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Unidirectional carbon fiber reinforced polyamide 6 (CF/PA6) laminates after tensile tests exhibited two different types of fracture modes, namely, splitting and step-like. In this study, the mechanical properties and fracture behavior of the unidirectional CF/PA6 laminates were simulated and analyzed by using finite element simulation of a tensile test. The simulations considered the actual deformation and damage mechanism (matrix plastic deformation, stochastic fiber fracture, and interfacial debonding between the fiber and the matrix) experimentally observed in previous research. Results of the numerical simulations were in good with the experimental stress–strain curves. Moreover, the above-mentioned two types of fracture mode were well simulated by changing the interfacial properties. The simulated results showed that the unidirectional laminates with weak interface properties exhibit the splitting fracture mode. Meanwhile, the unidirectional laminates with strong interface properties show the step-like fracture behavior.

Cellulose nanofibers (CNF) have recently attracted attention as one of the reinforcements for composite materials. However, the same as other nanofibers, CNFs can be easily agglomerated and the nano-level defibration is necessary to obtain their outstanding properties in polymer matrices. To overcome this issue, two major defibration methods, chemical and mechanical defibration have been considered. The chemically-defibrated CNF is expensive and so prevents the practical realization of CNF as general-purpose products. Therefore, the mechanical defibration method by a clean low-cost water jet was focused on. Mechanically-defibrated CNF reinforced epoxy resin matrix (Epoxy-CNF) composite was fabricated via mechanical mixing and the variation of their tensile and flexural properties with different CNF volume fractions was evaluated. The tensile and flexural moduli of the epoxy resin were increased by CNF addition, while the fracture elongation was decreased. The calculated ultimate tensile strength (UTS) and ultimate flexural strength (UFS) of the Epoxy-CNF composites, using the aspect ratio of the agglomerated CNF clusters, indicated the validity of the random dispersion model considering the agglomerated CNF as one whole fiber. It seems that epoxy resin is enhanced by the mechanical interaction when the CNF volume fraction is higher than 0.37 vol.%.

Understanding the damage mechanisms of fiber-reinforced polymer matrix composite materials under high voltage conditions is of great significance for lightning strike protection and high voltage insulation applications of composite structures. In this paper, we investigated effects of the lightning impulse (LI) voltage and high voltage alternating current (HVAC) puncture on damage modes of the electrically nonconductive glass fiber-reinforced polymer (GFRP) matrix composite materials through experimental tests and numerical simulations. The LI and HVAC tests represent the lightning strike and high voltage insulation cable puncture conditions, respectively. Our experimental examinations showed that GFRP composite specimens subjected to the LI voltage test exhibited distinct damage modes compared with those in the HVAC puncture test. The GFRP composite material suffered more charring and fiber vaporization in the HVAC puncture test, whereas less matrix charring and fiber vaporization but severe fiber breakage and delamination in response to the LI voltage tests. The findings indicate that the thermal effect dominates the damage of GFRP composites inflicted by the HVAC puncture test, whereas the mechanical impact effect governs the GFRP composite damage in the LI voltage test. In addition, the electric arc plasma formation during the puncture of the GFRP composite material was modeled through solving Maxwell’s equations and the heat generation equations using finite element analysis. Simulation results provided insights on the effects of duration and intensity of the high voltage electric discharge on the composite damage.

In this paper, different mechanical responses of polymer matrix composite rings under thermal cycling are analyzed. The polymer matrix composite rings are classified to un-notched and open-hole specimens and tested based on the naval ordnance laboratory-ring tensile test method. Internal pressure, fracture and surface degradation changes as a function of the hole diameter and number of cycles are investigated. Experimental results suggest that specimens with larger hole radius have more mechanical property variations during thermal cycling. Also, it is revealed that the mechanical properties of laminates degrade as a function of the number of cycles and the hole diameter. The results also demonstrate that long splitting fiber breakage is the main mode for the failure of un-notched specimens, whereas fiber breakage and fiber matrix debonding are the two main modes for the failure of open-hole specimens. Finally, a multiple regression model is proposed to predict the tensile response of un-notched and open-hole polymer matrix composite rings subjected to thermal cycling condition.

A great deal of research and development work has been recently conducted on natural fiber-reinforced polymer matrix composite for its abundancy, low density, excellent damping characteristic, and good mechanical properties. However, the low strength of natural fiber composite has limited its use to only low stress applications. The purpose of this work is to develop a natural fiber hybrid material with both enhanced strength and failure strain using a novel approach and study the effect of the processing temperature on its microstructure and performance. High-strength ultra-high molecular weight polyethylene fabrics are co-molded onto the surfaces of a kenaf fiber high-density polyethylene-based composite material by single-step compression molding. The status of the ultra-high molecular weight polyethylene fabrics at different processing temperatures is investigated using microscopic analysis. The tensile strength and impact strength of the hybrid material are evaluated. It is found that its tensile strength is increased by more than 90% with only 8% ultra-high molecular weight polyethylene fiber reinforcement added and its low density is maintained.

This study investigates the feasibility of using embedded optical fibers in polymer matrix composite laminates to characterize delaminations caused by low-velocity impacts with energies between 30 J and 50 J. Impact damage can occur in composite structures during manufacture, in-service, storage and routine maintenance. Because of their small size and light weight, optical fibers can be embedded in composite structures during the manufacture of composite parts, allowing the structure to be monitored for impact-induced delaminations without being removed from service. In this study, optical fibers are embedded in a grid configuration at four selected locations (one-third from impact surface, midplane, two-thirds from impact surface, and farthest ply from impact) in thick autoclave-cured graphite/epoxy laminates. Low-velocity impact testing is performed at four energy levels. Manufacturing procedures for embedding the optical fibers within the composite laminates are investigated. The strain distribution from the optical fibers is correlated with ultrasonic C-scans of the laminates in which they are embedded. X-ray computed tomography scan images are also compared to those from ultrasonic C-scans. Results indicate that embedded optical fibers can provide post-impact strain responses and delamination area from each embedded site within the impacted laminates.

The acceptability of polymer-based composite materials is increasing with time due to superior mechanical and chemical properties. Still, scope of advancement in mechanical behavior of these materials motivated research fraternity to develop new materials. The present article aims to analyze the effect of environment conditions on newly fabricated glass fiber epoxy composites reinforced with variable SiC particles as secondary reinforcement. The evaluation of mechanical strength and effect of environmental conditions on composite performance was analyzed using tensile, compression, flexural, and impact strength testing on universal testing machine. The mechanical strength characterization demonstrated the effect of size and weight fraction of reinforced SiC particles on mechanical strength of composites. The surface morphology of samples was examined by field emission scanning electron microscopy with energy dispersive spectroscopy. Based on mechanical strength evaluation results, an exponential equation was developed to predict the composite life.

A comprehensive investigation is carried out into the elastic behavior of carbon nanotube-reinforced polymer nanocomposites using two combined analytical micromechanical methods. A unit cell-based micromechanical method is developed to model the random distribution of carbon nanotubes within the polymer matrix. Also, the Eshelby method is used for modeling the random orientation state of carbon nanotubes within the matrix. Two fundamental aspects affecting the mechanical behavior of carbon nanotube/polymer nanocomposites, including the carbon nanotube waviness and the interphase formed due to the non-boned interaction between the carbon nanotube and the surrounding polymer, are considered in the unit cell method. Comparisons between the results of present method and experimental data reveal that for more realistic predictions, five important factors including, random orientation and random distribution of carbon nanotubes, interphase, waviness and transversely isotropic behavior of carbon nanotube should be considered in the modeling of carbon nanotube-reinforced polymer nanocomposites. The effects of volume fraction, number of waves and waviness factor of carbon nanotube as well as the type of random distribution of CNTs within the matrix on the elastic modulus of the polymer nanocomposites are studied.

This study focuses on understanding and prediction of short-term thermal degradation of polymer matrix composites. One sided irradiation of two commercial composites (HexPly® 8552/IM7 and M18-1/G939) is carried out on specimens of various thickness (2, 4, 6 mm) at different heat fluxes (50 and 80 kW/m2) for various exposure times prior to ignition. The aim is to correlate the amount of the applied thermal energy with the heat damage and the residual mechanical strength. Among the two primary components of each matrix the epoxy resin is observed to degrade faster than the thermoplastic under thermal load, as measured by IR spectroscopy. A correlation is achieved between the interlaminar shear strengths and the relative amount of the residual matrix components. The interlaminar shear strengths and degradation processes are assessed in dependence of the applied energy per volume. The derived relationships and a chemometric analysis of IR spectra, can be used to rapidly estimate mechanical properties, as well as other properties of specimens with unknown thermal preload. Degradation processes are discussed in detail.

This paper investigates the repair of polymer matrix composites and validates the repair by static testing. Scarf repair is carried out on the laminates and cured under vacuum. It is observed that 80% of tensile strength is recovered due to this process. Therefore, this investigation highlights the significance of atmospheric pressure plasma treatment on repair of graphite epoxy laminate. It is observed that the surface energy of graphite epoxy laminate has improved significantly due to atmospheric pressure plasma treatment. Atmospheric pressure plasma treatment results in noteworthy increase in oxygen functionalities as detected by X-ray photoelectron spectroscopy, as well as surface roughness as detected by atomic force microscopy. The improvement in adhesion properties is correlated with lap shear strength of adhesive bonded joints and mode of failure has been analyzed by scanning electron microscopy. The plasma-treated laminates, when used for repair, provided an additional 12% in the tensile strength and thereby retaining a strength increase up to 92%.

Geometric unconformities may arise due to residual stresses during the manufacturing process of polymer matrix composite materials. Compaction of the laminate, due to resin squeeze, is another phenomenon that occurs and it influences the mechanical behaviour of the laminate itself. In the past, models for spring-in or for compaction were developed separately, whereas in the present work a new kind of analysis is developed to take into account both residual stresses and compaction that take place during cure simultaneously. This analysis is composed by two different steps: in the former the compaction is predicted, in the latter the cure state and the residual stress condition are determined. Finally, the relationship between different degrees of compaction and the corresponding values of spring-in is studied, and it was found that a higher value of thickness reduction produces the smallest residual stresses for the composite structures with angled cross sections.

In this work, the thermomechanical viscoelastic response of a high temperature polymer matrix composite system made up of T650-35 graphite fibers embedded in PMR-15 resin is studied through a micromechanical model based on the assumptions of simplified unit cell method within a temperature range of 250–300℃ corresponding to aerospace engine applications. The advantage of this particular micromechanical model lies in its ability to give closed-form expressions for the effective viscoelastic response of unidirectional composites as well as each of their constituents. Using the experimental data of the creep behavior of thermostable PMR-15 polyimide, the micromechanical model is first calibrated to account for the effect of temperature. The resulting elastic and viscoelastic responses are found to be in good agreement with the existing experimental data. The validated model is then used to predict the behavior of the composite material under different combinations of thermal and mechanical loadings. The results clearly demonstrate the importance of accounting for the viscoelastic effect of the matrix material as the temperature increases. Current works on modeling temperature-dependent viscoelastic behavior of polymer matrix composites are mainly based on the assumption of thermorheologically simple material. However, through the present approach where the matrix is modeled as a thermorheologically complex material, the effect of temperature on the elastic and viscoelastic response of the composite system can be individually investigated.

As the application of fibre-reinforced polymer composite material continue to increase day by day, so the knowledge about the impact behaviour of fibre-reinforced polymer composite structures in the areas such as automotive and aerospace is very much needed. This article attempts a comprehensive review of recent literature in the broader area of impact damage. Testing methods and standard parameters as well as discussion of important aspects such as impactor shape, weight of impactor, velocity of impact, environment in which impact takes place are presented. Furthermore, the damage area, energy absorbed, contact time and many other considerations are discussed. Finally, an effort is made to review the research work by considering all aspects related to impact on such type of composite materials.

Aluminum nitride reinforced glass fiber epoxy resin composite was prepared by simple hand lay-up technique and its mechanical as well as erosion wear behavior were investigated. The interactive influence of various operational variables on specific wear behavior of composite materials has been studied thoroughly. It was observed that with increasing percentage of filler particles, there is a decline in tensile strength, but there is a significant improvement in hardness and erosion wear performance. Among all the factors, impact velocity is the most significant factor followed by filler percentage and impingement angle, while temperature has the least significance on erosion of the hybrid composite. Taguchi’s orthogonal arrays were used to identify the controlling factors influencing the erosion wear rate. Scanning electron microscopy studies were conducted to understand the erosion mechanism involved during the material removal process.

This work provides techniques to separately determine temperature and duration of a thermal pre-load on a polymer matrix composite with a focus on long-term load (max. 400 days) without massive polymer degradation. The aim is a non-destructive, rapid, robust and precise in-service method to characterize incipient heat damage. A commercially available composite 8552/IM7 is investigated. Infrared spectroscopy of the surface and bulk material traces thermal degradation of the polymer. A multivariate (chemometric) data analysis was performed. The reliability of the calculated values for time (±19 d) and temperature (± 12℃) is increased by including other parameters with various degradation velocities such as mass loss of the composite and color changes and binder degradation of a typical top coat. The residual strength of a composite with unknown thermal history can be predicted and thermal loads such as heating in an oven and hot air are compared.

A 2D woven reinforced composite is a typical engineering material used in aerospace applications. For the thickness of these laminated composites is no longer thin, the stress state cannot be considered planar. Therefore, it is necessary to focus on the material’s mechanical properties in the through-thickness (abbreviated TT) direction to fully evaluate their mechanical properties. This article considers the tensile properties of a carbon fiber 2D woven reinforced polymer matrix composite in the TT direction under quasi-static and impact loadings. The quasi-static and impact tensile experiments were performed using a material testing machine and split Hopkinson tensile bars, respectively. The micrographs of the fracture surfaces were examined using scanning electron microscope (SEM) analysis. The results show that, before fracture, the material deforms in the linear elastic mode; the TT tensile strength and ultimate strain are very low; the tensile strength increases significantly with increasing loading rates, and the breakage mode changes. An interpretation on the different breakage modes observed is proposed from the point of view of crack propagation. A simple numerical simulation based on ABAQUS is performed to prove that the mesostructure and layer displacements are the two key factors that affect the TT tensile properties. Additionally, an unexpected result, a two-peak stress–strain curve, was found in one of our experiments. By combining the results with the SEM images, a reasonable explanation is proposed.

This study presents a new diffusion model named the gradient-dependent model. It is based on the strain-dependent model, a modification of Fick’s diffusion model, and is employed to solve penetrant diffusion in a 2D-infinite plate with finite thickness. An immersion test is performed and analyzed for studying moisture diffusing through the surfaces of a polymer matrix composite laminate into the material and the induced in-plane expansion is measured. The hygric expansion is proportional to the moisture concentration, which can be derived from the gradient-dependent model involving parameters standing for the hygric property of the material. By fitting the theoretical solution of the hygric expansion to the data, the parameters for locating the penetrant front can be obtained and they are very important in describing the hygric behavior of the material.

A hydrodynamic method has been employed to align short, reinforcing fibers in polymer matrix composite materials. Samples of composite materials were prepared and tested two at a time (one with randomly oriented fibers and the other with aligned fibers) to isolate and directly measure the effect on mechanical properties of aligning the fibers. Data were collected on the ultimate tensile strength, modulus of elasticity, and ultimate tensile strain of the composite samples prepared. The results show that the aligned fibers were approximately twice as effective as randomly oriented fibers at stiffening and strengthening the composite in the alignment direction.