Parametric Study on Impact-Echo Phase
Date Issued
2016
Date
2016
Author(s)
Hsu, Ying-Yan
Abstract
The impact-echo test can be adopted to detect the defects or inclusions in concrete structures. The conventional impact-echo analysis applies the Fourier transform to the surface response of the target structure due to an impact of a steel ball. Then, the magnitude spectrum is used to determine the frequency of the echo signals. Although the magnitude spectrum may disclose the existence of an interface in the structure, it cannot tell whether the interface comes from a crack or a rebar. Such information is crucial in the safety assessment of the structure. It is also necessary in the determination of interface depth because the impact-echo equations for crack and rebar differ. This research examines the feasibility of the impact-echo phase analysis in the recognition of rebars and cracks. The idea is based on the phase change of the reflected wave. As a wave encounters a hard interface, it reflects with a phase change of π, but as it encounters a soft interface, there is no phase change. Hence, the phases of the echo signals from a crack and a rebar should disagree. Firstly, Fourier transform was applied to the idealized signals. It is found that the phases of crack echoes are indeed close to 0, while the phases of rebar echoes are close toπ/2 . Then, numerical and model tests were conducted to verify the results. The influence of the impact source, depth of interface, and the size of inclusion on the impact-echo phase are also studied. It is found that the phase offsets of crack and rebar echoes mostly fall within two separated ranges, ( -π,π/4) and ( π/4 ,π), respectively. By the parametric study, it is found that the impact-echo phase method works under the following conditions. Firstly, when the echo frequency equals to central frequency of the impact source, one would get satisfactory results. However, if the echo frequency exceeds twice central frequency of the impact source, the phase offsets would get worse results. Secondly, as the length-depth ratio of the crack exceeds 0.2 or the radius-depth ratio exceeds 0.4, one would get worse phase offsets. Hence, it is suggested that the impact-echo test be conducted using proper impact ball. Construct both the magnitude and phase spectra in the Fourier analysis. Use the magnitude spectrum to determine the echo frequency and use the phase spectrum to determine the echo phase. Then, useπ/4 as the decision line to judge the type of inclusion. As such, one can get both the type and depth of the inclusion correctly.
Subjects
Impact echo method
Nondestructive test
Phase
Crack
Reinforcing Bars
Type
thesis
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