Finite Element Modeling of Impact-Generated Stress Wave Propagation in Concrete Plates for Non-Destructive Evaluation

Abstract Impact-echo (IE) is a widely accepted and applied non-destructive evaluation technique for quality control and defect characterization in concrete structures. IE is an acoustic method based on the propagation of impact-generated stress waves that are reflected by material defects or interfaces. Numerical simulations using finite element method (FEM) have been used to study the impact-echo response of structural component with complex geometry and boundary conditions.The present study focuses on developing a modeling methodology to simulate stress wave propagation in concrete plates. The significant modeling parameters that influenced the wave propagation and behavior were identified based on the literature review. A set of thirteen simulations were run to study the effect of each parameter. Time histories were recorded at four different locations to represent the waveform in an IE test. Fast Fourier transform (FFT) using Matlab was performed to transform the time-domain displacement histories to frequency-domain amplitude spectra.The impact of steel ball on concrete plate was modeled using a force-time function with amplitude Fmax for a given duration tc. A gradually increasing force-function such as a half-cycle sine cubed was used to reduce the high-frequency ringing observed due to an abruptly increasing force. Similarly, distributing the force over two or more nodes eliminated the localized deformation of the concrete plate surface.The unwanted wave reflections from the mechanical boundaries of a semi-infinite deteriorated the displacement waveform and had an adverse effect on thickness-mode frequency fT. Absorbing layer with increased damping (ALID) boundaries were used to effectively absorb these wave reflection and give a more accurate amplitude spectrum. By implementing these absorbing boundaries, a significant reduction in model and computation cost was achieved. Wave reflections from the free edges are important when modeling stress wave propagation in bounded sections. The foundation layer approach, where the concrete plate is supported by a flexible foundation layer, was used for modeling bounded concrete plates. The relative difference in the stiffness of concrete and foundation layer RK dictated the dynamic response of the system which primarily vibrated in piston-mode. A higher value of RK was useful in separating the piston-mode from the thickness-mode frequencies to obtain a more accurate spectrum. The relative difference in the plate and foundation layer impedances RZ influenced the behavior of propagating wave at the interface. A high value of RZ was used to increase the coefficient of reflection "R " of the interface such that most of the incident compression wave reflected back as tension wave in the concrete plate