Internal delamination in fiber-reinforced composites is difficult to detect and nearly impossible to repair by conventional methods. Bioinspired, microvascular self-healing strategies based on sequestering reactive liquids within hollow conduits have shown promise for repairing internal damage and extending lifetimes of laminated composites. However, difficulties in achieving in situ mixing of two-part healing agents, polymerization times on the order of hours/days at room temperature, and propensity for flow blockages from cross-contamination are research challenges that have limited the adoption of this technology. This paper describes an interdisciplinary research effort to develop self-healing microvascular composites using a one-part, photo-reactive chemistry. The new platform employs microvascular networks for functional fluid transport combined with optical waveguides for rapid, light-activated healing. An epoxy-based, cationic photochemistry was developed to polymerize upon sub-hour exposure to visible light. The one-part cationic chemistry is able to achieve higher recovery ( 75%) in mode-I fracture toughness compared to a commercial free-radical photochemistry ( 55%) and two-part epoxy/amine system ( 65%). In addition to providing targeted light delivery for photo-conversion, the optical waveguides are also intended to serve dual purpose as a self-sensing conduit to relate internal polymerization with mechanical self-recovery. Towards this in situ sensing target, Raman spectroscopy is conducted to show chemical conversion along with fracture recovery increases with increasing light exposure. This novel approach to integrated self-sensing/healing in structural composites outlines a new paradigm in multifunctional material design.
The aim of this work is to introduce both passive and active approaches for achieving advanced computational properties for enabling new metamaterial applications. More specifically, these approaches achieve computational capabilities via (i) passive architectures that consist of simple micro-sized beam elements organized within multi-stable switching mechanisms that mimic mechanical logic-gates, or (ii) active architectures that consist of embedded actuators and sensors that utilize external power and control to train the architecture's properties as a mechanical neural network. Applications include power-free compact materials that can perform calculations in extreme environments that would damage conventional electronic chips or aircraft wings that can learn to exhibit optimal air foil shapes as they are exposed to differing flight conditions in real-time.
Recent catastrophic events involving fires in multi-storey buildings such as the Grenfell Tower fire (UK, 2017) involving combustible aluminium composite panels (ACPs) have led to substantial loss of lives. Combustible panel cores and insulation materials, mostly polyethylene, drastically facilitates the propagation of fire along the height of the structure which imposes challenges in fire safety. The purpose of this research was to investigate the propagation of temperature vertically in modern facade systems with the use of ACP cladding subjected to both static and dynamic heat fluxes. Three different types of ACP with different combustibility namely FR, PE and NC, were tested in the experiments. It was found that the degradation of PE samples for increasing heating regime started at a heat flux lower than that for static conditions. Moreover, the total energy reaching the surface of the sample by that time for increasing regime was lower (6542 kJ/m2) than for static (7600 kJ/m2).
The effect of water absorption on the explosive air blast response of composite laminates representative of the materials used in naval ships is investigated. The materials examined were woven carbon fibre-vinyl ester and glass fibre-vinyl ester laminates. The laminates were submerged in artificial seawater for increasing periods of time up to and beyond saturation. Subsequently, their deformation response when impulsively loaded by the airborne shock wave generated by an explosive charge was determined. The effect of increasing the peak overpressure and impulse of the explosive blast on the amount and types of damage to the laminates before and after seawater immersion was investigated. The amount of blast-induced damage sustained by the laminates was dependent on the types of fibre. The glass fibre laminate experienced much less delamination damage compared to the carbon fibre composites for different seawater immersion conditions. The deterioration to the blast resistant of both types of laminate was due to plasticisation of the polymer matrix and fibre-matrix interface, which reduced the mechanical properties.
This paper presents the effect of multiple ballistic impacts on the structural performance of composite honeycomb sandwich panels. Carbon fibre reinforced plastic and the Nomex honeycomb core were utilised to manufacture the sandwich specimen. Three different projectile calibres were fired to conduct ballistic impact tests in a shooting range. Damage inflicted by small weapons and anti-aircraft ammunition fragments were simulated by firing live (multiple) projectiles into the sandwich specimens. The non-intrusive techniques such as an optical microscopy, X-ray inspections, and computed tomography techniques have been applied to analyse the degree and patterns of damage. Inplane edgewise compression tests were conducted to investigate the structural performance of the intact and ballistically damaged panels. The reduction in the load-resistance ability caused by the multiple ballistic damage has been assessed for the panels under consideration and compared with intact sandwich panels.
The high mechanical properties to density ratio of composite materials make them very attractive for transportation industries, where an important key factor is the energy consumption and the capability of increase capacity. In this regard, the use of light materials as composites could decrease the weight obtaining energy savings and make possible the increase of payload. The main problem to implement composite materials within the railway sector is that each component and assembly in the complete train needs to fulfil the Fire, Smoke and Toxicity (FST) requirements described in standard EN 45545. At present, there are few resins that comply with this regulation, due to the recent incorporation of this standard, which was approved in 2013. The resins, which comply with this standard, are highly doped increasing the viscosity. A high viscosity matrix is more difficult to process, making that the fabrication of the laminates becomes a challenge. A preliminary selection of materials developed for railway industry was done in this work. Furthermore, an extensive characterisation was made to find the best material as possible for the different railway applications. Using prepreg-based materials, a comparison between two different processes (autoclave and furnace) was carried out. The main aim is to get a preliminary evaluation and compare the different materials between each other.
Infrared thermography is one of the effective non-destructive testing methods for damage characterization and identification in structural materials. Infrared thermography induces a temperature variation on the specimen and monitors the surface temperature to detect defects deep inside the structure. It offers advantages such as being non-contact inspect method, scanning large surface area and recording in real time. Although various techniques have been developed for infrared thermography, lock-in thermography (LT) and pulse thermography (PT) are the most preferred ones due to their rapid detection, in-service applicability. LT method uses a sinusoidal heat waves in different frequencies whereas PT, employs an instantaneous heat pulses to excite the specimen temperature and monitor its evolution to identify the defects and manufacturing flaws. In this study, both lock-in and pulse active thermography methods are used to detect different type of defects namely delamination, liquid ingress and debonding in a glass/phenolic prepreg with NomexTM honeycomb core sandwich composites which is a widely used material in aviation industry. The results are presented comparatively on the basis of advantages and disadvantages.
Fibre-resin-interface is an important composite parameter for the mechanical properties of fibrereinforced plastics. An insufficient interface between fibres and resin leads to early debonding between both materials and therefore premature failure in the overall composite. Moreover, the interface characteristic is essentially driven by the fibre sizing. Here, several fibre sizings exist for carbon fibres, which are just suitable either for thermosets or for thermoplastics. It is becoming exciting, when a thermoset as well as a thermoplastic is implemented in one common composite and an additional carbon fibre woven fabric lays in between both resin system. Then, a trade-off regarding the carbon fibre including the fibre sizing need to be done to optimize the hybrid composite system. In an experimental testing study, the fibre-pull-out test was used to make a statement regarding the interface properties of different fibres and resin systems, especially thermoplastics like PEI, PVDF and PA6. Here, the testing devices, FIMABOND and FAVIMAT+, were used to realize the tests. To evaluate the ideal fibre embedding conditions, rheological tests were done with the thermoplastics. Furthermore, the tensile strength of the fibre material should be known, because it can influence the embedding depth. Some embedded fibres were scanned by an x-ray microscope to ensure the quality of the specimens and to optimize the embedding cycle. Here, design recommendations were given for the ideal fibre embedding with FIMABOND.
Placing electronics sensor modules at high temperature regime is getting increasingly important for accurately monitoring performance life of aircraft engines or other sensitive thermal components. Electronics sensor modules capable of operating at high temperature also eliminates parasitic electronics cooling accessories and thus enables both weight and cost savings [1]. Usually in high temperature electronics modules, the electronics chips (dies) are bonded to the packaging board (heat sink) via a metalized dielectric substrate (heat spreader) in between, known to be are the directly bonded copper (DBC) or directly bonded alumina (DBA). In this work, a materials solution for the CTE mismatch issues in DBC is presented with two-phase amorphous alloy composition via Molecular Dynamics (MD) simulation. A two-phase Cu/Zr amorphous alloy composition of 0-45% Cu composition appears to show promise to match with the CTEs of ceramic substrates (such as AlN). This type of hybrid material composition is thus expected to provide a possible desirable material solution towards enhancing the fatigue life in high temperature electronics modules.
Especially in the automotive industry, continuous fiber reinforced thermoplastics are currently experiencing a steadily increasing demand. Thermoplastic matrix systems promise shorter cycle times and improved joining and recycling characteristics when compared to fiber reinforced thermosets. In order to process continuous fiber reinforced thermoplastics, semi-finished parts are usually produced first. These are afterwards heated and post-processed in a back injection molding process. This means that design elements such as load transfer elements and ribs are injected onto the usually flat semi-finished part. The effect of Joule heating can be exploited to heat the part. The electrically conductive carbon fibers are heated by an applied voltage. The temperature is then distributed through heat conduction into the part's thermoplastic matrix. By using Joule heating, high heating rates with low energy consumption are possible. For the industrial use of Joule heating of continuous fiber reinforced thermoplastics, further correlations between influencing parameters must be investigated and specified. The part's shape in particular has a decisive influence on the quality of heating. Within the scope of this study, heating experiments are compared using a thermographic image system. In principle, rounded corners and small changes in shape result in homogeneous temperature distributions. Complex shapes should be avoided and design elements such as gaps, thickness steps and corners designed small compared to the overall area of the part.
Textile Reinforced Concrete (TRC) is an innovative composite material that possesses excellent properties including ease of manufacturing, highly compatible to other existing inorganic-based construction materials and most importantly fire resistant which offers a potential material for building facades. Textiles such as glass, carbon, basalt are used as the internal reinforcement in a thin concrete matrix to form TRC. In TRC, the matrix fails before the tensile capacity of the fibres is reached. Fibres will help to bridge the cracks formed in the matrix, thus improving the serviceability capacity and limiting the crack width. This research investigated, for the first time, the behaviour of high-performance textile reinforced concrete (HSTRC) with different mixes and reinforced configuration. The HSTRC samples were also exposed to different elevated temperatures to examine its response to flexural deflection. Results showed that the layers of reinforced textile improved the flexural strength by 28% while the exposure to elevated temperature significantly affected HSTRC flexural strength. Further investigation on the interfacial bonding between the reinforcing textile and the cementitious matrix at elevated temperature is recommended.
Transverse Young’s modulus of carbon fibres is an important material property for micromechanical modeling and design of carbon fibre reinforced composites. To accurately measure their transverse Young’s modulus is of special importance for applications in novel multifunctional devices, such as structural composite batteries. However, experimental measurement of their transverse Young’s modulus is still largely lacking due to experimental challenges. In this study, we successfully prepared high quality longitudinal cross sections from a commercial carbon fibre using precision ion milling in a combined focused ion beam and scanning electron microscope (FIB/SEM) instrument. These cross sections were then directly used in an atomic force microscope (AFM) and a nanoindentation equipment to measure the transverse Young’s modulus. Here, the entire procedure is described in detail. In particular, the most critical aspects for specimen preparation are identified and discussed.
Combustibility of cladding materials in the facade system is the main issue in the prefabricated building envelopes as combustible materials spread the fire rapidly. Investigation of new nanocomposite material to be replaced the existing combustible cladding's material is the purpose of the current study. Sepiolite-phosphate (SepP) has been made and different contents of it (3%, 5%, and 10%) synthesized with resole Phenolic resin in order to study the dispersion of the SepP nanopowders in the phenolic resin nanocomposite. Helium ion microscope (him) and Scanning Electron Microscope (SEM) showed that the SepP has been made successfully and the SepP 3% had a better dispersion in the Phenolic nanocomposite.
Short fiber reinforced thermoplastics are used extensively due to their high mechanical properties and low processing costs. Long fiber reinforced thermoplastics (LFT) show an even more interesting property profile and are used more and more often for structural parts. However, processing is not as simple and their anisotropic properties resulting from the fiber microstructure pose a challenge with regard to the engineering design process. To reliably predict the structural mechanical properties of LFT, it is necessary to investigate a method that allows comprehensive consideration of the existing fiber microstructure within the engineering design process. For this purpose, a micromechanical approach developed at IKT has been extended to include the necessary process variables and the associated models.
Filament preparation is a prerequisite procedure in composites 3D printing based on fused deposition modelling (FDM). However, the impact of filament preparation on the properties of resultant FDM products has not been reported. In this work, the short glass fibre/polylactide (GF/PLA) composites were made into filaments via two different workflows: a 3-step workflow of melt compounding - granulation - extrusion, or a 2-step workflow of material blending and extrusion. The filaments were then used for FDM. For the 3-step workflow, fibres were apparently shortened after melt compounding, especially for the 4 mm fibre bundles that were shortened by > 90%. The 2-step workflow worked smoothly to prepare filaments containing 0.1 mm fibres. However, the 4 mm fibre bundles led to jammed extrusion and the 2-step workflow was frequently interrupted. In contrast to 3-step workflow, the 2-step workflow only led to FDM products made with different filaments were examined for the fibre length/orientation as well as their flexural properties. More than 55% of short fibres aligned with the FDM extrusion paths, and over 80% fibres were parallel with the horizontally deposited layers. The FDM products made with 2- step filaments showed superior strength and stiffness, as higher degree of fibre alignment and greater fibre length led to improved flexural properties. Greater fibre length was beneficial for higher strength and stiffness of the additive-manufactured polymer composites, and the fibre length was more influenced by the filament preparation than the final additive manufacturing process.
The sustainable filler biocarbon was added into nylon-6/polypropylene blends, where the composites were successfully prepared by melt extrusion. The influence of the biocarbon content and size on the morphology and properties of these composites were evaluated. Using scanning electrical microscropy (SEM), it was found that the biocarbon was primarily distributed in the nylon-6 phase due to the similar polarity of the biocarbon. Therefore, the selective distrubution of biocarbon greatly influenced the morphology and properties of these formulations. Results indicated that the high loading of biocarbon in the nylon-6 phase resulted in a decreased coefficient linear of thermal expansion (CLTE) and increased viscosity of the nylon-6/polypropylene blends.
This paper investigates the effect of tape placement speed and process temperature on the wedge peel strength of unidirectional CF/PA6 laminates manufactured in a laser-assisted tape placement process. Placement trials were performed at three different speeds, 100 mm/s, 200 mm/s and 400 mm/s, with varying process temperatures between 250 degreesC and 400 degreesC. The process temperature was increased steadily by 30 degreesC increments in this range. The wedge peel strength was determined with a wedge with a thickness of 3.2 mm, a wedge angle of 30 degrees and a displacement rate of 1 mm/s. The maximum wedge peel strength (6.99 N/mm) was obtained at a tape placement rate of 100 mm/s and tape temperature set point of 340 degreesC. At increasing processing speeds the impact of the nip point temperatures reduces. Thus, the scatter of the wedge peel strength values is reduced by 60.8% for doubling the speed and by 67.1% for quadrupling the speed. The effect of the applied polyimide film for the wedge peel testing was found to reduce the process temperature for the third ply. Also, higher process speeds lead to a more evenly distributed temperature history without overshooting in temperature and therefore overheating the incoming tape.
This work aims at developing thermoplastic laminates combining structural and heat management functions. The laminates are composed of continuous carbon fibers as the reinforcement, microencapsulated paraffin as the phase change material (PCM), and the newly developed thermoplastic liquid acrylic resin Elium, processable as a thermosetting polymer. The characterization aims to study how the paraffin microcapsules influence the thermo-mechanical properties and thermal management performance of the resin and the carbon laminates. For the resin/PCM systems, the phase change enthalpy increases with the experimental PCM concentration up to 101 J/g. The melting enthalpy of the laminates also increases with the microcapsule amount, up to 66.8 J/g, which indicates that the mild conditions applied in the processing of the liquid resin allow the preservation of the integrity of the microcapsules. This is also confirmed by the improved thermal management performance observed through thermal camera imaging. However, the microcapsules are preferentially distributed in the interlaminar zone, which justifies the decrease in the interlaminar strength and the flexural properties. These results show potential for the future development of multifunctional thermoplastic composites with high thermal energy storage capabilities.
A hybrid repair solution combining a stress optimised cut-out and a bonded patch is an effective approach for primary structure repairs, in which the optimised cut-out enhances the residual strength to meet certification requirement while the bonded patch provides further stress reduction in the repaired structure and achieve significant additional fatigue life enhancement. In this study, a cracked specimen was repaired with an optimised cut-out at the crack tip and two boron/epoxy patches bonded back-to-back. The patches were positioned adjacent and some distance away from the cut-out and thus potential crack re-occurrence/growth can be visually monitored without the patch concealing the area. The effectiveness of the hybrid repair was assessed experimentally by comparing to three other specimen configurations namely specimens with a crack, a standard stop drill at the crack tip, and an optimum cutout hole at the crack tip. Finite element modelling results indicated that compared with the standard stop-hole specimen, the stress concentration factors of the specimens with optimum cut-out hole and hybrid repairs were reduced by 63% and 73%, respectively. Based on a simple cubic law, these stress reductions would result in the fatigue life before crack re-initiation to be 20 and 52 times longer. Fatigue tests under variable amplitude loads confirmed the significant extension of fatigue initiation life, where 28 and 53 times of enhancement were achieved for the optimum cut-out hole and hybrid repairs, respectively. The fatigue test results further showed that that for the stop drill and optimum cut-out configurations, once the crack was reinitiated, its growth was relatively fast, whilst in the case of the specimen with the hybrid repair the crack growth rate was significantly slower.
