The use of composite materials of various types and forms has become a significant engineering solution for manufacturing a range of mechanical, aerospace and automotive structures, leading to a significant increase in payload, weight reduction, speed, fire resistance, manoeuvrability and durability in comparison with traditional structural materials [1, 2]. However, the investigation of their mechanical behaviour under high-velocity impact loads is particularly challenging owing to the generation of simultaneous and multiple failure phenomena with an inevitable detriment of residual in-plane properties . In this paper, experimental tests and finite element methods (FEM) were performed on plates and T-stiffened laminates under transient dynamic loading, aiming at better understanding the damage tolerance, failure phenomena and impact damage of these composites. The 3D explicit model, based on continuum damage approach, was built using a DYNA3D suite on a framework of an orthotropic constitutive behaviour with stress-based and energetic failure criteria. Numerical results were validated using visual and ultrasound (C-scan) evaluation from dynamic tests. Furthermore, based on the results of the experimental campaign and the numerical model, a numerical study on the optimisation of the structure of the laminate was carried out, considering the inclusion of a secondary hybrid layer within the layup sequence. The mechanical characteristics and geometry of this secondary phase were studied in order to find the best configuration to optimise damage tolerance and improve dynamic response to out-of-plane loads. Several examples with different initial conditions were carried out on the new hybrid material to demonstrate the excellent predictive capability of the numerical model and to study the influence of new hybrid characteristics on the impact responses and impact-induced damages. Numerical results of analyse and their trend were then presented, showing a decrease of -22%, -33% and -94% in damaged area, a decrease of -33%,-57%, - 58% in maximum indentation and an increase of +62%, +134% and +156% in rebound velocity of projectile due to the presence of TPU layers with 0.25 mm, 0.5 mm and 1 mm of thickness respectively.
The large diffusion of Carbon Fibre Reinforced Polymers (CFRP) over the last three decades in multiple industrial sectors is due to their excellent in-plane mechanical properties and their exceptional strength-to-weight-ratio. As the use of CFRP moved from non-structural parts to primary structures however, the intrinsic layered nature of these materials and their consequent weak resistance to out-of-plane solicitations has changed the safety approach used for traditional ductile materials, shifting the design paradigm towards more severe safety margins. This zero-damage manufacturing strategy, necessary to prevent catastrophic failures, led to overdesigned composite parts, preventing the full exploitation of their unique characteristics and limiting their use in harsh environments. Based on this premise, the possibility to manufacture composite laminates able to respond with a pseudo-ductile behaviour when subjected to an out-of-plane load is of crucial importance as it would eliminate the need of overdesigned parts and extend the range of applications available to composite structures. This project is aimed to the design, manufacturing and characterisation of a bioinspired CFRP laminate in which the pseudo-ductility arises from an ordered pattern of discontinuities which are created over the surface of the different layers before the curing reaction. The presence of this carved pattern creates a hierarchical interplay of high-strength carbon fibre segments and elastic soft matrix-rich areas which resembles the interaction between the β-sheets crystalline domains and amorphous helical and β-spiral structures typical of spider silk and other biological structures (e.g. cellulose, hair) which enables a combination of high mechanical strength and elasticity. The effect of different geometrical parameters of the carved pattern such as critical length, shape and dimensions, on the mechanical properties of the laminate have been analysed via Finite Element Analyses in order to identify the optimal configuration of the discontinuities, finding the best trade-off between in-plane and out-of-plane mechanical properties. Samples with different carved patterns were then manufactured and their properties were assessed by subjecting them to three-point-bending test. The internal distribution of damaged areas was assessed via different Non Destructive Techniques and was compared with the behaviour of traditional CFRP. Results showed that the presence of the artificial discontinuities is able to induce pseudoductile behaviour into the CFRP, improving the energy absorption mechanism during out-of-plane solicitations without severely affecting the in-plane properties.
The large diffusion of structural parts made of carbon fibres reinforced polymers (CFRP) in the aerospace and
automotive sectors has highlighted the importance of developing hybrid multifunctional materials characterised by
improved mechanical properties and coupled with non-structural features. Indeed, while due to their high specific
strength and light weight, composite systems are characterised by very high mechanical properties in the in-plane
direction, their intrinsic layered structure makes them very susceptible to low-velocity impacts resulting in Barely Visible
Impact Damage (BVID) that can lead to the critical failure of primarily structures. Based on these premises, the
development of a multifunctional hybrid system can overcome this drawback by tackling this issue from two different
points of view, enhancing the total reliability of light-weight composite parts in order to improve fuel efficiency and
optimise the footprint of new generation aero-structures. Indeed, by including an additional metallic phase within the
structure of a traditional laminate it is possible to develop a smart multifunctional system in which the hybrid phase acts
simultaneously as a reinforcement to enhance the out-of-plane properties of the material and as an intelligent embedded
sensor system able to communicate information about the health status of the part and detect impact events or critical
This work is focused on the design, manufacturing and testing of a hybrid CFRP (H-CFRP) in which the hybridisation is
obtained by including an array of Shape Memory Alloys (SMA) or Copper wires within the laminate. The electrical
properties of the hybrid network is exploited to design a smart sensing system which can be interrogated to monitor the
load distribution on the part and detect critical solicitations in critical points. The low-power system, controlled by an
Arduino microcontroller, is able to monitor the integrity status of the part using each wire as a linear probe to scan
complex structures at a certain frequency, measuring the local change in the electrical resistance from which it is
possible to build a map of the stress distribution. The position of the metallic network along the laminate’s thickness was
determined by analysing the response of different configurations of hybrid samples subjected to Low Velocity Impacts
(LVI) in order to optimise the design of the H-CFRP and enhance the energy absorption. Using the same Arduinocontrolled
Multiplex the smart wires array was exploited as heat source to scan the sample inner structure and
monitoring the variation of the superficial apparent thermal variation with an Infra-Red (IR) Camera, a simulated
delaminated area was detected.
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.
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.
The use of dielectric elastomer (DE) for the realisation of new generation actuators has attracted the interest of many
researchers in the last ten years due to their high efficiency, a very good electromechanical coupling and large achievable
strains [1-3]. Although these properties constitute a very important advantage, the industrial exploitation of such systems
is hindered by the high voltages required for the actuation  that could potentially constitute also a risk for the
In this work we present a DE based active layer that can be used in different macro-scaled parts of industrial equipment
for roto-flexographic printing substituting traditional mechanical devices, reducing manufacturing costs and enhancing
its reliability. Moreover, the specific configuration of the system requires the driving voltage to be applied only in the
mounting/dismounting step thus lowering further the operative costs without posing any threat for the workers.
Starting from the industrial requirements, a complete thermo-mechanical characterisation using DSC and DMA was
undertaken on acrylic elastomer films in order to investigate their behaviour under the operative frequencies and
solicitations. Validation of the active layer was experimentally evaluated by manufacturing a DE actuator controlling
both prestrain and nature of the complaint electrodes, and measuring the electrically induced Maxwell’s strain using a
laser vibrometer to evaluate the relative displacement along the z-axis.
The purpose of this paper is to analyse the possibility to manufacture and verify the self-sensing capability of
composite materials plates with an embedded network of NiTi shape memory alloys (SMA) used as
transducers for structural integrity. Firstly, the thermo-electrical material properties of SMAs were
investigated to assess their capability to sense strain within. The results showed that the electrical resistance
variation provided by the shape memory alloys network enables a built in and fast assessment of the stress
distribution over the entire structure. Then, by transmitting a low amperage current, results in an electric and
thermal flow through the entire SMA network. Using an IR Camera it is possible to capture the emitted
thermal waves from the sample and create an image of the thermal field within the material. Consequently,
analysing the behaviour of the heating curves on different points of the sample, it is possible to identify
potential variation in the apparent temperature of the composite, leading to the identification of damages
within the composite structure.