Accelerated Life Testing of Thermoplastic Polyurethane Encapsulants Used in Underwater Acoustic Sensor

The lifetime of thermoplastic polyurethane (TPU) encapsulants used in underwater acoustic sensor (UAS) was predicted using accelerated life testing (ALT). The TPU specimens for tensile and tear strength tests were aged at six different temperatures, and the strengths of the aged specimens were then measured. The measured tensile and tear strengths as a function of aging temperature were used as ALT data for the analysis of the TPU encapsulant lifetime using ReliaSoft's ALTA software. For the analysis of ALT data, the acceleration models and the life distribution models in the ALTA software were combined. The maximum likelihood estimation (MLE) method was used to find the most reliable acceleration-life distribution model when interpreting ALT data for tensile and tear strengths. It reveals that the Arrhenius-Weibull distribution model best fits the ALT data of the TPU encapsulant. The lifetime of the TPU encapsulant for tensile and tear strength was estimated using the Arrhenius-Weibull distribution model. The estimated results exhibited that the lifetime for tear stress was much shorter than that for tensile stress at 25 degrees C. Tensile and tear stresses are applied simultaneously to the TPU encapsulant during UAS operation. Thus, its replacement time should be determined by its tear strength lifetime.