F2010C010
Fatigue Life Estimation of Joints in Vehicle Body Development Based on a Local Approach and Mesh-independent Modelling
Computational evaluation of load-bearing structures with respect to stiffness and operational strength is a permanent feature of vehicle body development, in the same way NVH simulation and crash simulation are. The efforts to lightweight design increase the requirements to joints and joining techniques. Furthermore, lightweight design and lightweight materials require new joining techniques such as laser welding and self-piercing riveting, for example. Due to the development time available, it is necessary to assess fatigue life also by means of virtual testing. Along with the behaviour of single components this applies especially for their joints.
This lecture presents a method to evaluate spot-wise joints by simulation with respect to stiffness and operational strength. Based on the simulation processes for structure independent spot welds, thoroughly proven in practice, this method has been developed for short laser seam welds (Robscans) and self-piercing riveting.
The target is not only to show global and local stiffness in an appropriate way but to determine also local stress in order to allow the fatigue life estimation on the basis of local stress.
The FEM model shows the spot-wise joints as stiffness model by means of bar and hexahedron elements. The FEM models of the components remain unchanged, while the elements of the joints are applied additionally to the FE model by coupling degrees of freedom. These interconnections (MPC = Multi-Point Constraints) comply with the basic interpolation functions of the elements of the components that are to be joined. Representing correctly the main types of stress of spot wise joints, such as peel tension and/or cross tension and shear tension, is the essential criterion in model validation.
For the fatigue life estimation the local stress is calculated on the basis of the force distribution via the joint. Therefore the forces, that have been calculated in the complete model of the vehicle body, are divided into unit load cases, that are stored in a database, as factors for the linear superposition. The stresses for each unit load case are calculated by parameter models for the corresponding joint and are stored in a joint database. Besides the sheet metal thickness of the components that are to be joined, major parameters of the database are the geometrical form and the material combination at the joint.
Thus it is possible to calculate the stress-time history at the critical notches for each joint by superposition of the stresses corresponding to the unit load cases in the joint database. The subsequent fatigue life estimation is performed using the "critical plane technique". The material fatigue is evaluated by means of Rainflow analysis, hypothesis of damage accumulation and considering the material behaviour (such as micro and macro supporting effect, influence of mean stress).
The presented method is compared to test results with respect to stiffness behaviour and material fatigue.
This abstract is supplemented by a PDF, which can be viewed here.