Experimental investigation of impact on composite laminates with protective layers

This paper presents an experimental study of low energy impacts on composite plates covered with a protective layer. In service, composite materials are subjected to low energy impacts. Such impacts can generate damage in the material that results in significant reduction in material strength. In order to reduce the damage severity, one solution is to add a mechanical protection on composite structures. The protection layer is made up of a low density energy absorbent material (hollow spheres) of a certain thickness and a thin layer of composite laminate (Kevlar). Energy absorption ability of these protective layers can be deduced from the load/displacement impact curves. First, two configurations of protection are tested on an aluminium plate in order to identify their performance against impact, then the same are tested on composite plates. Test results from force–displacement curves and C-scan control are compared and discussed and finally a comparison of impact on composite plates with and without protection is made for different configurations.     

Response of pultruded composite tubes subjected to dynamic and impulsive axial loading

The energy absorption of circular pultruded composite tubes subjected to axial crush load, transmitted by a small attached mass accelerated by means of an explosive load is presented in this paper. Different masses of explosive are used to provide a range of transmitted impulse and crushed distance of the pultruded composite tubes. The influence of the mass of the explosive on the tube response is investigated with regard to crushed distance, the average crushing force and the specific energy absorption (SEA). The crushing distance increases with increasing transmitted impulse. The results and failure mode are also compared with compression tests carried out on a servo-hydraulic machine (type: MTS-309).     

Study on quasi-static compressive properties of aluminum foam-epoxy resin composite structures

Closed cell aluminum foam (AF) has extensive application prospects due to its extended plateau stress region and high energy absorption capacity. As one of the most important manufacturing routes for aluminum foams, the gas injection method still does not guarantee an excellent energy absorption performance. In order to improve the energy absorption capacity while remaining the plateau region extended, epoxy resin (ER) was infiltrated into the aluminum foams in various composite forms. In this paper, different AF-ER composite structures were designed and their uniaxial quasi-static compressive behaviors were investigated. The experimental results indicate that the plateau stress and energy absorption capability of the AF-ER composite structures increase with increasing amount of epoxy resin. Additionally, both the stress fluctuation and the peak stress in the plateau region become insignificant, which is beneficial for energy absorption applications. The composite form is also confirmed to have great effect on the compressive property of the AF-ER composite structures. At last, the Young's modulus of the composite structure is theoretically deduced while the plateau stress and the energy absorption capacity are fitted with the composite parameters by considering the contribution of aluminum foam, epoxy resin and the reciprocity of these two materials. The present model is found to have good agreement with experimental data.     

Prediction of residual strength of CFRP after impact

It was found that the residual strength of CFRP after impact decreases as the impact energy increases when the energy is larger than the threshold impact energy. If the impact energy is sufficiently large, the influence of the mass of an impactor on the residual strength of the composite materials can be disregarded. Also, when the specimen is placed on a rigid plane, it was seen that the residual strength decreases as the diameter of the impactor nose increases. The residual strength after impact can be estimated by measuring the size of the permanent impression on the surface of composite materials after impact and applying the prediction equation for the residual strength proposed in this study.     

Experimental study of impact energy absorption by reinforced braided composite structures: Dynamic crushing tests

High speed dynamic loadings such as small engine fragments, bird strike, tyre impact or ice debris are a concern for many aeronautical structures, as they can create severe damages raising safety issues. A strategy to develop dedicated mechanisms for energy absorption of high speed dynamic impact debris at sub-component level is therefore proposed by means of several reinforced foam-woven composite structures. Among the tests for evaluating the mechanical performances, dynamic crushing tests were performed on a slice of such reinforced composite structures to evaluate their energy absorption. Using simultaneously load signal and fast camera imaging, the tests were analyzed to provide important informations such as damage mechanisms and displacement-load-energy absorption values. At the end, quantitative criterions are presented in order to distinguish the designs that have a good potential for absorbing shock energy and for getting a better understanding for designing reinforced composite structures.     

The effect of hybridization on the ballistic impact behavior of hybrid composite armors

In the present study, effect of hybridization on the hybrid composite armors under ballistic impact is investigated using hydrocode simulations. The hybrid composite armor is constructed using various combinations and stacking sequences of fiber reinforced composites having woven form of fibers specifically high specific-modulus/high specific-strength Kevlar fiber (KF), tough, high strain-to-failure fiber Glass fiber (GF) and high strength/high stiffness Carbon fiber (CF). Different combinations of composite armors studied are KF layer in GF laminate, GF layer in KF laminate, KF layer in CF laminate and CF layer in KF laminate at various positions of hybridized layers for a fixed thickness of the target. In this article the results obtained from the finite element model are validated for the case of KF layer in a GF laminate with experimental predictions reported in the literature in terms of energy absorption and residual velocity and good agreement is observed. Further, the effect of stacking sequence, projectile geometry and target thickness on the ballistic limit velocity, energy absorbed by the target and the residual velocity are presented for different combinations of hybrid composite armors. The simulations show that, at a fixed thickness of the hybrid composite armor, stacking sequence of hybridized layer shows significant effect on the ballistic performance. The results also indicate energy absorption and ballistic limit velocity are sensitive to projectile geometry. Specifically, it is found that arranging the KF layer at the rear side, GF layer in the exterior and CF layer on the front side offers good ballistic impact resistance. The hybrid composite armor consisting of a CF layer in KF laminate acquires maximum impact resistance and is the best choice for the design compared to that of other combinations studied.     

Strain energy release rate in shaft-loaded blister tests for composite repairs on steel

The design of composite repairs of corroded oil and gas pipelines must take into account the strength of the interface adhesion between composite and metal. A shaft-loaded blister test is a common method to measure interface fracture toughness and energy release rate. The study aimed on evaluating shaft-loaded blister tests as replacements for more complex pressure blister tests. Specimens investigated were thick fibre-reinforced plates bonded on metal disks as substrates containing a circular through-hole defect. This paper presents the influence of different punch head geometries on the resulting energy release rates and compares the results with blister tests using fluid pressure. Test and simulation results are presented and analytical solutions were derived and evaluated to establish best fitting formulations. It was shown, that significant variations between the different means of loading exist.     

Effect of trigger configuration on the crashworthiness characteristics of natural silk epoxy composite tubes

In the current study, the quasi-static compression test over natural silk epoxy composite tubes was performed using two different trigger mechanisms. The natural silk epoxy composite tubes used in this study consisted of 12 layers of woven natural silk as reinforcement and a thermoset epoxy resin as matrix. The natural silk epoxy composite tubes had lengths of 50 mm, and they were associated with external triggers, including four steel pieces located on the downside flat plate fixture and a plug trigger. The failure modes of the natural silk epoxy composite tubes were investigated using representative photographs taken during the quasi-static compression test. In addition to the load–displacement graphs, the crashworthiness characteristics of the natural silk epoxy composite tubes were exported. The results showed that the four-piece trigger mechanism changed the manner in which failure progressed i.e. from catastrophic to progressive. Plug trigger caused a significant reduction in load carrying capacity and energy absorption capability of specimens. The four-piece trigger retained energy absorption capability of specimens similar to the non-triggered ones, while both the reduction of peak load and increase in crash efficiency of these were observed to be significant.     

Impact and compression after impact experimental study of a composite laminate with a cork thermal shield

The aim of this paper is to present an experimental study of impact and compression after impact (CAI) tests performed on composite laminate covered with a cork thermal shield (TS) intended for launchers fairing. Drop weight impact tests have been performed on composite laminate sheets with and without TS in order to study its effect on the impact damage. The results show the TS is a good mechanical protection towards impact as well as a good impact revealing material. Nevertheless, totally different damage morphology is obtained during the impact test with or without TS, and in particular at high impact energy, the delaminated area is larger with TS. Afterwards, CAI tests have been performed in order to evaluate the TS effect on the residual strength. The TS appears to increase the residual strength for a same impact energy, but at the same time, it presents a decrease in residual strength before observing delamination. In fact, during the impact tests with TS, invisible fibres’ breakages appear before delamination damage contrary to the impacts on the unshielded sheets.     

Strain-rate effects in Ni/Al composite metal foams from quasi-static to low-velocity impact behaviour

Metal foams are used as absorbers for kinetic energy but predominantly, they have only been investigated under quasi-static load-conditions. Coating of open-cell metal foams improves the mechanical properties by forming of Ni/Al hybrid foam composites. The properties are governed by the microstructure, the strut material and geometry. In this study, the strain-rate effects in open-cell aluminium foams and new Ni/Al composite foams are investigated by quasi-static compression tests and low-velocity impact. For the first time, drop weight tests are reported on open-cell metal foams, especially Ni/Al composite foams. Furthermore, size-effects were evaluated. The microstructural deformation mechanism was analysed using a high-speed camera and digital image correlation. Whereas pure aluminium foams are only strain-rate sensitive in the plastic collapse stress, Ni/Al foams show a general strain-rate sensitivity based on microinertia effects and the rate-sensitive nano-nickel coating. Ni/Al foams are superior to aluminium foams and to artificial aluminium foams with equal density.     

Embedding thin-film lithium energy cells in structural composites

This paper reports on the recent progress towards the development of power composite structures capable of energy harvesting and storage in addition to load bearing. The process of physically embedding all-solid-state thin-film lithium energy cells into a carbon fiber reinforced plastic (CFRP) and the performance of the resulting power composites are reported. The embedded thin-film lithium-ion energy cells did not significantly alter the mechanical properties of the composite (modulus and strength) under quasi-static uniaxial loading conditions. The embedded energy cells performed at baseline charge/discharge levels up to a loading of about 50% of the CFRP tensile strength.     

Designation of Mode Mix in Orthotropic Composite Delamination Problems

In interfacial fracture modeling of composite delamination, mode mix is typically specified in terms of energy release rates. Other near-tip quantities can be used to designate mode mix, however. This paper considers the designation of mode mix in terms of energy release rates, stress intensity factors, stresses ahead of the crack tip and crack face displacements and the consequences of using different near-crack-tip quantities to designate mode mix in analyzing composite delamination. The problem addressed is two-dimensional debonding between plies or ply groups modeled as in-plane orthotropic materials; however, the conclusions discussed apply to general composite delamination problems. It is shown that use of different quantities to designate mode mix can give significantly different results in matching composite applications to mixed-mode toughness tests. For cases where measured interfacial toughness increases with increasing mode II deformation, it is demonstrated that use of a mode mix designation based on energy release rates could be non-conservative. Based on these findings, it is suggested that practitioners consider the differences in failure load predictions that would result if different near-tip quantities were used to relate composite applications to measured toughnesses. To this end, methods for converting mode mix designations in terms of energy release rates into designations in terms of other fracture quantities are outlined and applied. This revised version was published online in August 2006 with corrections to the Cover Date.     

Vibration analysis of laminated composite plates with damage using the perturbation method

The present work focuses on vibration characteristics of damaged laminated composite plates. Damage is considered as a local reduction of anisotropic plate stiffness, and three damage factors (representing the damage severity, damage anisotropy, and damage location/area, respectively) are defined to describe damage status in the laminated composite plates. The analytical solutions are obtained by the perturbation method. A numerical analysis is conducted on the vibration of damaged laminated composited plates, and the effect of damage factors on the vibration characteristics is discussed. Results indicate that three damage factors have different influences on the vibration characteristics. Also, the modal curvatures and strain energy show higher damage sensitivity than the natural frequencies and displacement mode shapes. The perturbation-based vibration analysis developed in this study can be used to effectively evaluate the effect of damage on the vibration behavior of anisotropic plates and potentially identify the damage in the laminated plates.     

Analytical and experimental studies on the low-velocity impact response and damage of composite laminates under in-plane loads with structural damping effects

In this paper, low-velocity impact response and damage of composite laminates under in-plane loads are analytically and experimentally investigated. The authors recently proposed a modified displacement field of plate theory, considering the effect of initially loaded in-plane strain, and used a finite element program to analyze the structural behavior of the composite laminate. In this study, the program is upgraded to account for the structural damping effect of the laminate. A pendulum type impact test system and an in-plane loading fixture are constructed for the experimental study. The analytical and experimental impact behaviors are compared at different impact energy levels for cases with an initial in-plane tensile load and a compressive load, as well as cases without the initial in-plane load. The results show good correspondence between the analytical and experimental impact force histories. The effect of the initial in-plane load reduces for higher impact energies. The numerical estimation of the damaged area is in good agreement with the results from C-scanning experiments.     

Experimental analysis of the through-thickness compression properties of z-pinned sandwich composites

This paper presents an experimental investigation into the flat-wise compression properties, strengthening mechanisms and failure modes of sandwich composite materials reinforced with orthogonal z-pins. The compression modulus of the sandwich composite increases rapidly with the volume content of z-pins due to their high longitudinal stiffness, however acoustic emission monitoring and X-ray computed tomography reveal that some z-pins are damaged during elastic loading. The compression stress to induce core crushing is increased greatly by z-pinning (up to nearly 700%), although a large percentage of the z-pins fail close to the elastic stress limit by longitudinal splitting and/or kinking. The total absorbed compressive strain energy of the sandwich composite is also improved greatly by z-pinning (more than 600%) due to the z-pins resisting core crushing, even though they are severely damaged. The results and observations presented in this paper have implications on the mechanical modelling of sandwich materials reinforced with brittle z-pins.     

Effects of stacking thickness on the damage behavior in CFRP composite cylinders subjected to out-of-plane loading

The purpose of this study is to evaluate effects of stacking thickness on the microscopic damage behavior in a filament wound carbon fiber reinforced plastics (FW-CFRPs) composite cylinder subjected to impact or quasi-static out-of-plane loading. From both tests, thicker CFRP improved the stiffness of the cylinder and decreased the resultant plastic deformation due to indentation. From the cross-sectional observation, it is clarified that fiber breakages were localized for the specimens with impact tests more than 10-layers and specimens with quasi-static tests more than 15-layers. In order to discuss the relation between the damage and the absorbed energy, damage depth ratio was defined as fiber damage depth per unit CFRP thickness. To normalize the effect of thickness, absorbed energy ratio was also defined as absorbed energy per unit CFRP thickness. Absorbed energy ratio as a function of absorbed energy ratio was expressed as one master curve regardless of loading conditions.     

Mechanical response of barium-titanate/polymer 0–3 ferroelectric nano-composite film under uniaxial tension

Ferroelectric polymer based 0–3 composite films are attractive for applications such as capacitors and electric energy storage devices. In this paper, deformation and fracture behavior under uniaxial tension is characterized for BaTiO₃/poly(vinyledene fluoride-trifluoroethylene) (abbreviated as BT/P(VDF-TrFE)) ferroelectric composite film. Compared with the pure P(VDF-TrFE) copolymer film, the composite film with a small volume fraction of BT powders shows an enhanced ductility in accompany with reduced stiffness and fracture strength. Scanning electron microscope (SEM) observation and X-ray diffraction (XRD) analysis are carried out to examine the morphology and microstructure change during uniaxial tension. It is demonstrated that addition of a small amount of BaTiO₃ powders into the copolymer matrix inhibits the growth of the crystallite size, causes reduction in the crystalline content and a loosely packed molecular chain structure. Consequently, the fracture strain increases while the stiffness and fracture strength decreases for the composite films.     

Numerical analysis of thermoplastic composites laser welding using ray tracing method

Laser technology is a good alternative for continuous joining of thermoplastics composites structures. Presence of continuous fibers at a high fiber volume fraction (superior to 30%) does not allow using traditional development as for pure thermoplastic materials, due to the presence of fiber clusters or polymer rich areas. Those heterogeneities induce macroscopic light scattering through the structure, reducing the resulting energy level absorbed at the welding interface. The study proposed here takes into account the real microstructure of the composite in order to evaluate changes in local energy diffusion directly linked with local fiber arrangements. The objective of this work is to develop an affordable numerical simulation of the laser welding process modeled with adapted physics mechanism and taking into account the microstructure heterogeneity of the considered materials regarding optical and thermal properties. To model the optical path of the laser beam through the composite fibrous structure, a simulation tool based on geometrical optic is developed. Weldability is considered on composites with different thicknesses, showing the non linear relationship between welding energy and substrate thickness.     

Strength and damage tolerance of composite–composite joints with steel and titanium through the thickness reinforcements

Today’s aeronautic, automotive and marine industry is in demand of structurally efficient, low weight alternatives for composite–composite joints which combine the advantages of low weight input of adhesively bonded joints and high damage tolerance of through the thickness bolted joints. In the present work, composite–composite joints are reinforced through the thickness by thin metal inserts carrying cold metal transfer welded pins (CMT pins). The influence of pin alignment and type of pin on the damage tolerance of single lap shear (SLS) composite–composite joints is investigated. The use of titanium reinforcements is evaluated and compared to stainless steel reinforced, adhesively bonded and co-cured specimens. A detailed analysis of the stress–strain behavior is given and the stiffness and energy absorption of the SLS joints during tensile loading is assessed. The results show that joints reinforced with CMT pins absorb significantly higher amounts of energy, when compared to adhesively bonded and co-cured joints.     

Effect of kneading process conditions on the particle fineness and the polydispersity in nanocomposites

This paper is part of a series of publications that cover the entire process chain to produce nanocomposites. The associated papers are published in the chronological sequence and broach the issues of: “production and dispersing of nanoparticles”, “characterisation of the liquid and reactive matrix” as well as “resulting composite properties by experimental and simulation methods”. Nevertheless, all resulting composite properties are strongly dependent on the method of particle incorporation and on the particle size distribution. Therefore, this study focuses on the optimisation of the dispersion referring to finest particles, smallest particle size distribution, shortest dispersing time and lowest specific energy. In order to prepare the matrix suspension, nano-fillers were dispersed conducting shear mixing techniques in a high performance laboratory kneader. As carrier fluid epoxy resin and a corresponding anhydride hardener system were chosen. Tests were performed using neat and surface modified alumina particles at different levels of particle concentrations. The particle size distribution was determined using dynamic light scattering directly after the dispersing process. Additionally each sample was characterised after 1, 3 and 7 days. Since similar examinations were performed for all formulations, a statement on the influence of re-agglomeration processes and the role of surface modification can be derived. By correlating the progress of the dispersing process to the mass fraction and the particle size distribution, the dispersion process can be evaluated regarding the dispersing time, specific energy and product quality. However, an optimum polydispersity can be found between 25 and 30 wt.%, even if the finest average particle size is reached at higher mass fractions around 45 wt.%. Silane modified alumina particles in epoxy resin constitute the most stable system against re-agglomeration, although the finest particles and the smallest specific energy are attained in non-modified systems. Moreover it can be concluded, that resulting properties of the cured composite are strongly related to the aspired optimisation, e.g. product fines, particle size distribution, required energy input and stability.