Lifetime-Oriented Structural Design Concepts- P7: At the beginning of 1996, the Cooperative Research Center SFB 398 financially supported by the German Science Foundation (DFG) was started at Ruhr-University Bochum (RUB). A scientists group representing the fields of structural engineering, structural mechanics, soil mechanics, material science, and numerical mathematics introduced a research program on “lifetimeoriented design concepts on the basis of damage and deterioration aspects”. | 138 3 Deterioration of Materials and Structures Fig. . Development of strains in bending 662 Fig. . Degradation process of relevant concrete properties due to tensile loadings 429 process in metals consists of a microcrack initiation and afterwards microcrack propagation phase. The fatigue degradation may culminate into macrocracks and cause fracture after an adequate number of fluctuations or loads cycles. The part of fatigue modeled here is the propagation of the microcracks. Altough most materials on a scale of a few dozen grains are still anisotropic and microplasticty certainly plays a role in the propagation of such microcracks in most cases it is tried to build up a phenomenological continuum damage model based on the small scale yielding approach of linear elastic fracture mechanics. So the propagation of microcracking can be described with stress intensity factors of the cracks near tip field embedded in an isotropic material with the properties of the macroscopic scale. Phenomena of Material Degradation on Various Scales 139 Related cycle ratio VZVf Fig. . Degradation process of relevant concrete properties due to flexural loadings 866 Fig. . Stiffness reduction by high cycle fatigue Brittle Damage by Microcracks As in this context only damaging processes caused by microcracks which are triggered by elastic stresses are regarded no macroscopic plasticity has to be considered. Imagine such a member with growing microcracks undergoing a process in which it is deformed by a total deformation a certain part of this will be elastically recoverable and another part can be induced by damage. When these loads are released the member will have in contrast to plasticity not any remaining permanent deformation. Nevertheless the state of the member could have changed its elastic stiffness could have been reduced by the growth of microcracks. For a process which involves no further damaging the total deformation is an elastic one but starting from
