Influence of Stress State on the Failure Behavior of Cracked Components Made of Steel

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Abstract
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.

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