Lightening structures by metal replacement: From traditional gym equipment to an advanced fiber-reinforced composite exoskeleton

A redesign procedure used for introducing new functional properties in innovative gym equipment is here reported. It is based on a metal replacement action where a tempered steel was firstly replaced by an aluminum alloy and then by high strength-to-weight fiber-reinforced polymers. The effect of fiber properties (as strength and volume ratio) and plies stacking sequences (as thicknesses and orientation) were investigated. Numerical analyses, done by Ansys ACP, allowed evaluating the stress-strain behavior in realistic boundaries and quasi-static loads, comparing materials and layouts in terms of stiffness. The single-layered shell method with additional integration points was preferred as a technique for discretizing composite laminates. Maximum Principal Stress and Maximum Distortion Energy (Tsai-Hill) were applied as anisotropic failure criteria. Changes in geometry were also considered given their relevant effects on parts and processes. Specifically, this paper is focused on a representative component of the main kinematic chain (the ‘forearm’) and details the different redesign phases for that part. The chosen solution consisted of 14 layers of unidirectional and bidirectional carbon fiber-reinforced pre-pregs, offering a 68 % weight reduction with respect to a solid aluminum component with equal stiffness. The part was manufactured by hand lay-up and cured in autoclave. This redesign practice was extended to the rest of the equipment allowing its transformation into an exoskeleton.