Accelerated Test for Suspension Components Durability based on Vehicle Dynamics

One of the most critical subsystems in vehicles are suspensions because they are subjected to high transient loads in different directions. The development of a methodology to asses suspension components durability requires a mixture of different background knowledge, for instance, vehicle dynamics, fatigue, fem analysis, mathematical statistics together with practical experience in industry field. For this reason, it is the result of a collaboration with different departments in AVL List GmbH. The interdepartmental collaboration drew on the need to reduce the duration and the cost of vehicle developments but ensuring the same level of reliability and robustness. To accomplish this task, it’s no more enough to assume a high level of static safety factor and then testing durability when a prototype will be available. To predict suspension component’s life in an early design phase it is necessary to focus on modelling and analyses technique. Suppliers need to have a very precise knowledge of the operating environment of the vehicle and make a design that meets the needs of the customers. This requires a better study on loads and lead to focus on load analysis to predict the damageability of manoeuvres. The necessity to evaluate operating conditions severity without detailed information about the geometry of the structure requires analyses on strains, accelerations, displacements and forces coming from similar vehicle and from multibody simulations. For this reason, the first part of this work focuses on the combination of dynamic modelling (Adams Car), Finite Element Analysis (MSC Nastran) and finite element method fatigue (FEMFAT channelmax). To create a load case database a Toyota Yaris fully flexible multibody model is built in Adams Car and it is simulated across 36 different manoeuvres which characterize the target customer usage of a B-segment passenger car. The damage in the lower control arm and in the upright of the MacPherson suspension is computed for each load case using a quasi-static approach, e.g. inertia relief, because vibrational effects don’t occur. In the second part is illustrated a way to shorten the amount of data collected in time domain to scalar values, which are still able to describe the severity of the load and which allow the use of statistical methods for comparing different operating conditions. Particular attention is paid on the pseudo damage calculation, which consists in a rainflow cycle count of a time signal (e.g. strain, displacement, acceleration, force) and then the pseudo damage is computed considering an idealized Wöhler curve. In the third part is presented a tool to replicate the loads previously simulated in a proving ground on a suspension assembly multiaxial test rig, in order to speed up the durability evaluation. The approach is based on virtual iteration which consists in linearizing the dynamic model and using the obtained transfer function to reconstruct the input from the output evaluating the results through the pseudo damage. In the fourth and last part is presented the ability of track mixing optimization. It defines the most economical test schedule, in terms of track repeats and time, that meets the customers usage profile. The thesis provides an accurate analysis of the proposed methodology to evaluate in accelerated way the durability of suspension components as well as it highlights the advantage of the latter.