The correct prediction of a composite parts’ final performance is of paramount importance during the initial design phase of the manufacturing process. To this end the correct evaluation of the most effective process parameters and their influence on the parts performance is key for the success of the manufacturing process. Our aim with this paper is to provide methodologies for the prediction of the temperature field in thermoplastic composites during thermoforming and to propose a strategy for process parameter selection. We measured the temperature variations over the different thermoforming stages and compared these values with analytical and finite element results. Our results show the accuracy of the predictions and the importance of the correct laminate temperature with respect to the prediction of the parts’ spring-in angle. We discuss the essential features needed for accurate predictions of the temperature fields over the whole thermoforming process at an early design stage and the potential of a Machine Learning procedure based on Artificial Neural Network to aim for the optimum range of process parameters for a desired part performance outcome. In conclusion, we provide essential guidelines for blank temperature predictions, and the benefit of a machine learning-based tool over traditional approaches.

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Fibre metal laminates (FML) are hybrid materials perspective for wind-turbine, containers and marine objects, besides the aerospace industry. During the manufacturing process some faults can occur and can be hazardous for the reliability of FML structures. One of the most critical defects are kissing bonding due to their lack of detectability and strength compared to traditional delamination defect. The quantitative explanation were under consideration, such as loads effects; material properties; prediction of response; fracture analysis. The purpose of this work is the evaluation the impact of this type of defect on the part in-plane and the out-of-plane mechanical properties. It was presented that even responsive NDT methods are not able to detects the kissing bonding defect in FML components. Simultaneously, the kissing bonding impact on mechanical properties in FML is significant. In the case of FMLs with the orientation of the fibre perpendicular to the peel direction there is one failure pattern which is interlayer fracture. Whereas in the case of FMLs with the direction of the fibres longitudinal to the peel direction two failure patterns occur which is interlayer fracture and translaminar fibre crack. Depending on the kissing bonding area width the interlayer fracture in the composite can be observed until kissing bonding defect area and then transmission of the crack to the metal/composite interface through the fibres. In the case of low extension of poor adhesion area, the two parallel interlaminar cracking can be seen, one at the metal/composite interface in poor adhesion area, the second continuous in the composite layer.

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The paper addresses the role played by the cure stage of a vacuum assisted resin transfer molding process in residual stresses generation. The Airstone 780E epoxy resin and Hardener 785H system broadly used in the wind turbine blade industry has been used in this study. The viscous–elastic properties of the resin have been characterized and implemented in a thermo-mechanical FE model. The model has been validated against manufactured [0/90]4 asymmetric laminates. Analysis of residual stresses generation highlighted that compressive stresses generation occurs when the cure is shrinkage dominated and tensile stresses when expansion dominated in the 0° plies. The finding points out that 10% reduction in warpage and 33% reduction in process time can be obtained by selecting cure cycle parameters that allow tensile stresses development during the cure process in the 0° plies.@en

Press forming of Fibre Reinforced Thermoplastics (FRTP) is a widely used manufacturing process. However, in order to boost innovation in FRTP production, new manufacturing strategies have to be implemented. In this context, the multistep forming process represents a promising concept for achieving a higher level of performances by means of customized fibre orientations, fibre types, fibre architecture, and thicknesses, but also higher product functionalities through the combination of different polymers in the same product. In order to improve the functional efficiency of FRTP components, this paper investigates the optimization of a multistep forming process of glass fibre fabric combined with polyetherimide (GF/PEI). The deformation mechanisms encompassing the multistep forming process is here analysed, along with the understanding of the effects of process parameters (e.g. temperature and pressure) over part quality. In particular, the feasibility of the reduction of the cycle time of the process is evaluated by means of active cooling. In addition, in order to foster future industrial application of multistep forming processes, robust and reliable process simulations are presented aiming at reducing development times and improving the overall cost-effectiveness. @en

The manufacturing of Fibre Metal Laminates, consisting of alternating layers of pre-preg fibre and metal is 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 laminate. In this paper/research, non-destructive testing based on ultrasonic inspections are performed on Glare with pre-preg gaps in order to extend and improve the current state of the art to different pre-preg gap widths, depths, and 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 ultrasonic testing. Finally, images of the laminate cross-section have been collected in order to provide a meaningful evidence of the ultrasonic-based analysis. Results from different Glare 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.@en

The quality control of Glare panels manufacturing is an important and complex process including the evaluation of the quality of the basic constituents, namely the aluminium sheets, and the glass fibre reinforced pre-preg. In particular, the handling of aluminium sheets is one of the most critical steps for the manufacturing of Glare laminates. The unintentionally induced deformations, referred to as kinks, can significantly affect the geometry and the mechanical properties of the final laminate. This paper considers the effects of kinks in Glare laminates by developing a comprehensive investigation based on controlled kink manufacturing in aluminium sheets, non-destructive and destructive evaluation of laminates with kinks, and the impact on the compressive ultimate strength. The results contribute to the understanding of the kink induced defects and to define thresholds for improving future automated laminate manufacturing.@en

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 non-destructive 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.@en

During the automated manufacturing of fibre reinforced laminates, defects can be produced. Gaps and overlaps between adjacent prepreg layers can be produced in composites during the tape-layup process. However, the topic is not yet studied for hybrid materials, in which metal sheets and thin prepreg layers lead to different effects due to the defects than in full composites. Here, the effect of gaps and overlaps on the mechanical properties of the Fibre metal laminates (FML) is evaluated. Specimens are manufactured with a specified width of gaps/overlaps and the mechanical performance of the panels is evaluated by some selected mechanical tests. Gaps show to have a considerable effect on the mechanical performance of FML. Compression strength of samples with overlaps was rather increased. Discussions are presented on the influence on each mechanical property according to the severity of the defect (gaps/overlap) and the failure mode(s) under consideration.

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Gaps in pre-preg plies, in a case when adjacent plies do not align perfectly, may represent a significant issue in an automated fibre placement manufacturing process and can be detrimental to the properties of the final laminates. In order to tackle the consequences of gaps in the composite material, a comprehensive research has to be performed. Starting from existing manufacturing processes, the effects of gaps were investigated in different ways, including the first indication of their impact on the mechanical properties. The focus of this paper is to extend the state of the art in gaps in composite materials to hybrid composite materials such as Glare, a member of Fibre Metal Laminates (FMLs). The investigation started with the manufacturing of Glare specimens having gaps with different widths, followed by a non-destructive ultrasonic inspection. Also, an optical evaluation of the gaps was performed by microscope image analysis of the cross sections of the specimens. Results from the ultrasonic inspections revealed the presence of areas corresponding to the gaps areas. The optical evaluations supported the ultrasonic results and showed the presence of fibre waviness and delaminations. Finally, the ultimate tensile strength of comparable specimens was determined, proving the detrimental effect of gaps on the strength of the final laminate.@en

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.@en

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.@en

The detection of delaminations in composite structures due to impacts is a critical issue for any structural health monitoring (SHM) programme. In this work, an experimental campaign is carried out to investigate the presence of delamination induced in carbon-fibre-reinforced-plastic (CFRP) plates by low velocity impacts (LVI). A reduced number of piezoelectric devices for actuation and sensing purposes are employed. The proposed study starts with the choice of the proper positions of the piezoelectric sensor and actuator through the analysis of the numerical curvature mode shapes of the composite laminated plate. A wavelet packet transform (WPT)-based algorithm is applied on the vibrating response of the plates to extract wavelet-based damage sensitive features. Linear Discriminant Analysis (LDA) is then applied on the extracted damage sensitive features to enhance the performance of the proposed delamination identification procedure. Results show the effectiveness of the developed SHM routine when a reduced number of sensors is either desirable or mandatory.@en

The detection of delaminations in composite structures due to impacts is a critical issue for any structural health monitoring (SHM) programme. In this work, an experimental campaign is carried out to investigate the presence of delamination induced in carbon-fibre-reinforced-plastic (CFRP) plates by low velocity impacts (LVI). A reduced number of piezoelectric devices for actuation and sensing purposes are employed. The proposed study starts with the choice of the proper positions of the piezoelectric sensor and actuator through the analysis of the numerical curvature mode shapes of the composite laminated plate. A wavelet packet transform (WPT)-based algorithm is applied on the vibrating response of the plates to extract wavelet-based damage sensitive features. Linear Discriminant Analysis (LDA) is then applied on the extracted damage sensitive features to enhance the performance of the proposed delamination identification procedure. Results show the effectiveness of the developed SHM routine when a reduced number of sensors is either desirable or mandatory.@en

The detection of delaminations in composite structures due to impacts is a critical issue for any structural health monitoring (SHM) programme. In this work, an experimental campaign is carried out to investigate the presence of delamination induced in carbon-fibre-reinforced-plastic (CFRP) plates by low velocity impacts (LVI). A reduced number of piezoelectric devices for actuation and sensing purposes are employed. The proposed study starts with the choice of the proper positions of the piezoelectric sensor and actuator through the analysis of the numerical curvature mode shapes of the composite laminated plate. A wavelet packet transform (WPT)-based algorithm is applied on the vibrating response of the plates to extract wavelet-based damage sensitive features. Linear Discriminant Analysis (LDA) is then applied on the extracted damage sensitive features to enhance the performance of the proposed delamination identification procedure. Results show the effectiveness of the developed SHM routine when a reduced number of sensors is either desirable or mandatory.@en