Development of Accelerated Life Testing Method for Bone Conduction Devices

Abstract

This thesis investigates the development of an accelerated lifetime testing (ALT) methodology for Cochlear’s bone conduction hearing devices (BCDs). An ALT is a testing method in which a product is exposed to higher-than-normal stress conditions to accelerate ageing and failure mechanisms. The purpose is to evaluate long-term reliability and predict product lifetime within a significantly shorter testing period. The work was motivated by Cochlear’s need for a robust, data-driven ALT methodology. The implementation of this framework provides significant engineering and business value by reducing testing costs, accelerating time-to-market for future products, and supporting verification of regulatory compliance standards. Several theoretical ALT methods and models were evaluated. The aim was to develop theoretical acceleration models together with practical experimental methods capable of identifying long-term performance degradation caused by age-related failure modes in adhesives, nylon thread-locking materials, and piezoelectric components. An ALT concept based on thermal and electrical overstressing was selected. Individual overstress limits for each component were then determined through tailored practical experiments designed around the specific failure mechanisms of each component. A total of 16 experiments were conducted, with a total sample count exceeding 80. The experiments identified safe accelerated operating ranges of 2-5 VRMS and 40-70 °C. Due to time constraints, obtaining long-term ALT degradation results was outside the scope of this thesis. However, validation tests was initiated using conservative inputs of 4.3 VRMS at 70 °C to verify the functionality and stability of the developed test setup. The developed ALT framework contributes to more reliable predictions of long-term actuator performance and supports earlier validation of future design changes. Lastly, the methodology is highly scalable, allowing multiple actuator variants or models to be tested simultaneously.