Rigidity, strength and damage tolerance of fiber-reinforced components can be analyzed precisely with the aid of complex calculations. The calculation methods often focus on thin-walled structures on the basis of laminate theory and semi-empirical design tools for a number of special situations. Strength is highly dependent on the areas of a component where loads are introduced into the component, around drilled holes or reinforcing structures, where strong load concentrations and multi-axial stresses arise. Component failure occurs in the form of small cracks that can ultimately lead to an accumulation of critical damage. If damage development and its microstructural causes are understood in detail, it then becomes possible to load the material safely to its limits. In many practical cases, the load-bearing capacity of a material can be increased appreciably through specific microstructural designs. New material and component models deal directly with microstructural damage in the form of discrete micro and meso cracks within the unidirectional fiber layers or as minuscule delaminations between layers of differing orientation. This is the only way to accurately handle the mechanical interactions between the heterogeneous material structure and the developing crack system in all their complexity and to have a detailed effect on them.

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Compared to classical strength calculations, fracture mechanics evaluations involve the assumption that there is a defect in the component. These defects are a result of the manufacturing process (casting, welding) or of component use (fatigue), and can be postulated in advance if the component has been designed to be fault-tolerant based on the threshold limits determined in non-destructive tests. Fracture mechanics evaluation concepts are constantly under further development and have now become a fixed part of national and international guidelines (SINTAP, FITNET, FKM guidelines). Current fracture mechanics approaches and computer programs apply these concepts to a wide range of technology issues. The Fraunhofer IWM has taken this on board in the development of advanced analytical solutions for crack issues and calculation tools, and provides licenses to users in the fields of research and industry. The VERB fault evaluation program is particularly relevant to the energy technology, plant construction and aeronautics industries. Other programs – FracSafe and ERWIN – have been developed together with representatives of the VDMA and the rail industry and modified to solve specific issues.

IWM VERB 8.0 fault evaluation program

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Special elements known as analogous models and the relevant input data for these analogous models are needed in order to reliably describe the failure of spot-welded and lasered joins in crash simulations. The IWM has developed a simple, efficient analogous model and a process by which to determine the failure parameters for spot-welds. A pressure-dependent, viscoplastic material model that deals with failure according to the Johnson-Cook approach models adhesive bonds in which the parameters are defined by sample substances and joins and has been calibrated for an analogous model. The models have been verified by simulating tests on spot-welded and glued component-like samples. A detailed damage-based model has also been developed in addition to the analogous spot-weld failure models.

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