Ruhr University Bochum
Multiscale Durability Mechanics of Materials
The durability of civil engineering infrastructure made of porous materials is considerably affected by the accumulation of damage induced by time variant external loading in conjunction with physico-chemical processes (e.g. freeze-thaw action, chemical dissolution processes, chemical expansive reactions, reinforcement corrosion). These phenomena are, to a large extent, controlled by the transport of fluids and ions within the (cracked) porous structure, which is characterized by a large range of spatial scales from the nm to the mm scale. Adequate computational models in durability mechanics need to account for the multiscale and multiphase character of these phenomena along with their mutual interactions, considering the large range of spatio-temporal scales involved in the description of transport, physical and chemical processes as well as fracture.
For the modeling of transport in porous materials with distributed microcracks, a multi-level approach is adopted. At the level of the multiphase porous material with its hierarchical pore structure, methods of continuum micromechanics are used to compute effective transport properties. A cascade scheme, based on the concept of self-similarity, is proposed. In a subsequent level, incorporating the homogenized solution for the porous material, effects of distributed micro-cracks, their orientation and density on the transport process are accounted for. To enable model-based material design, a multi-level model is developed for fiber reinforced cementitious composites. Current extensions include multi-level micromechanics damage models for prognosis of Alkali-Silica Reaction in concrete structures. For the modeling of propagating macro-cracks, we are investigating various methods including Embedded Crack methods, Extended Finite Element Methods (XFEM), variational interface models and peridynamics methods.
The cascade micromechanics approach to model the overall properties of intact and microcracked porous materials allows to characterize the threshold volume fraction of microcracks beyond which there is a sharp increase in the overall transport properties and loss of material integrity with regards to the load carrying capacity. Model predictions confirm the experimentally observed increase in water uptake of cyclically loaded concrete structures in which the overall increase is attributed to distributed microcracking. Application of continuum micro-mechanics fracture models at multiple scales of concrete has recently showed, that the ASR induced deterioration of concrete structures is characterized by the existence and interplay of two damage mechanisms at the microcscale. Propagating cracks are predicted with high fidelity by means of a novel variational interface crack model, in which the fracture path is obtained from energy minimization.
Methods of micromechanics in conjunction with advanced poromechanics models and discretization techniques constitute a key step towards i) more re-liable predictions of long-term deterioration and life time of structures subject to chemo-physical actions, ii) more efficient rehabilitation methods and iii) the performance-based design of engineered cementitious materials with improved durability properties. As specific modeling components, the cascade micromechanics scheme provides explicit formulae for pore-space topology and micro-crack dependent ion diffusion in porous materials; poromechanics models allow for the description of interactions between transport, phase change processes such as freezing, deformations and damage of porous materials. The new variational interface crack models allows for the coupling with fluid transport in discrete fractures and was successfully applied to the analysis of hydraulic fracture.