Fatigue delamination behaviour of unidirectional carbon fibre/epoxy laminates reinforced by Z-Fiber® pinning

Z-Pin reinforced carbon-fibre epoxy laminates were tested under Mode I and Mode II conditions, both quasi-statically and in fatigue. Test procedures were adapted from existing standard or pre-standard tests. Samples containing 2% and 4% areal densities of carbon-fibre Z-pins (0.28 mm diameter) were compared with unpinned laminates. Quasi-static tests under displacement control yielded a dramatic increase of the apparent delamination resistance. Specimens with 2% pin density failed in Mode I at loads 170 N, equivalent to an apparent G_IC of 2 kJ/m². Fatigue testing under load control showed that the presence of the through-thickness reinforcement slowed down fatigue delamination propagation.     

Thermomechanical behaviour of interfacial region in carbon fibre/epoxy composites

A study of the thermomechanical stability of the fibre-matrix interphase in carbon/epoxy composites has been carried out. The thermodynamic work of adhesion has been evaluated at room temperature by wetting measurements. The interfacial shear stress transfer level τ for sized and desized carbon fibre has been measured as a function of temperature by means of a single-fibre fragmentation test. As the test temperature increased τ values were found to decrease, with values being higher for the desized carbon fibre. The dependence of interfacial shear stress transfer on bulk matrix mechanical properties (modulus and shear strength) has also been discussed. Dynamic mechanical measurements performed on single-bundle composites confirmed the better thermomechanical stability of the desized fibre interphase.     

Aligned short glass fibre/epoxy composites

An orientation system based on the principles of the ERDE glycerine process was built and used to prepare short glass fibre mats of known orientation distributions. The mats were subsequently impregnated with epoxy, B staged to yield partially cured prepregs and finally compression moulded. Mechanical characterization and comparison with theoretical formulations were performed on longitudinal, transverse and off-axis specimens. The mechanical characteristics of specimens with different orientation patterns were compared with analytical predicitions based on ‘Laminate analogy’ methods. Fracture surfaces were studied by optical and scanning electron microscopy techniques.     

Comparative study of dynamic and static delamination behaviour of carbon fibre/epoxy composite laminates

Dynamic and static delamination characteristics of two unidirectional carbon fibre-reinforced epoxy composite laminates (Hercules MI 1610 and Torayca T300) have been studied under impact and low-speed (2 mm min¹) test conditions. The influence of interlaminar reinforcement with chopped Kevlar fibres on toughness has also been examined. The quasi-static or low-speed delamination tests were conducted with the usual double cantilever beam, end-notched flexure and mixed-mode flexure specimens. To determine the corresponding mode I, mode II and mixed-mode toughnesses under the impact condition, a special specimen design has been adopted and tests were performed with a Charpy impact machine. The novel aspect of the test scheme in the present study is that a single-plane delamination surface with a well-defined fracture mode has been obtained. The dynamic and static delamination characteristics of the same fracture mode were then studied by scanning electron microscopy, and special features were compared. While interlaminar reinforcement with a small amount of chopped Kevlar fibres resulted in an appreciable increase in the quasi-static delamination toughness, it was less effective under the impact condition.     

Fatigue behaviour of glass fibre reinforced epoxy composites enhanced with nanoparticles

Nanoparticle reinforcement of the matrix in laminates has been recently explored to improve mechanical properties, particularly the interlaminar strength. This study analyses the fatigue behaviour of nanoclay and multiwalled carbon nanotubes enhanced glass/epoxy laminates. The matrix used was the epoxy resin Biresin® CR120, combined with the hardener CH120-3. Multiwalled carbon nanotubes (MWCNTs) 98% and organo-montmorillonite Nanomer I30 E nanoclay were used. Composites plates were manufactured by moulding in vacuum. Fatigue tests were performed under constant amplitude, both under tension–tension and three points bending loadings. The fatigue results show that composites with small amounts of nanoparticles addition into the matrix have bending fatigue strength similar to the obtained for the neat glass fibre reinforced epoxy matrix composite. On the contrary, for higher percentages of nanoclays or carbon nanotubes addition the fatigue strength tend to decrease caused by poor nanoparticles dispersion and formation of agglomerates. Tensile fatigue strength is only marginally affected by the addition of small amount of particles. The fatigue ratio in tension–tension loading increases with the addition of nanoclays and multi-walled carbon nanotubes, suggesting that both nanoparticles can act as barriers to fatigue crack propagation.     

Role of fibre surface—matrix combination in carbon fibre reinforced epoxy composites

The effect of surface treatment of carbon fibres with concentrated as well as dilute nitric acid on the mechanical properties of carbon fibres has been reported. The role of the fibre—matrix interface in carbon fibre reinforced epoxy resin composites has been studied. Composites have been made both with untreated and surface treated carbon fibres and epoxy resin Araldite LY556 with different hardeners. Mechanical properties as well as fracture behaviour of these composites suggest that it is the physical interlocking between the fibres and the matrix, along with some chemical bonding between the two, and not the pure chemical bonding which yield better composites.     

Effects of fibre content on mechanical properties and fracture behaviour of short carbon fibre reinforced geopolymer matrix composites

Geopolymer matrix composites reinforced with different volume fractions of short carbon fibres (C_f/geopolymer composites) were prepared and the mechanical properties, fracture behaviour and microstructure of as-prepared composites were studied and correlated with fibre content. The results show that short carbon fibres have a great strengthening and toughening effect at low volume percentages of fibres (3·5 and 4·5 vol.%). With the increase of fibre content, the strengthening and toughening effect of short carbon fibres reduce, possibly due to fibre damage, formation of high shear stresses at intersect between fibres and strong interface cohesion of fibre/matrix under higher forming pressure. The property improvements are primarily based on the network structure of short carbon fibre preform and the predominant strengthening and toughening mechanisms are attributed to the apparent fibre bridging and pulling-out effect.     

Investigations on d.c. conductivity behaviour of milled carbon fibre reinforced epoxy graded composites

This paper reports the d.c. conductivity behaviour of milled carbon fibre reinforced polysulphide modified epoxy gradient composites. Milled carbon fibre reinforced composites having 3 vol. % of milled carbon fibre and poly sulphide modified epoxy resin have been developed. D.C. conductivity measurements are conducted on the graded composites by using an Electrometer in the temperature range from 26°C to 150°C. D.C. conductivity increases with the increase of distance in the direction of centrifugal force, which shows the formation of graded structure with the composites. D.C. conductivity increases on increase of milled carbon fibre content from 0·45 to 1·66 vol.%. At 50°C, d.c. conductivity values were 1·85 × 10⁻¹¹, 1·08 × 10⁻¹¹ and 2·16 × 10⁻¹² for samples 1, 2 and 3, respectively. The activation energy values for different composite samples 1, 2 and 3 are 0·489, 0·565 and 0·654 eV, respectively which shows decrease in activation energy with increase of fibre content.     

Impact behaviour of carbon fibre reinforced epoxy and non-isothermal cyclic butylene terephthalate composites manufactured by vacuum infusion

Carbon fibre reinforced cyclic butylene terephthalate (CF-pCBT) composites non-isothermally processed by vacuum infusion starting from a one-component system have been successfully manufactured. Both the micro-structure and low-energy impact properties of the CF-pCBT have been investigated and compared with an epoxy system (CF-epoxy). The CF-pCBT composite presents higher solidification shrinkage than the CF-epoxy one, and its void content doubles that of the CF-epoxy. The CF-epoxy composite’s critical delamination threshold energy is slightly higher than that of the CF-pCBT, whereas the CF-pCBT composite absorbs twice as much energy before being penetrated. The lower interlaminar shear strength of the CF-pCBT composite is suggested to be the origin of its higher energy absorbing capability and less brittle behaviour.     

Failure behaviour of carbon fibre/epoxy composites in pin-ended buckling and bending tests

T300 carbon fibre/epoxy composite specimens with unidirectional and cross-ply orientation and G802 woven fabric/epoxy specimens were tested in a pin-ended rig at various specimen geometries. The rig was designed so that a combination of bending and compression was possible. Failure initiation and failure sequences at the midspan were monitored and established through photography. In some specimen geometries, high compressive strains of the order of 1.85% for pin-ended tests, accompanied by a higher loadbearing capacity than in conventional bend tests, were observed. Fracture characterization and failure analysis were carried out with the aid of a scanning electron microscope.     

Mechanical behaviour of carbon and glass hybrid fibre reinforced polyester composites

Mechanical behaviour of carbon fibre/glass mat/polyester resin hybrid composites of sandwich construction is studied through tension, flexure, impact and post-impact tension tests. Tensile and flexural strength, modulus and failure strain values are compared to the calculated values. Total impact fracture energy and residual (after impact) tensile strength values of hybrid composites are analysed with regard to corresponding values of carbon/polyester composites. Failure of tested coupons was analysed by visual inspection and observation by scanning electron microscopy.     

The effect of fibre length on fracture toughness and notched strength of short carbon fibre/epoxy composites

The effect of radius of curvature on the tensile notched strength of random short carbon fibre/epoxy composites containing 1, 5 and 15 mm length fibres is studied. The strength of all laminates showed a sensitivity to the radius of curvature, with the tensile strength decreasing at smaller radii of curvature. A model is developed to predict notched strength based on assumed evolution and propagation of damage from the tip of the notch. The predictions of the model depend principally on two material properties: the unnotched tensile strength and fracture toughness. Reasonable agreement is achieved between the predicted notched strength and experimental data.     

Numerical prediction of delamination onset in carbon/epoxy composites drilling

Despite the fact that their physical properties make them an attractive family of materials, composites machining can cause several damage modes such as delamination, fibre pull-out, thermal degradation, and others. Minimization of axial thrust force during drilling reduces the probability of delamination onset, as it has been demonstrated by analytical models based on linear elastic fracture mechanics (LEFM).A finite element model considering solid elements of the ABAQUS^® software library and interface elements including a cohesive damage model was developed in order to simulate thrust forces and delamination onset during drilling. Thrust force results for delamination onset are compared with existing analytical models.     

Fractographical analysis on the mode II delamination in woven carbon fiber reinforced epoxy composites

Mode II delamination phenomena of woven fabric carbon/epoxy composites were investigated by scanning electron microscopy. End notch flexural (ENF) test was used to examine the mode II delamination. Woven fabric composites showed two peculiar crack propagation patterns due to the complexity of woven geometry. In warp yarn region, crack propagated with forming a shear band and breaking the fiber/matrix interface. In fill yarn region, however, no shear band was observed. Considering these crack patterns, matrix shear property and fiber/matrix interfacial strength played an important role in enhancing the delamination properties of woven fabric carbon/epoxy composites. Due to the woven geometry, matrix rich positions, which are interstitial and undulated region, were formed in woven carbon/epoxy composite. In these regions, matrix fracture and complex crack path were mainly observed.     

Mode I fatigue delamination of Zanchor-reinforced CF/epoxy laminates

The Zanchor process is a novel through-thickness reinforcement technique in which in-plane yarns are entangled with each other using special needles. Mode I interlaminar fatigue crack growth behavior was investigated in carbon fiber (CF)/epoxy cross-ply laminates with Zanchor reinforcement. The laminates were molded with a Zanchor-reinforced CF dry fabric through resin film infusion (RFI). Delamination fatigue tests were carried out using double cantilever beam (DCB) specimens. The threshold values of the maximum energy release rates, G_Imaxth, under R = 0.1 were 70 J/m² for Zanchor 0 (base laminate without Zanchor reinforcement) and 240 J/m² for Zancor 2 (the density of Zanchor reinforcement is twice as high as the unit density), respectively; those under R = 0.5 were 80 J/m² for Zanchor 0 and 400 J/m² for Zancor 2, respectively. Thus, the threshold values for Zanchor 2 were about 3.4–5 times higher than those without Zanchor reinforcement. This increase induced by Zanchor reinforcement is almost the same or higher than that obtained under static loading (3.5 times). It is common that the increase in the fracture toughness, G_Ic, induced by replacing the matrix resin with a tougher system only partially contributes to the increase in the fatigue threshold, G_Imaxth. On the other hand, the increase in G_Ic induced by Zanchor reinforcement was fully translated to the increase in G_Imaxth. This is why Zanchor 2 gives one of the highest fatigue threshold values among existing toughened composite material systems. The difference between the reinforcing effects under static and fatigue loadings was discussed in conjunction with the microscopic fracture mechanism.     

The environmental fatigue behaviour of carbon fibre reinforced polyether ether ketone

The fatigue behaviour of carbon fibre/PEEK composite is compared with that of carbon/ epoxy material of similar construction, particularly in respect of the effect of hygrothermal conditioning treatments. Laminates of both materials were of 0/90 lay-up, and they were tested in repeated tension at 0° and at 45° to the major fibre axis. The superior toughness of the polyether ether ketone and its better adhesion to the carbon fibres results in composites of substantially greater toughness than that of the carbon/epoxy material, and this is reflected in the fatigue behaviour of the carbon fibre/PEEK. The tougher PEEK matrix inhibits the development of local fibre damage and fatigue crack growth, permitting a 0/90 composite with compliant XAS fibres to perform as well in fatigue as an epoxy laminate with stiffer HTS fibres. Hygrothermal treatments have no effect on the fatigue response of either material in the 0/90 orientation. The fatigue response of a cross-plied carbon/PEEK laminate in the ±45° orientation is much better than that of equivalent carbon/epoxy composites, again because the superior properties of the thermoplastic matrix.     

Interface chemistry in carbon fibre/epoxy matrix composites

Microcomposite test methods were used to measure the properties of the interphase of HTA/922 carbon/epoxy composites. The shear strength of the interphase resin is lower than that of the bulk resin. It is suggested that the discrepancy arises from changes in resin chemistry at the fibre/matrix interface. Bulk resin samples where the proportions of the constituents had been altered were tested. Resin with a reduced level of hardener matched the mechanical behaviour of the interphase resin. It is concluded that, for the system examined, the interphase resin had a lower hardener concentration than the bulk resin.     

Mode I interlaminar fracture toughness of commingled carbon fibre/PEEK composites

Carbon fibre/poly (ether-ether-ketone) (PEEK) composites were fabricated from plain weave cloth using the commingled yarn of carbon fibres with PEEK filaments. The undirectional specimen was made from the warp of commingled yarn and the weft of PEEK yarn, while the two-dimensional specimen was made from commingled yarns both of the warp and the weft. During the hot-pressing process, PEEK filaments melt to form the matrix of the composite. The interlaminar fracture toughness of the commingled composite was measured and compared with that of the prepreg composite. The critical strain energy release rates,/'G _Ics, obtained for the commingled composites were higher than the prepreg composite. In particular, the two-dimensional composite exhibited higherG _Ic than the unidirectional commingled composite. Factors increasing the fracture toughness of commingled composites have also been investigated by SEM observation of the fractured surface.     

Influence of alkali treatment and fibre length on mechanical properties of short Agave fibre reinforced epoxy composites

Composites based on short Agave fibres (untreated and alkali treated) reinforced epoxy resin using three different fibre lengths (3 mm, 7 mm and 10 mm length) are prepared by using hand lay up and compression mould technique. The materials were characterized in terms of tensile, compressive, flexural, impact, water absorption properties and machinability behaviour. All mechanical tests showed that alkali treated fibre composites withstand more fracture strain than untreated fibre composites. As evidenced by the dynamic mechanical analysis (DMA) tests, the thermo-mechanical properties of the composite with alkali treated Agave fibre were considerably good as alkali treatment had facilitated more sites of fibre resin interface. The machinability and atomic force microscope (AFM) studies were carried out to analyze the fibre–matrix interaction in untreated and alkali treated Agave fibre–epoxy composites.