In this paper we report experimental results on the connection between the strength reduction due to accumulated damage in elastic electrically conductive material, resistance variation across the damaged specimen and changes in electro-mechanical impedance (EMI) response. We compare behavior of a cyclically loaded specimen (44,000 loading cycles with the stress amplitude near 0.5 of the yield stress) and intact one (nonloaded). Electro-mechanical impedance testing of the specimens was carried out and the electrical resistances of different parts of the specimens were measured. Finally, both the specimens were tested for ultimate strength under uniaxial tension. We observed qualitative correlation between changes in EMI response, electrical resistance and strength reduction in damaged specimen.
Embedded ultrasonics has demonstrated considerable utility in structural health monitoring of aeronautical vehicle. This active sensing approach has been widely used to detect and monitor cracks, delaminations, and disbonds in a broad spectrum of metallic and composite structures. However, application of the embedded ultrasonics for active sensing of incipient damage before fracture has received limited attention. The aim of this study was to investigate the suitability of embedded ultrasonics and nonlinear acoustic signatures for monitoring pre-crack fatigue damage in aerospace structural material. A harmonic load was applied to structural specimens in order to induce fatigue damage accumulation and growth. Specimens of simple geometry were considered and piezoelectric active sensors were employed for generation and reception of elastic waves. The elastic wave signatures were analyzed in the frequency domain using nonlinear impedance and nonlinear resonance methods. A relationship between fatigue severity and linear as well as nonlinear acoustic signatures was investigated and considered in the damage classification procedure. Practical aspects of the active sensing of the fatigue damage before fracture were discussed and prospective avenues for future research were suggested.
Dynamic measurements are widely used for structural condition assessment and damage detection. A wide range of studies are available on vibration-based detection and identification of fatigue cracks in simple and complex structures. This research explores the application of the electromechanical impedance method and nonlinear resonance measurements to high frequency detection of incipient fatigue damage in aluminum alloy specimens. The electromechanical impedance method relies on the coupling between the mechanical properties of a structure and the electrical properties of attached piezoelectric wafer active sensors (PWAS). This coupling allows structural properties to be inferred from the electrical impedance signature of the sensor. In this study, the electromechanical impedance method is utilized for assessment of material deterioration under cyclic fatigue loads. Aluminum specimens were subjected to increasing fatigue cycles at stress amplitudes below the yield point, and electromechanical impedance signatures were taken at discrete levels of fatigue damage. Linear and nonlinear features of the impedance signatures were compared for different damage conditions. The results show a downward frequency shift of impedance peaks with increasing fatigue load. This frequency shift is observed before visible crack development and fracture. Nonlinear resonance tests were applied to fatigued aluminum samples. PWAS were utilized for transmission and reception of elastic waves at increasing amplitude levels. Variations in structural dynamic characteristics were considered for different excitation conditions and increasing damage severity. This paper discusses damage detection capabilities of each method and provides perspectives for utilizing information on incipient damage for predicting structural performance under known operational loads.
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