In the last decades, advanced thermoplastics matrix composites (TPCs) were recognised as a valid alternative to thermosetting matrices for many advanced applications. One of the advantages in the use of TPCs is the capability to perform fusion bonding which avoids the use of external joints. Induction welding of carbon fiber reinforced TPCs has gained large interest thanks to the minimum surface preparation required, high efficiency and capability to localise heat at the welding surface. This study relies on a thermal wave technique for the in-situ and real-time evaluation of defects during electromagnetic induction welding of TPCs. The technique is based on a methodology which analyses thermal images acquired in real-time during the welding process to reveal discontinuities from variations in heat distribution. Furthermore, the proposed apparatus is used to conduct post-welding inspections on the damaged area for more detailed defects characterisation. An induction welding device is used to perform the bonding process and different kinds of defects were tested and evaluated. Real-time thermal images of the welding process of TCP samples were obtained by using Infrared (IR) cameras. The recorded data were elaborated and used to locate and evaluate the different kinds of damaged samples. A post-welding analysis of a detected damaged region was performed using heating parameters optimised for the thermography scan. Results show the reliability of the method in detecting and characterising the presence of defects during the welding process using the available heating source without altering the process parameters.
Nonlinear ultrasonic methods for non-destructive evaluation and damage detection rely on the measurement of nonlinear elastic effects, such as amplitude of the second harmonic in the frequency response of the sample, to reveal the presence of surface and internal cracks of various scale and nature. These methods require an amplification system and a high sensitivity ultrasonic transducer to measure nonlinear features, since harmonics are typically an order of magnitude lower than the fundamental frequency. In this work, we investigated various geometrical filters to amplify nonlinear signals and improve nonlinear air-coupled inspections: hyperbolic, cylindrical and conical duct. A 40 kHz ultrasonic speaker and standard Air Coupled ultrasound system arranged with 88 transmitting elements and 1 receiving element were used to conduct the experimental test. The results show that the passive hyperbolic-shaped filter was able to increase the second harmonic response of the damage region of 5db, compared to standard nonlinear inspections, and increases the signal to noise ratio of the measured signal of 11 db. Results shows that higher harmonics generated from instrumentation highly decrease as the wave propagates through the converging horn. Air-coupled inspection confirms the increase at the damage location of the second harmonic of the wave propagating though the diverging horn. The proposed setup could allow more accurate nonlinear air-coupled inspection of complex materials.
Damage such as micro cracks, layer delaminations, corrosion or barely visible impact damage (BVID) could irreparably affect the integrity of the structure. These defects are not ever detectable by the common inspection techniques based on the ultrasonic wave propagation. However, a number of techniques based on nonlinear wave behaviour have been recently developed to improve the sensitivity of ultrasonic methods. The nonlinear acoustic approach proposed in this work relied on generation of new frequency generation due to defects. The spectral changes are caused by nonlinear local dynamics of defects of various scale and nature due to contact between crack surfaces. A standard Air Coupled ultrasound (ACU) system arranged with 88 transmitting elements and 1 receiving element focused on the same point (N=88 mm) with a central frequency (f0) of 41 KHz was used to excite corroded samples. Results showed that the intact parts of the material outside the defect vibrate linearly, i.e. with no greatly frequency variation in the output spectrum, whilst a small cracked defect behaves as an active radiation source of a new frequency component (2f0). For the nonlinear ultrasonic testing, the second order nonlinear parameter (β) was chosen as the nonlinear feature to damage identification. In conclusion this research work demonstrated that nonlinear techniques are suitable for numerous classes of defects, such as fatigue cracks and corrosion (micro-cracks).