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.     

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.     

Ultimate tensile properties of elastomers. VI. Strength and extensibility of a styrene–butadiene rubber vulcanizate in equal biaxial tension

The ultimate properties of an unfilled styrene-butadiene rubber vulcanizate in equal biaxial tension were determined by inflating a circular membrane into a bubble. Tests were made at several extension rates (evaluated at the pole) from about 0.15 to 4 min^?1 and at temperatures from ?43 to 90°C. The stress in the vicinity of the pole when rupture occurred was evaluated from the pressure, the radius of curvature, and the extension ratio λ, the latter two quantities being obtained from photographic data. Below 70°C, the ultimate extension ratio λ_b is approximately 5.2 and is essentially independent of extension rate and temperature, in striking contrast to the behavior in simple and constrained biaxial tension (pure shear). Likewise, the rupture stress is manyfold greater than in either simple or constrained biaxial tension. From the extremum points of failure envelopes, the maximum extension ratio (λ_b)_max in equal biaxial tension is 5.7 and in simple tension is 7.2. An examination of ruptured membranes showed that, except at 70 and 90°C, rupture began away from the pole in a region where the stress state is unequal biaxial tension. Hence, values of the ultimate properties in truly equal biaxial tension are no doubt somewhat greater than those obtained from the membrane tests. However, it is shown that (λ_b)_max in truly equal biaxial tension must be lower than that in simple tension by at least 10%. A consideration of rupture data in simple, constrained biaxial, and equal biaxial tension leads to the conclusion that no simple failure criterion is applicable for interrelating data obtained under the several states of combined stress. The rupture patterns and factors that affect the site of rupture initiation and the mode of crack growth are also discussed.     

Investigation of flexural properties and failure behaviour of biaxial braided CFRP

In this research, the experimental tests of quasi-static three-point bending and three-point bending fatigue were carried out for a ±25° biaxial braided carbon fibre reinforced polymer (CFRP) manufactured using vacuum assisted resin transfer moulding (VARTM). A finite element (FE) model was also set up for quasi-static testing and the prediction results revealed that local fibre volume fraction (FVF) is a primary source affecting the mechanical properties of braided CFRP. The fatigue of the braided CFRP was defined as three different stages according to the flexural modulus results. The damage modes of the test specimens were observed via a digital microscope and scanning electron microscope (SEM) and the process-induced defects were summarised. With compiled results and observations, this study provides a better understanding of failure and fatigue behaviour of biaxial braided composites and their flexural properties which offers a good basis for any further research in fibre volume fractions, structure design and manufacturing for braided CFRP.     

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.     

碳纤维三向织物/环氧树脂复合材料的制备与力学性能

选择不同纱线间距[即二经(纬)纱之间的中心距]尺寸的碳纤维三向织物, 采用热压成型技术制备了碳纤维三向织物/环氧树脂复合材料. 研究了纱线间距及样品裁剪角度等对力学性能的影响, 并与碳纤维二向织物/环氧树脂复合材料的力学性能进行对比. 结果表明, 随着纱线间距尺寸从2 mm增加到6 mm, 0°方向断裂强度从221.7 MPa下降到148.1 MPa, 撕裂强力从1000 N下降到600 N; 90°方向断裂强度从50.0 MPa下降到22.1 MPa, 撕裂强力从330 N下降到100 N; 顶破强力从424 N下降到216 N. 这些力学性能的逐渐降低是单位面积的碳纤维增强体含量减少和织物的孔洞增大共同作用的结果. 纱线间距为2 mm的碳纤维三向织物复合材料在0°(以纬纱为基准), 30°, 45°, 60°和90°方向的断裂强度分别为221.7, 48.5, 44.3, 227.7和50.0 MPa, 即断裂强度在0°和60°方向大于在30°, 45°及90°方向. 由三向织物的编织原理可知, 0°与60°方向完全相同, 因此其断裂强度相似, 且样品中有一组纱线与外加载荷平行, 对形变破坏具有一定的约束作用, 而30°, 45°和90°方向样品中三组纱线所在的方向均和外加载荷存在一定的夹角, 有效分散外加载荷的能力减弱. 对比碳纤维三向织物/环氧树脂复合材料与碳纤维二向织物/环氧树脂复合材料的断裂强度、 撕裂强力和顶破强力发现, 前者的综合性能明显优于后者, 碳纤维三向织物/环氧树脂复合材料的断裂强度、 撕裂强力、 顶破强力分别为221.7 MPa, 1000 N和424 N, 三向织物的优势得到凸显, 为后续制备多种类型的碳纤维三向织物复合材料及应用奠定了基础.     

Self‐reinforced composites based on commercial PP woven fabrics and a random PP copolymer modified with quartz

Self‐reinforced composites based on commercial polypropylene (PP) woven fabrics and a random PP copolymer modified with quartz were obtained by film stacking. The effect of the incorporation of quartz on the materials fracture and failure behavior was studied through uniaxial tensile tests and quasi‐static fracture experiments. Acoustic emission analysis was also performed in situ in the tensile tests. A higher consolidation quality was obtained for the composites containing quartz. In the composite with random PP modified with 5 wt% quartz, the higher consolidation and the better dispersion of quartz particles positively impacted on the materials tensile and fracture behavior. From the results of acoustic emission analysis, fiber fracture appears as the dominant failure mechanism in the investigated composites. Copyright © 2016 John Wiley & Sons, Ltd.     

Impact resistance of all-polypropylene composites composed of alpha and beta modifications

The impact behaviour of self-reinforced polypropylene (PP) composites was studied. α and β polymorphs of isotactic PP homopolymer and random copolymer (with ethylene) were used for matrix materials, whereas the reinforcement was a fabric woven from highly stretched split PP yarns. The composite sheets were produced by the film-stacking method and consolidated by hot pressing at 5 and 15 °C above the melting temperature (T_m) of the matrix-giving PP grade. The composite sheets were subjected to static tensile, dynamic falling weight impact and impact tensile tests at room temperature. Dynamic mechanical thermal analysis (DMTA) was also performed on the related composites and their constituents. The results indicated that the β-modification of the PP homopolymer is more straightforward than that of the PP copolymer. Stiffness and strength usually increased while the toughness (tensile impact strength, perforation impact energy) decreased with increasing temperature of consolidation. This was assigned to differences in the failure mode based on fractographic results.     

Self‐reinforced polypropylene composites based on low‐cost commercial woven and non‐woven fabrics

In the present paper, different self‐reinforced polypropylene (PP) composites based on low‐cost commercial woven (w) and non‐woven (nw) fabrics were obtained. Hot compaction (HC) and film stacking (FS) followed by compression molding were used to prepared the composites. The fracture and failure behavior of the different materials was determined under different testing conditions through quasi‐static uniaxial tensile tests, Izod impact experiments and by means of fracture mechanics tests on mode I double‐edge deeply notched tensile specimens. In the case of the composite obtained by film stacking + compression molding (rPP/nw/w‐FS) and the hot‐compacted composite (nw/w‐HC) containing simultaneously woven and non‐woven fabrics, the acoustic emission technique was applied in situ in the tensile tests to determine their consolidation quality and to identify the failure mechanisms responsible for their fracture behavior. It was observed that both composites exhibited relatively similar high consolidation quality. However, the hot‐compacted composite presented a more uniform distribution of failure mechanisms (debonding and fiber fracture) than the film‐stacked composite. The hot‐compacted composite containing both types of reinforcements exhibited the best combination of mechanical (tensile, impact, and fracture) properties. Therefore, this composite appeared as the most promising for structural applications among the different composites investigated.     

Investigations on the recycling of hemp and sisal fibre reinforced polypropylene composites

Microscopic, mechanical, rheological and thermal tests were carried out in order to determine the recycling behaviour of PP/vegetal fibre composites. Different composites using hemp and sisal were characterized. All results were compared with PP-g-MA/hemp composites and PP/glass fibre composites.The results prove that mechanical properties are well conserved with the reprocessing of PP/vegetal fibre composites but that there is poor adhesion between the fibres and PP without any treatment. The addition of PP-g-MA shows an improvement of the bonding evidenced by MEB pictures. Vegetal fibres induce an increase in the percentage of crystallinity χ_c and in the crystallization temperature T_c which can be explained by the nucleating ability of the fibres improving crystallization of PP. The Newtonian viscosity η₀ decreases with cycles, indicating a decrease in molecular weight and chain scissions induced by reprocessing. The decrease of fibre length with reprocessing could be another reason for viscosity decrease.     

Thermal behavior of flax and jute reinforced in matrix acrylic composite

Natural fabric such as flax and jute was considered in biaxial plain reinforcement in matrix of acrylic resin, and the composite is prepared in hand layup techniques. Fabric mass fraction of 7% was used in the matrix of composite. The samples were treated at r.t and 60 °C for the final fabrication. Scanning electron microscopy was carried out to support the microstructure effect of composite in terms of thermal change. Thermogravimetric and differential thermogravimetric analysis and residual compositional analysis with FTIR were carried out for the composite and matrix samples. The mechanical and viscoelastic properties, as well as the influence of frequency and fibers types, were evaluated, in flexural mode, by means of dynamical mechanical analysis. Glass transition (T _g) and initial decomposition (T _i) temperatures increase with incorporation of fibers into the matrix. While T _i of flax and jute composite was similar, T _g in case of flax improves than jute fabric-reinforced composite. This type of composites can be used in the automotive sector, in exterior and exterior components.     

Mechanical and thermal properties of poly-ether ether ketone reinforced with CaCO3

CaCO₃/PEEK (poly-ether ether ketone) composites were prepared on a twin-screw extruder with different mass ratio of CaCO₃/PEEK from 0% to 30%. Four types of particles were used as filler in PEEK matrix. The influence of surface treatment with sulfonated PEEK (SPEEK) of the particles on the mechanical and thermal properties of the composites was studied. The experiments included tensile tests, flexural tests, notched Izod impact tests, TGA, DSC and SEM. The modulus and yield stress of the composites increased with CaCO₃ particles loadings. This increase was attributed to the bonding between the particles and the PEEK matrix, as can be proved by the SEM pictures of tensile fracture surface of the composites. The impact strength of the composites was modified by the SPEEK coated on the CaCO₃ particle surface. DSC experiments showed that the particle content and surface properties influenced the glass transition temperature (T_g) and melting temperature (T_m) of the composites. The T_g increased with the content of fillers while T_m decreased. In this study the fillers treated were found to give better combination properties, which indicated that SPEEK played a constructive role in the CaCO₃/PEEK composites.     

Effect of electrospun polyamide 6 nanofibres on the mechanical properties of a glass fibre/epoxy composite

Recently, several types of nanoparticles are frequently incorporated in reinforced epoxy resin composites. A homogeneous dispersion of these nanoparticles is still a problem. Thermoplastic nanofibrous structures can tackle this dispersion issue. Therefore, this paper investigated the effect of electrospun polyamide 6 nanofibrous structures on the mechanical properties of a glass fibre/epoxy composite. The nanofibres were incorporated in the glass fibre/epoxy composite as stand-alone interlayered structures and directly spun on the glass fibre reinforcement. Both ways of nanofibre incorporation have no negative effect on the impregnation of the epoxy. Moreover, the nanofibres remain well dispersed within the matrix. Incorporation of nanofibres increases the stress at failure in the 0°-direction, the best results are obtained when the nanofibres are directly electrospun onto the glass fibres. Optical microscopic images also demonstrate that nanofibres prevent delamination when a 90° crack reaches a neighbouring 0° ply. Furthermore, mode I tests showed a small improvement when a thin nanofibrous structure is deposited directly onto the glass fibres. When the composites are loaded under 45°, it is proven that, for an identical stress, the glass fibre composite with deposited nanofibres has less cracks than when interlayered nanofibrous structures are incorporated. Generally, it can be concluded that the addition of polyamide 6 nanofibres improves some mechanical characteristics of a glass fibre/epoxy composite.     

A fracture criterion of rubber-like materials under plane stress conditions

In this work, we attempt to derive a fracture criterion for filled and unfilled elastomer vulcanizates and thermoplastics from a set of experimental data. Firstly, fracture criteria reported in the literature have been applied to experimental data obtained from tests including various loading modes (simple tension, equal biaxial tension and biaxial tension) and performed on four materials: a natural rubber (NR), a styrene butadiene rubber (SBR), a polyurethane (PU) and a thermoplastic elastomer (TPE).

Then, a new failure criterion based on an equivalent elongation concept is proposed. This equivalent elongation seems to be linearly dependent on a given biaxiality ratio n=(ln(λ_2b)/ln(λ_1b)), which leads to expressing the principal elongations at break as functions of both the biaxiality n and two experimental parameters. Quite good agreement is highlighted when comparing the failure experimental data with the proposed criterion for the tested elastomers.     

Strength predictions by applied effective volume theory in short-glass-fibre-reinforced plastics

Uniaxial stresses were evaluated under flexural, tensile and torsional tests with various fibre volume fractions, v_f, and specimen sizes to develop simple and effective predictions on the quasi-static strength of compression-moulded short-glass-fibre-reinforced phenolic resin. The distributions in fibre orientation and fibre length were represented by Weibull and normal distributions, regardless of v_f. The size effect was characterized by uniaxial stress testing and Weibull statistical analysis. The effective volume theory (EVT) was used to predict the quasi-static strength for v_f of 0.0 with a scale parameter of 108.2 MPa and a shape parameter of 8.92. Strength prediction while considering V_eff and v_f was proposed by applying the EVT of the resin matrix to a method that combined a modified rule of mixtures and the Thai-Hill failure criterion. The measured value agreed well with the prediction for each V_eff and v_f, regardless of manufacturing defects in the matrix resin.     

Effect of relative humidity on mechanical properties of a woven thermoplastic composite for automotive application

The purpose of this work is to characterise the influence of moisture content in a woven glass fibre reinforced polyamide 6,6 composite. Two different stacking sequences are studied: [(0/90)₃] and [(±45)₃] as well as the neat PA6,6 matrix. Samples have been conditioned through three ways: either water immersed, left at ambient temperature and humidity or dried in 35 °C oven. A one dimensional Fick's law has been used to model the water uptake in immersed samples. The glass transition temperature is highly affected by the presence of water and has been measured using modulated DSC technique. Finally, the effects of water on these composite materials have been investigated through tensile tests instrumented with acoustic emission monitoring (AE). Mechanical properties are highly affected by the presence of water in the composite. This result is even more visible on [(±45)₃] sample because of the preferential loading of the matrix.     

Mechanical properties of oriented polyvinylchloride composites filled with ultrafine particles

The tensile stress-strain behaviour of undrawn and drawn polyvinylchloride (PVC) composites filled with ultrafine SiO₂ and micron sized glass particles were discussed as functions of filler content and size. For the undrawn PVC composites filled with ultrafine SiO₂ particles, Young's moduli, yield and breaking stresses increased with filler content and decreasing filler size. Whereas for the composites filled with micron sized glass particles, their Young's moduli slightly increased with filler content but both the yield and breaking stresses decreased. Oriented PVC composites were made by uniaxially drawing to × 2.5 at 100 °C. Anisotropic mechanical properties of oriented specimens were discussed in terms of compliance tensor and yield stress measured in the direction of 0°, 45° and 90° to the original stretching direction at room temperature. In the case of 70A SiO₂, all the compliance decreased with filler content, whereas those of 65μ glass, this relation was reversed. The yield stress of the oriented PVC composites showed filler size dependence similar to Young's moduli. The anisotropic yield stress of oriented PVC composites were reasonably analysed by Hill's yield criterion.     

Analysis of fracture behaviour of fibre reinforced polypropylene using R-curve concept

Experimental results for investigation of dynamical crack resistance curves in the instrumented Charpy impact test on polypropylene (PP)/glass fibre composites are presented. For this purpose the multiple specimen R-curve method, stop-block technique is used. With the aid of J-integral versus stable crack growth (δa) curves the influence of a special coupler system on crack toughness as resistance against stable crack growth is discussed. It is shown that it is possible to quantify different energy dissipative processes with the new fracture mechanical material value J × T₇ (T₇ - tearing modulus). The problems of determining physical crack initiation values for short fibre composites are discussed. The physical material background for using the ‘plastic hinge’ model to describe the deformation behaviour of PP/glass fibre composites is shown, using the example of selected crack opening displacement (σ) versus δa curves.     

Deformation investigation on iPP/SiO2 composites: Influence of stretching temperature and particle size on morphology evolution and crystalline structure of thin films

 In the present work, structure changes during stretching of isotactic polypropylene (emPP) and emPP/silicon dioxide (SiO₂) composites have been investigated systematically. The α-form crystal structure of both iPP and emPP/SiO₂ composites is destroyed and transforms into the mesophase as the samples are stretched at a low temperature (35℃), while stretching at high temperatures (90℃ and 120℃) can restrain the appearance of defects and keep the perfection of crystal structure. FTIR results reveal that the stretching temperatures show no obvious difference of the effect on the orientation of pure iPP, however, the orientation of emPP/SiO₂ composites is greatly changed by the tensile temperature. In the case of micron-sized SiO₂ particles (average particle diameter d>1 μm), the orientation of the composites is lower than that of pure iPP at all stretching temperatures. The above results suggest that the stretching temperature and the SiO₂ particle size have great influence on the structure variation and orientation behavior of emPP/SiO₂ composites.     

Finite element modelling of mode I delamination specimens by means of implicit and explicit solvers

The simulation of delamination using the Finite Element Method (FEM) is a useful tool to analyse fracture mechanics. In this paper, simulations are performed by means of two different fracture mechanics models: Two Step Extension (TSEM) and Cohesive Zone (CZM) methods, using implicit and explicit solvers, respectively.TSEM is an efficient method to determine the energy release rate components G_Ic, G_IIc and G_IIIc using the experimental critical load (P_c) as input, while CZM is the most widely used method to predict crack propagation (P_c) using the critical energy release rate as input.The two methods were compared in terms of convergence performance and accuracy to represent the material behaviour and in order to investigate their validity to predict mode I interlaminar fracture failure in unidirectional AS4/8552 carbon fibre composite laminates.The influence of increasing the loading speed and using mass scaling was studied in order to decrease computing time in CZ models.Finally, numerical simulations were compared with experimental results performed by means of Double Cantilever Beam specimens (DCB).Results showed a good agreement between both FEM models and experimental results.