Concept and design
In the concept and design development of elastomeric elements, the focus must be on the interfaces in addition to the required stiffness and service life for the component. In particular, the axles should be able to bear static special load cases without breaking. Here, the boundary conditions should be realistically depicted, and in particular the screw pre-tensioning force and the correct relative stiffness of brackets are of immense importance for the analysis. Furthermore, the contact conditions between the components are considered to obtain details regarding the realistic deformation and stress states in the analysis. For special load cases, an elastic-plastic analysis can be carried out in order to exploit the load carrying capacity of the component.
For drawing the Haigh diagrams, mostly single-stage, uniaxial Wöhler tests are carried out with test specimens. The specimens should have an appropriate size and a clear damage point to ensure a high reproducibility of the results. The Haigh diagram can be viewed as a Wöhler line set under various loads (mean load and amplitude). As strength hypothesis for elastomers, with which the stress on the component is converted into a reference value which is in direct relation with the material failure, the concept of the greatest principal stress and elongation has proved successful, wherein the method of critical cutting plane can be brought to application.
The damage calculation is based on the application of the local concept of fatigue strength calculation taking into account the FEM with the possibility of fully calculating a given load-time function (measurement signal), or synthetic signals from MKS. The key points of the IB-CM calculation routine are thus the strength hypothesis, the physical material behavior, the knowledge of the load capacity of the material, as well as a suitable calculation method for stochastic signals. The analysis of the damage of multi-axial stress signals is carried out for all nodes and integration points of the FE model and only the use of this method makes it possible to close the gap between the requirements of function, space and load.
For the production of prototypes an accurate 3D model of vulcanization geometry is created; in particular the shrinkage of the elastomer during the cooling process after the vulcanization process must be taken into consideration in the numerical calculation in this case. The first production-based prototypes present an important milestone in the development process; on the one hand the theoretical design values are compared to the real measured values for the first time and on the other hand, the approval is issued by the Quality Management division for series production. The accurate prediction of the spring characteristics within the specified tolerances for all six degrees of freedom in a given space is one of the core competencies of IB-CM. Deviations at this stage have a serious impact on the schedule.
Service life test
The most important milestone in the development process is the service life test. In particular, for safety-critical components, it is a common practice today to perform multi-axial dynamic service life tests, which should help ensure that the component may be used for field operation. The test may extend over a period of several days to a few weeks, whereby the smallest of errors in the design may lead to a premature failure of the components. For the statistical validation, at least two components have to be tested, often with an analysis of the test results according to the Weibull method, which is also offered and conducted by IB-CM (keywords: 'Success Run', lifespan, failure slope, reliability, confidence level of the entire set).
Strong competitiveness is characterized by short turnaround times in product development, especially in new developments. The 'trial and error' method often pursued in the past cannot be applied here in any case. Iterative loops, especially in case of premature failure of components during the service life test, must be reduced to an absolute minimum. The application of the damage analysis is thus a MUST for the risk minimization in the run-up to the the test planning, in particular for service life tests with multi-axial stochastic signals. Any weak points of the design can be eliminated with relatively little effort, thereby ensuring an increase in the product quality. IB-CM is ready to assist in trial planning and execution as a competent partner.
Residual service life test
Usually, elastomeric suspension elements can be used for a limited period of time. Usually 6 years and or a mileage corresponding to this time period is expected (in kilometers). Often, however, there is the demand for a higher period of use or mileage, which would bring about a considerable cost benefit for the operator. However, an extension of the maintenance interval cannot be realized without further testing. Even here, the application of the damage analysis has enormous potential. With its help, the reserves of the component can be calculated numerically and the service life tests can be planned for ensuring the mileage still required..