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The effect of microcracks ahead of a macrocrack on the post-peak behavior of concrete-like quasi-brittle material is studied. The critical length of a microcrack is estimated by considering a small element near the macrocrack tip and defining the critical crack opening displacement of the microcrack that exist in the interface region between the aggregate and cement paste. A fracture model is proposed to predict the post-peak response of plain concrete. This model is validated using the experimental results for normal-strength, high-strength and self-consolidating concretes available in the literature. Through a sensitivity analysis, it is observed that the elastic modulus of concrete and the fracture toughness of the interface have a substantial influence on the critical microcrack length.

Simulation has become a prerequisite in engineering and science today for visualization of ideas and concepts. In non-destructive evaluation, simulation is increasingly used to show how an inspection method functions with regard to the component to be inspected and is even used for determining the probability of detection of a respective flaw with regard to the inspection method applied. Probability of detection in non-destructive evaluation is optimized in a way that the best sensor positions, as well as sensor tracking paths, can be found through simulation. In classical non-destructive evaluation, a transducer or transducer array can be virtually moved over the surface of a component to be inspected until a full capture of the component's surface and hopefully volume is achieved in terms of the inspection process. However, with structural health monitoring, no movement of the transducers is possible in case those become an integral and hence fixed part of the component considered. Determining the optimum position of a respective structural health monitoring transducer network can therefore only be achieved through optimization procedures, where numerical simulation is possibly the only viable solution to get this done. Establishing a numerical simulation platform for structural health monitoring purposes has been the major objective of the recently completed INDEUS (Integration of Non-Destructive Evaluation-based Ultrasonic Simulation) project, which is described in this article. The open simulation platform includes different simulation tools, where the requirements and options for further extension of those tools and different test cases applied for validation so far are described. The target is to even simulate real complex structures such as applied in civil, aeronautical, and other engineering disciplines made of metallic and polymer-based monolithic and composite materials where the digital models are inherited from traditional computer-aided design and finite element-based designs. This lays the ground for determining the probability of damage for a given loading condition and structure, and the propagation of guided waves in the structure considered for an undamaged and a damage tolerant condition. From those simulation results, the determination of an optimum configuration of sensing transducers for a given set of actuating transducers is then shown for a guided wave-based structural health monitoring system solution to be designed allowing the tolerable damage to be detected reliably.

A linearized stress intensity factor (SIF) is derived for concrete through a multiscale approach by considering the predominant process zone mechanisms such as aggregate bridging and microcracking. This is achieved by considering a bridging zone and a microcrack at the macrocrack tip. The bridging zone resists the crack growth through aggregate bridging mechanism. The SIF thus derived is further used in developing an analytical model which predicts the entire crack growth curve for plain concrete by making use of the concepts of dimensional analysis and self similarity in conjunction with the human population growth model. This model is validated using experimental data reported on normal strength, high strength and self consolidating concrete. Through sensitivity analyses it is shown that the specimen size plays an important role in the fatigue crack growth process of concrete. (C) 2017 Elsevier Ltd. All rights reserved.

The fracture process of concrete is characterized by various toughening mechanisms that exist at the macro crack tip. In this study, the crack growth resistance due to the bridging of aggregates (defined as bridging stress) is evaluated by relating the crack opening displacements at the macroscopic scale to the mesoscopic one by considering the fracture toughness and the elastic modulus of the interface between the coarse aggregate and the mortar. The influence of specimen size and the stress ratio on the bridging stress is studied. The effect of the bridging stress on the fatigue crack growth rate is predicted and the results are found to agree well with the experiments for normal and micro concrete. The residual strength of a damaged beam is computed in terms of the moment carrying capacity by considering the bridging resistance offered by the coarse aggregates. (C) 2016 Elsevier Ltd. All rights reserved.