Fracture Properties Of Composites Research Articles

Fatigue life assessment of hybrid bio-composites based on self-heating temperature

An increasing interest to hybrid bio-composites leads to a necessity of their fatigue life assessment, which is a challenging task considering the high influence of selected reinforcing fibres and their composition, as well as anisotropy of natural fibre used more and more often in load-bearing applications. In this study, the influence of hybridization is investigated for flax, glass/flax, Kevlar/flax, and carbon/flax bio-composites in terms of evaluation of fatigue limit by means of the critical self-heating temperature and acoustic emission approaches as well as microscopic characterization used for justification of the observed phenomena analysing fracture mechanisms. It was shown that the hybridization opens new possibilities of steering fatigue and fracture properties of composites. Furthermore, it was confirmed that the critical self-heating temperature determined during fatigue tests can be used as an alternative indicator of the fatigue limit of a composite structure, which was justified by microscopic examination of structures at various levels of fatigue degradation.

Influence of cluster shaped defects on fracture process of fiber bundle

Defects that exist inside composites have an important effect on the tensile fracture properties of composites. The fiber bundle model is a theoretical model commonly used to study the tensile fracture properties of disorder materials. Existing work on fiber bundle models with single fiber defects shows that after single fiber defects are introduced into the fiber bundle model, the defects have a significant effect on the tensile fracture properties of the model. Since there are more complex microscopic defect structures in actual materials, such as voids, gaps, impurities, dislocations, micro-cracks, etc, it is necessary to build a multi-size defect model. In order to study the defects of different sizes and damage degrees existing in actual materials, the spatial size of the defect, the degree of defect and the distribution of fiber damage levels within the defect and other influencing factors are introduced to construct an extended fiber bundle model with cluster shaped defects. For the model, it is first assumed that the degree of defect of the fiber inside each cluster decays linearly from the center to the outside in two spatial attenuation forms: exponential decay and constant degree of defect. In the fiber bundle model of this cluster-shaped defect, the two most important factors are the number of defects <i>α</i> and the upper limit of defect size <i>β</i>. The numerical simulation method is used to analyze the influence of the number of defects, the upper limit of defect size, and spatial distribution of degree of defective fibers inside defect on the macroscopic mechanical properties and statistical properties of fracture when the model is subjected to quasi-static load borne under the nearest neighbor stress redistribution. Through the simulation analysis, it is found that owing to the overlapping competition mechanism of the defect spatial distribution, when the upper limit <i>β</i> of the defect size is large, the influence of the number of defects on the system load capacity trends to saturation. Since the defect degree of the defect center fiber is proportional to the defect size, with the upper limit <i>β</i> of the defect size increasing, its influence on the load capacity of the model becomes more and more significant. When large size defects exist, even if the number of defects is small, the load bearing performance of the material will be significantly reduced. The spatial distribution function of the damage degree of fiber inside the defect has no substantial influence on the above rules, and only changes the specific value of each fracture property. The simulation analysis results in this paper have certain theoretical significance in improving the mechanical properties of composite materials.

Experimental investigation on graphene oxides coated carbon fibre/epoxy hybrid composites: Mechanical and electrical properties

In this article, Mode I interlaminar fracture toughness, flexural and interlaminar shear strength (ILSS) properties of graphene oxides (GOs) coated carbon fibre reinforced epoxy composites have been studied. Here, a simple electrophoretic deposition technique was employed to coat the GO and reduced graphene oxide (rEGO) fillers onto the carbon fibre (CF) surfaces. To partially reduce the coated GOs, the GOs coated CFs were thermally annealed at 200 °C for 2 h. Vacuum assisted resin transfer moulding (VARTM) technique was employed to fabricate hybrid epoxy composites with three variants of GOs (namely, GO, rEGO, TrGO) coated CFs. The modified composite laminates have demonstrated substantial improvement in ILSS properties, where composites based on GOs coated CFs exhibited the highest enhancement of 47%. Similarly, flexural and Mode I fracture properties of composites also delivered significant improvements in their values, gaining at 25% and 14% respectively. Interesting observations were made from the fracture surface images of double cantilever beam (DCB) test specimens of composites. Through-thickness electrical conductivity of thermally reduced GOs coated and rEGOs coated CF composites have shown a substantial increment of 127% and 90% compared to uncoated composites.

Effect of POSS and chain extender on tensile and fracture properties of neat and short glass fiber reinforced polyamide 6 composites

Polyamide 6 (PA6) and short glass fiber (SGF) reinforced PA6 composites toughened with glycidylisobutyl oligomeric silsesquioxane (G-POSS) and 1.1′-carbonyl-bis-caprolactam (CBC) were prepared by melt-extrusion compounding process and then compounds were formed to 2mm thick double edge notched tension samples (DENT) by injection moulding process. Fracture properties of composites were analyzed by the essential work of fracture (EWF) method. Incorporation of G-POSS and CBC with 1% weight ratio to PA6 matrices individually caused a remarkable enhancement on specific essential work of fracture, we, value of neat PA6. It was also found that the EWF method is incapable to characterize fracture response of both 10 and 20wt% SGF reinforced PA6 composites without G-POSS and CBC due to brittle fracture event. But the incorporation of G-POSS and CBC into the SGF reinforced PA6 matrix led to a transition on the fracture event from brittle to ductile. Accordingly, the EWF method became an applicable model for G-POSS and CBC modified, SGF reinforced PA6 composites. Increase on the tensile strength of SGF reinforced PA6 composites with 1wt% CBC is also another outstanding enhancement in this study.

Effect of CNT on the Delamination Resistance of Composites

Polymer composite materials reinforced by continuous fibers have excellent in-plane strength but are usually weak against delamination. The current study has provided a FEM model with using carbon nanotubes to improve the interlaminar fracture properties of composites. A full 3-D multiscale finite element model consisting of a carbon nanotube, inter-phase, surrounding polymer and cohesive layer was built using ANSYS commercial package. The cohesive layer was constructed using cohesive zone method (CZM), which is a commonly used method to investigate delamination analysis. The inter-phase region was represented by van der Waals interactions. In addition, the critical energy release rates of mode-I delamination (GIC) were characterized for nanocomposite specimens and for those with pure resin. Significant improvement in GIC was consistently observed.

Simulation of crack growth in ductile materials

During crack growth of real materials, the total energy released can be partitioned into elastic and dissipative terms. By analyzing material models with mechanisms for dissipating energy and tracking all energy terms during crack growth, it is proposed that computer simulations of fracture can model crack growth by a total energy balance condition. One approach for developing fracture simulations is illustrated by analysis of elastic–plastic fracture. General equations were derived to predict crack growth and crack stability in terms of global energy release rate and irreversible energy effects. To distinguish plastic fracture from non-linear elastic fracture, it was necessary to imply an extra irreversible energy term. A key component of fracture simulations is to model this extra work. A model used here was to assume that the extra irreversible energy is proportional to the plastic work in a plastic-flow analysis. This idea was used to develop a virtual material based on Dugdale yield zones at the crack tips. A Dugdale virtual material was subjected to computer fracture experiments that showed it has many fracture properties in common with real ductile materials. A Dugdale material can serve as a model material for new simulations with the goal of studying the role of structure in the fracture properties of composites. One sample calculation showed that the toughness of a Dugdale material in an adhesive joint mimics the effect of joint thickness on the toughness of real adhesives. It is expected, however, that better virtual materials will be required before fracture simulations will be a viable approach to studying composite fracture. The approach of this paper is extensible to more advanced plasticity models and therefore to the development of better virtual materials.

Micromechanical properties of high fracture performance SiCp–6061Al/6061Al composite

A SiCp–6061Al bar reinforced 6061Al matrix composite (SiCp–6061Al/6061Al) exhibited a higher fracture performance than a conventional SiC particle reinforced 6061Al composite (SiCp/6061Al) by greatly altering particle distribution. Compared with the conventional composite, the fracture toughness of the SiCp–6061Al/6061Al composite increased by 34.1%. The variation in particle distribution affects the local micromechanical properties, such as residual stress and strain hardening, which can influence the overall fracture properties of composites. X-ray diffraction with a ∅ 30 μm collimator was used to measure the residual stresses in the matrix between SiCp–6061Al bars in the SiCp–6061Al/6061Al composite. In addition, atomic force microscope-based nanoindentation, as a qualitative measurement, was used to estimate the residual stress and strain hardening level in both composites. It is found that the low residual stress and strain hardening level in the matrix improves the matrix ductility, which is good for the fracture toughness of the SiCp–6061Al/6061Al composite.

Effect of fibres coating on fracture properties of composite

In the present paper, the ultimate tensile strength, fracture toughness and surface fracture energy of continuously coated, uncoated fly-ash filled unidirectional glass fibre reinforced epoxy resin have been studied. The results show that the tensile strength and fracture toughness of fly-ash filled glass fibre reinforced epoxy resin can be increased by the continuous coating of fibre tows with epoxy resin prior to chopping into moulding pellets.

The influence and detection of processing induced defects in roving based composites

The fracture properties of composites are strongly dependent on defects originating from the heterogeneous structure of the composite, the imperfections of the processing methods and the properties of the raw materials. The influence of the glass content, the fibre distribution, the binders etc on the debonding, intra-sttrand crack growth and the final catastrophic crack growth through the inter-strand matrix rich regions has been studied for roving based composites, using a series of chopped strand mat, glass fibre/polyester laminates as model materials. The studies include measurements of stress-strain relations, acoustic emission, hysteresis, mechanical loss factor, laser light scattering, electron and visible light microscopy.