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This study explored the impact response of two bio-inspired composite laminates with linear and non-linear helicoidal fibre architectures. The helicoidal laminates, together with quasi-isotropic and cross-ply control samples, were fabricated using heterocyclic aramid fibres and tested against a spherical projectile under high impact velocities. The results revealed that helicoidal lay-ups with small rotation angles led to inferior perforation resistance and energy absorption capacity when compared to the quasi-isotropic and cross-ply counterparts. The cross-ply configuration was confirmed as the optimal fibre architecture for impact perforation. Post-impact inspections highlighted that the failure mechanisms of laminates were significantly affected by their lay-ups. The number of fractured fibres was found to reduce with the decrease of inter-ply angle. This was due to the small rotation angles promoted a wedge-in mechanism. Moreover, a smaller angle mismatch resulted in diminished overall delamination area in a laminate. Due to these two effects, therefore, helicoidal lay-up configurations with small inter-ply angles are not recommended for impact-resistant laminates reinforced by tough fibres.

Benzoxazine and phthalonitrile-based resins play an important role in fields of advanced materials because of their excellent properties. In this work, a kind of matrix resin with various ratios of phthalonitrile and benzoxazine were designed and prepared via co-reaction between aniline and 3-aminophenoxy phthalonitrile in the presence of biphenol and formaldehyde. Curing behaviors and dynamic rheology of matrix resins were investigated with DSC and DRA. Results indicated that obvious double curing stages can be assigned to ring-opening polymerization of oxazine rings and ring-forming polymerization of nitrile groups. Possible polymerization mechanisms were discussed in this work. Thermal stability of cured resins was evaluated by TGA testing and an integral decomposition temperature (IPDT). Then, fiber-reinforced composite laminates were prepared and their mechanical properties were investigated and confirmed with SEM images of fracture surfaces. Results demonstrated that this work proposed an improved matrix resin system with outstanding thermal stability and mechanical properties that broadened the foundation and ideas for subsequent researches.

A Distributed fiber Optical Sensor (DOS) was embedded inside and attached to the surface of a multi-directional continuous fiber composite laminate coupon, along with the traditional foil of Strain Gages (SG) and Digital Image Correlation (DIC), to compare the DOS measure strain response to traditional strain acquisition methods. Tensile and bending tests were performed on an Open Hole composite laminate coupon to study different modes of loading and the performance of the DOS in the presence of a strain gradient. Tensile test results show good agreement between the SG, the DIC, and the DOS strain measurements in the region far from the strain concentration. Near the hole, the DOS repeatably measured the strain concentration during tensile and bending tests. The DOS was able to measure the residual strain in the composite laminate coupon through the hole-drilling method. Finite Element Analysis was performed to obtain the theoretical distribution of axial strain along the locations corresponding to the physical placement of the DOS in the coupon. As the results indicate, the DOS readings agree well with the theoretical results for the strain distribution in the test coupons. The results obtained during these evaluations provide confidence that the DOS strain measurement method and equipment are a robust strain acquisition option with the capability to overcome some of the limitations of the traditional foil SG and DIC methods.

This paper presents experimental and numerical studies of the progressive damage in composite laminates. Firstly, damage initiation or first ply failure (FPF) is predicted by comprehensively investigating several failure criteria. Then, a modified method for the progressive damage modeling of composite laminates by employing a combination of gradual and sudden reduction rules is proposed for predicting the last ply failure (LPF). In the method employed, progressive damage is modeled with various exponential material softening laws and appropriate exponents for exact simulation of the damaged ply derived from experimental tests. Finally, for investigating the performance and capability of the proposed method for predicting progressive damage in composite materials, a series of experimental tests in Glass/Epoxy composite laminates with a variety of stacking sequences are presented. A comparison of the numerical and experimental results shows that the proposed method can accurately simulate the progressive damage in composite laminates.

Nowadays, many components are made of composite laminates assembly by bolted or riveted joints. Thus, the drilling of laminates is a usual machining operation in many industries. Due to the stress concentration around the hole, different damage modes can occur during the service. In this study, the effect of interleaving Nylon 66 nanofibers between glass/epoxy laminate on decreasing damage under the three-point bending test is considered. The results showed that applying nanofibers caused a 57% decrease in the delaminated area. On the other hand, the Acoustic Emission (AE) technique was used to consider the effect of Nylon 66 nanofiber on other damage modes, matrix cracking, fiber/matrix debonding, and fiber breakage. AE signals were analyzed with three different methods, the outcomes showing that these damages significantly decreased.

Bismaleimide (BMI) resins are gaining popularity as matrix materials in carbon fiber composites, especially in high temperature applications, due to their very high glass transition temperatures. Extended elevated temperature exposure can cause a decrease in the matrix dominated properties of BMI-matrix composites due to thermal damage mechanisms that can be chemical (crosslinking, oxidation) or physical (microcracking, delamination) in nature. The chemical damage begins at lower thermal exposures than the physical damage; however standard ultrasonic testing (UT) techniques detect only the physical damage, which is not apparent until a significant loss in matrix dominated properties, such as shear strength, have already occurred. In this study, a thermal damage detection method using a handheld Fourier transform infrared (FTIR) spectrometer to detect chemical changes caused by thermal exposure of Solvay 5250-4/IM7 BMI composite laminates was investigated. Infrared spectra were collected from samples with varying levels of thermal exposure. The spectra were analyzed using multivariate analysis techniques. The FTIR measurements were combined with mechanical property changes measured using short beam strength testing (ASTM D2344-16 ) to develop a model which identified the onset and extent of damage by predicting the change in interlaminar shear strength based on IR spectral changes. We compare our model's performance to ultrasonic inspection as a method for identifying the onset of thermal damage. The FTIR based method detected statistically significant decreases in interlaminar shear strength at thermal exposures well below those causing UT-detectable damage.

A riveting process was developed to fasten carbon fiber/epoxy composite laminates. This process heats a thermoplastic composite rivet blank placed into the joint hole, above melt temperature, to form a rivet fastening the joined components. This process poses a threat to the integrity of the structure's epoxy matrix. To verify the state of matrix degradation, a pin-loaded mechanical test was applied on composite test pieces heat treated at various temperatures. The heat treatment was specifically designed to recreate the riveting process thermal cycle. A finite element model of the heat treatment was produced to obtain the time and temperature exposure of the test piece during the heat treatment. For use in the finite element model, emissivity was characterized through infrared image analysis, and thermal conductivity was characterized with an inverse method. The results show that a riveting process at nominal temperature of 350 °C will not affect the bearing properties of the composite laminates. However, the process at 450 °C does affect the bearing properties.

Composite structures are particularly vulnerable to impact which drastically reduces their residual strength [1–3]. In this work, the influence of laminate thickness, ply-stacking sequence and scaling technique on the impact damage tolerance of CFRP laminates is investigated by means of compression after impact (CAI) tests. Drop-weight impact tests were carried out to determine impact response of the different composite laminates. CAI tests were performed in a non-standard CAI device in order to obtain the compression residual strength. In addition, open-hole compression (OHC) tests were performed for comparative purposes. Ultrasonic C-scanning and cross-sections of CAI samples were examined to assess failure mechanisms of the different configurations. It was observed that damage tolerance decreases as impact energy increases. In addition, thicker laminates show higher CAI strength due to higher bending stiffness. Furthermore, angle-ply laminates depict better performance in terms of damage tolerance. Finally, the results obtained demonstrate that introducing ply blocking had a negative effect on the damage resistance but, conversely, an improvement of the CAI strength.

A major concern affecting the efficient use of composite laminates is the effect of low velocity impact damage on the structural integrity [1–3]. The aim of this study is to characterize and assess the effect of laminate thickness, ply-stacking sequence and scaling technique on the damage resistance of CFRP laminates subjected to low velocity impact. Drop-weight impact tests are carried out to determine impact response. Ultrasonic C-scanning and cross-sectional micrographs are examined to assess failure mechanisms of the different configurations. It is observed that damage resistance decreases as impact energy increases. In addition, thicker laminates show lower absorbed energy but, conversely, a more extensive delamination due to higher bending stiffness. Thinner laminates show higher failure depth. Furthermore, quasi-isotropic laminates show better performance in terms of damage resistance. Finally, the results obtained demonstrate that introducing ply clustering had a negative effect on the damage resistance and on the delamination area.

A three-dimensional differential viscoelastic model combining the effects of curing degree, thermal expansion, chemical shrinkage and stress relaxation for composite laminates was established and well verified. The evolution of strain and stress of composite laminates during cure was numerically simulated using the validated model. Also, fiber Bragg grating temperature and strain sensors were adjacently embedded in the composite laminate to in situ monitor the temperature and strain evolution. The monitored strain was evaluated by comparison with the corresponding simulated strain. The results reveal that the monitored temperature can reflect the actual temperature evolution in composite laminates, whereas the monitored strain cannot accurately characterize the actual strain evolution in composite laminates at the early stage of cure. However, when the resin/grating interfacial bond strength increases enough to transfer resin strain effectively, the changes in the monitored strain can match well with the actual strain changes.

In the present work, dynamic compression response of polypropylene (PP) based composites reinforced with Kevlar/Basalt fabrics was investigated. Two homogeneous fabrics with Kevlar (K3D) and Basalt (B3D) yarns and one hybrid (H3D) fabric with a combination of Kevlar/Basalt yarns were produced. The architecture of the fabrics was three-dimensional angle-interlock (3D-A). Three different composite laminates were manufactured using vacuum-assisted compression molding technique. The high strain rate compression loading was applied using a Split-Hopkinson Pressure Bar (SHPB) set-up at a strain rate regime of 3633–5235/s. The results indicated that the dynamic compression properties of thermoplastic 3D-A composites are strain rate sensitive. In all the composites, the peak stress, toughness and modulus were increased with strain rate. However, the strain at peak stress of Basalt reinforced composites (B3D, H3D) decreased approximately by 25%, while for K3D specimens it increased approximately by 15%. The K3D composites had a higher strain rate as compared to the B3D and H3D composites. In the case of K3D composite, except strain at peak stress, remaining dynamic properties were lower than the B3D composite, however, hybridization increased these properties. The failure mechanisms of 3D-A composites were characterized through macroscopic and scanning electron microscopy (SEM).

The main objective of this paper is to investigate the effect of intra-ply hybrid patches based on glass and Kevlar woven fabrics on the local bending response of adhesive bonded external patch repairs in damaged glass/epoxy composite laminates. In intra ply hybrid patches glass and Kevlar fibre reinforcements are combined in the same layer. The intention, in using these hybrid patches, is to combine the excellent mechanical properties of glass fiber as a brittle reinforcement with the superior high elongation to failure property of Kevlar fiber as a ductile reinforcement. Five different kinds of plain weave woven fabrics with different ratios between glass and Kevlar fibers (100/0, 75/25, 50/50, 25/75 and 0/100) were used as the external patches. The undamaged virgin specimens were taken as a reference for the comparison of residual mechanical properties. Multiple quasi-static indentation tests were carried out on repaired glass/epoxy specimens, and their ultimate indentation load, stiffness and permanent deformation were estimated. Failure mechanisms of repaired glass/epoxy specimens under indentation loads were investigated using online Acoustic Emission (AE) monitoring technique. The indentation loads required for the occurrence of various failure modes were measured to illustrate the chronology of progression of different damage modes with increasing load and the kinetics of the various damage modes individually defined in real time. The use of different hybrid patches had a significant effect on the local bending response of the repaired glass/epoxy specimens. In practice, specimens repaired with patches including equal volume fraction of glass and Kevlar fibers presented a more favorable indentation response than virgin ones and other repaired specimens by exhibiting balanced mechanical properties (i.e., high deflection to ultimate failure associated with superior patch-parent laminate bond strength).

This paper presents an experimental study on the impact behaviour of composite laminates made of a Dyneema® woven fabric and four different resin matrices. Three thicknesses of each kind of resin laminate were subjected to impact by a spherical steel projectile in a velocity regime ranging from 100 to 200 m/s. The results revealed that the laminates having flexible matrices performed much better in perforation resistance and energy absorption, but had a greater extent of deformation and damage than the counterparts with rigid matrices. It was found that the matrix rigidity played a crucial role in controlling the propagation of transverse deformation, and thereby the local strain and perforation resistance of laminates. The more rigid matrix restrained the laminate's transverse deformation to a smaller area at a given time, which led to higher local strain and lower perforation resistance. Fibre failure in tension was identified as the dominant failure mechanism for the tested laminates.

This paper investigates the characterization of in-plane shear properties of thermoplastic composites reinforced with Kevlar/basalt fabrics. Different fabrics had architectures of two dimensional plain woven (2D-P) and three dimensional angle-interlock (3D-A). Intralayer hybridization was performed during the weaving of the fabrics with the combination of Kevlar and basalt yarns. Five 2D-P and three 3D-A composite laminates were manufactured with polypropylene (PP) as a matrix, using compression molding. Iosipescu shear tests were carried out to evaluate the in-plane shear properties. The experimental results revealed that the shear properties including shear modulus, shear strength and shear failure strain of homogeneous composites were improved by 6.5–14.9%, 4.3–19.7%, and 3.2–46.7%, respectively. Similarly, change in the fabric architecture from 2D-P to 3D-A also enhanced the shear strength and shear failure strain by 32.0–41.6% and 7.2–22.5%, respectively. Intralayer hybrid composites had better in-plane shear properties than the interlayer hybrid composites. The fracture morphologies of the specimens were examined by scanning electron microscopy (SEM).

This paper investigates the effects of thermal cycling on mechanical degradation of polymer matrix composites (PMCs). Un-notched and open-hole specimens are tested using developed thermal cycling apparatus and tensile test machine. In addition, the hole-size effect of open-hole tension glass/epoxy composite laminates is investigated. The tensile strength, mass loss and surface degradation of the specimens were obtained during 250 cycles. Experimental results showed that the holes diameter is the main parameter to control the thermal cycling effects on open hole structure. Also, it is found that laminates with smaller holes have higher tensile strength variation than those with larger holes. The results showed that increment of the hole diameter and number of cycles decreases the tensile strength.

Digital Image Correlation (DIC), which exploits non-contact advantages and full-field analysis, provides more data in-situ that are not possible with traditional techniques. In this work, elevated temperature digital image correlation techniques were applied to a glassy polymer network via thermal expansion and contraction experiments to study volumetric behavior during the curing process. A glassy epoxy network was tested in both cured and under-cured states and heated to the ultimate cure temperature. Matrix volume changes due to both thermal expansion and cure shrinkage were quantified. Concurrently, the thermal expansion of aerospace-grade composite laminates was also observed in matrix and fiber-dominant directions. Additionally, the strain-free temperature of a non-symmetric composite laminate was identified through thermal compensation of process-induced curvatures. Finally, laminate dimension changes were related to the strain-free temperature as means to probe process-induced strains within composite laminates. Thermal properties of the neat matrix and composite laminates were compared to traditional techniques, validating the benefit of elevated temperature digital image correlation for composite matrix qualification.

Mechanical properties of unidirectional (UD) and woven fabric glass/epoxy composites under off-axis tensile loading were experimentally investigated. A number of off-axis tests considering different fibre orientations were performed to study the character and failure mechanisms of the composite laminates. The experimental results indicated that both off-axis elastic moduli and strength degrade with increasing off-axis angle in all cases, and the woven fabric composites present nonlinear stress-strain behaviour under off-axial tension loading. The Tsai-Wu criteria used for failure analysis of the UD and woven fabric composites were compared and discussed, especially considering different values of interaction coefficient F12. The prediction results demonstrated that the Tsai-Wu criterion can be used successfully to analyse failure properties of the woven fabric composites under multiaxial stress conditions, where the criterion with the modified coefficient F12 obtained from the 45° off-axial tension tests is better and has higher accuracy. Finally, the specific failure modes were compared in the UD and woven fabric composites. The selected fracture surfaces were also observed by scanning electron microscopy (SEM), and the corresponding failure mechanisms of the woven fabric composites under off-axis tensile loading were identified.

In this work, two types of self-healing composite laminates with embedded 6 and 12 wt% microcapsules respectively, were prepared. The guided waves were activated and captured using piezoelectric transducer in a pitch-catch approach. Mode I interlaminar fracture tests were conducted using double cantilever beam specimens, and the effects of concentration of microcapsules on the interlaminar properties of specimens were investigated. The interlaminar fracture surfaces of self-healing specimens were observed to support the analysis of mechanical tests and guided wave detecting results. The results show that the self-healing efficiency of specimens with high-concentration microcapsules is less than that of specimens with low-concentration microcapsules. Meanwhile, low-concentration microcapsules reduce the interlaminar properties of specimens. Two guided-wave parameters, namely signals peak value and waveform similarity, were set and correlated with the self-healing capability of specimens. When the specimens were damaged, the peak value of response signals decreases, and the waveform difference between the response signal and reference signal becomes larger. When the specimens were healed, the peak value of response signals rises, and the waveform difference between them becomes smaller.

Fibre-matrix adhesion affects fibre-reinforced composites' mechanical properties, a process which can be improved by applying appropriate sizing on the fibre. Transverse bending tests and Scanning Electron Microscopy (SEM) can help quantify this effect This paper investigates if modal damping measurements are a reliable alternative for quantifying fibre-matrix adhesion. When a composite sample is vibrating, part of the dissipated energy is due to the internal friction. More internal friction and slipping at the fibre-matrix interface is expected with a weaker fibre-matrix bond, hence increasing the amount of dissipated energy, which in turn is proportional to the modal damping value. This paper researches two different cases to validate this hypothesis. In the first case, we will use two composite samples of flax fibre, one with and one without sizing. In the second case, we will compare flax and carbon fibre laminates. If the only variable is fibre sizing, better adhesion is related to significantly lower damping and higher resonance frequencies. If composite laminates with different fibre and matrix type are compared, lower adhesion is not necessarily related to increased damping and lower resonance frequencies. However, when combining the damping result with SEM microscopy, it is possible to assess the relative contribution to the internal energy dissipation of the fibre, the matrix and the fibre-matrix interface individually. (C) 2016 Elsevier Ltd. All rights reserved.

Graphene oxide (GO) was functionalized using three different diamines, namely ethylenediamine (EDA), 4,4′-diaminodiphenyl sulfone (DDS) and p-phenylenediamine (PPD) to reinforce an epoxy/glass fiber (EP/GF) composite laminate, with the aim of improving the overall composite mechanical performance. Different mechanical characterization techniques were used to determine the mechanical performance, including: tensile stress strain, double cantilever beam (DCB) mode-I fracture toughness and dynamic mechanical thermal analysis (DMTA). Scanning electron microscopy (SEM) was used to support the results and conclusions. The results demonstrated remarkable enhancements in the mechanical performance of EP/GF composite laminates by incorporation of functionalized graphene oxide (FGO) nanofiller, whilst the mechanical performance of the GO reinforced composite only improved marginally. Finally, the mechanical performance of the EP/GF/FGO multi-scale composites was found to be dependent on the type of FGO functional groups; of which EDA exhibited the highest performance. These investigations confirmed that the EDA-FGO-reinforced EP/GF composites possess excellent potential to be used as multifunctional engineering materials in industrial applications.

The effect of through-thickness compression on in-plane tensile strength of glass/epoxy composites with random microstructure was investigated experimentally. The studied composite laminates were manufactured with a self-regulating Resin Transfer Moulding device. Their mechanical behaviour was assessed in pure in-plane tensile and through-thickness compressive tests, followed by biaxial tests combining both loading modes; indenters with a radius ranging from 5 to 25 mm were used to impose a compressive mode. The obtained results demonstrate a nonlinear decreasing trend for the in-plane tensile strength under the growing through-thickness compressive stress. All the failed specimens showed catastrophic brittle failure with a specific fracture orientation that mainly exhibited a tensile mode of fibre fracture for smaller radii of indenters and a combination of matrix crack, fibre fracture and typical shear failure for larger radii.

It is widely known that damage induced by low-velocity impact events will significantly affect the damage tolerance and integrity of composite structures. This study investigates the damage evolution and failure mechanism of impacted laminates under fatigue loadings using the infrared thermography and ultrasonic methods. Low-velocity impact damage was introduced by a drop weight with a hemispherical impactor, and compression-after-impact (CAI) tests were conducted to determine the residual compressive strength. Four stress levels were selected to conduct compression-compression fatigue tests with a stress ratio R = 10 based on the static CAI strength. During fatigue tests, the surface temperature variations were monitored online by an infrared camera, and the delamination damage evolution was observed using ultrasonic C-scan methods. The thermal images, delamination contours and stiffness degradation were analyzed. The maximum temperature increase and the stiffness degradation show the characteristic with three stages “rapid-slow-rapid”. For evolution of damage area, two stages were observed. The damage area extends from the impact indentation to two outer free edges in the perpendicular direction until the final collapse of the specimen.

Today composite structures are widely used in different industries. Since these structures are sometimes experienced impacts of low-velocity objects, evaluating the intensity of the damage due to the impact in these structures seems important and necessary. In this article, the effectiveness of the thermography method with three different thermal excitation sources, including the ultrasonic vibrothermography, infrared thermography with the heating element, and infrared thermography with hot airflow, for damage evaluation in CFRP and GFRP composite plates subjected to low-velocity impacts are investigated. The ultrasonic vibrothermography method is introduced as the most promising technique for damage evaluation in CFRP and GFRP composite plates subjected to low-velocity impacts, and the effect of the excitation power and excitation frequency on the effectiveness of this method is studied. To find the damage location in the thermography images in which the damage is not clear enough to be identified, the wavelet transform is applied while it is not suitable for calculating the damaged area. Finally, finite element (FE) simulations are carried out for the case of an internal defect. The results show the acceptable accuracy of the FE simulation in comparison with the experimental results.

Flax-PP based thermally bonded roving (TBR) has a unique structure where the flax fibres remain twist-free and fully aligned along the roving axis. The present study describes an experimental investigation on the low velocity impact (LVI) behaviour of the TBR based woven fabric composites and compares the same with plain woven glass fabric reinforced PP composites (GRPC). Two different fabric architectures namely plain woven (PW) and unidirectional (UD) are fabricated using flax/PP based TBR. These TBR based woven fabrics and the glass fabric/PP sheets are consolidated in a compression moulding machine and the resultant composite-laminates are tested for their LVI behaviour. The impact test results revealed that the glass/PP composites absorb more energy and exhibit a higher peak load than both TBR based PW and UD fabric composites. However, the specific load and energy of all flax/PP composites are higher than the glass/PP composite. The damage tolerance of all composite laminates are evaluated by comparing their flexural strength before and after the impact. It is observed that the proportionate loss in flexural strength due to impact thrust is larger in case of glass/PP composites than all flax-PP composites.

Carbon/glass hybrid composite (CGHC) laminates are some of the most promising composites for lightweight applications. Sometimes these laminates are used in warm environment, such as aircraft frame structures, and this may affect their performance. In order to investigate this issue, the present research aims to study the effect of temperatures on the impact behavior and pseudo-ductile behaviour of CGHC in presence of different types of thermosets “epoxy” and thermoplastic “acrylic poly-methyl methacrylate-PMMA”. The experiments were started with making of CGHC laminates from different stacking sequences of unidirectional carbon and woven glass fibre layers, using a vacuum-assisted resin transfer method followed by curing treatment. In addition to CGHC laminates, four other neat batches (Carbon/epoxy, Carbon/PMMA, Glass/epoxy, Glass/PMMA) were prepared for comparison. The low velocity impact behaviour of the fabricated panels was evaluated at high temperatures (60 °C and 80 °C) according to ISO 6603-2 standard, using drop tower, while pseudo-ductile behaviour and ductility index (DI) of the specimens were estimated based on the measured total energy and elastic energy. Also, the low-velocity impact response was modeled mathematically based on a modified energy-balance model to predict the absorbed energies. Finally, the failure mechanisms were examined using optical microscope to determine the influence of these damage growth on DI of the composites under different temperatures. The results showed that the impact energy response of both hybrid composites i.e. epoxy and PMMA was stable even as the temperature rose, however, carbon/glass/PMMA exhibited better performance compared with carbon/glass/epoxy with an increase in impact energy response estimated at 50% (25 °C) and 53% (80 °C). Also, the pseudo-ductile phenomenon was strongly evident, which facilitates the predictablility of failure.

The composite laminates are susceptible to delamination between reinforcing plies during their long-term service. In this paper, we propose a modified carbon fiber/epoxy composite laminate with embedded clustered dual-component microcapsules in order to increase the interlaminar fracture toughness of the lamina. The details of microcapsules were illustrated using scanning electron microscope (SEM). The modified CF/EP composite laminates were fabricated using hot-compaction technique. Mode I interlaminar fracture tests were conducted using double cantilever beam specimens, then the values of opening fracture toughness GIC were calculated to evaluate the toughening effect of modified laminates. The toughening mechanism was revealed and discussed through micrographs of the fracture surfaces obtained by ultra-depth microscope and SEM. The results show that clustered microcapsules after polymerization are equal to special Z-pinning, significantly enhancing the ability of crack arrest, and largely and roundly improved the GIC values of resultant composite laminates. Meanwhile, the clustered microcapsules and matrix resin formed a second-phase material layer, which also absorbed the fracture energy and suppressed the expansion of cracks.

An experimental and semi-analytical study of distortion of asymmetric composite laminates with different cooling rates and lay-ups has been presented. In this study, thermomechanical constitutive equations of thin composite laminates are developed using basic viscoelastic constitutive law considering chemical and thermal effects with time-temperature dependent material properties. To solve a fully scouple problem, both the thermochemical and thermomechanical constitutive equations are formulated. The general heat conduction equation known as the Fourier-Biot equation, viscoelastic laws, Boltzmann superposition principle and composite equations are utilized to formulate thin composite laminates. A static model with constant properties in ambient temperature and a transient model by obtaining constitutive equations are simulated. Results are compared with experimental data. Changing lay-up from cross-ply to angle-ply and then quasi isotropic will increase the value of maximum distortion. Results indicated that the increasing cooling rate will increase the value of the maximum distortion. The differences between FEM results with static analysis of different lay-ups and experimental specimens that cooled in the oven, environment and refrigerator is about 3%, 35% and 55% respectively. The differences between FEM simulation with transient analysis of different lay-ups and experimental specimens that cooled in the environment and the refrigerator is less than 9%.

During the last decade, interest in using natural fibre reinforced composites has increased due to their good specific mechanical properties. The results of an experimental study of the influence of temperature and impact velocity on the low velocity impact response of jute fiber reinforced unsaturated polyester (UP) composite laminates is presented. A series of low velocity impact tests were performed using an instrumented falling weight test system at three different temperatures (30, 50 and 75 °C) and two impact velocities (1.5 and 2 m/s). The results indicate that jute/UP composites were able to withstand higher loads when tested at 30 °C. The composites were subsequently subjected to three-point bending tests in order to investigate the flexural strength after impact (FAI) and it was found that FAI significantly decreased with increase in test temperature. Impact damage characterised by AE revealed delamination as a major failure mode.

A new test configuration is proposed for measuring transverse compressive strength of unidirectional composite laminates by three-point bending. A cross-ply configuration [90 m /0 n ] has been chosen in order to get failure due to transverse compressive stresses. Thermal and mechanical stresses have been calculated based on hypotheses of classical beam theory. The optimal cross-ply configuration has been obtained by maximizing the ratio between maximum compressive stresses at the 90° layer and maximum tensile stresses at the 0° layer. Cross-ply carbon/epoxy strips with different thicknesses have been tested in three-point bending until failure. The appearance of the transverse compressive damage has been found to agree well with that found in the literature for end-loaded compression tests.

The aim of this work is to propose a new method for determining both the in-plane shear modulus and the in-plane shear strength of composite materials by using three-point flexure tests on [±45] angle-ply laminates. Symmetric and anti-symmetric configurations have been analysed by Classical Laminated Plate Theory. The anti-symmetric configuration has been considered more suitable than the symmetric one, due to the absence of bending/twisting coupling effects. Three-point bending tests have been performed on [±45] anti-symmetric laminates of AS4/8552 carbon/epoxy material. Experimental results obtained by flexure tests in anti-symmetric laminates agree with the values attained by tensile tests on symmetric laminates, and with values reported in literature. Experimental conditions for determining in-plane shear properties by using the anti-symmetric three-point bending method are recommended.

Structural health monitoring (SHM) and in situ monitoring of structural integrity are of primary importance in many engineering fields such as automotive, aerospace and complex civil structures. Furthermore, the ever growing use of composite materials has raised the need for reliable and cost-effective health monitoring systems. This work presents an experimental investigation of a piezopolymeric based in situ damage detection technique based on acoustic emission (AE) with poly(vinylidene fluoride) (PVDF) sensors as AE sensors. These sensors have been surface-mounted and embedded in glass/epoxy laminates and used to monitor the laminates while being subjected to three types of loading, namely tension, three-point bending and post-impact three-point bending. A parametric off-line analysis of signals detected and scanning electron microscopy (SEM) analysis allowed assessing the ability of PVDFs to monitor the damage growth and development in these laminates. The results confirm that PVDFs can be applied in real time monitoring studies of composite laminates.

A new test method is proposed for measuring longitudinal compressive strength of composite laminates by three-point bending of cross-ply laminates. Optimal cross-ply configuration has been designed in order to get compressive stresses higher than tensile stresses. Thermal and mechanical stresses have been calculated based on the hypothesis of Classical Beam Theory. Cross-ply carbon/epoxy strips with different thicknesses and spans have been tested by three-point bending. Failures on the compressive side have been observed in all cases and no evidence of transverse cracking has been found. Experimental results agree well with compressive strength reference values. The possibility of determination of compressive and tensile strengths by three-point bending from a unique cross-ply lay-up has been also analyzed.

peer-reviewed The present study deals with the quasi-static and high strain rate characterization of Kevlar-129 based thermoplastic composites. Two different thermoplastic matrices, namely, Polypropylene (PP) and Polyetherimide (PEI) were used to manufacture composite laminates. Quasi-static compression tests were performed at strain rates of 0.041 s−1 and 0.045 s−1. High strain rate tests were performed using a split Hopkinson pressure bar apparatus within the strain rates ranging from 2548 s−1 to 4379 s−1. Stress-strain relations reveals the rate-sensitive behaviour of composites. Kevlar/PP (K-PP) showed higher peak stress under quasi-static loading as compared to the high strain rate test. Comparable peak stresses were revealed under quasi-static and high strain rate loading for Kevlar/PEI (K-PEI) composite. Also, high strain rate compression properties such as peak stress, peak strain and toughness of K-PP were 25%, 27% and 6% higher than that of the K-PEI composite. The failure mechanisms of both the composites were characterized through macroscopic and scanning electron microscopy. K-PP failed majorly due to matrix crush and fibre failure while K-PEI failed due to shear cracking. Damage study reveals that a single fibre based composite system can be tailored to act as an energy-absorbing or dissipating material system by varying the thermoplastic matrix materials