One of the decisive factors influencing the safety of components is the capacity for plastic deformation of the material employed. This depends not only on the actual material properties, such as reduction of area or notch impact energy, but also on the stress conditions prevailing in the component. With sufficiently sharp transitions of geometrical form, or at cracks, such high multiaxial stress states can arise in components, that in spite of excellent plastic deformation capability of the malterial, practically deformationless fractures are inevitable. If one generates from the principal normal stresses (σ1, σ2, σ3) the multiaxiality quotient q, which represents a characteristic quantity for the degree of multiaxiality of the stress state, the effect of the stress states on the strength and deformation behavior of a component can be estimated. With the aid of the Sandel fracture theory, which includes the von Mises yield theory as a special case, the critical q value qc, which characterises the stress conditions leading to cleavage fracture if q < qc, can be calculated. The fracture mechanics evaluation of the sharply notched specimens of dimensions similar to components shows no dependence of the effective crack initiation value on the specimen size or stress state, since at the load free crack tip, plane stress conditions generally prevail. The further failure process after crack initiation in the form of stable crack extension is very strongly controlled by the stress state. This phase could also be estimated from consideration of the pattern of the q value in the remaining cross section. The investigations have shown that the multiaxiality quotient q, which characterizes the degree of multiaxiality of the stress state, represents a characteristic quantity with which, in combination with fracture mechanics methods, the failure behavior of components may be estimated, even with respect to stable crack extension.

The characterization of interlaminar fracture—with the goal to obtain a database that can be used in design—is demonstrated for a first generation graphite/epoxy composite widely used by European aircraft manufacturers. Critical energy release rates for Mode I and Mode II failure have been obtained from static tests using double cantilever beam, end notched flexure, and transverse crack tension specimens. An interaction criterion for the mixed mode case is formulated based on the results from mixed mode bending tests. Fatigue tests have been carried out to determine the Paris law parameters for pure Mode I, pure Mode II, and mixed mode conditions as well as threshold energy release rates that could be used as a design limit in a no-growth concept. In accordance with the static case, an interaction criterion is formulated for the crack growth rate under mixed mode conditions. Delamination progression in more complex specimens has been measured, and mixed mode energy release rates have been computed along the delamination fronts. Results lie well within the scatter band of the Paris law as obtained by the specimens employed for characterization. This confirms that the data obtained from the characterization of interlaminar fatigue growth can be applied for predictions in design.

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The strength and deformation behavior of specimens and components is, on the one hand, influenced by the local state of stress and strain, and on the other hand, by the chemical composition and the microstructure of the material used. Using two different steels, it was investigated how far it is possible to predict the failure behavior of specimens and components qualitatively and quantitatively by means of local approaches. For this purpose, two methods differing considerably from the basic idea were chosen. For the description of the failure behavior, so-called damage models were used. These damage models try to describe numerically the process developing microscopically and finally leading to fracture by means of continuum mechanical approaches in order to calculate the macroscopical failure behavior. The results show that for ductile materials, the damage models allow a very accurate calculation of smooth and notched specimens and components. The efforts presently required for the calculation are, however, still very high. Analyses using fracure mechanics approaches (J-integral) in combination with the local stress states (multiaxiality) were performed to describe the failure behavior. With this approach, it was attempted to calculate crack initiation and maximum load of pre-cracked specimens and components. The fracture mechanics methods are preferred for cracked components if an engineering estimation of crack initiation and maximum load only is required since the calculation efforts of the fracture mechanics methods are much lower than those of the damage models.

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