Fulvio Pinto

Multifunctional SMArt composite material for in situ NDT/SHM and de-icing

The past few decades have seen significant growth in the development and application of multifunctional media for the enhancement of material properties, thermo-mechanical and sensing properties. This research work reports a novel approach in which a multifunctional material, herein referred to as SMArt composite, can be employed as a structural health monitoring system for strain sensing and damage detection (SMArt sensing and SMArt thermography), but also as an embedded ice protection tool for structural applications (referred as SMArt de-icing). Such a material, obtained by embedding shape memory alloy (SMA) wires within traditional carbon reinforced plastic composites, relies on the possibility of using the wires both to increase the mechanical properties of composites panels and to exploit their intrinsic electrothermal properties. The electrical resistance variation and the internal power resistive heating source provided by the SMA network, enable a built in and fast assessment of the strain distribution and in situ damage visualization via thermographic imaging. The efficiency of these techniques was experimentally validated on a number of SMArt composite laminates with single and multiple internal defects at various depths. The results showed that strain sensing and damage detection were achieved with high spatial resolution and accuracy, without the need to use large external heaters or complex signal processing techniques.

Recent Advances in Active Infrared Thermography for Non-Destructive Testing of Aerospace Components

Active infrared thermography is a fast and accurate non-destructive evaluation technique that is of particular relevance to the aerospace industry for the inspection of aircraft and helicopters’ primary and secondary structures, aero-engine parts, spacecraft components and its subsystems. This review provides an exhaustive summary of most recent active thermographic methods used for aerospace applications according to their physical principle and thermal excitation sources. Besides traditional optically stimulated thermography, which uses external optical radiation such as flashes, heaters and laser systems, novel hybrid thermographic techniques are also investigated. These include ultrasonic stimulated thermography, which uses ultrasonic waves and the local damage resonance effect to enhance the reliability and sensitivity to micro-cracks, eddy current stimulated thermography, which uses cost-effective eddy current excitation to generate induction heating, and microwave thermography, which uses electromagnetic radiation at the microwave frequency bands to provide rapid detection of cracks and delamination. All these techniques are here analysed and numerous examples are provided for different damage scenarios and aerospace components in order to identify the strength and limitations of each thermographic technique. Moreover, alternative strategies to current external thermal excitation sources, here named as material-based thermography methods, are examined in this paper. These novel thermographic techniques rely on thermoresistive internal heating and offer a fast, low power, accurate and reliable assessment of damage in aerospace composites.

Mechanical response of shape memory alloy–based hybrid composite subjected to low-velocity impacts

One of the most common problems with composite materials is their low resistance to impacts with foreign objects because of their tendency to dissipate impact energy through internal delamination, weakening a large area of the structure. One of the possible solutions to increase impact resistance is to use of shape memory alloy wires in order to exploit their unique superelastic behaviour and the hysteresis that characterises their stress–strain curves. In this study, composite laminates were hybridised by embedding a network of shape memory alloy within the laminate structure and were subjected to low-velocity impact in order to analyse their response in comparison with a traditional composite. Ultrasonic C-scan analysis was undertaken on the samples after the impact in order to estimate the extension of the internal delamination. Results show that the shape memory alloy wires embedded in the laminate are able to absorb a large amount of energy, reducing the extension of the internal delamination.

Preparation and thermomechanical characterisation of graphene nanoplatelets/low-density polyethylene composites

The thermomechanical properties of Graphene Nanoplatelets (GNPs)/low-density polyethylene (LDPE) composites were investigated and characterised to understand the effect of nanoscaled reinforcement in the thermoplastic matrix. Results show that the presence of the filler does not produce a change in the microscopic structure of the polymer. However, on a macroscopic scale, graphene platelets limit the mobility of the polymer chains, resulting in an increase in stiffness and in some cases, strength of the composite. Orientation of Graphene Nanoplatelets in the LDPE matrix was evaluated by testing composites made with two different manufacturing techniques (compression moulding and blown extrusion). A comparison between experimental data and predictions using the Halpin–Tsai model shows that the orientation of the nanoplatelets due to the extrusion process leads to better mechanical properties than those obtained with the randomly oriented graphene resulting from the compression moulding technique.

Characterisation of Ductile Prepregs

This study is focused on the analysis of micro-perforated prepregs created from standard, off the shelf prepregs modified by a particular laser process to enhance ductility of prepregs for better formability and drapability. Fibres are shortened through the use of laser cutting in a predetermined pattern intended to maintain alignment, and therefore mechanical properties, yet increase ductility at the working temperature. The increase in ductility allows the product to be more effectively optimised for specific forming techniques. Tensile tests were conducted on several specimens in order to understand the ductility enhancement offered by this process with different micro-perforation patterns over standard prepregs. Furthermore, the effects of forming temperature was also analysed to assess the applicability of this material to hot draping techniques and other heated processes.

In situ damage detection in SMA reinforced CFRP

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.

Bioinspired pseudo-ductile composite laminates with hierarchical energy absorption mechanism

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.

Impact resistant smart hybrid laminates

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 loads. 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.

Bioinspired twisted composites based on Bouligand structures

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.

Modeling of thermal wave propagation in damaged composites with internal source

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.