A BSTRACT . This paper describes an experimental study developed to characterize the mode I and mode II fracture toughness of carbon/epoxywoven composites, using DCB and ENF tests, respectively. The laminates were manufactured using an epoxy resin and twelve woven balanced bi-directional layers of carbon fibres, all of them with the same orientation (0/90º). Significant instantaneous delaminations were observed particularly for the DCB specimen, which were responsible for an oscillatory behaviour of G I versus crack length. The
Composite structures are widely used in many industrial applications like aerospace and defence industry due to their inherently high specific mechanical properties. In many situations, these composite structures are subjected by high and low velocity impact. Composite structures are very sensitive to non-visual damage that strongly influence their residual load bearing capability. Lack of knowledge of the impact effects on composite structures is a factor in limiting the use of composite materials (Abrate, 1998). To understand the responses of composite structures under high velocity projectile impacts, various experimental and numerical studies have been conducted. Cantwell and Morton (1989) examined the initiation and development of damage in composite structures with a series of low and high velocity impact tests. Guoqi et al., (1992) investigated experimentally the response of woven Kevlar/polyester laminates of varying thicknesses to quasi-static and dynamic penetration by projectiles. Cheng et al., (2003) developed a model based on hydrodynamic finite element code for high velocity impact on thick composites. This model was based on a continuum approach which was built on the basis of an orthotropic constitutive behaviour with stress-based failure criteria and a simplified degradation model of the failure of composites. Silva et al., (2005) investigated experimental and numerical simulation of ballistic impact on Composite structures made of Kevlar-29. Cerioni (2009) presented a numerical simulation of delamination in composite materials under static and fatigue loading by cohesive zone models. Zhao et al., (2010) experimentally reported the failure modes of T300/epoxy composite laminates at different impactor velocities of 10-300 m/s on the different stacking sequence. Khalili et al., (2011) proposed a verified finite element model using ABAQUS finite element for composite laminates and shell structures subjected to low-velocity impact. Gonzalez (2011) investigated the damage induced in composite plates under drop-weight impact loading by analytical description and experimental test plan for assessment of the virtual test performance by finite element simulation. Ramadhan et al., (2013) investigated the high velocity impact response of composite laminated plates, both experimentally and numerically.
The internal damage developed inwoven fabrics laminates sub- mitted to fatigue loading is shown in Figures 8b and 8d, where one can note that overall crack density is considerably smaller than that displayed by the cross-ply unidirectional tape array, for an equivalent number (approximately 1.5 million) of applied cycles. As a result of the smaller inter-ply surface area, if compared to the counterpart tape array, delamination is very much limited. On the other hand, there is a clear trend of transverse cracking to extend along the whole thickness of individual laminas. This can partially explain why the fabric laminate invariably fractured much earlier than the carbon tape array. It can be argued that delamination is an effective toughening mechanism whereby fracture energy dissipates (and crack arrests) at expenses of trans-laminar cracking conducting to premature failure of the notched laminate.
Abstract- The main objective of this paper is to investigate the effect of properties of resin and fibre on the performance of Glass/Epoxylaminates, a fibre-reinforced composites. Compression failure of composite structures previously damaged by an impact event is due to the propagation of impact-induced damage mechanisms such as interlaminar bonding, constituent micro cracking, sub laminate buckling, as well as the interactions between these mechanisms. This requires fabrication of woven glass fibre epoxy matrix laminates with 1% graphite fillers. Fabrication is done by vacuum bag method followed by curing for specified time in hot air oven. The impact test was carried out for a specific height with given mass and diameter of hemispherical shape impactor. The compression loading was given to the impacted specimen. The damage size was analyzed. Ultimate Compressive residual strength was then calculated.
For all conditions analysed, the damaged area (delaminations) was higher for Kevlar/epoxylaminates. In fact, the aramid fibres do not fail by brittle cracking, as do glass or carbon fibres, but they fail essentially by a series of small fibril failures . According with Aktas et al  the main energy absorption mechanism for carbon-fibre reinforced composite is fibre breakage mode, while for Kevlar composites it is essentially by delaminations. On the other hand, for a woven laminate, delamination starts at the centre of impact and propagates to the directions of warp and fills fibres . Additionally, aramid fibres are strongly hygroscopic and often with values higher than the matrices. A maximum moisture content of 6% is reported for Kevlar-49 at room temperature 96% RH . Studies developed by Reis et al  shown that, after 100 days, the system SR 1500 epoxy resin and SD 2503 hardener presents around 0.9% of moisture content, but when the Kevlar fibres are added to the resin this value increases 2.5 times. In this context, the combined action of water and corrosive fluids leads to matrix expansion, with consequent occurrence of micro- cracks and/or development of microstresses in the composites [70-73]. These micro-cracks, according with some researchers, play an important role at the onset of delaminations, with consequent increase of the damaged area [50, 74]. Mahmoud et al , for example, associated the drop of mechanical properties with the absorption, penetration and reaction that occur between the solutions and the composite constituents (matrix and fibres). For Hammami and Al-Ghilani  the degradation takes place via two stages. In the first stage resin is attacked under the combined action of water diffusion and the presence of H + . In the
Polymers are substances made up of a persistent structural unit which can be regarded as resulting from a definite compound called a monomer. Polymers in whatever form they exist (natural or synthetic) usually have the desirable property of being light weight, flexible, ease of production, good mechanical properties and resistant to corrosion attack. Synthetic polymers can be classified into Thermoplastic (e.g. Polyethylene), thermoset (e.g. epoxy resin or polyesters) and Elastomers. Polymers can be considered as engineering materials and have therefore been the subject of research to improve its property and expand its usage in the engineering profession. Polymers have low stiffness, low strength and some polymers (e.g. resin) have low resistance to crack initiation (Dong et al., 2004). Reinforcing of polymer have being in existent for sometimes, polymers reinforcement using both synthetic and natural fibrous material such as glass fibre, wood, kenaf, rice husk, carbon fibre etc. have being carried out, with significant improvement in the mechanical properties of polymer being achieved and much more so when the fibre reinforcing efficiency is further enhanced by chemical modification (Hornsby, 1997).
RAM application can be done in both civilian and military sectors. Considering applications of these materials in military area it can say that the energy scattered from a target (eco-radar), that would be used for its detection by radar, is attenuated and the object coated with RAM becomes stealth or, as reported in the literature, “invisible” to radar. In civilian applications, the RAM use brings beneits in different areas, such as in telecommunication area coating cell phones and radio transmitting antennas; in medical sector coating, for example, electronic pacemakers and equipment; in anechoic chambers used for research and industrial purposes; in home appliances in general, in electromagnetic shielding and control of interference, among other applications 8-10 .
Three common 3D (Three Dimensional) Glass woven structures were studied to analyze the tensile behavior. Each type of strand (Warp, weft and binder) of 3D woven structure was studied in detail. Crimp percentage of those strands was measured by crimp meter. Standard size samples of each 3D woven structure were cut in warp and weft direction and were stretched by Instron Tensile testing computerized machine. Results reveal that hybrid possesses lowest crimp in core strands and higher strength in warp as well as weft direction. Layer to layer woven structure appeared with lower strength and higher strain value due to highest crimp percentage in core strands.
The main aim of the paper is to study the wave propagation in thin orthotropic laminate panel loaded in-plane by stress pulse. Experimental solution utilizes for noncontact measurements a laser vibrometer. Experiments were performed on in-plane loaded square composite panels with dimensions 501 mm × 501 mm × 2.2 mm. Panels have several fiber orientations. They were loaded by in-plane impact of steel sphere (diameter 4 mm). The impact area was on the edge, exactly 150 mm from to left corners of the panels.
The specimens after testing are presented in Figures 3 and 4. Mechanical properties obtained from compressive tests performed on iber reinforced epoxy polymer concrete and commercial concretes found in the market are presented in Table 3. The results presented in Table 3 show that iber reinforcement improves the compressive strength of epoxy polymer concrete. While carbon iber reinforcement shows an improvement by 16% in the compressive strength, only 8.7% increase improvement is observed for glass iber reinforcement. The behavior is different when elastic modulus is analyzed. Both carbon iber reinforcement and glass iber reinforcements do not improve the compressive elastic modulus of the composites; in fact, a slight decrease is observed incarbon iber reinforced composite. On the other hand, the values of Poisson’s ratio do not show a clear trend. The random distribution of ibers means that anisotropic feature has its value averaged through all possible orientations.
A composite is usually made up of at least two materials out of which one is binding material called as matrix and other is a reinforcement material known as fiber. For the past ten years research is going on to explore possible composites with natural fiber like plant fibers and animal fibers. The important characteristics of composites are their strength, hardness light in weight. It is also necessary to study about the resistance of the composites for deferent chemicals. In the present work, composites prepared with epoxy (Araldite LY-556) as resin and „emu‟ bird feathers as fiber have been tested for chemical resistance. The composites were prepared by varying fiber loading (P) of „emu‟ feathers ranging from 1 to 5 and length (L) of feather fibers from 1 to 5 cm. The composites thus prepared were subjected to various chemicals (Acids, Alkalis, solvents etc.). Observations were plotted and studied. The results reveal that there will be weight gain for the composite samples after three days, when treated with Hydrochloric acid, Sodium carbonate, Acetic acid, Sodium hydroxide, Nitric acid and Ammonium hydroxide. Weight loss was observed for all the samples including pure epoxy when treated with Benzene, Carbon tetra chloride and Toluene.
drop or raise was considered as a potential defect). As it can be observed from these results, both laminates presented a similar level of attenuation. In blue color, it is represented the high attenuation, which means a lack of resin when compared to green-colored regions (region of low attenuation, around 55%). Those different attenuations can not be considered as defects so far, like delaminations or dry spots. The discrepancies in attenuation signals of the laminates may be attributed to variations of the applied pressure during the manufacturing process, which did not keep the same impregnation along the plate.
Tribological studies revealed that the addition of NT decreases wear and friction coefficient. The best result was achieved with 0.25 wt. (%) MWCNTs, with 12 hours of UV-A curing, which yielded 24% and 17% reduction in wear rate and friction coefficient, respectively. However, for larger MWCNT content, wear and friction coefficient may increase, especially when CNT clusters (non-homogeneous dispersion) in the matrix occurs. The wear mechanisms found for the nanocomposites were: adhesive wear, fatigue and delamination. These mechanisms are commonly found in most epoxy materials.
Sandwich beams loaded in three points bending may fail in several ways including tension or compression failure of facings. In this paper , The effect of the transverse shear on the face yielding and face wrinkling failure modes of sandwich beams loaded in three points bending have been studied, the beams were made of various composites materials carbon/epoxy, kevlar/epoxy, glass/epoxy at sequence [+θ/-θ] 3s, [0°/90°] 3s. . The stresses in
A mesh convergence test was performed in order to find the best element type and size to be used in the numerical models developed in the following chapters of this dissertation . The results indicate that 8 nodes brick element, using full integration, with a size of 0.3 𝑚𝑚 was the adequate element to use, leading to a mesh with 4128 elements per ply. This is an element size capable of providing accurate results and allows its deactivation. First, it is the biggest element size that still provides accurate results (there are smaller element sizes, which provide the same results with a higher computational cost) in less time and, second, it is a small enough element, allowing the deactivation of the elements. Once a strategy using elements deactivation, in order to simulate the fiber and/or matrix failure, its size needs to be as small as possible in order to observe conveniently the damage propagation.
show the importance of residual stresses due to solid state bonding (thermo-compression). All the works on this subject recommend to take into consideration residual stresses during the material analysis. From a maintenance technique point of view, composite materials are used for reinforcement of undamaged plates and structures or to repair cracks in aeronautical metallic structures. These stresses have a great effect on the debonding between the fiber and the matrix. In this study a finite element method is used to compute and analyze the residual stresses particularly near the fiber/matrix interface where they are the most important.
can be broken, where q is a positive number. It means that the probability of rupture for the material does not depend on the number of fibers in the fiber bundle. This assumption is in agreement with the observation that systems with different sizes must have the same rupture features for the same external conditions (temperature and traction velocity). Obviously the force and the energy needed to break the bundle must depend on the system size but not the stress vs. strain diagrams or the size of the cracks that arises in the breaking processes. This assumption makes also possible the appearance of cracks in different parts of the material for the same deformation. Let us consider a chosen fiber. The breaking probability is evaluated and compared with a random number in the interval [0, 1). If the random number is less than the breaking probability, the fiber breaks. The load spreads to the neighbour fibers and the breaking probability of them increases because of the decreasing of the parameters n i−1 and n i+1 . This procedure describes the propagation of the crack through the fiber
There was a reduction in the values of tensile strength and modulus and an increase in the elongation at fracture of modified- CTBN resin which might be attributed to the lowering in cross- linking density of the polymeric network as the rubber occupies the reaction sites. The fractography study reveals that the presence of CTBN particles led the material to fracturein a way akin to ductile materials. SEM analysis showed also the good adherence between the elastomeric phase and the epoxy matrix which was evidenced by IR analysis. The dynamic mechanical thermal analysis of the polymeric systems showed a reduction in T g value with rubber addition which
According to Ávila et al. (2011), carbon based nano-structures, i.e. carbon nanotubes (CNT) and graphene nano sheets (GN), present remarkable mechanical, electrical and thermal properties. CNT capabilities have been observed experimentally and verified by numerical simulations. Although carbon nanotubes have great potential for applications in a large variety of usages, e.g. aerospace industry, medical and electronic devices, there is no consensus about their exact mechanical properties. Moreover, the high cost of CNT is an issue that cannot be discarded for industrial applications. Even though a large number of researchers have being using carbon nanotubes for reinforcement of composites laminates with good results. Among those researchers are Kim and collaborators (2009), whom described no significant increase on tensile properties of the addition of CNTs to carbon fibers/epoxylaminates. Nonetheless, they noticed an enhancement on flexural modulus (≈12%) and strength (≈18%) with the addition of 0.3 wt. % of CNT to the epoxy system. This increase can be attributed to changes into flexural failure mechanisms. Following the same idea, Chou et al. (2010) discussed the influence of CNTs into the failure of laminated composites. They even proposed the concept of a hybrid inter- laminar architecture that can bridge inter-laminar cracks. Wicks and colleagues (2010) actually produced the hybrid nano reinforced laminated composites proposed by Chou et al. (2010). As mentioned by Wicks, aligned CNTs bridge the plies interfaces, which can lead to an increase on toughness, for the steady state condition, 76%
Novel carbon/glass hybrid thermoplastic composite rods have been developed consisting of a PAN-based carbon fiber, an E-glass fiber, and a thermoplastic matrix. Three types of hybrid rods with different carbon/glass ratios were then fabricated. The surface and cross-sectional morphologies of the hybrid rods were observed using a digital microscope. Additionally, the volume fractions of carbon/glass fibers, matrix, and void were determined by specific gravity measurements and thermogravimetric analyzes. The glass fibers in the hybrid rods display a braided structure of the 2/2 twill weave. The braid angle (defined as the orientation angle of the interlacing yarns with respect to the longitudinal axis of the rod) ranged from 22.3° to 35.2°. The various volume fractions were in the ranges of 24.6–46.2% for the carbon fiber, 23.2–39.8% for the glass fiber, 23.4–25.5% for the matrix, and 7.3–10.2% for the voids. The tensile properties and fracture behavior of the hybrid rods under static and fatigue loading were also investigated. For the static tests, the stress applied to the specimen was nearly linearly proportional to the strain until the failure of all-hybrid rods. The tensile modulus, strength, and failure strain of the hybrid rods were measured in the ranges of 65–91GPa, 1.42–1.84 GPa, and 2.1–2.2%, respectively. The tensile modulus and strength increased as the volume fraction of the carbon fiber increased. However, the failure strain decreased as the volume fraction of the carbon fiber increased. For fatigue tests, the maximum applied stress-number of cycles to failure (S-N) curves for all-hybrid rods were obtained from 0.1 of the stress ratio (minimum/maximum stress) and 10 Hz of the loading frequency. The fatigue strength at 10 7 cycles for all-hybrid rods was less than 30% of ultimate breaking stress. The fatigue performance of the hybrid rods was significantly lower than that of conventional carbon fiber reinforced polymer matrix composites and steel rods. The voids in the hybrid rods affected the fatigue tensile properties.