Fracture behaviour of model toughened composites under Mode I and Mode II delaminations

The fracture behaviour of two toughened epoxy composite systems was investigated using various microscopy techniques. The Mode I delamination fracture toughness,G _IC, Mode II delamination fracture toughness;G _IIC, and compression after impact (CAI) strength of these model composite systems were also measured. Under Mode I fracture, it was found that these composites exhibit nearly identical toughening mechanisms to those of the rubber-modified neat resins. The composites differ primarily in having smaller damage zones than the neat resin equivalents. Under Mode II fracture, the typical hackles were found to initiate from inside the resin-rich interlaminar region due to the presence of the toughener particles. The CAI strength, based on the present study as well as the work conducted by others, appeared to be related to, but not necessarily strongly dependent on, the interlaminarG _IC andG _IIC, the thickness of the interlaminar resin-rich region, and the type of the interlaminar toughener particles. Approaches for improving theG _IC,G _IIC, and CAI strength of high-performance toughened composites are discussed.     

Interlayer toughened unidirectional carbon prepreg systems: effect of preformed particle morphology

A new production process was used to change the surface morphology and reduce the water sensitivity of a commercial preformed rubber particle used for interlayer composite toughening. These particles were incorporated in a model epoxy resin and impregnated into unidirectional carbon fibers. The mode II fracture toughness of a laminate made with experimental particles was 250% higher than the control system and 100% higher than a laminate made with the commercial preformed rubber particles. The new particles were also found to reduce the damage area resulting from impact; however, the ultimate laminate compression strength after impact was lower for the experimental particle modified composite than the commercial particle modified laminate. The composites were also subjected to hot–wet conditions and the initial water absorption rate was less for the experimental particle modified laminates than the laminate containing the commercial material. Yet, after 6400 h, the laminates made with experimental particles were found to absorb more water than the other materials.     

Novel carbon fiber/epoxy composite toughened by electrospun polysulfone nanofibers

Novel carbon fiber/epoxy composite toughened by electrospun polysulfone (PSF) nanofibers was prepared to enhance fracture toughness of the composite, and compared the morphology and toughness to those of composite toughened by PSF films prepared by solvent method. Polysulfone nanofibers with the average diameter of 230 nm were directly electrospun onto carbon fiber/epoxy prepregs to toughen the composite. SEM observations of the polysulfone nanofibers toughened composite revealed that polysulfone spheres with uneven sizes presented uniform dispersion through interleaves of the composite, which was different from those of composite toughened by PSF films. Mode I fracture toughness (G_IC) of the nanofibers toughened composite was 0.869 kJ/m² for 5.0 wt.% polysulfone nanofibers content, which was 140% and 280% higher than those of PSF films toughened and untoughened composite due to the uniform distribution of polysulfone spheres.     

Pre-yield tensile set of a semi-crystalline polymer,its blend and composite

Tensile set was studied at low strains on polypropylene, aliphatic polyketone, rubber toughened blends and CaCO₃ particle toughened composites. The rubber in the rubber toughened blends had a particle size of 0.7 μm. The CaCO₃ particles had a size of 0.7 μm and had been coated with stearic acid. Step-cyclic loading was applied in 1% strain incrementals at a strain rate of 10⁻² s⁻¹. The maximum strain applied was 20%. The temperature of the test bar was studied with an infra-red camera. Pre-yield deformation is normally assumed to take place in a nonlinear elastic manner. However, for polypropylene and polyketone elastoplastic deformation starts at low strains. For PP the onset of tensile set is at very low strains and increases with strain. The tensile set at the yield point was only 50% and at the drawing strain 100%. Polyketone had a similar tensile set development but shifted to slightly higher strains. Here too the tensile set at the yield point was about 50% and at the drawing strain 100%. The temperature of the non yielded material was found to rise in polyketone a 7 °C. The rubber toughened blends had at low strains a higher tensile set, but after the yield strain the set was similar to the base polymer. At 5% strain the tensile set increased with rubber content. The sub micron CaCO₃ particle toughened composites increased the tensile set too. The tensile set is a simple technique for studying the pre-yield behaviour of multi phase systems.     

Influence of moisture and resin ductility on delamination

The critical strain energy release rate, $\text{G}{}_{\mathrm{c}}$, at edge delamination has been measured indirectly for angle-ply laminates with standard resin and toughened resin under mixed-mode loading at various K_III/K_I ratios. It is shown that $\text{G}{}_{\mathrm{c}}$ increases with increasing K_III/K_I ratio. The critical energy release rate is significantly higher in the toughened resin laminate than in the standard resin laminate. Moisture absorption does not affect the critical energy release rate for laminates with standard resin, whereas it reduces the critical energy release rate for laminates with toughened resin. In confirmation of this, it has been observed that the hackle-dominated brittle resin fracture occurring in the standard resin laminate is replaced by semi-ductile fracture in the toughened resin laminate. When the toughened resin laminate is moisturized prior to testing the fracture surface appearance returns to being hackle-dominated.     

Mixture of fuels approach for the solution combustion synthesis of Al2O3-ZrO2 nanocomposite

A modified solution combustion approach was used for the first time in the preparation of nanosize zirconia toughened alumina (ZTA) composite. ZTA-1 with an average particle size of ∼37 nm was prepared using corresponding metal nitrates and urea. ZTA-2 with an average particle size of <10 nm was prepared by using mixture of fuels such as ammonium acetate, urea and glycine. The products formed were characterised by powder X-ray diffractometry, Transmission electron microscopy and BET surface area analysis. By using mixture of fuels, the energetics of the combustion reaction and eventually the properties of the combustion product have been changed. A series of combustion reactions were carried out to optimise the fuel ratio combinations required to obtain <10 nm ZTA particles. The microstructure of ZTA consisted of crystallites of Al₂O₃ and ZrO₂ both of which were nanocrystalline as evident from TEM.     

Effects of oxidation of Cr3C2 particulate-reinforced alumina composites on microstructure and mechanical properties

Al₂O₃ can be strengthened and toughened by incorporating Cr₃C₂ particles through hotpressing. For instance, an Al₂O₃-10 vol% Cr₃C₂ composite exhibits fracture strength and toughness of 600 MPa and 5.5 Mpa m^0.5, respectively. An annealing treatment in air from 1000–1200C may further substantially strengthen the same composite to give _f = 800 MPa andK _IC = 9.5 MPam^0.5. Possible oxidation reactions and toughening mechanisms are discussed in terms of oxygen diffusion, the formation of micropores beneath the exposed surface, as well as the fracture mode.     

Characterization of SiO2/Ag composite particles synthesized by in situ reduction and its application in electrically conductive adhesives

In the study, SiO₂/Ag composite particles with silver coating onto the surface of silica have been successfully prepared via a novel and facile approach (Oxidation–Reduction Method). In this approach, the SiO₂ particles were first modified with 3-ammoniatriethoxysilane (APTES) and glyoxalic acid (GA) through two-step reaction, the aldehyde group (CHO) were anchored onto the surfaces of silica spheres via electrostatic attraction, these [Ag(TEA)₂]⁺ ions in the solution were then reduced by the CHO and coated onto the surface of silica to obtain SiO₂/Ag composite particles. The effects of the reaction conditions on silver content and synthetic mechanism had also been discussed. The structure, morphology and optical properties of the SiO₂/Ag composite particles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV–vis spectroscopy. The results showed the surface of SiO₂ was surrounded by pure silver nanoparticles, and the silver nanoparticles had face-centered-cubic structure, the SiO₂/Ag composite particles with core–shell morphology and special optical properties. And the small content SiO₂/Ag composite particles applied in electrically conductive adhesives (ECAs) improved the electrical bulk resistivity and tensile shear strength.     

Particle distribution and interfacial reactions of Al–7%Si–10%B<Subscript>4</Subscript>C die casting composite

Aluminum–boron carbide particle reinforced composite is an advanced material which can be used in applications such as neutron-shielding components, aircraft, and aerospace structures. In the microstructural characterization of an Al–7%Si–10%B₄C die casting, attention is particularly focused on particle distribution and interface reaction products between B₄C particles and the aluminum matrix. The quantitative analysis results show that, in a cross-section of the cast part, more particles concentrate in the center and fewer particles are present in the wall regions. Moreover, some particle segregation bands have been observed. The mechanisms of the particle migration are proposed to describe the phenomenon. However, the average particle fraction in any cross-section of the cast part is almost the same. A barrier layer consisting of several sublayers was detected on the surface of B₄C particles. Using electron diffraction in selected areas, it is found that these sublayers are composed of Al₃BC crystals, TiB₂ crystals, Si crystals, and coarse stick-shaped TiB₂ particles. In addition, it is observed that Si plays an important role in the formation of a dense barrier layer. The barrier layer can limit B₄C decomposition and improve B₄C stability in the aluminum melt.     

The mechanical behavior of optical fiber sensor embedded within the composite laminate

Mechanical behavior, such as tensile and fatigue strength, of the optical fiber sensor embedded within the composite laminate was investigated. Tensile and fatigue tests were performed to evaluate the static and fatigue characteristics of optical fibers embedded within three types of laminated composite specimens, [0₆/OF/0₆]_T, [0₂/90₄/OF/90₄/0₂]_T and [0₃/90₃/OF/90₃/0₃]_T. The initiation of damage and fracture of the optical fiber were detected by observation of the intensity drop-off of laser signal transmitted through the optical fiber during test. Experimental results showed that the fatigue strength of optical fiber embedded within the cross-ply laminate is much lower than the fatigue strength of optical fiber within the unidirectional ply laminate. It was also found that the optical fiber embedded within unidirectional ply laminate fractured due to the fatigue damage accumulation of internal defects of optical fiber itself. However the optical fiber embedded within the cross-ply laminate fractured due to the growth of transverse matrix crack of host composite laminate.     

Simulation‐Experimental Approach to Investigate the Role of Interfaces in Self‐Replenishing Composite Coatings

To investigate self‐replenishing on surface‐structured composite coatings a dual simulation‐experimental approach is employed to study the decisive role of polymer‐air and polymer‐particle interfaces. Experimentally, the composite system consists of a cross‐linked polymer network with fluorinated‐dangling chains, embedding colloidal SiO₂ nanoparticles which are incorporated in the network via covalent bonding. These particles provide the desired surface structure at the air‐interface before and after damage. Any damage replicates the rough surface, while the polymer layer on top of the particles serves as source of low surface energy groups which are able to reorient towards the new air‐interfaces. Using coarse‐grained simulations details of these self‐replenishing composite systems are revealed such as the minimum thickness of the polymer layer necessary for providing optimal self‐replenishing ability and the distribution profile of the dangling chains at the various interfaces. The principles and dual approach reported here may be applied to other self‐healing composite systems with applications in self‐cleaning, anti‐fouling or low adhesion materials.     

Interlaminar fracture toughness and associated fracture behaviour of bead-filled epoxy/glass fibre hybrid composites

To investigate enhancement of matrix-dominated properties (such as interlaminar fracture toughness) of a composite laminate, two different bead-filled epoxies were used as matrices for the bead-filled epoxy/glass fibre hybrid composites. The plane strain fracture toughness of two different bead-filled epoxies have been measured using compact tension specimens. Significant increases in toughness were observed. Based on these results the interlaminar fracture toughness and fracture behaviour of hybrid composites, fabricated using bead-filled epoxy matrices, have been investigated using double cantilever beam and end notch flexure specimens for Mode I and Mode II tests, respectively. The hybrid composites based on carbon bead-filled matrix shows an increase in both G _IC initiation and G _IIC values as compared to a glass fibre reinforced plastic laminate with unmodified epoxy matrix. The optimum bead volume fraction for the hybrid composite is between 15% and 20%. However, the unmodified epoxy glass-fibre composite shows a higher G _IC propagation value than that of hybrid composites, due to fibre bridging, which is less pronounced in the hybrids as the presence of the beads results in a matrix-rich interply region.     

Enhancement of crack propagation resistance in epoxy resins by introducing poly(dimethylsiloxane) particles

The physical and mechanical properties of polyepoxy DGEBA /DDA/Diuron networks toughened with Poly(dimethylsiloxane) particles have been studied. Blends have been realized with two kinds of dispersion tools: a high-speed stirrer and a twin-screw extruder. The dispersion state quality is discussed using transmission spectroscopy image analysis. Poly(dimethylsiloxane) suspension in an epoxy prepolymer was used as a toughening agent. Different particle quantities were introduced: 4, 8, 15% by weight. Static mechanical tests were performed in tension and compression on these poly(dimethylsiloxane) modified materials. A slight decrease of Young's modulus and an increase in plastic deformation capacity were noticed as the volume fraction of the modifier increased. Using linear elastic fracture mechanics (LEFM), an improvement in the fracture properties (K_IC, G_IC) was shown. Fatigue crack growth propagation studied for the blends demonstrated that the Paris law can be used to describe the behavior of the materials. Increasing the volume fraction of the modifier leads to an improvement of fatigue crack propagation resistance. Finally a decrease in the wear rate and the friction coefficient with the increase of particle quantities has been shown (in a pin on disk configuration). Toughening mechanisms are discussed with SEM fracture surfaces. This revised version was published online in September 2006 with corrections to the Cover Date.     

Effect of particles on interlaminar crack growth in cross-plied carbon-fibre/epoxy laminates

This paper discusses the effect of particulate additions on the mode-I and mode-II interlaminar fracture toughness of a cross-plied, carbon-fibre-reinforced, epoxy-resin laminate. Particles of graphite, silicon carbide and polyethylene were mixed with the epoxy resin prior to laminating with woven carbon-fibre cloth. Tests have been performed on double cantilever beam (DCB) and end-notched-flexure (ENF) specimens to obtain the critical-strain energy-release rates,G _IC andG _IIC, for the laminates with and without particulate additions. The dependences of the values ofG _IC andG _IIC on the crack length are also considered. The results indicate that the interlaminar-fracture-toughness (mode-I and mode-II) values of the CFRP laminate increase with increases in the particle content up to about 3%, and thereafter they decrease with further increases in the particle content. This was thought to be due to an increase in multiple-crack formation in the middle region of the cracked-plate samples. Furthermore, mode-I tests indicate that the propagation values ofG _IC are dependent on the crack length.     

MMCs with controlled non-uniform distribution of submicrometre Al2O3 particles in 6061 aluminium alloy matrix

Abstract

By employing spherical agglomerate of submicrometre Al₂O₃ particles, a novel composite with controlled non-uniform reinforcement distribution was produced by the squeeze casting technique. The resulting composite has a microstructure in which composite spheres, consisting of Al reinforced by small Al₂O₃ particles, are uniformly distributed in a particle free Al matrix. The response of mechanical properties to the controlled non-uniform distribution of the reinforcement was investigated and the fracture mechanism was discussed in order to understand experimentally the influence of particle clustering on the deformation characteristics. It is demonstrated that a composite with such a controlled microstructure exhibits significant increase in ultimate tensile strength (UTS), elastic modulus, and also a slight increase in ductility compared to composite with a homogeneous microstructure at a given volume fraction. It is proposed that the composite spheres act as units of reinforcement when subjected to deformation and contribute positively to the modulus, UTS, and hardness. The higher ductility is attributed to the extensive ductile deformation of the Al matrix. In addition, the highly restrained deformation observed in the interagglomerate region can also be beneficial to the high strength.     

Strategical parametric investigation on manufacturing of Al–Mg–Zn–Cu alloy surface composites using FSP

Friction stir processing (FSP) is a unique approach being presently researched for composite fabrication. In the present investigation, Al-B₄C surface composite was fabricated through FSP by incorporating B₄C powder particles into Al–Mg–Zn–Cu alloy (AA 7075) matrix. The influence of varying powder particle reinforcement strategies on the microstructure, powder distribution, microhardness, and wear resistance of the surface composite is reported. In addition, AA 6061/B₄C composites were prepared using the same parameter set and the powder distribution in the composite was compared to that in the AA 7075/B₄C composite. More homogeneous dispersion of B₄C powder was observed in AA 6061 as compared to AA 7075 substrate. Among the prepared AA 7075/B₄C composites, the best B₄C powder distribution was detected in samples processed using fine powder and incorporating the change in stirring direction between passes. The hardness and wear resistance of the prepared composites were almost doubled attributing to several strengthening mechanisms and B₄C powder distribution in the AA 7075 matrix.     

RMS matrix strains in transformation toughened alumina

The Warren Averbach technique was used to determine the RMS strain profile in the Al₂O₃ matrix surrounding ZrO₂ particles in zirconia toughened alumina. The X-ray domain size was found to be 100nm in all but the most severely microcracked sample. The RMS strain, averaged over all domains, decreased nearly linearly from the edge of the domain (presumably from the ZrO₂ particle), except when microcracking was detected and then the RMS strain level was reduced near the surface of the domain. The maximum RMS strain level increased with increasing ZrO₂ particle size up to the point where microcracking occurred then decreased. The results were explained by noting that the maximum RMS strain was directly related to the monoclinic content of the sample.     

Einfluß der Laminatkomponenten von CFK-Werkstoffen auf das statische und das dynamische Verhalten

Influence of Different Laminate Components on the Static and Dynamic Properties of CERP The static and the dynamic properties of a conventional and a modified, toughened matrix system reinforced with two high strength carbon fibres of a different strain to failure was investigated. Three different laminates were chosen, an unidirectional laminate [0₈], a cross ply laminate [0₂, 90₂, 0₂, 90₂]_s, and a nearly quasi-isotropic laminate [0₂, ±45, 90 ]_s. It is shown, that the damage development under static as well as dynamic loading is delayed when using a toughened matrix system. This delay in damage development results in improved mechanical properties. However, when using a toughened matrix system together with fibres of a high strain to failure, the improvement of the static properties does not lead in the same extent to an improvement of the dynamic properties.     

Effect of delamination failure in crashworthiness analysis of hybrid composite box structures

In the present paper the effects of delamination failure of hybrid composite box structures on their crashworthy behaviour will be studied and also their performance will be compared with non-hybrid ones. The combination of twill-weave and unidirectional CFRP composite materials are used to laminate the composite boxes. Delamination study in Mode-I and Mode-II with the same lay-ups was carried out to investigate the effect of delamination crack growth on energy absorption of hybrid composite box structures. The end-loaded split (ELS) and double-cantilever beam (DCB) standard test methods were chosen for delamination studies. In all hybrid composite boxes the lamina bending crushing mode was observed. Regarding the delamination study of hybrid DCB and ELS the variation of the specific energy absorption (SEA) versus summation of G_IC and G_IIC were plotted to combine the effect of Mode-I and Mode-II interlaminar fracture toughness on the SEA. From this relationship it was found the hybrid laminate designs which showed higher fracture toughness in Mode-I and Mode-II delamination tests, will absorb more energy as a hybrid composite box in crushing process. The crushing process of hybrid composite boxes was also simulated by finite element software LS-DYNA and the results were verified with the relevant experimental result.     

Identification of failure mechanisms of metallised glass fibre reinforced composites under tensile loading using acoustic emission analysis

In this work, failure mechanisms of metallised glass fibre reinforced epoxy composites under tensile loading were investigated using acoustic emission analysis. Sandblasting with Al₂O₃ was used to pre-treat the composite surface prior to metallisation, and therefore to improve adhesion. The sandblasting time was varied from 2 s to 6 s. A two-step metallisation process consisting of electroless and subsequent electroplating was used for depositing the copper coating on the pre-treated composite surface. The mechanical pre-treatment had no significant negative effect on the mechanical properties of the composite laminate. The acoustic emission (AE) from the metallised composite was recorded during tensile testing in order to investigate the failure mechanisms. AE-Signals were analysed using pattern recognition and frequency analysis techniques. A correlation between the cumulative absolute AE-energy and the mechanical behaviour of uncoated and coated specimens during tensile testing was successfully observed. It was shown that a stronger adhesion between substrate and coating leads to a lower release of mechanical elastic energy, which could be recorded by means of AE analysis. Furthermore, differences in peak frequency, frequency distribution and the use of pattern recognition techniques allowed classifying the signal into three failure mechanisms for the uncoated samples and four failure mechanisms for the coated samples, namely matrix cracking, fibre-matrix interface failure, fibre breakage and substrate-coating interface failure. Waveform and frequency analysis of the classified signals supported the identification of the failure mechanisms. Furthermore, optical investigation and SEM images of the tested samples and fracture surfaces confirmed the identified mechanisms evaluated by acoustic emission analysis.