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Advanced sandwich composite structures that incorporate foams or honeycombs as core materials, have been extensively investigated and used in various applications. One of the major limitations of the conventional materials used is their weak impact resistance and their end-of-life recyclability and overall sustainability. This paper is focused on the study of the production and mechanical characterization of hybrid sandwich panels using hemp bi-grid cores that were manufactured with an ad hoc continuous manufacturing process. Bi-grid structures were stratified in multiple layers, resulting in cores with different thicknesses and planar density. Sandwich panels made with carbon fibers skins were then subjected to Low Velocity Impact, compression and indentation and the damaged panels were investigated via CT-Scan. Results show that the high tailorability of the failure modes and the very good energy absorption properties of the hybrid material open new exciting perspectives for the development of new sandwich structures that can extend the use of natural fibers into several industrial applications.

Thanks to their mechanical properties and lightness, composite materials, are widely used as structural parts in marine, automotive and aerospace. However, even though they are characterised by high in-plane properties, their out of plane performances are not remarkable, therefore during operating conditions, they can be involved in accidental impacts by runaway debris or tool drop in maintenance operations that can cause barely visible impact damage (BVID). This type of damage is usually related to a particular category of impact events defined as Low Velocity Impact (LVI), characterised by velocities up to 10 m/s [1] . In order to overcome the aforementioned low impact resistance of composites, research is focused on improving the damage tolerance of this class of materials. One solution proposed in literature is the hybridisation process that merges synthetic and natural fibres aiming to obtain a composite with good in-plane and out-of-plane properties [2] , [3] , improving impact performance while guaranteeing resistance and stiffness due to the higher modulus of the synthetic fibres. The aim of this work is to evaluate the impact behaviour of Hemp-Carbon composite panels manufactured by hybridisation, under LVI events. Damping tests and low velocity impact tests at 10 J and 20 J were carried out in order to evaluate the Hybrid configuration damping properties and impact behaviour. Moreover, in order to have a better understanding of the internal damage for each impact energy, non-destructive analyses were performed. Results showed improved damping properties for the Hybrid configuration, which is characterised by a more ductile behaviour if compared to traditional Carbon fibre reinforced polymers (CFRP). Non-destructive tests carried on the Hybrid composite showed, at the same absorbed energy level, a smaller extension of the damaged area (−40% at 10 J and −10% at 20 J) localised at the interface between hemp and carbon fibres.

Due to the introduction of highly restrictive safety and pollution legislations in the railway industry, weight reduction has become an increasingly important topic over the last decade. Carbon fibre-reinforced polymers (CFRPs) constitute an excellent alternative to traditional materials, due to their highly specific in-plane mechanical properties. Their use in railway industry, however, is currently hindered by their weak out-of-plane properties. Bogies and underframes are often subjected to impact loadings caused by objects and debris surrounding the tracks (i.e. ice, ballast) that become airborne during the train transit and impact lower part of the carriage. While metal structures absorb impact energy via plastic deformation, barely visible impact damage can occur in CFRP, weakening the component, and often leading to catastrophic failures. This work proposes a method for the improvement of impact absorption performance of railway composite structures via the addition of a thermoplastic polyurethane (TPU) coating to CFRP laminates. The thermomechanical behaviour of the thermoplastic layer was investigated with dynamic mechanical analysis and differential scanning calorimetry analysis to optimize the manufacturing process, while damping tests were carried out to demonstrate its unaltered energy absorption ability in the final manufactured structure. TPU/CFRP plates (150 × 100 mm2 of in-plane size) were subjected to 2, 3 and 5 J impacts, and the results were compared with those of traditional CFRP laminates. Non-destructive test (NDT; i.e. C-scan, phased array) and compression-after-impact test were carried out on the impacted samples to assess the damaged area and residual in-plane mechanical properties. Results show that the TPU layer modifies the energy absorption mechanism, preventing the propagation of damage within the CFRP and resulting in undamaged samples even at the highest energy. To predict the TPU/CFRP impact behaviour and identify the best process parameters to optimize impact energy absorption, a finite element model was developed and validated using experimental data. The comparison showed good correlation, and a fine approximation of the different impact mechanisms was observed with a maximum error of 5% between experimental and simulated output values. The experimental and numerical results show that the TPU/CFRP laminates constitute a novel solution for the manufacturing of lighter and safer railway composite structures.

Ultrasound non-destructive testing (NDT) is a common technique used for defect detection in different materials, from aluminium to carbon-fiber-reinforced polymers (CFRPs). In most cases, a liquid coupling medium/immersion of the inspected component is required to maximize impedance matching, limiting the size of the structure and materials. Air-coupled inspection methods have recently been developed for noncontact inspections to reduce contact issues in standard ultrasonic inspections. However, transmission of ultrasound in air is very inefficient because of the enormous impedance mismatch between solids and air, thus requiring a signal amplification system of high-sensitivity transducers. Hence, the captured signal amplitude may not be high enough to reveal any wave distortion due to defects or damage. This work presents a design of a holey-structured metamaterial lens with a feature size of λ/14 aiming at improvement of acousto-ultrasonic imaging using air-coupled transducers. The required effect is obtained by matching geometrical parameters of the proposed holey-structured metamaterials and the Fabry–Perot resonance modes of the structure. Transmission tests have been conducted on different fabricated metamaterial-based structures, to assess the frequency component filtering of the proposed method in both acoustic (f = 5 kHz, 20 kHz) and ultrasonic range (f = 30 kHz, 40 kHz). Results showed an improved sensitivity of damage imaging, with an increase in amplitude of the design frequencies of the lens by 11 dB. Air-coupled inspections were conducted on a stress-corrosion cracked aluminum plate and impacted CFRP plate using the holey-structured lens. Results showed an improvement in the damage-imaging resolution due to a wave-amplitude increase across the defective features, thus demonstrating its potential as an efficient and sensitive inspection tool for damage-detection improvement in geometrically complex components of different materials.

Composite materials structures are particularly susceptible to the damages induced by low-velocity impacts that may result in Barely Visible Impact Damages (BVIDs), which can hardly be identified through visual inspection. These damages are particularly dangerous, since they can critically reduce the mechanical properties of the impacted structures. In this work, the damage induced in impacted long and short fibre composite specimens has been experimentally evaluated by means of Non-Destructive Technique (NDT) inspections. The damages size and location have been evaluated by means of ultrasonic testing to assess the influence of fibres aspect ratio (long and short fibres), fibres material (carbon and glass), volume fraction, and impact energy for low velocity impacts on composite specimens. Considerations about the failure mechanisms arising as a consequence of the impact event and their interactions have been finally introduced.

This work is focused on the design and production of hybrid laminates in which the reinforcement is a combination of carbon and hemp fibres. Four different stacking sequences were considered and pure hemp and pure carbon laminates were fabricated as references. Three-point bending, interlaminar shear, damping and impact tests at different energy levels in the range of 5–20 J were carried out to understand how the presence and the position of hemp fabrics affects the mechanical response. It was pointed out that it is possible, for example, to tune the flexural modulus, the absorbed energy and the damage extension after impact very close to the one of pure carbon, by placing the hemp layers close to the neutral axis. Otherwise, to improve the absorbed energy in both flexural and impact dynamic conditions, the best result was achieved when the hemp layers are placed on the laminate top side.

The paper investigates experimentally and numerically the response of a smart hybrid thermoplastic composite plate subjected to low-velocity impact. The response of the composite plates embedded with shape memory alloy (SMA) wires embedded is investigated. The SMA wires were embedded within a fabric layer of the composite laminate placed in the middle plane of the laminate. By embedding SMA wires into composites an increase of the damage resistance and ductility of composites structures was observed with respect to conventional composites structures. Moreover, a clear increase of the composites toughness and higher energy levels of absorbed energy before failure was also observed.

The coupling between structural support and protection makes biological systems an important source of inspiration for the development of advanced smart composite structures. In particular, some particular material configurations can be implemented into traditional composites in order to improve their impact resistance and the out-of-plane properties, which represents one of the major weakness of commercial carbon fibres reinforced polymers (CFRP) structures. Based on this premise, a three-dimensional twisted arrangement shown in a vast multitude of biological systems (such as the armoured cuticles of Scarabei, the scales of Arapaima Gigas and the smashing club of Odontodactylus Scyllarus) has been replicated to develop an improved structural material characterised by a high level of in-plane isotropy and a higher interfacial strength generated by the smooth stiffness transition between each layer of fibrils. Indeed, due to their intrinsic layered nature, interlaminar stresses are one of the major causes of failure of traditional CFRP and are generated by the mismatch of the elastic properties between plies in a traditional laminate. Since the energy required to open a crack or a delamination between two adjacent plies is due to the difference between their orientations, the gradual angle variation obtained by mimicking the Bouligand Structures could improve energy absorption and the residual properties of carbon laminates when they are subjected to low velocity impact event. Two different bioinspired laminates were manufactured following a double helicoidal approach and a rotational one and were subjected to a complete test campaign including low velocity impact loading and compared to a traditional quasi-isotropic panel. Fractography analysis via X-Ray tomography was used to understand the mechanical behaviour of the different laminates and the residual properties were evaluated via Compression After Impact (CAI) tests. Results confirmed that the biological twisted structures can be replicated into traditional layered composites and are able to enhance the out-of-plane properties without a dangerous degradation of the in-plane properties.

Nonlinear ultrasonic non-destructive evaluation (NDE) methods can be used for the identification of defects within adhesive bonds as they rely on the detection of nonlinear elastic features for the evaluation of the bond strength. In this paper the nonlinear content of the structural response of a single lap joint subjected to ultrasonic harmonic excitation is both numerically and experimentally evaluated to identify and characterize the defects within the bonded region. Different metallic samples with the same geometry were experimentally tested in order to characterize the debonding between two plates by using two surface bonded piezoelectric transducers in pitch-catch mode. The dynamic response of the damaged samples acquired by the single receiver sensor showed the presence of higher harmonics (2 nd and 3 rd ) and subharmonics of the fundamental frequencies. These nonlinear elastic phenomena are clearly due to nonlinear effects induced by the poor adhesion between the two plates. A new constitutive model aimed at representing the nonlinear material response generated by the interaction of the ultrasonic waves with the adhesive joint is also presented. Such a model is implemented in an explicit FE software and uses a nonlinear user defined traction-displacement relationship implemented by means of a cohesive material user model interface. The developed model is verified for the different geometrical and material configurations. Good agreement between the experimental and numerical nonlinear response showed that this model can be used as a simple and useful tool for understanding the quality of the adhesive joint.

SMArt Thermography exploits the electrothermal properties of multifunctional smart structures, which are created by embedding shape memory alloy (SMA) wires in traditional carbon fibre reinforced composite laminates (known as SMArt composites), in order to detect the structural flaws using an embedded source. Such a system enables a built-in, fast, cost-effective and in-depth assessment of the structural damage as it overcomes the limitations of standard thermography techniques. However, a theoretical background of the thermal wave propagation behaviour, especially in the presence of internal structural defects, is needed to better interpret the observations/data acquired during the experiments and to optimise those critical parameters such as the mechanical and thermal properties of the composite laminate, the depth of the SMA wires and the intensity of the excitation energy. This information is essential to enhance the sensitivity of the system, thus to evaluate the integrity of the medium with different types of damage. For this purpose, this paper aims at developing an analytical model for SMArt composites, which is able to predict the temperature contrast on the surface of the laminate in the presence of in-plane internal damage (delamination-like) using pulsed thermography. Such a model, based on the Green’s function formalism for one-dimensional heat equation, takes into account the thermal lateral diffusion around the defect and it can be used to compute the defect depth within the laminate. The results showed good agreement between the analytical model and the measured thermal waves using an infrared (IR) camera. Particularly, the contrast temperature curves were found to change significantly depending on the defect opening. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

This paper presents a nonlinear elastic wave tomography method, based on ultrasonic guided waves, for the image of nonlinear signatures in the dynamic response of a damaged isotropic structure. The proposed technique relies on a combination of high order statistics and a radial basis function approach. The bicoherence of ultrasonic waveforms originated by a harmonic excitation was used to characterise the second order nonlinear signature contained in the measured signals due to the presence of surface corrosion. Then, a radial basis function interpolation was employed to achieve an effective visualisation of the damage over the panel using only a limited number of receiver sensors. The robustness of the proposed nonlinear imaging method was experimentally demonstrated on a damaged 2024 aluminium panel, and the nonlinear source location was detected with a high level of accuracy, even with few receiving elements. Compared to five standard ultrasonic imaging methods, this nonlinear tomography technique does not require any baseline with the undamaged structure for the evaluation of the corrosion damage, nor a priori knowledge of the mechanical properties of the specimen.

The low-velocity impact response of a range of fiber metal laminate (FML) panels was investigated through testing and finite element simulations. The objective of this study was to understand the impact-damage resistance of these novel composites, so that they can be designed optimally for impact-resistant aircraft applications. The FML panels were made up of aluminum alloy 7475 T761 and unidirectional S2 glass/epoxy oriented in a cross-ply configuration. Experimental tests were performed using a free-fall drop dart testing machine. The plate specimens were constrained on a circular edge by the clamping fixture. The shape and the nature of the damage inflicted by impact were evaluated using both destructive cross-sectional microphotography and nondestructive ultrasonic techniques. The tests showed that FML laminates are capable of absorbing energy through localized plastic deformation and through failure at the interface between the layers. In particular, delaminations occurred in the back face of the aluminum-alloy sheet and its adjacent fiber-reinforced epoxy layer and in between adjacent fiber-reinforced epoxy layer. The finite element code, LS-DYNA3D, was used to perform numerical simulations of low-velocity impact to predict the complex damage propagations. The computed post-impact deformed shapes and damage patterns were found to be fairly close to experimental results.

Strengthening and repairing existing reinforced concrete structures is often more economical and sustainable than rebuilding them. Today the most commonly used technique here fore is the application of externally bonded Carbon Fibre Reinforced Polymers (CFRP). Still, they do not answer the need for a temperature resistant, fire safe and economical solution. High performance glass fibre Textile Reinforced Cements (TRC) can offer an answer to these drawbacks. A validation of this TRC technique against the existing and established one of CFRP is still needed. The Digital Image Correlation (DIC) and Acoustic Emission (AE) measuring techniques are applied to monitor the bending, cracking and failure behaviour of a reference beam, a similar beam with additional external CFRP reinforcement and a similar precracked and non-precracked beam externally reinforced with TRC. Four-point bending tests indicate the successful complementary use of DIC and AE and prove that TRC as an external reinforcement for concrete beams actually works.