Plastics have become increasingly important in the products used in our society, ranging from housing to packaging, transportation, business machines and especially in medicine and health products. Designing plastic parts for this wide range of uses has become a major activity for designers, architects, engineers, and others who are concerned with product development. Because plastics are unique materials with a broad range of proper- ties they are adaptable to a variety of uses. The uniqueness of plastics stems from their physical characteristics which are as different from metals, glasses, and ceramics as these materials are different from each other. One major concern is the design of structures to take loads. Metals as well as the other materials are assumed to respond elastically and to recover completely their original shape after the load is removed. Based on this simple fact, extensive litera- ture on applied mechanics of materials has been developed to enable designers to predict accurately the performance of structures under load. Many engineers depend on such texts as Timoshenko's Strength of Materials as a guide to the performance of structures. Using this as a guide, generations of engineers have designed ecomical and safe structural parts. Unfortunately, these design principles must be modified when designing with plastics since they do t respond elastically to stress and undergo permanent deformation with sus- tained loading.