Smith, R. and Petty, M., "Creep and Stress Relaxation Evaluation of Virgin and Thermally Aged Glass-Filled Poly(butylene terephthalate) used in Automotive Electrical Connector Applications for Electrically-Powered Vehicles by Dynamic Mechanical Analysis," SAE Int. J. Mater. Manf. 8(3):880-886, 2015, doi:10.4271/2015-01-0603.
Poly(butylene terephthalate) stock with 0, 15, and 30% glass fiber filler, used in the housings of electrical connectors on hybrid and electric vehicles, was examined by creep and stress relaxation for viscous deformation and modulus loss at 25, 75, 125, and 150°C. Sets of virgin and aged samples were examined within each set then compared to each other.Both the virgin and aged samples showed little viscous deformation or modulus reduction at 25°C, which was below the Tg of 40°C. Thermoplastic creep and stress relaxation was observed at the temperatures above Tg with, expectedly, more sample distortion and stiffness loss observed at higher test temperatures and lower filler contents both in virgin and unaged samples.Aged samples became stiffer, and more resistant to irrecoverable strain and stress relaxation than virgin samples indicating connectors to have a “self-healing” aspect through in-service use at elevated temperatures. This phenomenon was noted in literature and verified as arising from thermally-induced secondary crystallization in the aged samples by observation of increased crystalline content using DSC; and analysis of the variation of storage modulus vs. temperature in the aged vs. virgin samples.