主動式缺陷偵測之智能結構的初步探討 ─ 面外波傳

Abstract

本論文初步探討以應力波主動偵測結構系統內部損傷的可行性及波傳理論基礎。結構在使用期間受到外在負荷作用而致使材料內部損傷，使得結構的有效勁度減損，由勁度矩陣係數的減損量可以評估結構損傷的方向性及嚴重程度。本文將結構損傷區域視為均勻之異質內嵌散射體，以時諧波傳的散射公式及 Born 近似理論，計算散射體外部母材結構的散射波傳，模擬實驗量測資料。為簡化計算模型，本文以一維橫向 SH 剪力波在兩種介質組成之結構內的波傳為對象，應用簡單體法反算出散射體的幾何尺寸及剪力模數的減損。數值模擬結果顯示散射體的幾何尺寸會影響均質散射體之勁度矩陣係數的反算收斂值。由於 Born 近似的精確度與波長相對散射體特徵長度比值成正比，散射體尺寸愈大，Born 近似的精確度會下降，結構損傷所造成的勁度減損估計值也會較小，以致收斂誤差較大，因此結構損傷尺寸宜先行鑑別。

An investigation of the feasibility and theoretical background for active damage detection of smart structures by SH waves is presented in the thesis. Structural damages due to in-service loading often result in degradation of the effective stiffness. The degradation in stiffness components relates to severity and orientations of the damages. In this study, the damaged region was modeled as a homogeneous inclusion with material properties different from those of the matrix. The scattered waves from the heterogeneous inclusion carry information about stiffness and dimension of the obstacle. They were formulated by scattering formula for time-harmonic waves in two-component systems. An inversion scheme based on simplex was used to seek the optimal values of stiffness and dimension of the obstacle. The data for inversion were given by the scattered waves simulated by scattering formula. Numerical investigation showed that dimension of the scatterer had significant influence on the convergence of the stiffness by use of the Born approximation if both dimension and stiffness were inverse parameters. The Born approximation achieved better accuracy only for larger ratio of wavelength to characteristic length of the obstacle. Poor estimations of the degradation of stiffness were caused by large obstacle dimension. It concludes that the damaged area must be identified before inversion for stiffness degradation.