In this paper, the detection of delaminations in carbon-fiber-reinforced-plastic (CFRP) laminate plates induced by low-velocity impacts (LVI) is investigated by means of Auto-Regressive (AR) models obtained from the time histories of the acquired responses of the composite specimens. A couple of piezoelectric patches for actuation and sensing purposes are employed. The proposed structural health monitoring (SHM) routine begins with the selection of the suitable locations of the piezoelectric transducers via the numerical analysis of the curvature mode shapes of the CFRP plates. The normalized data recorded for the undamaged plate configuration are then analyzed to obtain the most suitable AR model using five techniques based on the Akaike Information Criterion (AIC), the Akaike Final Prediction Error (FPE), the Partial Autocorrelation Function (PAF), the Root Mean Squared (RMS) of the AR residuals for different order p, and the Singular Value Decomposition (SVD). Linear Discriminant Analysis (LDA) is then applied on the AR model parameters to enhance the performance of the proposed delamination identification routine. Results show the effectiveness of the developed procedure when a reduced number of sensors is available.
Gaps and overlaps between pre-preg plies represent common flaws in composite materials that can be introduced easily in an automated fibre placement manufacturing process and are potentially detrimental for the mechanical performances of the final laminates. Whereas gaps and overlaps have been addressed for full composite material, the topic has not been extended to a hybrid composite material such as Glare, a member of the family of Fibre Metal Laminates (FMLs). In this paper/research, the manufacturing, the detection, and the optical evaluation of intraply gaps and overlaps in Glare laminates are investigated. As part of an initial assessment study on the effect of gaps and overlaps on Glare, only the most critical lay-up has been considered. The experimental investigation started with the manufacturing of specimens having gaps and overlaps with different widths, followed by a nondestructive ultrasonic-inspection. An optical evaluation of the gaps and overlaps was performed by means of microscope image analysis of the cross sections of the specimens. The results from the non-destructive evaluations show the effectiveness of the ultrasonic detection of gaps and overlaps both in position, shape, width, and severity. The optical inspections confirm the accuracy of the non-destructive evaluation also adding useful insights about the geometrical features due to the presence of gaps and overlaps in the final Glare laminates. All the results justify the need for a further investigation on the effect of gaps and overlaps on the mechanical properties.
The manufacturing procedure of Fibre Metal Laminates (FMLs), such as Glare, consisting of alternating layers of pre-preg fibre and metal sheets is relatively complex, and defects can be introduced during manufacturing. The automation of the manufacturing steps is vital for future Glare production, and specific defects have to be considered. Among them, gaps between pre-preg plies represent a potential risk for the mechanical performances of the final laminate. In this paper/research, non-destructive testing (NDT) based on ultrasonic inspections are performed on Glare laminates with pre-preg gaps in order to extend and improve the current state of the art to different pre-preg gap widths, depths, and specimen lay-ups. Firstly, gaps were introduced in Glare specimens. Then, a conventional C-scan ultrasonic inspection is performed. In order to overcome the current limitations of a top or planar view of the laminates, the evaluation of the depth of the gaps in the laminates is performed by means of phased array pulse-echo ultrasonic testing (PAUT). Lastly, images of the laminate cross-section cut-outs have been collected in order to provide a meaningful evidence of the ultrasonic-based analysis. Results from different Glare lay-ups and gap widths and depths show the accuracy of the proposed investigation which is able to provide a detailed assessment of the gap occurrences also for very thin laminates, paving the way for portable and faster quality control strategies for future automated Glare manufacturing.
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