Biocomposites reinforced with natural fibers: 2000–2010

Studies on the use of natural fibers as replacement to man-made fiber in fiber-reinforced composites have increased and opened up further industrial possibilities. Natural fibers have the advantages of low density, low cost, and biodegradability. However, the main disadvantages of natural fibers in composites are the poor compatibility between fiber and matrix and the relative high moisture sorption. Therefore, chemical treatments are considered in modifying the fiber surface properties. In this paper, the different chemical modifications on natural fibers for use in natural fiber-reinforced composites are reviewed. Chemical treatments including alkali, silane, acetylation, benzoylation, acrylation, maleated coupling agents, isocyanates, permanganate and others are discussed. The chemical treatment of fiber aimed at improving the adhesion between the fiber surface and the polymer matrix may not only modify the fiber surface but also increase fiber strength. Water absorption of composites is reduced and their mechanical properties are improved.

Chemical Treatments of Natural Fiber for Use in Natural Fiber-Reinforced Composites: A Review

An engineering solution for mesh size effects in the simulation of delamination using cohesive zone models

A new decohesion element with the capability of dealing with crack propagation under mixed-mode loading is proposed and demonstrated. The element is used at the interface between solid finite elements to model the initiation and non-self-similar growth of delaminations in composite materials. A single relative displacement-based damage parameter is applied in a softening law to track the damage state of the interface and to prevent the restoration of the cohesive state during unloading. The softening law is applied in the three-parameter Benzeggagh-Kenane mode interaction criterion to predict mixed-mode delamination propagation. To demonstrate the accuracy of the predictions, steady-state delamination growth is simulated for quasi-static loading of various single mode and mixed-mode delamination test specimens and the results are compared with experimental data.

Numerical simulation of mixed-mode progressive delamination in composite materials

In the past decade, natural-fiber composites with thermoplastic and thermoset matrices have been embraced by European car manufacturers and suppliers for door panels, seat backs, headliners, package trays, dashboards, and interior parts. Natural fibers such as kenaf, hemp, flax, jute, and sisal offer such benefits as reductions in weight, cost, and CO2, less reliance on foreign oil sources, and recyclability. However, several major technical considerations must be addressed before the engineering, scientific, and commercial communities gain the confidence to enable wide-scale acceptance, particularly in exterior parts where a Class A surface finish is required. Challenges include the homogenization of the fiber's properties and a full understanding of the degree of polymerization and crystallization, adhesion between the fiber and matrix, moisture repellence, and flame-retardant properties, to name but a few.

Natural-fiber-reinforced polymer composites in automotive applications

A three-dimensional finite element model is developed to predict damage progression and strength of mechanically fastened joints in carbon fibre-reinforced plastics that fail in the bearing, tension and shear-out modes. The model is based on a three-dimensional finite element model, on a three-dimensional failure criterion and on a constitutive equation that takes into account the effects of damage on the material elastic properties. This is accomplished using internal state variables that are functions of the type of damage. This formulation is used together with a global failure criterion to predict the ultimate strength of the joint. Experimental results concerning damage progression, joint stiffness and strength are obtained and compared with the predictions. A good agreement between experimental results and numerical predictions is obtained.

A progressive Damage Model for Mechanically Fastened Joints in Composite Laminates

Natural fibres have long been used as cost-cutting fillers in the plastics industry. Nowadays, they are considered to be a potential replacement of glass fibres for use in composite materials. However, although natural fibres have many advantages, the most important being their low cost and low density, they are not totally free of problems. A serious problem of natural fibres is their strong polar character, which creates many problems of incompatibility with most thermoplastic matrices (especially polyolefins). Surface treatments, although having a negative impact on economics, are potentially able to overcome the problem of incompatibility. The present study focuses on the development, optimisation and characterisation of two such treatments; acetylation and stearation. The two treatments were applied on two grades of flax fibres (green and dew retted flax), the results are discussed in terms of process variables, such as temperature, time of treatment, recycling of reactants, etc. Three characterisation techniques were applied on the treated and untreated fibres; X-ray diffraction, scanning electron microscopy, and inverse gas chromatography. It was found that both treatments result in a removal of non-crystalline constituents of the fibres, and alter the characteristics of the surface topography. It was also found that both treatments change the fibre surface free energy, with acetylation increasing it and stearation decreasing it.

Engineering and characterisation of the interface in flax fibre/polypropylene composite materials. Part I. Development and investigation of surface treatments

This article describes the application of interface elements to the prediction of progressive delamination in composite materials. The work has been implemented in two separate commercial finite-element computer packages, ABAQUS [1] and LUSAS [2]. In the former case, this has been achieved via a "user subroutine", while in the latter case the elements are embedded directly within the system, either in the standard "release version" of the code or in a research version, to which the authors have access and in which they have implemented various algorithms directly. Comparisons are made with a range of experimental results for composite material specimens. In addition to the finite-element solutions, closed-form results are also presented that are based on a corrected beam theory.

Predicting Progressive Delamination of Composite Material Specimens via Interface Elements

Cellulose fibres have long been used in the plastics industry as cost-cutting materials. Nowadays they are recognised as a potential replacement for glass fibres for use as reinforcing agents in composite materials. They have a number of certain advantages over glass fibres, such as low cost, high strength-to-weight ratio, biodegradability and ease of processing. In this study crystallisation from the melt of two different isotactic polypropylene matrices (iPP) in the presence of flax ( Linum usitatissinum ) fibres of four different types (green flax, dew retted flax, Duralin ® treated flax and stearic acid sized flax) was examined. The effect of processing parameters such as crystallisation temperature and cooling rate was investigated using hot stage optical microscopy. Differential scanning calorimetry (DSC) was used to investigate the inner morphology of the transcrystalline (TC) layer. Scanning electron microscopy (SEM) and X-ray diffraction were used in an attempt to identify the origin of the TC layer in connection with the structural characteristics of the fibres. The effect of transcrystallinity upon the mechanical properties of the interface was assessed using the single fibre fragmentation test. It was found that the interfacial adhesion is improved by the presence of a TC layer.

A study of transcrystallinity and its effect on the interface in flax fibre reinforced composite materials

A three-dimensional finite element model is created to simulate a mechanically fastened joint in a composite laminate Using the results from the finite element model, a spline approximation for the through-thickness stress is applied in order to determine the stress state at the interfaces between the layers A delamination onset criterion is then applied at each interface After validation, the model is used to assess the effects of stacking sequence and clamping pressure on the delamination onset loads and surfaces It is concluded that "blocked" laminates lead to lower delamination onset loads and larger initial delaminated regions Finger-tight and clamped joints have higher delamination onset loads and smaller initial cracked regions than pin-loaded joints These results are in agreement with experimental observations, but no significant difference is detected between finger-tight and clamped joints It is therefore concluded that the through-thickness pressure has an effect not only on damage initi

F.L. Matthews

Papers

A progressive Damage Model for Mechanically Fastened Joints in Composite Laminates

Engineering and characterisation of the interface in flax fibre/polypropylene composite materials. Part I. Development and investigation of surface treatments

Predicting Progressive Delamination of Composite Material Specimens via Interface Elements

A study of transcrystallinity and its effect on the interface in flax fibre reinforced composite materials

Delamination Onset Prediction in Mechanically Fastened Joints in Composite Laminates