Durability engineering on component level
Fraunhofer ITWM
The fatigue life estimation of components essentially relies on the stresses and strains induced by the loads acting on the component (section forces). To determine the section forces methods from Multibody simulation (MBS) and Mechatronics are often used, whereas the local stresses and strains may be calculated by FEA methods. Especially in cases, where multiple non-proportional time dependent loads are acting on the component, a pure static analysis is not sufficient or the assessment of the fatigue life. Instead, the local stress-strain histories are needed.
Further Information
Depending on the component itself, the static or modal superposition approach can be used here. The separation of time and space variables enables the calculation of stress-strain histories even for large structures and long periods. However, linear behavior of the structure is required for that procedure. In some cases (e.g. contact problems) the separation principle can be extended to non-linear problems.
If the superposition approach is not applicable transient FE analysis is needed. Here geometric non-linear behavior as well as non-linear material laws can be taken into account. Of course, the computational effort increases rapidly. The simulation of a misuse event like driving over a cleat with a high speed might serve as an application example.
The final fatigue life estimation itself is based on various concepts like the so called stress life approach or the local strain approach.
The methods for the calculation of local stresses and strains can also be used for the design of a rig test setup. By looking at the inverse model and in combination with suitable optimization procedures, we can estimate the minimum number and orientation of hydraulic actuators required for a certain component test.
- Besides the application of such methods in the fatigue life assessment specific features of our work are
- Extension of the fast superposition methods to non-linear applications
- Loading of bolted connections
- Notch stress correction schemes for Low Cycle Fatigue (LCF)
Projects
Designing simplified tests for suspension components
When testing vehicle components on servo-hydraulic test rigs, it has to be made sure that the excitation is as close as possible to the excitation of the component in the driving vehicle. This should be accomplished using a rig configuration which is as simple as possible, e.g. needing as few actuators as possible.
The mathematical modelling and simulation of the component and the rig configurations enable the application of optimisation methods to solve the problem by varying the mount points, actuators and excitation signals.
In order to achieve this goal, the ITWM has developed suitable methods. For statically determinate mountings, the approach even avoids time consuming repetitions of MBS and FE simulations for each new rig configuration using inertia relief modes.
For each configuration, excitation signals are determined and the responses of the component are compared. If necessary, e.g. for more involved fatigue life estimations such as seam or spot weld calculations, dedicated fatigue solvers can be integrated. The assessment of the different rig configurations does not only take the desired hot spots into account, but also avoids the synthetic generation of new hot spots which do not exist in the vehicle configuration.
The automated calculation and assessment of different rig configurations enables the derivation of a good compromise between realistic loading of the component and simplicity of the rig. The approach has been successfully applied in various industry projects so far and is currently further developed with the aid of PhD project together with the ITWM department for optimisation.
