Experimental investigation on mechanical properties of unidirectional and woven fabric glass/epoxy composites under off-axis tensile loading

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.     

Damage resistance of carbon fibre reinforced epoxy laminates subjected to low velocity impact: Effects of laminate thickness and ply-stacking sequence

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 study on correlating reduction in Poisson's ratio with transverse crack and delamination through acoustic emission signals

During the uniaxial loading of fiber reinforced polymer (FRP) composites, Poisson's ratio (ν_xy), which is a constant elastic property for isotropic materials, decreases significantly. Micro-damage created within FRP composites as a result of an applied stress causes this decrease. As the level of micro-damage increases, a greater level of reduction in Poisson's ratio occurs. FRP composites, in general, show three main micro-damage types under uniaxial tensile loading, namely, transverse crack, delamination and fiber rupture. To determine micro-damage types which dominantly affects the relevant reduction in Poisson's ratio, glass fiber reinforced cross-ply laminates with three different off-axis ply content are produced and then tested under a uniaxial tensile loading. The Acoustic Emission (AE) signals are concurrently recorded and grouped into three clusters in accordance with their frequency, which is either associated with transverse crack, delamination or fiber rupture. The frequency based clustering of AE signal facilitates detailed investigation of delamination onset and effect of different micro-damage types on Poisson's ratio. It is proven that stacking sequences with a higher number of transverse cracks and delaminations, quantified based on AE signals, show a greater reduction in Poisson's ratio.     

Experimental observation of tensile behavior of patch repaired composites

This work concerns notched composite plates repaired with external circular patches. Their load carrying capability and failure process under tensile loading were investigated experimentally. In the case of a strong adhesive joint, the stiffness of the patches has to be optimized to release high stress concentration and obtain the best repair performance. It was found that the damage progression of the repairs depends not only on the patch stiffness but also on the stacking sequence. With visual inspection and acoustic emission, three modes of damage and failure process for patch repaired specimens were identified. Theses damage models are essential for the development of a numerical design tool for the optimization of external patch repairs.     

Scaling effect on the tensile properties of [±45/0/±45/0/±45] Polypropylene/Twaron laminates

The effects of scaling on the mechanical response under tension of balanced nonsymmetrical laminates were investigated for a thermoplastic composite: Polypropylene reinforced with Twaron^® fibers. The composite baseline was an 8-ply laminate which consisted of unidirectional plies arranged in the sequence [±45/0/±45/0/±45]. The influence of specimen size on the tensile properties was studied for one (thickness), two (in-plane) and three (volume) dimensional scaling. The stress-strain curves suggested some variation in laminate behavior owing to the dimensional scaling; nevertheless, a further analysis with the classical lamination theory demonstrated that the observed effect was due to small variations in the fiber volume fraction of the laminates. It was concluded that the mechanical properties of these thermoplastic laminates do not exhibit scaling effects. The failure mechanism of the laminates was studied at macroscopic level; a scale effect of the fracture mechanism was observed.     

Effects of fabric architectures on mechanical and damage behaviors in carbon/epoxy woven composites under multiaxial stress states

The mechanical properties and damage evolutions of carbon/epoxy woven fabric composites with three different fabric architectures, including one plain weave and two twill weave patterns, are experimentally investigated under multiaxial stress states. In particular, the effects of weave patterns are investigated by monotonic and cyclic off-axis tension tests. Both elastic modulus and strength degrade remarkably with increasing off-axis loading angle, while Poisson's ratio is much higher than that measured from on-axis tests and increases with loading strain gradually. Different fabric architectures show limited effects on the modulus and strength under multiaxial stress states, and they are well predicted by transformation equation and Tsai-Wu failure criteria, respectively. However, significantly different failure behaviors are observed in three fabric composites, and microstructure observation shows that fabric architecture affects the stress concentration and the damage development. Smaller crimp ratio and compacted structure postpone the damage development but result in more abrupt failure under multiaxial stress states.     

Coupling coefficients of glass/epoxy laminates under off-axis tensile conditions: Experimental verification

A comprehensive experimental study on the coupling coefficients of unidirectional (UD) and woven fabric glass/epoxy laminates under off-axis tensile loading was conducted in comparison with the theoretical prediction. The capability of the off-axis test to evaluate the elastic constants in the loading direction was reported. Four coupling coefficients were obtained from tests and discussed in comparison with the theoretical prediction. A further comparison of coupling compliance coefficients in compliance matrix was made in order to generalize the influence of off-axis angle on the compliance coefficients. The theoretical prediction agreed well with the experimental data. It is shown that non-monotonic and symmetry phenomena can be observed in the curves of the coupling and compliance coefficients. The results of this experimental study provide a data base of the coupling coefficients of glass/epoxy laminates for engineering application.     

High strain rate compression testing of intra-ply and inter-ply hybrid thermoplastic composites reinforced with Kevlar/basalt fibers

In this study, the influence of hybridization on the compression response of thermoplastic matrix-based composites under high strain rate loading was investigated. The intra-ply and inter-ply hybrid composites were manufactured with Kevlar/Basalt yarns as the reinforcements with Polypropylene as a matrix. Cylindrical composite specimens were laser cut from the flat compression moulded laminates. The composite specimens were loaded under high strain rate using split-Hopkinson pressure bar setup at strain rates ranging from 2815/s to 5481/s. The study revealed differences in the rate-dependent growth of peak stress, peak strain and toughness with the strain rate. Intra-ply hybrid composites with alternate weaving of Kevlar and basalt yarns exhibited highest peak stress as compared to the Inter-ply hybrid composites (alternate layers of Kevlar and basalt fabrics) and another intra-ply composite containing Kevlar in the warp and basalt in the weft direction. Whereas in inter-ply hybrid composite, with Kevlar as the loading face attained higher stress, while composite with Basalt as the loading face attained higher strain. SEM micrographs revealed that Kevlar on the loading face can bear the impact with lesser delamination as compared to the Basalt on the loading face. Damage studies revealed that Kevlar fiber surface loading results in higher stress as compared to basalt (brittle) surface loading with lower overall damage.     

Failure analysis of plain woven glass/epoxy laminates: Comparison of off-axis and biaxial tension loadings

An experimental study was focused on investigation of the failure properties of plain woven glass/epoxy composites under off-axis and biaxial tension loading conditions. Four fibre orientations (0°, 15°, 30° and 45° with respect to the load direction) were considered for off-axis tests and two biaxial load ratios for biaxial tests to study failure characteristics and mechanism. Four classical polynomial failure criteria - Tsai-Hill, Hoffman, Tsai-Wu and Yeh-Stratton - were analysed comparatively to predict off-axis and biaxial failure strength of the composites. For failure prediction of the plain woven composites under multiaxial tension loads, the Tsai-Wu criterion was modified by introducing an interaction coefficient F₁₂ obtained from 45° off-axis or biaxial tension tests and the Yeh-Stratton criterion was modified with the interaction coefficient B₁₂ = 0 or obtained from the biaxial tension test. The former criterion was found to have higher accuracy. Finally, according to macroscopic and microscopic studies, the failed specimens showed mostly distinct failure with a specific fracture orientation, mainly exhibiting fibre or fabric tensile fracture mode and a combination of matrix cracking and delamination, both in off-axis and cruciform samples.     

Effect of laminate thickness on impact behavior of aramid fiber/vinylester composites

Aramid fiber/vinylester composites were fabricated to investigate the effect of laminate thickness on impact behavior of the composites. The impact energy and the delamination area of composites were examined as a function of laminate thickness and surface treatment of aramid fiber. The laminate thickness and surface treatment changed the impact absorption mode from plate bending stress to local stress. The absorption mechanism of impact energy changed at a thickness between three-layer composites and four-layer composites. The impact energy of thin laminates was dominated by a large displacement and delamination area, whereas that of thick laminates was controlled by maximum load. The trend of total delamination area was similar to that of total impact energy in both untreated and treated composites. In spite of low delamination area, thick composites exhibited the higher impact energy through increase of maximum load.     

Effect of strain rate on the dynamic tensile behaviour of UHMWPE fibre laminates

Ultra-high molecular weight polyethylene (UHMWPE) fibre has great potential for strengthening structures against impact or blast loads. A quantitative characterization of the mechanical properties of UHMWPE fibres at varying strain rates is necessary to achieve reliable structural design. Quasi-static and high-speed tensile tests were performed to investigate the unidirectional tensile properties of UHMWPE fibre laminates over a wide range of strain rates from 0.0013 to 163.78 s⁻¹. Quasi-static tensile tests of UHMWPE fibre laminates were conducted at thicknesses ranging from 1.76 mm to 5.19 mm. Weibull analysis was conducted to investigate the scatter of the test data. The failure mechanism and modes of the UHMWPE fibre laminates observed during the test are discussed. The test results indicate that the mechanical properties of the UHMWPE fibre laminate are not sensitive to thickness, whereas the strength and the modulus of elasticity increase with strain rate. It is concluded that the distinct failure modes at low and high strain rates partially contribute to the tensile strength of the UHMWPE fibre laminates. A series of empirical formulae for the dynamic increase factor (DIF) of the material strength and modulus of elasticity are also derived for better representation of the effect of strain rate on the mechanical properties of UHMWPE fibre laminates.     

Progressive failure numerical simulation and experimental verification of carbon-fiber composite corrugated beams under dynamic impact

To explore the axial impact energy absorption capacity of bidirectional carbon pre-impregnated (prepreg) composite corrugated beams, a solid 3D finite element model with different trigger mechanism settings and different ply designs was established. Numerical simulation of dynamic impact was performed on the model. An in-plane damage model considering shear failure was created based on continuum damage mechanics and Hashin's criteria, and a stiffness degradation model of damage failure for G803/5224 is proposed. The cohesive zone model is used and the bilinear traction-separation constitutive model is considered to simulate inter-laminar delamination failure, thereby accurately reflecting the anisotropic progressive damage characteristics of bidirectional carbon-fiber prepreg composite corrugated beams. The results show that progressive failure and damage occur under impact loading of corrugated beams. The energy-absorbing load-displacement curve and specific energy absorption were obtained through simulation. Simulation results were validated by comparison with test results. With the maximum relative error of its average crushing load less than 11%, the damage morphology and test results of the beam has improved in uniformity. Furthermore, the validity of 3D finite element models considering inter-laminar delamination damage has been validated.     

Mechanical characterization of 3D angle-interlock Kevlar/basalt reinforced polypropylene composites

The present work reported the mechanical characterization of novel polypropylene (PP) composites reinforced with three-dimensional angle-interlock (3D-A) Kevlar/basalt fabrics. Two homogeneous fabrics with Kevlar (K3D) and basalt yarns (B3D), and a hybrid fabric (H3D) with a combination of both Kevlar and basalt yarns were produced. Three types of two layer 3D-A composites were manufactured using vacuum-assisted compression molding method. Static tensile and in-plane compression tests were carried out on the manufactured composites. The mechanical behavior of the three 3D-A composites was compared in terms of stress-strain response, elastic modulus, strength and failure strain. Influence of hybridization on the mechanical behavior of the 3D-A composites was also studied. Significant improvement in the tensile behavior of 3D-A homogeneous composites was observed due to hybridization. Meanwhile, there was no considerable improvement in in-plane compression behavior. The damage patterns for in-plane compression loading were examined through scanning electron microscopy (SEM) to explore the possible damage patterns such as matrix cracking, fiber failure, delamination and deformation. Numerical simulations were carried out using ABAQUS/Standard, by implementing a user-defined material subroutine (VUMAT) based on the Chang-Chang linear orthotropic damage model. Good agreement between experimental and numerical simulations was achieved in terms of damage patterns.     

Characterization of nonlinear response in quasi-unidirectional E-glass fabric reinforced polypropylene composites under off-axis tensile loading

The present paper addressed the nonlinear stress-strain response in quasi-unidirectional E-glass fabric reinforced polypropylene composites under off-axis tensile loading. A series of monotonic and cyclic loading-unloading tensile tests were carried out. Both irreversible strains and stiffness degradation were observed in cyclic loading-unloading tests, which indicate that the nonlinear response of composites was induced by a combination of damage and plasticity. A coupled damage-plasticity model was employed to describe the nonlinear off-axis tensile stress-strain relation of materials. In this model, a plastic potential function together with associated plastic flow rule were adopted to assess the evolution of plastic strains. The damage variables in forms of stiffness degradation were expressed as a Weibull function of the effective stress. A full suite of model parameters was experimentally determined from cyclic loading-unloading tensile tests. The stress-strain curves predicted by this model agreed well with experimental results.     

Absorption plasticization of alumina сeramic fiber

The dependence of tensile strength and elongation at failure strain on the change in oiling-emulsion concentration was studied. A correlation between the tensile strength and elongation values at failure strain with the change in the surface activity and specific electrical conductivity of the oiling emulsion was observed with a change in the oiling-emulsion concentration from 2 to 20%. This indicates that electro-physical processes play an important role in the adsorption mechanism: free electrical charges on the fiber surface under mechanical loading.     

Acoustic emission of a crazing polymer

The acoustic emission from a crazing polyvinyltoluene in a tensile and bending experiment is described. Acoustic emission appears as a series of bursts which most likely correspond to the initiation and growth of crazes. The emission intensity is characterised by acoustic activity (pulse rate) measured by the ring-down technique. The average activity increases with strain. During repeated loading the acoustic activity shows a measurable intensity and significant rise only beyond the maximum strain of the former runs. This is equivalent to Kaiser's effect in metals. Acoustic emission during the creep experiment occurs in three characteristic periods. They are characterized as the relaxation, fatigue, and breakdown periods. Visual observations indicate that the relaxation period corresponds to the initiation, and the fatigue period to the growth of crazes. In the breakdown period a macroscopic crack develops and the sample fails.     

On the strain measurement and stiffness calculation of carbon fibre reinforced composites under quasi-static tensile and tension-tension fatigue loads

The applicability of different strain measurement techniques for carbon/epoxy laminates under quasi-static tensile and tension-tension fatigue loads was studied. Strain gauges, mechanical extensometers, digital image correlation and 2 D camera systems were applied on laminates tested at angles of 0°, 45°, 60°, 90° and ±45°. In addition, displacements recorded by the servo-hydraulic piston were monitored and compared to local strain measurement techniques. Representative examples that illustrate characteristics and limits of each technique in quasi-static and fatigue tests are discussed. Influences of the respective method of strain measurement, the specimen surface, fibre direction and processes in the specimens during tests on the recorded stress-strain behaviour and on the calculated stiffness are presented. Recommendations for accurate strain measurement of anisotropic laminates based on the results are made.     

High strain rate in-plane shear behavior of composites

Studies are presented on in-plane shear properties of a typical plain weave E-glass/epoxy composite under high strain rate loading. In-plane shear properties were determined with ±45 degree off-axis compression and tension tests using a split Hopkinson pressure bar apparatus. In-plane shear properties are presented as a function of axial and shear strain rates. The range of axial strain rates for off-axis compression tests was 819–2003 per sec, and for off-axis tension tests was 91–180 per sec, whereas the range of shear strain rates for off-axis compression tests was 1388–3442 per sec and for off-axis tension tests was 153–303 per sec. In general, it was observed that in-plane shear strength was enhanced at high strain rate loading compared to that at quasi-static loading. Also, it was observed that in-plane shear strength increased with increasing strain rate within the range of strain rates considered.     

Acoustic emission evaluation of the fracture behavior of APS‐TBCs subjecting to bondcoating oxidation

Air‐plasma‐sprayed (APS) thermal barrier coatings (TBCs) under the conditions of without TBC, as‐sprayed and preoxidized, were tested under tensile loading, and their acoustic emission (AE) responses were monitored. AE parameters including event count, amplitude, and frequency spectrum realized by Fast Fourier Transformation (FFT) were analyzed for information regarding the fracture behavior of TBCs. On the basis of AE behavior, the fracture process of TBCs and corresponding cracking patterns in TBCs subjected to tensile tests were evaluated. AE results also showed that the bondcoating oxidation not only promotes the cracking but also changes the failure mode in TBCs. Copyright © 2007 John Wiley & Sons, Ltd.     

Fire behaviour of hybrid filament-wound single and adhesively bonded composites tubes under static pressure

The present work aims to experimentally investigate the fire behaviour of water-filled E glass reinforced thermoset resin hybrid filament-wound composites tubes under static pressure. Heretofore, fire endurance tests have been conducted on single and adhesively bonded tubes manufactured by CTRA Company. Furthermore, internal pressure tests until failure have been performed on the burnt single and burnt joined tubes in order to quantify their abilities to contain the fluid after being exposed to heat flux. A comparison between the pressure behaviour of exposed to fire (burnt) and non-exposed tubes (single and joined) was also inspected. The identification of the fire-induced damage mechanisms of the tubes was performed through optical microscopy, Scanning Electron Microscopy (SEM) and X-ray tomographic observations. Finally, the thermal analysis was carried-out on burnt specimens in order to better understand the multiphysical phenomenon taking place during the fire endurance tests. The experimental results have revealed that the combustion process of both single and joined tubes was described in four steps namely tube heating, resin degradation, ignition and flame decay. Moreover, it was found that no leakage was witnessed on the tubes (single and joined) outer surfaces during the fire endurance tests. The comparison between the pressure behaviour of the burnt single tube and the burnt joined one has proved that the single tube is much resistant under internal pressure loading than the burnt joined tube. Finally, the fire-induced damage included matrix cracking and delamination between the tube plies which was noticed from microscopic observations.