A study of fatigue loading on automotive and transport structures

Abstract

It is accepted that defective structural designs are mostly caused by insufficient knowledge of input data, such as material properties or loading, rather than inadequate analysis or testing methods. In particular, loads associated with automotive and transport (trucks, trailers, containers, trains) structures are nontrivial to quantify. Such loads arise from stochastic and ill-defined processes such as driver/operator actions and structure-terrain interaction. The fundamental processes involved with the determination of input loading are measurements, surveys, simulation, estimation and calculation from field failures. These processes result in design criteria, code requirements and/or testing requirements. The present study deals with methods for the establishment of input loading for automotive and transport structures. It is attempted to generalise and unify new and existing techniques into a cohesive methodology. This is achieved by combining researched current theory and best practices, with lessons learned during application on, as well as new techniques developed for, a number of complex case studies, involving road tanker vehicles, light commercial vehicles, industrial vehicles, as well as tank containers. Apart from the above, the present study offers four individual, unique contributions. Firstly, two methods, widely applied by industry, namely the Remote Parameter Analysis (RPA) method, which entails deriving time domain dynamic loads by multiplying measured signals from remotely placed transducers with a unit-load static finite element based transfer matrix, as well as the Modal Superposition method, are combined to establish a methodology which accounts for modal response without the need for expensive dynamic response analysis. Secondly, a concept named Fatigue Equivalent Static Load (FESL) is developed, where fatigue load requirements are derived from measurements as quasi-static g-loads, the responses to which are considered as stress ranges applied a said number of times during the lifetime of the structure. In particular, it is demonstrated that the method may be employed for multi-axial g-loading, as well as for cases where constraint conditions change during the mission of the vehicle. The method provides some benefits compared to similar methods employed in the industry. Thirdly, a complex analytical model named Two Parameter Approach (TPA) is developed, defining the usage profile of a vehicle in terms of a bivariate probability density distribution of two parameters (distance/day, fatigue damage/distance), derived from measurements and surveys. Based on an inversion of the TPA model, a robust technique is developed for the derivation of such statistical usage profiles from only field failure data. Lastly, the applicability of the methods is demonstrated on a wide range of comprehensive case studies. Importantly, in most cases, substantiation of the methods is achieved by comparison of predicted failures with ‘real-world’ failures, in some cases made possible by the unusually long duration of the study.