A hybrid embedded cohesive element method for predicting matrix cracking in composites

The complex architecture of many fibre-reinforced composites makes the generation of finite element meshes a labour-intensive process. The embedded element method, which allows the matrix and fibre reinforcement to be meshed separately, offers a computationally efficient approach to reduce the time and cost of meshing. In this paper we present a new approach of introducing cohesive elements into the matrix domain to enable the prediction of matrix cracking using the embedded element method. To validate this approach, experiments were carried out using a modified Double Cantilever Beam with ply drops, with the results being compared with model predictions. Crack deflection was observed at the ply drop region, due to the differences in stiffness, strength and toughness at the bi-material interface. The new modelling technique yields accurate predictions of the failure process in composites, including fracture loads and crack deflection path.

Reduction of metal leaching in brown coal fly ash using geopolymers

Current regulations classify fly ash as a prescribed waste and prohibit its disposal in regular landfill. Treatment of the fly ash can reduce the leach rate of metals, and allow it to be disposed in less prescribed landfill. A geopolymer matrix was investigated as a potential stabilisation method for brown coal fly ash. Precipitator fly ash was obtained from electrostatic precipitators and leached fly ash was collected from ash disposal ponds, and leaching tests were conducted on both types of geopolymer stabilised fly ashes. The ratio of fly ash to geopolymer was varied to determine the effects of different compositions on leaching rates.

Effect of the manufacturing process on the interlaminar fracture toughness of 2/2 twill weave fabric carbon/epoxy composites

A 2/2 twill weave fabric carbon fibre reinforced epoxy matrix composite MTM56/CF0300 was used to investigate the effect of different manufacturing processes on the interlaminar fracture toughness. Double cantilever beam tests were performed on composites manufactured by hot press, autoclave and 'Quickstep' processes. The 'Quickstep' process was recently developed in Perth, Western Australia for the manufacture of advanced composite components. The values of the mode I critical strain energy release rate (G1d were compared and the results showed that the composite specimens manufactured by the autoclave and the 'Quickstep' process had much higher interlaminar fracture toughness than the specimen produced by the hot press. When compared to specimens manufactured by the hot press, the interlaminar fracture toughness values of the Quickstep and autoclave samples were 38% and 49% higher respectively. The 'Quickstep' process produced composite specimens that had comparable interlaminar fracture toughness to autoclave manufactured composites. Scanning electron microscopy (SEM) was employed to study the topography of the mode I interlaminar fracture surface and dynamic mechanical analysis (DMA) was performed to investigate the fibre/matrix interphase. SEM micrography and DMA spectra indicated that autoclave and 'Quickstep' produced composites with stronger fibre/matrix adhesion than hot press.

Characterization and analysis of delamination fracture and nanocreep properties in carbon epoxy composites manufactured by different processes

Delamination resistance and nanocreep properties of 2/2 twill weave carbon epoxy composites manufactured by hot press, autoclave, and Quickstep^TM process are characterized and analyzed. Quickstep is a fluid filled, balanced pressure heated floating mold technology, which is recently developed in Perth, Western Australia for the manufacture of advanced composite components. Mode I and Mode II interlaminar fracture toughness tests, and nanoindentation creep tests on matrix materials show that the fast ramp rate of the Quickstep process provides mechanical properties comparable to that of autoclave at a lower cost for composite manufacturing. Low viscosity during ramping process and good fiber wetting are believed to be the reasons that this process produces composites with high delamination and creep-resistant properties. Nanocreep properties are analyzed using a Kelvin–Voigt model.

Investigation of failure mechanisms in aged aerospace composites

The effect of isothermal ageing on two high temperature, bismaleimide composite materials, a novel CSIRO CBR 320/328 composite and a commercial CIBA GEIGY Matrimid® 5292 composite, was examined at 204 and 250 °C. Delamination is a major cause of failure in composite materials, therefore, the Mode I interlaminar fracture toughness (GIC) of both materials was measured using the double cantilever beam (DCB) test. Chemical degradation of the matrix was monitored concurrently using Fourier transform infrared (FTIR) and Raman spectroscopy. Chemical changes at the core of both of these materials were found to occur concomitantly with the observed changes in interlaminar fracture toughness. FTIR analysis of both matrix materials revealed the predominant degradation mechanism to be the oxidation of the methylene group bridging two aromatic rings common to the structure of both resins, and was substantiated by the ingrowth of a broad peak centred at 1600 cm⁻¹ . In addition to this, the pyromellitic anhydride unit present only in the CBR 320/328 composites was found to be highly resistant to the effects of ageing, whereas the saturated imide, common to the cured structures of both materials, was observed to degrade. Raman spectroscopy indicated that the predominant degradation mechanism of the composites differed at the two ageing temperatures.

Manufacturing influence on the delamination fracture behavior of the T800H/3900-2 carbon fiber reinforced polymer composites

'Torayca' T800H/3900-2 is the first material qualified on Boeing Material Specification (BMS 8-276) which utilizes the thermoplastic-particulate interlayer toughening technology. Two manufacturing processes, the autoclave process and the fast heating rated Quickstep™ process, were employed to cure this material. The Quickstep process is a unique composite production technology which utilizes the fast heat transfer rate of fluid to heat and cure polymer composite components. The manufacturing influence on the mode I delamination fracture toughness of laminates was investigated by performing double cantilever beam tests. The composite specimens fabricated by two processes exhibited dissimilar delamination resistance curves (R-curves) under mode I loading. The initial value of fracture toughness GIC-INIT was 564 J/m2 for the autoclave specimens and 527 J/m2 for the Quickstep specimens. However, the average propagation fracture toughness GIC-PROP was 783 J/m2 for the Quickstep specimens, which was 2.6 times of that for the autoclave specimens. The mechanism of fracture occurred during delamination was studied under scanning electron microscope (SEM). Three types of fracture were observed: the interlayer fracture, the interface fracture, and the intralaminar fracture. These three types of fracture played different roles in affecting the delamination resistance curves during the crack growth. More fiber bridging was found in the process of delamination for the Quickstep specimens. Better fiber/matrix adhesion was found in the Quickstep specimens by conducting indentation-debond tests.

Cement composites reinforced with surface modified coir fibers

An experimental investigation of coir mesh reinforced mortar (CMRM) is conducted using nonwoven coir mesh matting. The main parameters in this study are the fiber volume fraction (number of mesh layers) and fiber surface treatment with a wetting agent. The composites are subjected to the four-point bending test. The short-term mechanical properties of CMRM are discussed. Scanning electron micrograph analysis is used to observe the fiber—matrix interfacial characteristics. The results indicate that the addition of coir mesh to mortar significantly improves the composite post-cracking flexural stress, toughness, ductility, and toughness index, compared to plain mortar materials. The Albatex © FFC wetting agent (2-ethylhexanol) can effectively improve water absorption of coir fiber and enhance the fiber—matrix bonding strength. These coir mesh reinforced composites may be useful in civil engineering applications.

The effect of cure cycle heating rate on the fibre/matrix interface

Development of civil aerospace composites is key to future “greener” aircraft. Aircraft manufacturers must improve efficiency of their product and manufacturing processes to remain viable. The aerospace industry is undergoing a materials revolution in the design and manufacture of composite airframes. The Airbus A350 and Boeing 787 (both due to enter service in the latter part of this decade) will push utilisation levels of  composite materials beyond 50% of the total airframe by weight. This  change requires massive investment in materials technology, manufacturing capability and skills development. The Quickstep process provides the ability to rapidly cure aerospace standard composite materials whilst providing enhanced mechanical properties. Utilising fluid to transfer heat to the   composite component during the curing process allows far higher heat rates than with conventional cure techniques. The rapid heat-up rates reduce the viscosity of the resin system greatly to provide a longer processing window introducing greater flexibility and removing the need for high pressure during cure. Interlaminar fracture toughness (Mode I) and Interfacial Shear Strength of aerospace standard materials cured using Quickstep have been    compared to autoclave cured laminates. Results suggest an improvement in fibre-matrix adhesion.

Preparation, characterization and dielectric properties of temperature stable SrTiO3/PEEK composites for microwave substrate applications

Poly(ether ether ketone) (PEEK) is a potential candidate for electronic applications due to its low permittivity, low loss, high melting point, better chemical resistance, excellent insulating properties and easy processibility. Present paper discusses the preparation and characterization of SrTiO3 filled PEEK composite for microwave substrate applications. The dielectric constant, dielectric loss and temperature variation of dielectric constant of the composites have been studied up to 1 MHz using an Impedance Analyzer. Different theoretical approaches have been employed to predict the effective permittivity of composite systems and the results are compared with that of the experimental data. The crystallinity of the bulk composite is studied by X-ray diffraction studies. Scanning electron microscopic technique has been employed to study the dispersion of the particulate filler in PEEK matrix. Vickers hardness of pure and filled PEEK composite has been measured using Microhardness Tester. The effect of particle size on the dielectric as well as mechanical properties of SrTiO3/PEEK composite system is also studied by incorporating micronsize and nanosize fillers. Present study shows that a temperature stable composite can be realized by judiciously selecting appropriate filler concentration in the PEEK matrix.

Conducting composite using an immiscible polymer blend matrix

Carbon black (CB) fillers were used to study the feasibility of achieving multiple percolation using an immiscible (polar) polymer blend matrix. By tailoring the morphology of the insulating dual phase matrix it has been shown that the percolation threshold (Ф_c) can be reduced over single-phase matrices. Cocontinuity in the polymer matrix is important in reducing Ф_c by either preferentially isolating the conducting filler at the interface of the two phases or within one particular continuous phase of the matrix thereby forming a continuous conducting network within a continuous network (multiple percolation). Actual melt processing time has been found to influence the dispersion of the fillers and hence Ф_c. Polarity of the matrix as well as the processing method has also been found to influence the dispersion of the filler within the host polymer.

Lithium-ion conducting ceramic/polyether composites

Composites of a lithium ion conducting ceramic with a lithium salt based polymer electrolyte matrix are described. Conductivity measurements as a function of the lithium ion conducting ceramic phase content in the composite show that there is a significant increase in conductivity at approximately 40 vol% of the ceramic. The room temperature conductivity above this ceramic content is enhanced by at least 100% over that of the polymer electrolyte phase alone. It is believed that this additional contribution is substantially lithium ion conduction. The major barrier to ion-motion in these materials appears to be the interface between the polymer and ceramic. This interfacial resistance is strongly moisture-sensitive.

Polymer-ceramic ion-conducting composites

Composites of a Li⁺ ion-conducting ceramic powder in a polyether-based elastomeric electrolyte matrix are described. At 66 wt.% of ceramic the composite can be prepared as a paste and cured into a coherent material having useful elastic and tensile properties. The total conductivity of the composite was found to be (1.9 ± 0.2) × 10⁻⁴ S cm⁻¹ at 40 °C which was approximately 1 order of magnitude higher than the polymer electrolyte component alone. The result was also approximately 1 order of magnitude higher than the total conductivity of the ceramic powders tested in this work.

Characterisation of out-of-autoclave carbon fibre composites

Out-of-autoclave processing parameters were tailored to investigate the effect of resin viscosity on mechanical performance. Faster heating rates improved the shear and fracture mechanisms of carbon fibre composites by improving their fibre to matrix adhesion, as a result of a decrease in resin viscosity.

Improving the dispersion and mechanical properties of epoxy/carbon nanotube composites

The incorporation and uniform dispersion of carbon nanotubes (CNTs) in polymer matrix could facilitate engineers to create high performance nanocomposites that potentially compete with most advanced materials in nature. The unique combination of outstanding mechanical, thermal, and electrical properties of CNTs makes them excellent nanofillers for the fabrication of advanced materials. Successful enhancement in mechanical properties via reinforcement is expected only when the nanofillers are well dispersed in the polymer matrix. Moreover, the orientation as well as the CNT/matrix interfacial strength also determines the effective physical properties of the nanocomposites. However, CNTs typically assemble to give bundles, which are heavily entangled to each other with a high aspect ratio and a large π-electronic surface. In this work, we outline some preliminary results in preparing high performance epoxy composites. Composites with fine dispersion and superior mechanical properties were prepared using epoxy and multiwalled carbon nanotubes (MWCNTs). The fine dispersion of the nanocomposites can be identified in the high resolution SEM image shown in Figure 1. This method can provide an alternative route for the preparation of new structural and functional nanocomposites.