Flexure and pressure-loading effects on the performance of structure-battery composite beams

The effects of sustained three-point bend loading and hydrostatic pressure on the mechanical and energy-storage performance of three structure-battery beam prototypes were experimentally investigated. The SB beams, designed for unmanned underwater vehicle applications, were fabricated using marine-grade structural composite constituents and commercial rechargeable lithium-ion "pouch" cells. Low-temperature cure materials and multistep processing were used in fabrication to avoid exposing the cells to temperatures above 60celcius. The results showed load relaxation (up to 6-18%) under constant displacement three-point bending within the elastic regime due to viscoelastic shear in adhesive bond layers between components and lamina. Concurrent cell charge-discharge during sustained load bending had a small effect on the load (similar to 1% change or less). Energy storage capacity under hydrostatic pressures up to 2 MPa, equivalent to 200 m ocean depth, showed a 6-8% decrease in capacity. The results highlighted the need for some design changes to improve structure-battery component performance including: exclusive use of high-temperature cure resins (epoxy or vinyl ester) to improve structural performance and enable single-step fabrication, and transverse (fiber) reinforcement to strengthen the interlayer bonds and embedded cell pockets to minimize load relaxation effects and maximize component bending strength.