PSI - Issue 37

Reza Soleimanpour et al. / Procedia Structural Integrity 37 (2022) 956–963 2 Reza Soleimanpour, Sayed Mohamad Soleimani and Naser Khaled Mohammad / Structural Integrity Procedia 00 (2019) 000 – 000 of concern for engineers. Detection of incipient damage enables tracking damage over the service time line and provides opportunity for decision making and time for logistical planning. 1.1 Guided waves applications Detecting of loose bolted joint has been a topic of interest for many years. Several techniques have been studied and proposed to address this issue such as ultrasonic technique, electromagnetic resonance and impedance technique and there have been a number of significant works in these fields concerning bolt loosening monitoring. Among these techniques, guided waves technique has shown many advantages over the other techniques such as reliability, ability to inspect large areas, sensitivity to small damages and accessibility to different parts of the structure. Over the past few years, guided waves techniques have been successfully tested and used in different structures such as rods, beams and plates for detection, localization and quantification of various types of damages such as notches, crack, delamination and debonding (Soleimanpour et. al (2015, 2016) ). The guided waves techniques are practically divided into two main categories; linear guided waves and nonlinear guided waves. Linear guided waves techniques rely on linear parameters of guided waves such as wave amplitude and wave velocity and attenuation whereas the nonlinear techniques rely on nonlinear parameters of guided waves such as side bands and higher harmonics. The term nonlinear is employed to indicate that the parameters of received signal at frequencies other than the excitation frequency are investigated. Linear guided waves techniques always rely on base line data which may be affected by environmental factors such as temperature whereas nonlinear guided waves techniques usually do not require base line data and rely on nonlinear features of the wave which can provide more information due its higher sensitivity to defects relative to linear features. Research into the use of nonlinear ultrasonic guided waves for nondestructive evaluation is expanding at a high rate because of the great potential benefit that they offer for early detection of defects (Wang et. al (2009)). Soldove et al. (2009) used a CAN model to explain the generation of higher harmonics due to interaction and clapping of the breathing cracks with waves. In this model the crack was modeled with a specific material with stepwise change in material stiffness. In this model when the crack opens, the stiffness for the compressional stress at the crack is zero which results in generation of higher harmonics of incident waves. Soleimanpour et al. (2017, 2018) studied higher harmonic generation of the fundamental asymmetric Lamb wave in delaminated composite beams using numerical and experimental approaches. It was shown that delminations can be detected and located by CAN. Moreover, a hybrid baseline free damage localisation approach was proposed. In another study conducted by the same group, scattering of nonlinear guided waves in composite laminates with delamination was investigated. It was shown that the scattering of nonlinear guided waves in composite laminate is independent of propagation direction of incident waves. One of the most commonly used nonlinear features of guided waves is second harmonic generation (SHG) due to the CAN where repetitive clapping between contact surfaces occur when the guided wave interacts with a breathing defect such as a bolt loosening. During the collisions, the compressive and tensile pressures of the wave close and open the defect, respectively. However, only the compressive pressure of the wave can penetrate through the closed contact surfaces. Therefore the wave becomes half modified or is literally distorted. The wave distortion changes the wavelength and therefore doubles the wave frequency which results in generation of second harmonic. Fig. 1 shows the schematic diagram of contact nonlinearity and the wave modification at a loose bolt joint. 957

Fig. 1 Schematic diagram of contact nonlinearity at a loose bolt