjected to periodic stress at operating temperature. Tests can be configured for pure tension or compression, single or dual cantilever, and three-point bend modes. DMA works for a wide range of materials from metals, ceramic, and composites to coatings and adhesives. But it's particularly useful for polymers because of their combined viscous and elastic response. DMA can be set to vary time, temperature, frequency, and deformation amplitude, all of which may adversely affect polymer mechanical properties. This helps designers predict how well a plastic part will perform in the field. DMA applies a sinusoidal stress to the sample and measures the resultant strain. In