Effects of Shear‐Transverse Coupling and Plasticity in the Formulation of an Elementary Ply Composites Damage Model,Part II: Material Characterisation

Abstract: In this two‐part study, we examine the effects of neglecting plasticity and shear‐transverse coupling in a continuum damage mechanics model for composites. In part I, two models were formulated: one in which plasticity was neglected, and one in which both plasticity and shear‐transverse damage coupling were neglected, and the predictive capabilities for both models were examined. In this second part of the paper, the procedure and results of the experimental test series carried out to determine input parameters for the above two models are presented. Two materials were tested: one a carbon fibre‐reinforced plastic, the other an S2‐glass fibre‐reinforced plastic. Both material systems are currently used in the aerospace industry so the experimental results should be of interest to that community. Both materials exhibited non‐linear intralaminar shear behaviour, whereas the S2‐glass fibre‐reinforced plastic also exhibited a significantly non‐linear transverse response. Tests on ±45º and 10º off‐axis coupons indicated that a reasonable estimate of shear strength could be obtained from the ±45º test specimens. Some further insight is provided into the model predictions that were presented in part I.     

Fatigue crack growth in thin notched woven glass composites under tensile loading. Part II: Modelling

Fatigue propagation of a through-the-thickness crack in thin woven glass laminates is difficult to model when using homogeneous material assumption. Crack growth depends on both the fatigue behaviour of the fibres and of the matrix, these two phenomena occurring at different time and space scales. The developed finite element model is based on the architecture of the fabric and on the fatigue behaviours of the matrix and the fibre, even if the pure resin and fibre behaviours are not used. That thus limits the physical meaning of this model. Basically, the objective of this simulation is to illustrate and to confirm proposed crack growth mechanism. The fatigue damage matrix is introduced with user spring elements that link the two fibre directions of the fabric. Fibre fatigue behaviour is based on the S-N curves. Numerical results are compared to experimental crack growth rates and observed damage in the crack tip. Relatively good agreement between predictions and experiments was found.     

Extraction and preparation of bamboo fibre-reinforced composites

Natural plant fibre composites have been developed for the production of a variety of industrial products, with benefits including biodegradability and environmental protection. Bamboo fibre materials have attracted broad attention as reinforcement polymer composites due to their environmental sustainability, mechanical properties, and recyclability, and they can be compared with glass fibres. This review classifies and describes the various procedures that have been developed to extract fibres from raw bamboo culm. There are three main types of procedures: mechanical, chemical and combined mechanical and chemical extraction. Composite preparation from extracted bamboo fibres and various thermal analysis methods are also classified and analysed. Many parameters affect the mechanical properties and composite characteristics of bamboo fibres and bamboo composites, including fibre extraction methods, fibre length, fibre size, resin application, temperature, moisture content and composite preparation techniques. Mechanical extraction methods are more eco-friendly than chemical methods, and steam explosion and chemical methods significantly affect the microstructure of bamboo fibres. The development of bamboo fibre-reinforced composites and interfacial adhesion fabrication techniques must consider the type of matrix, the microstructure of bamboo and fibre extraction methods.     

Experimental measurement and predictive modelling of bending behaviour for viscous unidirectional composite materials

It is widely accepted that the key deformation mechanisms during forming of viscous textile composite (prepreg) sheets are in-plane shear and out-of-plane bending. This paper focuses on the bending deformation mechanism, including experimental characterisation and theoretical modelling of bending behaviour during viscous composite forming. Experimental measurements are obtained by means of a large-displacement buckling test at a variety of displacement rates and temperatures. Some important aspects, such as viscoelastic behaviour, are also investigated. A bending model based on elastic theory combined with uniaxial continuum theory for ideal fibre-reinforced fluids for viscous shear deformation has been developed, using material parameters obtained from industrial manufacturers as input data, such as composite geometry, fibre properties, fibre volume fraction and matrix rheology. Model predictions demonstrate that the model can capture the main characteristics of material properties, such as rate dependence. This bending model can be used in formability analysis for viscous unidirectional composite materials, and might be applied in a finite element forming simulation to account for the bending stiffness.     

Effect of fibre volume fraction on fatigue behaviour of glass fibre reinforced composite

The aim of this paper is to study the fatigue behavior of GFRP composites manufactured by vacuum bagging process by varying the volume fraction. Constant‐amplitude flexural fatigue tests were performed at zero mean stress, i.e. a cyclic stress ratio R=?1 by varying the frequency of the testing machine. The relationship between stiffness degradation rate and fibre volume fraction, was observed, and the influence of volume fraction on the tensile strength was also investigated. The results show that, as the volume fraction increases the stiffness degradation rate initially decreases and then increases after reaching a certain limit for the volume fraction. Graph between volume fraction and Young's modulus shows that as the volume fraction increases Young's modulus also increases and reaches a limit and then it decreases with further increase in volume fraction, due to the increase in fibre content which changes the material properties of the composite material. The obtained results are in agreement with the available results.     

Finite element study of compressively loaded fibres fractured during impact

This paper examines the compressive behaviour of plies with fibres previously fractured during impact. The analysis is conducted using finite element (FE) modelling in ABAQUS 6.7. Two- and three- dimensional models are used to consider the possibility of fibre penetration and “brooming” of fractured fibres, or fibre buckling. A parametric study of the influence of the input parameters of the Drucker Prager plasticity model was also conducted, to enable a better understanding of the model. This was used to capture the triaxial stress state in the matrix surrounding the fibres. The results suggest that fibre buckling is more likely to occur due to the geometry of fibres and fibre spacing in carbon fibre composites, but fibre penetration could still occur in regions of low fibre volume fraction.     

Photoelastic study of the elastic stress distribution around discontinuous fibres

The mechanism by which load is transferred from a discontinuous fibre to surrounding unbroken fibres has been examined in some detail using two dimensional photoelastic models in which the applied load is aligned with the fibre axes. The gap in the broken fibre is assumed to have occurred during fabrication and is thus filled with the matrix material. Three different end gap configurations have been analysed and it is concluded that, for all practical purposes, the disturbance due to the broken fibre does not extend beyond the immediately adjacent fibres. Substantial shear stresses are developed in the matrix for some distance along the fibre from the discontinuity with a consequent early transfer of load. Paradoxically, this effect is not accompanied by the development of reduced maximum stresses at the broken fibre tip for any of the configurations included in these tests.     

Fatigue crack propagation assessment based on residual stresses obtained through cut-compliance technique

Crack growth rate versus crack length curves of heavily overloaded parent material specimens and fatigue crack propagation curves of friction‐stir‐welded aluminium samples are presented. It is shown that in both cases the residual stresses have a strong effect on the crack propagation behaviour under constant and variable amplitude loading. As a simplified engineering approach, it is assumed in this paper, that in both cases residual stresses are the main and only factor influencing crack growth. Therefore fatigue crack propagation predictions are performed by adding the residual stresses to the applied loading and by neglecting the possible effects of overloading and friction stir welding on the parent material properties. For a quantitative assessment of the residual stress effects, the stress intensity factor due to residual stresses K_res is determined directly with the so‐called cut‐compliance method (incremental slitting). These measurements are particularly suited as input parameters for the software packages AFGROW and NASGRO 3.0, which are widely used for fatigue crack growth predictions under constant and variable amplitude loading. The prediction made in terms of crack propagation rates versus crack length and crack length versus cycles generally shows a good agreement with the measured values.     

Characterization and Simulation of Tensile Deformation of Non‐Uniform Cellular Aluminium Until Damage

The uniaxial tensile modulus and strength of Alulight® foams are measured and simulated taking into account the non‐uniform mass density distribution characterized non‐destructively by X‐ray computer tomography. The density mapping method is employed for the reconstruction of the hard and soft regions in the samples investigated. A finite element (FE)‐model is introduced for simulations of the deformation of a continuum composed by domains of different local densities. Existing constitutive laws for cellular structures are incorporated for the numerical simulation of tensile deformation and the variance of the material parameters is determined with the aid of a scaling relationship. The experimental results for the stiffness, the ultimate strength, and the corresponding strain agree with the developed 3D FE simulations and are compared with the estimations according to scaling laws for uniform cellular structures. The non‐uniformity of the material distribution affects the strength and the ductility significantly. Simulations taking this into account provide conservative property predictions. The calculated positions of local strain concentration correspond with the observed locations of crack initiation. The material modelling and the simulation of the elasto‐plastic deformation up to damage are suggested for application to macroscopic components made of non‐uniform cellular metals.     

Characterization of moulded‐fibre packaging with respect to water vapour sorption and permeation at different combinations of internal and external humidity

A moulded‐fibre packaging system was characterized under conditions simulating real‐life packaging of food. A steady‐state moisture flux through the moulded‐fibre packaging was generated by subjecting the system to different combinations of internal humidity [33–97% r.h. (0.33–0.97a_w of contents), RH(i)] and surrounding humidity [33–97% r.h., RH(e)]. The objective was to resolve whether a hygroscopic fibre material absorbs moisture proportional to the rate of moisture transport, and the moulded‐fibre material was thus characterized with respect to accumulation of moisture in the fibre material, water vapour transmission rate (WVTR) and permeability (k/x). These steady‐state properties showed significant asymmetry depending on direction of moisture transport. When moisture was transported out of the system [RH(i) > RH(e)] the fibre material adsorbed moisture to a considerable lesser extent compared to when moisture was transported into the system [RH(i) < RH(e)], just as (k/x) increased by 15–20%. Taking both directions of moisture transport into account, the moisture content of the fibre material depended largely on surrounding humidity, even at high internal humidity. Moisture contents ranged from 5.5 g/100 g dry fibre at RH(e) 33% r.h. to 16.4–25.1 g/100 g dry fibre at RH(e) 97% r.h. The observed asymmetry was shown to derive from the experimental set‐up and not from the material itself. A minimal theory based on the various transport steps in the experimental set‐up was proposed in order to qualitatively explain this asymmetry. The rate of moisture adsorption in moulded‐fibre was described by the normalized response function H(t). Response times to reach equilibrium moisture contents were 6 and 8 h for RH(e) 33 and 53% r.h., and 40 and 41 h for RH(e) 75 and 97% r.h. Copyright © 2004 John Wiley & Sons, Ltd.     

A theoretical model on piezoelectric fibre pullout with electric input

A theoretical model of a single piezoelectric fibre pullout from an elastic matrix was developed to study the effect of an input electric field. The stress distributions in the fibre under both mechanical and electric loads are obtained. The relationships between pullout force, induced electric potential and deformation are evaluated by computer simulation. The effects of electric input, piezoelectric parameters and fibre volume fraction on the load- displacement curve for fibre pullout are discussed. Numerical results obtained in this study indicate that the pullout force can be adjusted by changing the value or the direction of the applied electric field. Also, the results show that piezoelectric parameters and fibre volume fraction play important roles on the pullout force in the piezoelectric fibre.     

Cohesive-zone modelling of the deformation and fracture of spot-welded joints

The deformation and failure of spot‐welded joints have been successfully modelled using a cohesive‐zone model for fracture. This has been accomplished by implementing a user‐defined, three‐dimensional, cohesive‐zone element within a commercial finite‐element package. The model requires two material parameters for each mode of deformation. Results show that the material parameters from this type of approach are transferable for identical spot welds in different geometries where a single parameter (such as maximum stress) is not. The approach has been demonstrated using a model system consisting of spot‐welded joints made from 5754 aluminium sheets. The techniques for determining the cohesive fracture parameters for both nugget fracture and nugget pullout are described in this paper. It has been demonstrated that once the appropriate cohesive parameters for a weld are determined, quantitative predictions can be developed for the strengths, deformations and failure mechanisms of different geometries with nominally identical welds.     

Mechanical and magnetic properties of metal fibre networks, with and without a polymeric matrix

Bonded networks of metal fibres are highly porous, permeable materials, which often exhibit relatively high strength. Material of this type has been produced, using melt-extracted ferritic stainless steel fibres, and characterised in terms of fibre volume fraction, fibre segment (joint-to-joint) length and fibre orientation distribution. Young’s moduli and yield stresses have been measured. The behaviour when subjected to a magnetic field has also been investigated. This causes macroscopic straining, as the individual fibres become magnetised and tend to align with the applied field. The modeling approach of Markaki and Clyne, recently developed for prediction of the mechanical and magneto-mechanical properties of such materials, is briefly summarised and comparisons are made with experimental data. The effects of filling the inter-fibre void with compliant (polymeric) matrices have also been explored. In general the modeling approach gives reliable predictions, particularly when the network architecture has been characterised using X-ray tomography.     

Effects of Fibre Geometry and Volume Fraction on the Flexural Behaviour of Steel‐Fibre Reinforced Concrete

Abstract: This work aims in studying the mechanical behaviour of concrete, reinforced with steel fibres of different geometry and volume fraction. Experiments include compression tests and four‐point bending tests. Slump and air content tests were performed on fresh concrete. The flexural toughness, flexural strength and residual strength factors of the beam specimens were evaluated in accordance with ASTM C1609/C1609M‐05 standard. Improvement in the mechanical properties, in particular the toughness, was observed with the increase of the volume fraction of steel‐fibres in the concrete. The fibre geometry was found to be a key factor affecting the mechanical performance of the material.     

The influence of fibre length and concentration on the properties of glass fibre reinforced polypropylene: 5. Injection moulded long and short fibre PP

We present results of a step by step comparison of the mechanical performance of injection moulded ‘long’ (LF-PP) and ‘short’ (SF-PP) glass fibre-polypropylene compounds. The study allows direct comparison of the mechanical performance of long and short fibre systems in the same resin at the same fibre diameter, and the effect of fibre diameter in short fibre compounds. Furthermore, the comparison of these three systems has been made over the 0–40 wt% fibre content range. At the same fibre diameter and fibre content LF-PP gives significant improvements in room temperature tensile and flexural strength, notched and unnotched impact resistance. The improvement in impact resistance is higher still at lower test temperature. LF-PP also gives increasingly higher modulus over SF-PP as the strain is increased. The effect of lowering the fibre diameter in SF-PP has been shown to increase both strength and unnotched impact, but not to the levels obtained with LF-PP at higher fibre diameter. Notched impact and modulus of SF-PP were relatively unaffected by reduction of the fibre diameter. The relative mechanical data are shown to conform well to available models. The results are discussed in terms of the relevant micro-mechanical parameters of these materials.     

A review of the tensile and fatigue responses of cellulosic fibre-reinforced polymer composites

ABSTRACT

The use of polymer-based composites has been gaining popularity in the industry over the last few decades. Their high strength to weight ratio and high fatigue resistance make these composites the preferred materials for a wide variety of applications. The current trend has inclined towards hybrid fibre reinforced composites owing to their outstanding characteristics compared to non-hybrid composites. Numerous research works have been conducted to study the fatigue life behaviour of such composite materials. This study addressed the monotonic and dynamic performance of non-hybrid and hybrid natural fibre based composite materials, and the factors that influence their fatigue performance, along with the stiffness decay of each composite material. Most studies have shown the superior potential of using natural fibres in place of synthetic fibres in those critical applications that involve tensile and cyclic loading.     

Phenomenological fracture model for biaxial fibre reinforced composites

This paper proposes a new mathematical fracture model (FM) applicable to a biaxial reinforced composite material. The mathematical model provides predictions about the limit state of composite material. It is applicable both in uniaxial and biaxial requests. The mathematical model is validated by comparing its predictions with the experimental data obtained by authors. The studied composite material is composed by carbon fibre in epoxy matrix. The process used for obtaining the composite materials plates is vacuum forming.     

Monitoring Elastic Strain and Damage by Neutron and Synchrotron Beams

Large‐scale neutron and synchrotron X‐ray facilities have been providing important information for physicists and chemists for many decades. Increasingly, materials engineers are finding that they can also provide them with important information non‐destructively. Highly penetrating neutron and X‐ray synchrotron beams provide the materials engineer with a means of obtaining information about the state of stress and damage deep within materials. In this paper the principles underlying the elastic strain measurement and damage characterisation techniques are introduced. The capabilities of the methods are illustrated through a number of practical applications including; mapping damage and stress transfer fibre by fibre in continuous fibre reinforced composites during loading, measurement of residual stresses in welding, the use of measurements to refine finite element models, and creep cavitation cracking in power plant steels.     

Development of durable cementitious composites using sisal and flax fabrics for reinforcement of masonry structures

Natural fibres are one of the most studied materials. However, the use of these fibres as reinforcements in composite materials for structural applications, especially for existing or historical masonry structures, remains a challenge. In this study, efforts were made to develop sustainable composites using cementitious matrices reinforced with untreated bi-directional fabrics of natural fibres, namely, flax and sisal fibres. The fibres were mechanically characterised by tensile tests performed on both single yarns and fabric strips. Ageing effects due to fibre mineralisation in alkaline cement paste environments may cause a reduction in the tensile strength of natural fibres. The matrices used to study fibre durability were a natural hydraulic lime-based mortar (NLM) mix with a low content of water-soluble salts and a lime-based grouting (NLG) mix containing natural pozzolans and carbonated filler. Tensile tests on impregnated single yarns subjected to wetting and drying cycles by exposure to external weathering were conducted at different ages to quantify these problems. Composite specimens were manufactured by the hand lay-up moulding technique using untreated fibre strips and an NLG matrix. The mechanical response of natural fibre reinforced cementitious (NFRC) composites was measured under tension, and the effect of the matrix thickness was also addressed. Both sisal and flax fibres showed good adhesion with the NLG matrix, making them capable of producing composites with ductile behaviour and suitable mechanical performance for strengthening applications in masonry structures.     

Oxide fibres and their composites with nickel and molybdenum matrices produced by internal crystallization

Complex oxide 2Al₂O₃·MgO·3CaO and a triple oxide eutectic TiO₂-MgO-CaO, both with relatively low melting points, were used to reinforce a nickel matrix by the internal crystalization method. Strength properties of the fibres were measured by testing composites with nickel as well as molybdenum matrices. It is shown that the fibre strength depended strongly on both the initial melt composition of the fibre material and the fabrication regimes.