Fatigue life prediction of lightweight electric moped frames after field load spectra collection and constant amplitude fatigue bench tests

Abstract

Modern mobility involves increasingly electric powered assisted vehicles; the structural integrity of a lightweight vehicle subjected to field loads for a given mission is a Fatigue Life Prediction (FLP) issue for the manufacturer: small motorcycle companies may prefer a more sustainable experimental approach to a complex a numerical approach.Aim of the present work was the fatigue life prediction of the frame of an innovative electric moped for urban commuting developed by a small manufacturer, in solo and dual riding. Given the mission adopted by the manufacturer, a load based Fatigue Life Prediction was carried out adopting an experimental approach. The moped frame and main components were strain gauged and statically calibrated to collect field load histories in solo and dual riding, including extreme driving events. Constant amplitude fatigue tests were performed on the welded tubular aluminum frame to obtain the fatigue curves at the steering tube node under horizontal loads (N = 8) and at the rear seat support node under vertical pulsated seat loads (N = 8). Measured field load spectra were extended to the design mission and used to predict the minimum required Virtual Load Life Curve (VLLC) able to guarantee the target mission life. A damage summation value appropriate to welded aluminum structures was adopted. The comparison of the Virtual Load Life Curve and the real component fatigue curves enabled to evaluate the frame durability safety factors in the two considered loading conditions and to check the validity of redesign solutions for the weakest component locations.