Due to the wide and ever increasing application of thermoplastic parts in the automotive industry, the measurement and interpretation of their properties must be thoroughly understood before anyone can hope to correctly utilize the results in material selection, product design, and performance analysis while all these can be greatly influenced by the end-use conditions.
Tensile properties of thermoplastics, such as stress and strain at yield, ultimate tensile strength, and Young's modulus, are among the most widely measured and cited mechanical properties for material evaluation, quality control, structure design, modeling, and failure analysis. This paper deals with several major challenges that an engineer may face when attempting to obtain accurate tensile property data for thermoplastics. One such challenge is the trend of automotive industry today to convert from ASTM to ISO procedures for thermoplastics evaluation and product certification.
Our study on the widely used, semi-crystalline polyamides (PA 6) based plastics indicates that, while in most cases the values of tensile properties produced using the two standards are close, difference does exists which might be a result of different specimen geometry, and in some cases different definitions of parameters.
Another challenge is the variation in the material's properties due to the changing environment. In this investigation the tensile properties of PA 6 were studied under two environmental conditions that have most influence on the structures in use: temperature (from -40°C to 150°C) and relative humidity (dry-as-molded (DAM) and 50% RH). The ultimate tensile strength and Young's modulus have been found to decrease significantly as the temperature or moisture level increases. However, the materials become less sensitive to the environment at elevated temperature or with high moisture content.
Results from this paper should help designers to accurately interpret tensile strength and deformation properties for the semi-crystalline thermoplastics in general and to utilize the material parameters at end-use conditions for the structural analysis of thermoplastic components.