Comparative Analysis of Strain based Fatigue Life Obtained from Uni-Axial and Multi-Axial Loading of an Automotive Twist Beam

Twist beam is a type of suspension system that is based on an H or C shaped member typically used as a rear suspension system in small and medium sized cars. The front of the H member is connected to the body through rubber bushings and the rear portion carries the stub axle assembly. Suspension systems are usually subjected to multi-axial loads in service viz. vertical, longitudinal and lateral in the descending order of magnitude. Lab tests primarily include the roll durability of the twist beam wherein both the trailing arms are in out of phase and a lateral load test. Other tests involve testing the twist beam at the vehicle level either in multi-channel road simulators or driving the vehicle on the test tracks. This is highly time consuming and requires a full vehicle and longer product development time. Limited information is available in the fatigue life comparison of multi-axial loading vs pure roll or lateral load tests. In this paper, an attempt is made to understand the effects of multi-axial loading vs uni-axial loading on the twist beam through strain gauges. This was then compared to the actual strains measured in the vehicle in service. Strain based fatigue life estimation has been used to predict the fatigue life. Predicted life of the component with uni-axial load, multi-axial loads and at the vehicle level have been compared. With this, a better validation of the twist beam can be done at an early phase of development.