PSI- Issue 9

K. Sobianin et al. / Procedia Structural Integrity 9 (2018) 215–220 Author name / Structural Integrity Procedia 00 (2018) 000–000

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1. Introduction Reinforced concrete is one of the most widely used building materials. The safety of reinforced concrete structures is mainly determined by their deformation state. Today automatic deformation monitoring systems is commonly used to ensure structural safety of these buildings and natural objects (Tsvetkov et al., 2013; Shardakov et al., 2014). As a rule, these systems incorporate experimental facilities to provide required information, the analysis of which makes it possible to estimate the deformation state of the structure regarding the development of subcritical and critical processes in its elements. In addition, the results of such analysis are used to make structural safety prognosis. In particular, among these devices, are the systems of sensors that record such deformation parameters as the components of the strain tensor at local points, the displacements and angles of rotation of the structural elements at characteristic locations, the distribution of the vertical settlements of the foundations, and so on. These measurements are generally taken in a quasistatic regime. The results of mathematical processing of these data and mathematical modeling of deformation processes allow researchers to analyze the criticality of the deformation state of the entire structure. At present, the deformation control systems are often supplemented with devices, which allow evaluation of the structure state based on the vibration measurements. Such devices include systems for recording acoustic emission (Merson et al., 2012; Carpinteri et al. 2002), as well as methods of shock-wave (vibration) diagnostics (Bykov et al. 2015). From the viewpoint of the assessment of incipient irreversible damage at local points of the structure, the informative value of these tools is rather high. They enjoy a wide-spread use due to the refinement of hardware (instrumentation pool), which makes it possible to register the parameters of the vibration processes occurring in the structure under monitoring. A very important component that ensures the effectiveness of these tools is a mathematical apparatus that provides an adequate interpretation of the measured vibration parameters (Carpinteri et al. 2002). The results presented in this article refer to the shock-wave vibrodiagnostics of reinforced concrete structures. Here the emphasis is placed on the vibration diagnostics in a "sparing mode", which implies that the force action on the structure during the diagnostics does not cause inelastic deformation in the elements of the inspected structure. This variant of vibrodiagnostics as a part of the deformation monitoring system, is implemented in the following way. The reinforced concrete structure is subjected at certain points to a local impulse force generated by a striker. The mechanical response of the structure to this impact is recorded by a set of sensors (accelerometers, velocimeters, etc.) located at different points of the structure. The response recorded at the time of installation of the monitoring system (and, even better, at the beginning of the lifecycle of the structure) is then compared with the results of measurements made at the current time. This comparison makes it possible to assess the degree of accumulation of irreversible defects and the corresponding changes in the material properties. The manner, in which this comparison is performed, and what criteria are used for this purpose, is a serious issue, which is beyond the scope of this article. The objective of this study is the identification of parameters of the local impulse force, which excites mechanical vibrations of the desired spectrum in the structure, and generates an elastic wave, which have necessary characteristics of the front. One of the main parameters of the impulse action that determine these characteristics is the duration of the impulse action. Therefore, an analysis of the dependence of the impulse duration on various factors and the evaluation of the possibility of controlling such force impulses is the focus of this study. Here, within the framework of the theory of elasticity, the results of the solution of the initial-boundary problem on the dynamic interaction of the

elements of the "striker-gasket-reinforced concrete beam" system are analyzed. 2. Mathematical formulation of the problem and its numerical implementation

The mechanical aspects of the problem studying the interactions between a striker, a gasket and a reinforced concrete beam are schematically represented in Fig.1. In the experiment, the fixed reinforced concrete beam interacted through the elastic gasket with the flying metal ball, which at the initial moment of its contact with the gasket had the velocity equal to V 0 and directed along the normal to the gasket surface. The contact interaction between the ball and the gasket gave rise to the force impulse, which initiated the shock-wave process in the reinforced concrete beam. The characteristic force impulse - time curve is given in Fig.1b.