Development and focusing enhancement of nonlinear air-coupled acoustic technique for damage characterization in materials

Abstract

In recent years, a number of studies within the field of structural health monitoring have focused on micro-damage location in materials. These non-invasive methods are widely used for damage characterization in materials employed in aerospace and automotive fields, such as steel, aluminum and carbon fiber–reinforced plastic (CFRP) composite. Damage such as micro cracks, layer delaminations, corrosion or barely visible impact damage (BVID) can compromise the integrity of structures. Common inspection techniques based on ultrasound cannot detect small defects. However, many techniques focused on nonlinear wave behavior have recently been developed to improve the sensitivity of ultrasonic methods. The nonlinear acoustic approaches proposed in this work rely on the production of further harmonics, which generally occur at multiples of the fundamental (driving) frequency. This phenomenon is related to the strong nonlinear dynamics of flaws due to the friction between crack surfaces. A damaged sample was excited by an Air-Coupled Ultrasound (ACU) device consisting of 88 transmitting sensors and 1 receiver element. The sensor has a central frequency of 40 KHz and focused at a certain point (F = 80 mm). Results confirmed a linear vibration of the intact part of the inspected sample, which results in no higher order harmonic production in the frequency spectrum. On the other hand, a small cracked defect behaves as an active radiation source of a new frequency, twice the value of the input frequency, known as second harmonic (2f0). For nonlinear imaging and damage characterization, the second order nonlinear parameter was studied to quantify these nonlinear features. In conclusion, this research work demonstrated that nonlinear techniques are suitable for numerous classes of defects, such as fatigue cracks and corrosion (micro-cracks).