Larissa Gorbatikh

Micro-scale finite element analysis of stress concentrations in steel fiber composites under transverse loading:

Steel fibers, with their high stiffness and high ductility, have a potential to provide a new range of properties in polymer composites, in comparison with carbon and glass fiber composites. However, the high stiffness contrast between the steel fiber and the polymer matrix plus the fiber’s non-circular cross-section are likely to generate high stress concentrations in a composite under transverse loading. In the present study, these stress concentrations are analyzed using finite element modeling and compared with the case of carbon and glass fiber composites. The study is performed for an isolated fiber and multiple fibers in hexagonal and random packings with 40% and 60% of fiber volume fractions. According to the results, in spite of a high contrast between the stiffness values of steel and glass fibers, no significant difference between the transverse stress concentrations was observed for steel and glass fibers in the hexagonal packing due to the difference in material properties. Differences in stress concentrations were noted for the case of randomly packed fibers. The polygonal cross-section of steel fibers was found to introduce extreme stress concentrations.

Damage development in woven carbon fibre thermoplastic laminates with PPS and PEEK matrices: A comparative study

In this work, we investigate the effect of the matrix on the mechanical performance of woven carbon fibre composites. More specifically, composites with the same 5-harness satin carbon fabric reinforcement and different thermoplastic matrices, PPS and PEEK, are compared in various mechanical tests (tensile, interlaminar fracture toughness and compression-after-impact tests). The results of tension tests show the influence of the matrix type on the development of cracks in yarns. The cracks in carbon fabric/PEEK composite appear later than in carbon fabric/PPS composite. Their density is also lower. A correlation between cumulative acoustic emission energy and transverse crack appearance in tensile tests is shown. The most evident difference is demonstrated for the Double Cantilever Beam tests and End Notch Flexure tests. The interlaminar fracture toughness for both mode I and mode II is more than 1.5 times higher for carbon fabric/PEEK laminates as compared to carbon fabric/PPS ones. The higher fracture toughness of carbon fabric/PEEK results in its higher residual compressive strength after impact (∼25%). Thus, the study concludes that the performance of textile composites is highly sensitive to the performance of the matrix. Matrices that have higher strength, ductility and fracture toughness lead to structural composites with lower crack densities, better performance in the bias direction, higher resistance to delaminations and higher residual strength after impact.

Localization of carbon nanotubes in resin rich zones of a woven composite linked to the dispersion state

Abstract The final position of carbon nanotubes (CNTs) in a fiber-reinforced composite and its mechanical properties can be influenced by the dispersion quality of CNTs in the resin. This topic is investigated here on the example of a woven glass fiber/epoxy composite. Two different localization states of CNTs in the composite are achieved by choosing matrices with two different dispersion states but the same CNT concentration. The two investigated states of dispersion are (1) a uniform dispersion with small CNT agglomerates and (2) an interconnected dispersion where CNTs form a network with large features. The uniform dispersion results in a better distribution of CNTs throughout the composite with CNTs also appearing inside the fiber bundles. The network-like dispersion, on the other hand, tends to localize CNTs in resin rich zones. The composite with CNTs in the resin rich zones has a higher strain-to-failure (by 10%) and a lower density of transverse cracks (by 29%) in comparison with a virgin composite. In the meantime, a lower strain-to-failure and about the same crack density are measured for the composite where CNTs appear in small individual agglomerates.

Penetration impact resistance of novel tough steel fibre-reinforced polymer composites

Conventional composites are susceptible to impact loading due to their low toughness. Novel annealed steel fibres possess a unique combination of high stiffness and high strain to failure. A range of composite laminates incorporating various steel fibre architectures with thermoplastic and thermoset matrices were produced and the low-velocity penetration impact resistance was characterized by falling weight impact tests, showing the high potential toughness of steel fibre composites. The effect of different parameters was determined. High matrix strain to failure allows a better exploitation of the steel fibre ductility. The higher absorbed energy up to penetration, in case of laminates with lower levels of fibre/matrix adhesion is attributed to fibre debonding, potential for more plastic deformation, and pull-out mechanisms. Thinner fibres are more sensitive to premature failure due to inclusions in the steel fibres. Cross-ply laminates show higher energy absorption, probably due to increased delamination, whereas woven fibre structures lead to more localized damage. A preliminary study shows that layer by layer (‘macro’) hybridization of carbon and steel fabrics leads to penetration impact resistance close to the pure steel fibre composite, in case the steel plies are placed on the outside surfaces, showing a significant positive effect of hybridization.

On incompressibility of a matrix in naturally occurring composites

The work illustrates that a soft matrix, which has the Poisson ratio close to 0.5 and is reinforced with a rigid-line inclusion, possesses an interesting behavior at the inclusion/matrix interface. It experiences a hydrostatic stress state and behaves as an incompressible fluid under longitudinal and transverse loads. The stress singularities are eliminated ahead of the inclusion tips, and when interface defects are formed, their effect on the composite compliance is minimal. These observations have far reaching applications when one is interested in mechanisms of multifunctional property improvement of composites (such as toughness and stiffness) learned from naturally occurring composites.

Tensile failure of hybrid composites: measuring, predicting and understanding

Fibre-hybrid composites are attracting an ever-increasing interest from academia and industry. It is therefore vital to develop a solid understanding of their basic mechanical properties. Measuring and predicting the tensile failure of hybrid composites however remains a challenging task. This paper describes how failure develops in unidirectional (UD) hybrid composites, and how this can be predicted using fibre break models. It also provides recommendations for experimental measurements of the hybrid effect, which is a synergetic increase of the failure strain of low elongation fibres when hybridised with higher elongation fibres. Finally, limitations of our understanding of the tensile failure of hybrid composites are discussed and recommendations for future research are proposed.

Analysis of Multiple Rigid-Line Inclusions for Application to Bio-Materials

Hard biological materials such as nacre and enamel employ strong interactions between building blocks (mineral crystals) to achieve superior mechanical properties. The interactions are especially profound if building blocks have high aspect ratios and their bulk properties differ from properties of the matrix by several orders of magnitude. In the present work, a method is proposed to study interactions between multiple rigid-line inclusions with the goal to predict stress intensity factors. Rigid-line inclusions provide a good approximation of building blocks in hard biomaterials as they possess the above properties. The approach is based on the analytical method of analysis of multiple interacting cracks (Kachanov, 1987) and the duality existing between solutions for cracks and rigid-line inclusions (Ni and Nasser, 1996). Kachanov’s method is an approximate method that focuses on physical effects produced by crack interactions on stress intensity factors and material effective elastic properties. It is based on the superposition technique and the assumption that only average tractions on individual cracks contribute to the interaction effect. The duality principle states that displacement vector field for cracks and stress vector-potential field for anticracks are each other’s dual, in the sense that solution to the crack problem with prescribed tractions provides solution to the corresponding dual inclusion problem with prescribed displacement gradients. The latter allows us to modify the method for multiple cracks (that is based on approximation of tractions) into the method for multiple rigid-line inclusions (that is based on approximation of displacement gradients). This paper presents an analytical derivation of the proposed method and is applied to the special case of two collinear inclusions.Copyright

Recent advances in fibre-hybrid composites: materials selection, opportunities and applications

ABSTRACT Fibre-hybrid composites are composed of two or more fibre types in a matrix. Such composites offer more design freedom than non-hybrid composites. The aim is often to alleviate the drawbacks of one of the fibre types while keeping the benefits of the other. The hybridisation can also lead to synergetic effects or to properties that neither of the constituents possess. Even though fibre-hybrid composites are attractive, they also pose more challenges in terms of materials selection than conventional, single fibre type composites. This review analyses the mechanisms for synergetic effects provides guidance on the fibre and matrix selection and describes recent opportunities and trends. It finishes by describing the current applications, and by contrasting how the industrial use is different from the academic research.

Stress distribution around a broken carbon fibre and how it is affected by carbon nanotubes in the interface region

ABSTRACT We performed a numerical study to evaluate the influence of carbon nanotubes (CNTs) on the stress build-up in and around a broken fibre in a single carbon fibre/epoxy composite loaded longitudinally. The CNTs were located in the interface region and mimicked one of the two deposition techniques: 1) direct growth on the fibre or 2) dispersion in the fibre sizing. These techniques are known to produce different alignment, orientation and concentration of CNTs in a composite, which affect stress redistribution upon the fibre failure. The study was performed using a two-scale finite element model based on the embedded regions technique. The most significant effect on the stress build-up was found for CNTs aligned in the fibre direction. In this case, the crack opening displacement and the ineffective length were reduced by 15% and 28%, respectively, thus making the stress recovery in the broken fibre faster. In addition, the shear stress at the fibre-matrix interface decreased by a factor of two. These results indicate that hybridization of microscopic fibres with nanotubes aligned in the fibre direction is a promising strategy to influence the process of fibre fragmentation in a composite. Graphical Abstract

Nano-engineered Carbon Fibre-Reinforced Composites: Challenges and Opportunities

Due to their exceptional mechanical properties, carbon nanomaterials such as carbon nanotubes (CNTs) have been intensively studied as additional reinforcements in structural composites. They have created opportunities to develop advanced composites with improved mechanical performance and new functionalities. CNTs are introduced in fibre-reinforced polymers via various routes. They can be dispersed in the matrix, deposited in fibre sizing, directly grown on fibres or assembled into fibres. Composites, which simultaneously combine nanoscale and micro-scale reinforcements, are frequently referred to as hierarchical or nano-engineered composites. In the present chapter, we highlight challenges and benefits for the use of CNTs in structural fibre-reinforced polymers. The focus is on the mechanical performance of composites with nano-modified matrices, interfaces and fibres.