In the Department of Mechanical Properties, mechanical properties of specimens and components are investigated and related to other material and component properties, including material behaviour under monotonic and cyclic loading on undamaged specimens, fracture mechanics specimens and components.
Accompanying microstructural investigations are carried out on the initial state and on the loaded state, with the focus on clarifying the failure mechanisms. Models and quantitative descriptions for component failure are derived from these studies, which allow to predict strength and durability under mechanical loading. In cooperation with other working groups of the institute, methods for increasing strength are developed and investigated.
On the one hand, the Department of Mechanical Properties carries out mechanical testing of materials and components within the framework of research projects and on behalf of industrial companies. The focus is on vibrational stress. On the other hand, the scientists of the department Structural Mechanics develop models based on these test results. The models serve to predict the material behaviour. In this respect, the department focuses on three fields of work:
Calculation of the stressability
Taking into account the existing microstructure, the local hardness and the existing residual stresses, a material model for stressability under vibrational stress is being worked on, which can be implemented in the simulation of the process chain with the help of common equivalent stress hypotheses. In the future, it should be possible to consider complex microstructures, such as those occurring in carbonitrided surface layers.
On the basis of the already successfully developed flaw modelling, which allows the calculation of the fatigue strength, a modelling is being worked on which allows the discrete life time prediction at a stress in the fatigue strength area. The locally existing strengths and stresses serve as a basis for the calculation.
Competing failure mechanisms
Furthermore, the description of different types of defects will be transferred to a common model in the future. Different materials fail under vibrational stress at different types of defects (pores, blowholes, precipitates, dispersions, structural inhomogeneities, ...). The model aims at a differentiated description of these defects based on the multiple flaw approach.
The common goal of these fields of work is to verify the generalization capability of the models with respect to material, stress and geometry in order to reduce the testing effort in the future and to implement the finished models as building blocks in simulation tools.
In further research projects the department is working on the investigation of interactions of stresses during the processing of components, on the development of test methods for unusual specimen geometries and on the elaboration of meaningful data structures for the structured storage of research data.
The mechanical properties department has 21 vibration testing machines in different load and frequency ranges and 5 rollover test stands. A wide range of testing capacities is offered, from the testing of large samples in a horizontal pulser with +/- 100 kN at 30 Hz to testing in an ultrasonic pulser with 20 kHz under alternating and pulsating loads. Our technical staff is experienced in performing stress and strain controlled vibration tests. The scientific staff provides support in test planning and evaluation of the tests. The emphasis is on the evaluation and classification of the results. Tests can be supported by digital image evaluation, local temperature measurement and a system for recording the crack growth rate.
On the basis of a well-preserved fracture surface, the history of a crack can be traced back in detailed detective work. Flanked by microstructure investigations and residual stress measurements, the causes of failure and remedial measures are revealed in practical application. In addition to investigations on samples tested in-house, the department also specializes in the assessment of damage caused by vibrational stress. From broken springs to large gearwheels, various cases have already been clarified in cooperation with the metallographic and physical analysis department.