Ductile deformation mechanisms and designing instructions for integrated woven textile sandwich composites

To suggest designing instructions for integrated woven textile sandwich composites (IWTSCs), anti-crush properties of IWTSC and the corresponding ductile deformation mechanism were investigated. Quasi-static out-of-plane crushing and dynamic impact tests were carried out. Typical deformation curves with a relative stable deformation plateau were obtained from tests. Failure of IWTSC is ductile through coupled compression–shear deformation. An analytical plastic model was proposed to explain ductile mechanism of IWTSC qualitatively, including densification caused by interactions among inclined piles. Combining with qualitative analysis, comparisons between two kinds of IWTSC panels with piles of different density and thickness reveal the key to design a ductile IWTSC.     

Flexural and impact response of woven glass fiber fabric/polypropylene composites

Impact tests with a falling dart and flexural measurements were carried out on polypropylene based laminates reinforced with glass fibers fabrics. Research has shown that the strong fiber/matrix interface obtained through the use of a compatibilizer increased the mechanical performance of such composite systems. The improved adhesion between fibers and matrix weakly affects the flexural modulus but strongly influences the ultimate properties of the investigated woven fabric composites. In fact, bending tests have shown a clear improvement in the flexural strength for the compatibilized systems, in particular when a high viscosity/high crystallinity polypropylene was used. On the contrary, the low velocity impact tests indicated an opposite dependence on the interface strength, and higher energy absorption in not compatibilized composites was detected. This result has been explained in terms of failure mechanisms at the fiber/matrix interface, which are able to dissipate large amounts of energy through friction phenomena. Pull-out of fibers from the polypropylene matrices have been evidenced by the morphological analysis of fracture surfaces after failure and takes place before the fibers breakage, as confirmed by the evaluation of the ductility index.     

Development of woven fabric reinforced all-polypropylene composites with beta nucleated homo- and copolymer matrices

In the present work self-reinforced polypropylene composites (SRPPC) were developed and investigated. As reinforcement a fabric, woven from highly stretched split PP yarns, whereas as matrix materials α and β crystal forms of isotactic PP homopolymer and random copolymer (with ethylene) were selected and used. The composite sheets were produced by film-stacking method and compression moulded at different processing temperatures keeping the holding time and pressure constant. The quality of the composite sheets was assessed by optical microscopy, density and peel-strength measurements. The SRPPC specimens were subjected to static tensile and flexural, and dynamic falling weight impact tests and the related results were analyzed as a function of processing temperature and polymorphic composition. Based on the results the optimum processing temperature was determined and found by 20–25 °C above the related matrix melting temperature. It was established that the β-modified PP homopolymer based one-component SRPPCs possessed similar attractive mechanical properties as the intensively studied α-random PP copolymer based two-component ones.     

Flexural and compression response of woven E-glass/polyester–CNF nanophased composites

A significant improvement in fiber reinforced polymeric composite (FRPC) materials can be obtained by incorporating a very small amount of nanofiller in the matrix material. In this work, an ultrasonic liquid processor was used to infuse carbon nanofiber (CNF) into the polyester matrix which was then mixed with catalyst using a mechanical agitator. Both conventional and CNF-filled glass-fiber reinforced polyester composites (GRPC) were fabricated using the vacuum assisted resin transfer molding (VARTM) process. Excellent dispersion of CNFs into the polyester resin was observed from the scanning electron microscopy (SEM) micrographs. Flexural and quasi-static tests were performed for investigating the mechanical responses. Fracture surface was examined using optical microscopy (OM) and SEM. Flexure tests performed on the conventional GRPC, 0.1–0.4 wt.% CNF-filled GRPC showed up to 49% and 31% increase in the flexural strength and modulus, respectively, compared to the conventional one with increasing loading of CNFs up to 0.2 wt.%. Similar trend was seen in quasi-static compression properties. SEM evaluation revealed relatively less damage in the tested fracture surfaces of the nanophased composites in terms of matrix failure, fiber breakage, matrix–fiber debonding, and delamination, compared to the conventional one. This might be the result of better interfacial interaction between matrix and fibers, due to the presence of CNFs.     

Micromechanics of hemp strands in polypropylene composites

The present paper investigates micromechanics of hemp strands. The main objective of the present work has been the determination of the intrinsic strength of hemp strands. Hemp strands have been used as reinforcement of Polypropylene composites. Different percentages of hemp strands and coupling agents (MAPP) have been tested to obtain a map of the mechanical properties of that kind of composites and the effect of the components on the final properties. Mechanical properties of the different specimens have been tested using standard experimental methods and equipment. Micromechanics of the strands have been obtained using Hirsch model, Bowyer–Bader methodology and Kelly-Tyson model.     

Effect of fabric compaction and yarn waviness on 3D woven composite compressive properties

3D-woven fabrics incorporate through-thickness reinforcement and can exhibit remarkable inter-laminar properties that aid damage suppression and delay crack propagation. However, distortions in the internal architecture such as yarn waviness can reduce in-plane properties, especially in compression. The degree of yarn waviness present in a 3D woven fabric can be affected by a range of factors including weave parameters and manufacturing-induced distortions such as fabric compaction. This paper presents a thorough analysis of the effect of fabric compaction and yarn waviness on the mechanical properties and failure mechanisms of an angel-interlock fabric in compression. Tests were conducted on coupons moulded to different volume fractions and data compared to previous measurements of local yarn angle. Major findings show the importance of yarn straightness on compressive strength and how this can be affected by optimising moulding thickness. Failure initiation was also found to be heavily influenced by weave style and yarn interlacing.     

Production of high strength Al-Al2O3 composite by accumulative roll bonding

Recently accumulative roll bonding has been used as a novel method to produce particle reinforced metal matrix composites. In this study, aluminum matrix composite reinforced by submicron particulate alumina was successfully produced and the effects of number of ARB cycles and the amount of alumina content on the microstructure and mechanical properties of composites were investigated. According to the results of tensile tests, it is shown that the yield and tensile strengths of the composite are increased with the number of ARB cycles. Scanning electron microscopy (SEM) reveals that particles have a random and uniform distribution in the matrix by the ARB cycles and a strong mechanical bonding takes place at the interface of particle-matrix. It is also found that the tensile strength of the composite, as a function of alumina content, has a maximum value at 2 vol.%, which is 5.1 times higher than that of the annealed aluminum.     

Mechanical and photocatalytic properties of hydroxyapatite/titania nanocomposites prepared by combined high gravity and hydrothermal process

Hydroxyapatite/titania nanocomposites of different ratios have been successfully synthesized by combined high gravity and hydrothermal methods. SEM and TEM observations showed that small spheres of TiO₂, identified as anatase crystals of 10–15 nm, were deposited on HAp rod-like crystals. EDAX analysis confirmed the presence of Ca, P, Ti and O. X-ray diffraction patterns indicated the presence of hydroxyapatite and anatase phase. More number of anatase peaks appeared in the XRD patterns with higher colloidal concentration of TiO₂ in the HAp/TiO₂ compound. Mechanical stability of the HAp/TiO₂ nanocomposites was determined by reinforcing them with high molecular weight polyethylene (HMWPE) and the tensile strength of the samples was analyzed. Photocatalytic activity of the HAp/TiO₂ particles was examined by decomposition of methyl orange (MO). The results showed that photocatalytic properties of HAp/TiO₂ composites are more effective than that of individual HAp and TiO₂ which implied that the HAp improved the photocatalytic activity of well known photocatalyst TiO₂.     

Flammability,thermal and physical-mechanical properties of cationic polymer/montmorillonite composite on cotton fabric

The flammability, thermal and mechanical properties on cotton fabric were improved by being finished with the composite containing montmorillonite. To this aim, polymer dimethyl diallyl ammonium chloride-allyl glycidyl ether (P_DMDAAC-AGE) was prepared and its structure characterized by Fourier transform infrared (FT-IR) and Nuclear magnetic resonance (¹H NMR). The quaternary ammonium salt copolymer/montmorillonite composite (P_DMDAAC-AGE/MMT) was obtained by polymer intercalation method. The X-ray diffraction (XRD) indicated that the MMT interlayer spacing increased after the polymer intercalation. Composite materials were loaded onto the cotton fabrics by a dip-pad-dry method. The thermo gravimetric analysis (TGA), vertical flame test and limiting oxygen index (LOI) results showed that the thermal and flammability properties of the cotton fabric were improved after it was finished with the composite. Tensile testing revealed an increase on mechanical properties of the finished fabric, but the physical properties hardly changed from the bending length and whiteness results. Scanning electron microscope (SEM) and energy disperse X-ray spectroscope (EDX) results verified the improvement of those properties due to the presence of montmorillonite in the composite.     

Stress-dependence and time-dependence of the post-fatigue tensile behavior of carbon fiber reinforced SiC matrix composites

The major objective of this paper is to phenomenally report the stress-dependence and time-dependence of fatigue damage to C/SiC composites, and to tentatively discuss the effects of the fatigue stress levels and the fatigue cycles on the post-fatigue tensile behavior. Results show that compared with the virgin strength of the as-received C/SiC specimens, the tensile strengths of the as-fatigued specimens after 86,400 cycles were increased by 8.47% at the stresses of 90 ± 30 MPa, 23.47% at 120 ± 40 MPa, and 9.8% at 160 ± 53 MPa. As cycles continued, however, the post-fatigue strength of the composites gradually decreased after the peak of 23.47%, at which the optimal strength enhancement was obtained because the mean fatigue stress of 120 MPa was the closest to thermal residual stress (TRS), and caused TRS relieve largely during the fatigue. Most interestingly, there was a general inflexion appeared on the post-fatigue tensile stress-strain curves, which was just equal to the historic maximum fatigue stress acted upon the as-fatigued specimens. Below this inflexion stress the tensile curves revealed the apparent linear behavior with little AE response, and above that nonlinearity with new damage immediately emitted highly increase rate of AE activities. This ‘stress memory’ characteristic was strongly relevant to damaged microstructures of the as-fatigued composites in the form of the coating/matrix cracks, interface debonding/wear, and fiber breaking, which resulted undoubtedly in reduction of modulus but in proper increase of strength via TRS relief.     

Evaluation of the mechanical behavior of epoxy composite reinforced with Kevlar plain fabric and glass/Kevlar hybrid fabric

In this study, composite plates were manufactured by hand lay-up process with epoxy matrix (DGEBA) reinforced with Kevlar fiber plain fabric and Kevlar/glass hybrid fabric, using to an innovative architecture. Results of the mechanical properties of composites were obtained by tensile, bending and impact tests. These tests were performed in the parallel direction or fill directions of the warp and in a 90° direction. FTIR was used in order to verify the minimum curing time of the resin to perform the mechanical tests, and scanning electron microscopy was used to observe reinforcement and matrix fractures. Composites with Kevlar/glass hybrid structure in the reinforcing fabric showed the better results with respect to specific mechanical strength, as well as bending and impact energy.     

Effect of frequency and environment on fatigue behavior of a CVI SiC/SiC ceramic matrix composite at 1200 °C

The fatigue behavior of a SiC/SiC CMC (ceramic matrix composite) was investigated at 1200 °C in laboratory air and in steam environment. The composite consists of a SiC matrix reinforced with laminated woven Hi-Nicalon™ fibers. Fiber preforms had boron nitride fiber coating applied and were then densified with CVI SiC. Tensile stress-strain behavior and tensile properties were evaluated at 1200 °C. Tension-tension fatigue tests were conducted at frequencies of 0.1, 1.0, and 10 Hz for fatigue stresses ranging from 80 to 120 MPa in air and from 60 to 110 MPa in steam. Fatigue run-out was defined as 10⁵ cycles at the frequency of 0.1 Hz and as 2 × 10⁵ cycles at the frequencies of 1.0 and 10 Hz. Presence of steam significantly degraded the fatigue performance. In both test environments the fatigue limit and fatigue lifetime decreased with increasing frequency. Specimens that achieved run-out were subjected to tensile tests to failure to characterize the retained tensile properties. The material retained 100% of its tensile strength, yet modulus loss up to 22% was observed. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Determination of strain rate dependent through-thickness tensile properties of textile reinforced thermoplastic composites using L-shaped beam specimens

The presented work focuses on a methodology to characterise strain rate dependent strength and elastic properties of textile reinforced composites in laminate through-thickness direction. Here, for the characterisation L-shaped beam specimens are used. The investigated composite is a fabric reinforced thermoplast made of hybrid E-glass/polypropylene yarns. The analytical solution for the determination of the through-thickness tensile strength as proposed by Lekhnitskii and Shivakumar is verified by means of an optical deformation analysis and is extended for thew determination of the through-thickness elastic modulus. Finally, the possibility of the strain rate dependent characterisation is investigated and a Johnson-Cook based modelling approach is used to represent the apparent strain rate dependency of the through-thickness failure onset. The methodology is successfully used to capture the material strain rate effects with the according strength values and model parameters over a strain rate range of 10 ⁻⁴ s⁻¹ to 10 s⁻¹ as well as the elastic modulus.     

On the effect of stacked fabric layers on the stiffness of a woven composite

Most micromechanical models for stiffness prediction of woven composites assume independence of the Q-matrix on the number of fabric layers in the composite. For example, the moduli of single and 10 layer composites are assumed to be equal in the case when all layers have the same in-plane orientation. Although this statement is likely to be true for isotropic materials or even for unidirectional laminated composites, it may not be valid in some cases of woven composites.

This paper contains experimental and theoretical investigations of plain weave carbon fiber/polyester composites. Specimens with one single and eight layers of fabrics are tested and observable differences of mechanical properties are obtained.

The theoretical part of this article consists of derivation and application of several micromechanical models on these particular composites. The use of those simplified models finally allows us to find the main mechanisms which cause the observed effects.     

Voids and their effect on the strain rate dependent material properties and fatigue behaviour of non-crimp fabric composites materials

The manufacture of composite structures is inevitably linked to the formation of voids. Several non-destructive techniques are potentially able of detecting defects, but just the exact knowledge of the effects of defects on the mechanical properties allows the definition of thresholds for the purpose of quality management. In this paper an experimental program for characterizing the effect of voids on the composite materials behaviour is presented. Therefore glass fibre non-crimp fabric reinforced epoxy composites were produced using vacuum assistant resin transfer moulding. For obtaining various void contents specially modified process parameters were used. Nominally defect free specimens are compared with flawed specimens. Tensile testing at different loading speeds and fatigue tests in tension-compression loading are performed.     

Local strain in a 5-harness satin weave composite under static tension: Part II - Meso-FE analysis

This paper presents the local strain analysis in a thermo-plastic 5-harness satin weave composite under uni-axial static tensile load using meso-FE simulations. In order to predict the local strain profiles as observed in the experiments (Part I) at various locations of the composite, different unit cell stacking models with appropriate boundary conditions are used for the FE analysis. Apart from the calculation of local strain values at different locations (inside/traction free surface) of the composite laminate, the aim of the numerical simulations is to understand the ‘shadowing’ effects of the internal ply shifting on the surface strain behavior of a 5-harness satin weave composite. Comparison of the experimental local strain values (Part I) at various locations of the satin weave composite reveals that the effects of local yarn constraints are negligible on the local longitudinal strain behavior of the composite.However, local stress-strain profiles obtained from unit cell meso-FE simulations indicate that the longitudinal strain and the transverse stress distribution in the weft yarn at the yarn crimp location is sensitive to the unit cell stacking as well as to the applied boundary conditions to the unit cell.     

Quasi-static and high strain rate response of aluminum matrix syntactic foams under compression

Aluminum alloy matrix syntactic foams were produced by inert gas pressure infiltration. Four different alloys and ceramic hollow spheres were applied as matrix and filler material, respectively. The effects of the chemical composition of the matrix and the different heat-treatments are reported at different strain-rates and in compressive loadings. The higher strain rates were performed in a Split-Hopkinson pressure bar system. The results show that, the characteristic properties of the materials strongly depends on the chemical composition of the matrix and its heat-treatment condition. The compressive strength of the investigated foams showed a limited sensitivity to the strain rate, its effect was more pronounced in the case of the structural stiffness and fracture strain. The failure modes of the foams have explicit differences showing barreling and shearing in the case of quasi-static and high strain rate compression respectively.     

Effects of fabric parameters on the tensile behaviour of sustainable cementitious composites

The mechanical behaviour of fabric-reinforced composites can be affected by several parameters, such as the properties of fabrics and matrix, the fibre content, the bond interphase and the anchorage ability of fabrics. In this study, the effects of the fibre type, the fabric geometry, the physical and mechanical properties of fabrics and the volume fraction of fibres on the tensile stress–strain response and crack propagation of cementitious composites reinforced with natural fabrics were studied. To further examine the properties of the fibres, mineral fibres (glass) were also used to study the tensile behaviour of glass fabric-reinforced composites and contrast the results with those obtained for the natural fabric-reinforced composites. Composite samples were manufactured by the hand lay-up moulding technique using one, two and three layers of flax and sisal fabric strips and a natural hydraulic lime (NHL) grouting mix. Considering fabric geometry and physical properties such as the mass per unit area and the linear density, the flax fabric provided better anchorage development than the sisal and glass fabrics in the cement-based composites. The fabric geometry and the volume fraction of fibres were the parameters that had the greatest effects on the tensile behaviour of these composite systems.     

Effects of camber height and boundary condition on energy absorption of arched composite laminates

The scope of this study on arched composite laminates subjected to low-velocity impact was two-fold. One was to investigate the effect of camber height on energy absorption and the other the effect of boundary condition on energy absorption. All specimens were made of the same prepreg tape material and a cross-ply stacking sequence of [0/90]_3s such that a comparison among flat panels and arched specimens with various camber heights can be made. Analysis on the load–deflection relation, the energy profile and the damage process were of primary interest as they provided the insight into the impact behavior of arched composites. Experimental results showed that the maximum deflection and the energy absorption increased significantly as the camber height increased while the peak load decreased slightly. It was also found that the boundary condition played an important role in the energy absorption process. Frame-clamped specimens experienced higher slippage, and hence higher energy absorption, than bolted specimens. This slippage was observed after impact because the specimen actually would pull out of the frame. Video footage also verified this phenomenon.