Studies of transient wave propagation in plates were carried out to establish a basis for the impact-echo technique as a nondestructive test for flaw detection in concrete. The surface displacements caused by stress waves generated by point impact on a plate were calculated using both the Green's function solution and the finite element method; displacement waveforms obtained by the two approaches showed good agreement. Displacement and stress fields in a plate were studied using finite element analysis. It was shown that transient point load applied normal to a stress-free boundary gives rise to P-and S-"wakes" -disturbances trailing the Pand S-waves. The displacement and stress fields in each wake resemble those in the preceding wave.
To develop a basis for a nondestructive test method for heterogenous solids such as concrete, analytical, numerical, and laboratory studies of transient stress wave propagation in solid plates and in plates containing flaws are presented. The technique, which is referred to as the impact-echo method, involves introducing transient stress waves into a test object by mechanical point impact and monitoring reflections of the waves from internal defects and external boundaries using a point receiver located close to the impact point . Introduct ion 1 1.2 Objectives and Scope of Research 1.2.1 Objectives 1.2.2 Scope CHAPTER 2: BACKGROUND 2.1 Basic principles of elastic wave propagation 2.1.1 Wave types 2.1.2 Wave velocity 2.1.3 Reflection and refraction 2.1.4 Diffraction at a crack tip 2.1.5 Attenuation and divergence 2.2 Stress pulses created by elastic impact 2.2.1 Hertz theory of elastic impact CHAPTER 3: REVIEW OF PREVIOUS APPLICATIONS OF ECHO TECHNIQUES TO CONCRETE 21 3.1 24 3.3.1 Pavements and bridge decks 24 3.3.2 Erosion cavities under slabs and behind walls 3.3.3 Dams 3.3.4 Piles 3.3.5 Reactor structures
The objective of the studies presented in this paper was to understand the transient response of thick circular and square bars subjected to transverse elastic point impact. It is shown that the transient response is composed of a number of resonant frequencies caused by cross-sectional modes of vibration. The individual cross-sectional modes and their corresponding natural frequencies were determined using plane strain and three-dimensional finite-element models of circular and square cross sections. The first few modes for both circular and square cross sections are shown. Subsequently, three-dimensional finite-element models of circular and square bars were used to determine the transient response caused by transverse point impact. To verify the results obtained from the numerical models, experimental studies were performed on a 0.4-m-diam circular bar and a 0.3-m square bar. These specimens were representative of full-size concrete columns—typical barlike structural elements. The results of these experimental studies were in excellent agreement with those obtained from the finite-element analyses. A future paper will present the results obtained from numerical and experimental studies on rectangular bars.
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