Abstract

Composite materials are increasingly used in various applications, especially in aerospace and automotive industries owing to their outstanding properties. However, everyone following the composites industry is waiting for a breakthrough in automation and scaling up to mass production. Advanced manufacturing processes of composites include many parameters and necessitates modern process monitoring and quality control techniques. On the other side, due to their heterogeneous and anisotropic nature, composite materials are susceptible to various complex and hidden damages and defects resulting in catastrophic failure. Since composite materials are high-valued structures used in critical applications, an advanced and efficient technique is needed to non-destructively inspect them. Furthermore, as many parameters are involved (process settings, change of material/supplier, etc.), characterization of composite materials is performed regularly. However, the traditional characterization methods are destructive and time-consuming which cannot be applied to in-situ investigations. Also, current inspection techniques are either manual and incapable of real-time and in-situ inspections or are time-consuming and unsuitable for inspecting large structures in industrial applications. Besides, different methods must be employed for process monitoring (e.g., cure monitoring), material characterization, and damage detection. Hence, a versatile approach is needed to perform precise, quick, real-time, and in-situ inspection and characterization of materials.

A novel approach called Laser Ultrasonics (LU) has been proposed to rise to the challenges above. Laser Ultrasonics is a non-contact and non-destructive technique for characterization and inspection of materials in which lasers generate and detect ultrasonic waves. The generation of waves by lasers is performed by optical absorption, followed by the photothermal effect (conversion of light to heat). Dynamic thermal expansion generates impulsive stresses triggering acoustic waves in the material. LU provides much information about the material because of the wide bandwidth and the generation of three different types of waves simultaneously.

However, several challenges and research gaps must be addressed to exploit the full potential of the method and achieve the envisioned goals of the research. The first challenge in the LU is the low signal-to-noise ratio (SNR). This issue stems from the limited laser power applied to the composite material due to the ablation threshold, the lower sensitivity of the laser interferometers, and the high attenuation of waves in composite materials. Another challenge that arises in applying the LU in composite materials is the frequency dependence of the phase velocity (dispersion), which has not been considered in Christoffel’s equation (the equation used for the determination of elastic constants based on phase velocity data). The receiving signals in the LU determine the wave group velocity data, and no closed-form solution relates the group velocity to the elastic constants. Moreover, the low SNR and reflected waves from the boundaries cause difficulties in signal processing of LU.

We propose a new approach using spatial modulation of the laser beam to make the generated acoustic waves converge to a certain point. Finding this particular laser beam shape determines the velocity of elastic waves in all directions from which all the elastic constants can be calculated. This laser beam shape generates strong waves at the convergence point where the laser interferometer detects the signals. Also, it establishes a signal pattern for the intact material allowing us to capture any minor changes in the material. Being a fast, accurate, non-contact, and real-time method, LU is a perfect match for the revolutionary Smart Industries.