The impact resistance of composite materials — a review

Elasto-Plastic Mechanical Properties and Failure Mechanism of Innovative Ti-(SiCf/Al₃Ti) Laminated Composites for Sphere-Plane Contact at the Early Stage of Penetration Process

A novel silicon carbide (SiC) continuous ceramic fiber-reinforced (CCFR) Ti/Al₃Ti Metal-Intermetallic-Laminate (MIL) composite was fabricated. A high-efficiency semi-analytical model was proposed based on the numerical equivalent inclusion method (NEIM) for analyzing the small-strain elasto-plastic contact in the early stage of the penetration process. The microstructure and interface features were characterized by the scanning electron microscopy (SEM). Quasi-static compression tests were performed to determine the contact response and validate the proposed model. A group of in-depth parametric studies were carried out to quantify the influence of the microstructure. The comparison between results under the sphere-plane and plane-plane contact load indicates that, under the first sphere-plane, the compressive strength and failure strain are both lower and the SiC reinforcement effect on strength is very clear while the effect on ductility is not clear. The maximum plastic strain concentration (MPSC) in the Al₃Ti layer is closest to the upper boundary of the central SiC fiber and then extends along the depth direction as the load increases, which are also the locations where cracks may initiate and extend. Moreover, the CCFR-MIL composite shows better mechanical properties when the center distance between adjacent SiC fibers is four times the fiber diameter and the volume fraction of Ti is 40%.

Dynamic shear-lag model for understanding the role of matrix in energy dissipation in fiber-reinforced composites

Lightweight and high impact performance composite design is a big challenge for scientists and engineers. Inspired from well-known biological materials, e.g., the bones, spider silk, and claws of mantis shrimp, artificial composites have been synthesized for engineering applications. Presently, the design of ballistic resistant composites mainly emphasizes the utilization of light and high-strength fibers, whereas the contribution from matrix materials receives less attention. However, recent ballistic experiments on fiber-reinforced composites challenge our common sense. The use of matrix with "low-grade" properties enhances effectively the impact performance. In this study, we establish a dynamic shear-lag model to explore the energy dissipation through viscous matrix materials in fiber-reinforced composites and the associations of energy dissipation characteristics with the properties and geometries of constituents. The model suggests that an enhancement in energy dissipation before the material integrity is lost can be achieved by tuning the shear modulus and viscosity of a matrix. Furthermore, our model implies that an appropriately designed staggered microstructure, adopted by many natural composites, can repeatedly activate the energy dissipation process and thus improve dramatically the impact performance. This model demonstrates the role of matrix in energy dissipation, and stimulates new advanced material design concepts for ballistic applications.

STATEMENT OF SIGNIFICANCE

Biological composites found in nature often possess exceptional mechanical properties that man-made materials haven't be able to achieve. For example, it is predicted that a pencil thick spider silk thread can stop a flying Boeing airplane. Here, by proposing a dynamic shear-lag model, we investigate the relationships between the impact performance of a composite with the dimensions and properties of its constituents. Our analysis suggests that the impact performance of fiber-reinforced composites could improve surprisingly with "low-grade" matrix materials, and discontinuities (often regarded as "defects") may play an important role in energy dissipation. Counter-intuitive as it may seem, our work helps understanding the secrets of the outstanding dynamic properties of some biological materials, and inspire novel ideas for man-made composites.

Analysis of CFRP Joints by Means of T-Pull Mechanical Test and Ultrasonic Defects Detection

Defects detection within a composite component, with the aim of understanding and predicting its mechanical behavior, is of great importance in the aeronautical field because the irregularities of the composite material could compromise functionality. The aim of this paper is to detect defects by means of non-destructive testing (NDT) on T-pull samples made by carbon fiber reinforced polymers (CFRP) and to evaluate their effect on the mechanical response of the material. Samples, obtained from an industrial stringer having an inclined web and realized with a polymeric filler between cap and web, were subjected to ultrasonic monitoring and then to T-pull mechanical tests. All samples were tested with the same load mode and the same test configuration. An experimental set-up consisting of a semiautomatic C-scan ultrasonic mapping system with a phased array probe was designed and developed, optimizing control parameters and implementing image processing software. The present work is carried out on real composites parts that are characterized by having their intrinsic defectiveness, as opposed to the previous similar results in the literature mainly obtained on composite parts with artificially produced defects. In fact, although samples under study were realized free from defects, ultrasonic mapping found defectiveness inside the material. Moreover, the ultrasonic inspection could be useful in detecting both the location and size of defects. Experimental data were critically analyzed and qualitatively correlated with results of T-pull mechanical tests in order to better understand and explain mechanical behavior in terms of fracture mode.

Enhanced durability of carbon nanotube grafted hierarchical ceramic microfiber-reinforced epoxy composites

As carbon nanotube (CNT) infused hybrid composites are increasingly identified as next-generation aerospace materials, it is vital to evaluate their long-term structural performance under aging environments. In this work, the durability of hierarchical, aligned CNT grafted aluminoborosilicate microfiber-epoxy composites (CNT composites) are compared against baseline aluminoborosilicate composites (baseline composites), before and after immersion in water at 25 °C (hydro) and 60 °C (hydrothermal), for extended durations (90 d and 180 d). The addition of CNTs is found to reduce water diffusivities by approximately 1.5 times. The mechanical properties (bending strength and modulus) and the damage sensing capabilities (DC conductivity) of CNT composites remain intact regardless of exposure conditions. The baseline composites show significant loss of strength (44 %) after only 15 d of hydrothermal aging. This loss of mechanical strength is attributed to fiber-polymer interfacial debonding caused by accumulation of water at high temperatures. In situ acoustic and DC electrical measurements of hydrothermally aged CNT composites identify extensive stress-relieving micro-cracking and crack deflections that are absent in the aged baseline composites. These observations are supported by SEM images of the failed composite cross-sections that highlight secondary matrix toughening mechanisms in the form of CNT pullouts and fractures which enhance the service life of composites and maintain their properties under accelerated aging environments.

Enhancing the Mechanical Toughness of Epoxy-Resin Composites Using Natural Silk Reinforcements

Strong and tough epoxy composites are developed using a less-studied fibre reinforcement, that of natural silk. Two common but structurally distinct silks from the domestic B. mori/Bm and the wild A. pernyi/Ap silkworms are selected in fabric forms. We show that the toughening effects on silk-epoxy composites or SFRPs are dependent on the silk species and the volume fraction of silk. Both silks enhance the room-temperature tensile and flexural mechanical properties of the composite, whereas the more resilient Ap silk shows a more pronounced toughening effect and a lower critical reinforcement volume for the brittle-ductile transition. Specifically, our 60 vol.% Ap-SFRP displays a three-fold elevation in tensile and flexural strength, as compared to pure epoxy resin, with an order of magnitude higher breaking energy via a distinct, ductile failure mode. Importantly, the 60 vol.% Ap-SFRP remains ductile with 7% flexural elongation at lower temperatures (−50 °C). Under impact, these SFRPs show significantly improved energy absorption, and the 60 vol.% Ap-SFRP has an impact strength some eight times that of pure epoxy resin. The findings demonstrate both marked toughening and strengthening effects for epoxy composites from natural silk reinforcements, which presents opportunities for mechanically superior and “green” structural composites.

Electrical and Electrochemical Properties of Conducting Polymers

Conducting polymers (CPs) have received much attention in both fundamental and practical studies because they have electrical and electrochemical properties similar to those of both traditional semiconductors and metals. CPs possess excellent characteristics such as mild synthesis and processing conditions, chemical and structural diversity, tunable conductivity, and structural flexibility. Advances in nanotechnology have allowed the fabrication of versatile CP nanomaterials with improved performance for various applications including electronics, optoelectronics, sensors, and energy devices. The aim of this review is to explore the conductivity mechanisms and electrical and electrochemical properties of CPs and to discuss the factors that significantly affect these properties. The size and morphology of the materials are also discussed as key parameters that affect their major properties. Finally, the latest trends in research on electrochemical capacitors and sensors are introduced through an in-depth discussion of the most remarkable studies reported since 2003.

Processing of Thermoplastic Polymers Using Reactive Solvents

The use of reactive solvents offers an interesting and flexible route to extend the processing characteristics of thermoplastic polymers beyond their existing limits. This holds for both intractable and tractable polymers. The first mainly applies for amorphous high- T g polymers where processing may be limited due to the high temperatures required which can cause problems related to degradation and where the solvent helps to decrease the processing temperature considerably. A prime example here can be found in the system poly(phenelyne–ether) PPE/epoxy. The second mainly holds for semicrystalline polymers and the attention here is focused on obtaining low viscosities in order to be able to apply alternative processing routes, like pouring or casting, for those polymers which are generally easy to process by more conventional techniques like injection moulding or extrusion. In this paper the model system poly(ethylene) PE/styrene is investigated. In both cases, based on intractable and tractable polymers, the solvent is polymerized after moulding, thus converting into a non-solvent, and becomes, after the concurrent phase separation and phase inversion, an integral and often structural part of the final product. Interestingly, specific morphologies, in terms of the size of the dispersed (previous solvent) phase formed or the position and thickness of in situ formed interlayers when polymerization occurs in the presence of a polar surface, can be obtained which can otherwise extremely difficult, or not at all, be realized. Moreover, flexibility of the choice of the reactive solvent creates tuneable mechanical (and, if requested, other) properties of those phases, varying from glassy, with a T g of typically 200°C, to complete rubbery, with a T g far below room temperature. Of course a disadvantage of the technique is that a polymerization step must occur after the shaping process. Compared to more standard reactive processing techniques, however, clear advantages can exist with respect to the occurrence of early vitrification—yielding a fast demoulding possibility—induced by the reaction induced phase separation, and the fact that the continuous thermoplast phase ultimately determines the main product properties, including the possibility of second-stage deformability and reprocessability.

Toughening of Epoxy Resins by Polyimides Synthesized from Bisanilines

A range of thermoplastic polyimides has been synthesized and used to modify a DGEBA-DDS epoxy thermoset. The influence of polyimide end-group functionality and particle size has been examined. Increases in fracture toughness of up to three times that of the neat resin have been achieved, together with no loss of modulus. Thermal capability is little affected across the range of modifying polyimides. The influence of copoly(imide–imide)s has also been studied. Morphological examination has revealed a range of structures in the blends, including some which are unusual. A particulate toughening mechanism is proposed for one high fracture toughness system.

Carbon/Kevlar/PEEK Thermoplastic Composites

Thermoplastic composites were developed from carbon/hybrid (1: 1)/Kevlar fabrics using a high-performance polyether ether ketone (PEEK) matrix. The mechanical properties and microstructure of the composites show that the type of fiber has a strong influence on the fiber-matrix bonding and fracture behavior of the composites. The fracture energy of the composites was of the order of -30 kJ m-2 for carbon/PEEK and hybrid/PEEK composites.

Plant Fibre: Molecular Structure and Biomechanical Properties, of a Complex Living Material, Influencing Its Deconstruction towards a Biobased Composite

Plant cell walls form an organic complex composite material that fulfils various functions. The hierarchical structure of this material is generated from the integration of its elementary components. This review provides an overview of wood as a composite material followed by its deconstruction into fibres that can then be incorporated into biobased composites. Firstly, the fibres are defined, and their various origins are discussed. Then, the organisation of cell walls and their components are described. The emphasis is on the molecular interactions of the cellulose microfibrils, lignin and hemicelluloses in planta. Hemicelluloses of diverse species and cell walls are described. Details of their organisation in the primary cell wall are provided, as understanding of the role of hemicellulose has recently evolved and is likely to affect our perception and future study of their secondary cell wall homologs. The importance of the presence of water on wood mechanical properties is also discussed. These sections provide the basis for understanding the molecular arrangements and interactions of the components and how they influence changes in fibre properties once isolated. A range of pulping processes can be used to individualise wood fibres, but these can cause damage to the fibres. Therefore, issues relating to fibre production are discussed along with the dispersion of wood fibres during extrusion. The final section explores various ways to improve fibres obtained from wood.

High resolution imaging of impacted CFRP composites with a fiber-optic laser-ultrasound scanner

Damage induced in polymer composites by various impacts must be evaluated to predict a component's post-impact strength and residual lifetime, especially when impacts occur in structures related to human safety (in aircraft, for example). X-ray tomography is the conventional standard to study an internal structure with high resolution. However, it is of little use when the impacted area cannot be extracted from a structure. In addition, X-ray tomography is expensive and time-consuming. Recently, we have demonstrated that a kHz-rate laser-ultrasound (LU) scanner is very efficient both for locating large defects and evaluating the material structure. Here, we show that high-quality images of damage produced by the LU scanner in impacted carbon-fiber reinforced polymer (CFRP) composites are similar to those produced by X-ray tomograms; but they can be obtained with only single-sided access to the object under study. Potentially, the LU method can be applied to large components in-situ.

Mechanical Characterization of the Tensile Properties of Glass Fiber and Its Reinforced Polymer (GFRP) Composite under Varying Strain Rates and Temperatures

Unidirectional glass fiber reinforced polymer (GFRP) is tested at four initial strain rates (25, 50, 100 and 200 s⁻¹) and six temperatures (−25, 0, 25, 50, 75 and 100 °C) on a servo-hydraulic high-rate testing system to investigate any possible effects on their mechanical properties and failure patterns. Meanwhile, for the sake of illuminating strain rate and temperature effect mechanisms, glass yarn samples were complementally tested at four different strain rates (40, 80, 120 and 160 s⁻¹) and varying temperatures (25, 50, 75 and 100 °C) utilizing an Instron drop-weight impact system. In addition, quasi-static properties of GFRP and glass yarn are supplemented as references. The stress–strain responses at varying strain rates and elevated temperatures are discussed. A Weibull statistics model is used to quantify the degree of variability in tensile strength and to obtain Weibull parameters for engineering applications.

Porphyrin–poly(arylene ether sulfone) covalently functionalized multi-walled carbon nanotubes: synthesis and enhanced broadband nonlinear optical properties

A porphyrin polymer functionalized MWNT hybrid was synthesized with enhanced NLO performance due to the synergistic effect of different mechanism and photo-induced ET/PET process.

Bi-phasic titanium dioxide nanoparticles doped with nitrogen and neodymium for enhanced photocatalysis

Bi-phasic or multi-phasic composite nanoparticles for use in photocatalysis have been produced by a new synthetic approach. Sol-gel methods are used to deposit multiple layers of active material onto soluble substrates. In this work, a layer of rutile (TiO2) was deposited onto sodium chloride pellets followed by an annealing step and a layer of anatase. After dissolving the substrate, bi-phasic nanoparticles containing half anatase and half rutile TiO2; with "Janus-like" characteristics are obtained. Nitrogen and neodymium doping of the materials were observed to enhance the photocatalytic properties both under UV and white light irradiation. The unique advantage of this synthetic method is the ability to systematically dope separate sides of the nanoparticles. Nitrogen doping was found to be most effective on the anatase side of the nanoparticle while neodymium was found to be most effective on the rutile side. Rhodamine B dye was effectively photodegraded by co-doped particles under white light.

Poly(arylene ether ketone)s with pendant porphyrins: synthesis and investigation on optical limiting properties

A series of poly(arylene ether ketone)s with porphyrin pendants were synthesized, which exhibited superior NLO, optical limiting and good thermal properties as novel polymer NLO materials.

Finite element modeling of laminated composite plates with locally delaminated interface subjected to impact loading

This paper investigates the effects of localized interface progressive delamination on the behavior of two-layer laminated composite plates when subjected to low velocity impact loading for various fiber orientations. By means of finite element approach, the laminae stiffnesses are constructed independently from their interface, where a well-defined virtually zero-thickness interface element is discreetly adopted for delamination simulation. The present model has the advantage of simulating a localized interfacial condition at arbitrary locations, for various degeneration areas and intensities, under the influence of numerous boundary conditions since the interfacial description is expressed discretely. In comparison, the model shows good agreement with existing results from the literature when modeled in a perfectly bonded state. It is found that as the local delamination area increases, so does the magnitude of the maximum displacement history. Also, as top and bottom fiber orientations deviation increases, both central deflection and energy absorption increase although the relative maximum displacement correspondingly decreases when in contrast to the laminates perfectly bonded state.

Therapy-resistant nephrolithiasis following renal artery coil embolization

Background

Transcatheter renal artery embolization is an effective and minimally invasive treatment option for acute renal bleeding. Early post-interventional complications include groin hematoma, incomplete embolization, coil misplacement and coil migration. Late complications are rare and mostly related to coil migration.

Case presentation

A 22-year-old woman with a history of recurrent stone disease and a lumbal meningomyelocele underwent bilateral open pyelolithotomy for bilateral staghorn calculi. Post-operatively, acute hemorrhage of the left kidney occurred and selective arterial coil embolization of a lower pole interlobular renal artery was performed twice.

Four years after this intervention the patient presented with a new 15.4 mm stone in the lower calyx of the left kidney. After two extracorporeal shock wave lithotripsy treatments disintegration of the stone was not detectable. Therefore, flexible ureterorenoscopy was performed and revealed that the stone was adherent to a partially intraluminal metal coil in the lower renal calyx. The intracalyceal part of the coil and the adherent stone were successfully removed using the holmium laser.

Conclusion

Therapy-resistant nephrolithiasis was caused by a migrated metal coil, which was placed four years earlier for the treatment of acute post-operative renal bleeding. Renal coils in close vicinity to the renal pelvis can migrate into the collecting system and trigger renal stone formation. Extracorporeal shock wave lithotripsy seems to be inefficient for these composite stones. Identification of these rare stones is possible during retrograde intrarenal surgery. It also enables immediate stone disintegration and removal of the stone fragments and the intraluminal coil material.

Towards high-performance bioinspired composites

Biological composites have evolved elaborate hierarchical structures to achieve outstanding mechanical properties using weak but readily available building blocks. Combining the underlying design principles of such biological materials with the rich chemistry accessible in synthetic systems may enable the creation of artificial composites with unprecedented properties and functionalities. This bioinspired approach requires identification, understanding, and quantification of natural design principles and their replication in synthetic materials, taking into account the intrinsic properties of the stronger artificial building blocks and the boundary conditions of engineering applications. In this progress report, the scientific and technological questions that have to be addressed to achieve this goal are highlighted, and examples of recent research efforts to tackle them are presented. These include the local characterization of the heterogeneous architecture of biological materials, the investigation of structure-function relationships to help unveil natural design principles, and the development of synthetic processing routes that can potentially be used to implement some of these principles in synthetic materials. The importance of replicating the design principles of biological materials rather than their structure per se is highlighted, and possible directions for further progress in this fascinating, interdisciplinary field are discussed.

The role of delamination in failure of fibre-reinforced composites

The mechanisms by which delamination contributes to the failure of fibre-reinforced composites are reviewed. Through-thickness failure owing to interlaminar stresses is considered first, and the effect of delamination in impact and compression after impact. The way in which in-plane failure can occur by delamination and matrix cracks joining up to produce a fracture surface without the need to break fibres is considered next. Examples of quasi-isotropic laminates loaded at different off-axis angles, and with different numbers and thicknesses of ply blocks show large differences in unnotched tensile strength controlled by delamination from the free edge. Similar mechanisms determine the strength of notched specimens and give rise to the hole size effect, whereby tensile strength increases with decreasing hole diameter owing to increased delamination and splitting. Open hole tension and over-height compact tension tests with constant in-plane dimensions show a transition in failure mode with increasing ply block thickness from fibre-dominated fracture to complete delamination. In all these cases, the critical factor controlling strength is the relative propensity to delaminate.

Chemical treatment of poly(lactic acid) fibers to enhance the rate of thermal depolymerization

When heated, poly(lactic acid) (PLA) fibers depolymerize in a controlled manner, making them potentially useful as sacrificial fibers for microchannel fabrication. Catalysts that increase PLA depolymerization rates are explored and methods to incorporate them into commercially available PLA fibers by a solvent mixture impregnating technique are tested. In the present study, the most active catalysts are identified that are capable of lowering the depolymerization temperature of modified PLA fibers by ca. 100 °C as compared to unmodified ones. Lower depolymerization temperatures allow PLA fibers to be removed from a fully cured epoxy thermoset resin without causing significant thermal damage to the epoxy. For 500 μm diameter PLA fibers, the optimized treatment involves soaking the fibers for 24 h in a solvent mixture containing 60% trifluoroethanol (TFE) and 40% H(2)O dispersed with 10 wt % tin(II) oxalate and subsequent air-drying of the fibers. PLA fibers treated with this procedure are completely removed when heated to 180 °C in vacuo for 20 h. The time evolution of catalytic depolymerization of PLA fiber is investigated by gel permeation chromatography (GPC). Channels fabricated by vaporization of sacrificial components (VaSC) are subsequently characterized by scanning electron microscopy (SEM) and X-ray microtomography (Micro CT) to show the presence of residual catalysts.

Composites reinforced in three dimensions by using low magnetic fields

The orientation and distribution of reinforcing particles in artificial composites are key to enable effective reinforcement of the material in mechanically loaded directions, but remain poor if compared to the distinctive architectures present in natural structural composites such as teeth, bone, and seashells. We show that micrometer-sized reinforcing particles coated with minimal concentrations of superparamagnetic nanoparticles (0.01 to 1 volume percent) can be controlled by using ultralow magnetic fields (1 to 10 milliteslas) to produce synthetic composites with tuned three-dimensional orientation and distribution of reinforcements. A variety of structures can be achieved with this simple method, leading to composites with tailored local reinforcement, wear resistance, and shape memory effects.

Fabrication of compound nonwoven materials for soft body armor

The primary objective of body armor research is the development of low-cost, lightweight, wearable garments that effectively resist ballistic impact. This study introduces a material intended to reduce nonpenetration trauma by absorbing energy from ballistic impacts. Layers of web were made by low-melting point polyester (LMPET) on unaligned fibers of high-strength polyamide 6 (HSPA6). A compound nonwoven fabric was made by laying high-strength Vectran filaments between two layers of HSPA6-LMPET web. The new fabric underwent needle punching and thermal bonding to form a composite sandwich structure. The new fabric was subjected to a falling weight impact test and a ballistic impact test. The results indicated that the material with the new design reduced maximum indentation depth by 8%. Furthermore, soft body armor made from the material with the new design would cost less to produce and would weigh 22.5% less than conventional soft body armor.

Ritualized fighting and biological armor: the impact mechanics of the mantis shrimp's telson

Resisting impact and avoiding injury are central to survival in situations ranging from the abiotic forces of crashing waves to biotic collisions with aggressive conspecifics. Although impacts and collisions in biology are ubiquitous, most studies focus on the material properties of biological structures under static loading. Here, we examine the mechanical impact properties of the mantis shrimp's telson, a piece of abdominal armor that withstands repeated, intense impacts from the potent hammer-like appendages used by conspecifics during ritualized fighting. We measured the coefficient of restitution, an index of elasticity, of the telson and compared it with that of an adjacent abdominal segment that is not impacted. We found that the telson behaves more like an inelastic punching bag than an elastic trampoline, dissipating 69% of the impact energy. Furthermore, although the abdominal segment provides no mechanical correlates with size, the telson's coefficient of restitution, displacement and impact duration all correlate with body size. The telson's mineralization patterns were determined through micro-CT (Computed Tomography) and correspond to the mechanical behavior of the telson during impact. The mineralized central region of the telson ‘punched’ inward during an impact whereas the surrounding areas provided elasticity owing to their reduced mineralization. Thus, the telson effectively dissipates impact energy while potentially providing the size-related information crucial to its role in conspecific assessment. This study reveals the mechanical infrastructure of impact resistance in biological armor and opens a new window to the biomechanical underpinnings of animal behavior and assessment.