Preparation and characterization of dense nanohydroxyapatite/PLLA composites

Synthetic bone graft substitutes based on PLLA have been largely studied during the past decade. PLLA/hydroxyapatite composites appear as promising materials for large bone defect healing. In this study dense PLLA/nano-hydroxyapatite composites were prepared by hot pressing. Dense samples were investigated rather than porous scaffolds, in order to shed light on possible correlations between intrinsic mechanical properties and nano-hydroxyapatite concentration. Hydroxyapatite deagglomerated by wet attrition milling, and further dispersed into chloroform was used (median diameter = 80 nm). Particle size distribution measurements and transmission electron microscopy show evidence that particle size and dispersion are maintained throughout the successive steps of composite processing. Mechanical properties were tested (uni-axial and diametral compression tests) as a function of nano-hydroxyapatite content. Increasing concentrations of nano-hydroxyapatite (0, 25 and 50 wt.%) increase the Young's modulus and the mechanical strength of the composite; at the same time, the failure mechanism of the material changes from plastic to brittle. Young's modulus over 6 GPa and uniaxial compressive strength over 100 MPa have been achieved. These values expressed in terms of intrinsic tensile and shear strengths indicate that 50 wt.% nano-hydroxyapatite containing samples develop properties comparable to those of cortical bone. PLLA/nano-hydroxyapatite composites are thus promising candidates to develop bioresorbable porous bone substitutes showing superior mechanical performance.     

Interfacial properties of fibrous composites

The processes for debonding and pull-out in parallel-sided as well as tapered fibre composites are described. Models which can predict and account for all the reported experimental debonding and pull-out behaviour are developed. The effect of the interfacial properties on the plot of maximum pull-out force against fibre embedded length is elucidated. Knowledge of the interfacial parameters of a composite allows proper characterization and leads to better prediction of the mechanical properties.     

Crack growth in hybrid fibrous composites

A preliminary analysis is made of the energetics of transverse crack growth in a brittle elastic matrix bridged by elastic fibres fictionally bonded to the matrix. Studies made of the stability of a crack of finite length in a brittle polymeric material reinforced with steel wires are found to be in reasonable agreement with the predictions of the theory. It is proposed that the stability of transverse cracks in a very brittle matrix could be increased substantially by the inclusion of a second fibre component designed specifically to increase the work of fracture of the matrix. This has been shown to be possible using a very small volume fraction of glass fibres as a matrix toughening component and it has also been observed that stable transverse matrix crack growth can be achieved with composite systems of this type. This principle might have applications in the design of hybrid composites utilizing either a brittle polymeric or ceramic matrix.     

Thermal stress development in fibrous composites

In this work, the thermal stress development in anisotropic fiber-reinforced polymer composites is investigated for temperatures below the glass transition temperature of the resin. By applying two independent experimental methodologies, it was found that the initial thermal (residual) strain in the reinforcing fibers is compressive of about − 0.04% at ambient temperatures. This is due to the mismatch of the thermal expansion coefficient between the polymer matrix and fiber, as the material is cooled down from the processing temperature. However, on reheating the composites the compressive stress in the fiber gradually diminishes and becomes zero at 50 °C. Further heating to 100 °C introduces tensile strains in the fiber of maximum of 0.13%. The conformity of these results to analytical models that relate the composite thermal strain to the thermal expansion coefficients of fiber and resin, as well as, the fiber volume fraction, is examined. Finally, the possibility of tailoring the sign (positive, negative or, even, zero) of the composite thermal expansion coefficient of certain advanced composites by simply varying the thermal expansion of the polymer matrix, is discussed.     

Giant barocaloric effects in natural graphite/polydimethylsiloxane rubber composites

Journal of Materials Science - Solid-state cooling based on caloric effects is considered a viable alternative to replace the conventional vapor-compression refrigeration systems. Regarding...     

The interfacial properties of fibrous composites

The dependence of the magnitude of the interfacial parameters for a glass-fibre-reinforced polypropylene on the thickness of the applied silane layer on the glass-fibre surface was investigated. The interfacial parameters studied included the interfacial-shear strength, the interfacial coefficient of friction, the interfacial-frictional stress and the shrinkage pressure. These parameters were evaluated from pull-out data using a recent model. The results indicate that the maximum interfacial-shear strength is obtained at a critical thickness of the silane layer on the treated fibre. Both the interfacial-frictional stress and the interfacial coefficient of friction decreased with increased thickness of the silane coat.     

Reliability-based optimization of fibrous laminated composites

This paper is concerned with the optimum design of multiaxial fiber reinforced laminate systems under probabilistic conditions of loads and material properties. A multiaxially laminated composite is treated as a structural system with each ply contained in the composite as one element. The Tsai-Wu failure criterion is adopted as the limit state function of a unidirectional ply. It is assumed that the system failure occurs when any one of the plies in a laminate system fails. The multiple-check-point method is successfully applied to evaluate the system reliabilities of multiaxial laminates under probabilistic in-plane stresses. An optimization problem is defined to find the optimal number of fiber orientation axes, optimum orientation angles, and optimum ply ratios which yield the highest system reliability.     

Debonding and pull-out processes in fibrous composites

The processes of debonding and pull-out in fibrous composites are described. Models predicting the debond length and the probability distribution of pull-out lengths of fibres and bundles are derived. These lengths are functions of the fibre, matrix and interface properties. Prediction is then compared with experiment and a simple relationship between pull-out and debond lengths is found. An understanding of the debonding and pull-out processes is important because they affect the fracture toughness of fibre composites.     

Micromechanical modeling of load transfer in fibrous composites

A unified analytical treatment is presented for the study of micromechanical stress distribution in unidirectional fibrous composites loaded with various thermal and mechanical loads. Two models are considered to represent the composite. Both use a concentric cylindrical system with the difference that one requires laterally free while the other requires laterally constrained outer boundaries, broadly describing situations of plane stress and plane strain, respectively. The present work has been motivated by the recent work of McCartney (McCartney, Proc. Roy. Soc. London, Ser. A 425 (1989) 215–244) who analyzed the laterally free system, and by our previous work (Nayfeh, Fibre Sci. Technol. 10 (1977)) in which we analyzed the laterally constrained one. For axisymmetric loading, and upon adopting some appropriate restrictions on the radial behavior of some field quantities, an elasticity-based procedure reduces the two-dimensional field equations, which hold in both the fiber and matrix components, together with the appropriate interface and boundary conditions, to a quasi-one-dimensional system. The resulting system is capable of identifying the stress distribution in each component as influenced by the other component via the readily identifiable interaction (transfer) terms. The model is general and applicable to a large variety of situations. These include situations of matrix cracking, fiber break and even regions of slip at the fiber–matrix interface. As a by-product, the model was capable of obtaining the classical Lamé solutions (the iso-strain case) as a degenerate case. Confidence in the modeling was gained when it identically reproduced all of the numerical examples presented by McCartney. Numerical results that parallel some of the ones presented by McCartney are included in the form of comparisons between results obtained based upon the laterally constrained and the laterally free systems.     

Repair of rabbit femoral condyle bone defects with injectable nanohydroxyapatite/chitosan composites

Repair of massive bone loss remains a challenge to the orthopaedic surgeons. Autologous and allogenic bone grafts are choice for bone reconstructive surgery, but limited availability, risks of transmittable diseases and inconsistent clinical performances have prompted the development of tissue engineering. In the present work, the bone regeneration potential of nanohydroxyapatite/chitosan composite scaffolds were compared with pure chitosan scaffolds when implanted into segmental bone defects in rabbits. Critical size bone defects (6 mm diameter, 10 mm length) were created in the left femoral condyles of 43 adult New Zealand white rabbits. The femoral condyle bone defects were repaired by nanohydroxyapatite/chitosan compositions, pure chitosan or left empty separately. Defect-bridging was detected by plain radiograph and quantitative computer tomography at eight and 12 weeks after surgery. Tissue samples were collected for gross view and histological examination to determine the extent of new bone formation. Eight weeks after surgery, more irregular osteon formation was observed in the group treated with nanohydroxyapatite/chitosan composites compared with those treated with pure chitosan. 12 weeks after surgery, complete healing of the segmental bone defect was observed in the nanohydroxyapatite/chitosan-group, while the defect was still visible in the chitosan-group, although the depth of the defect had diminished. These observations suggest that the injectable nanohydroxyapatite/chitosan scaffolds are potential candidate materials for regeneration of bone loss.     

Brittle coating studies on fibrous composites

A brittle coating stress analysis technique applicable to orthotropic materials has been developed. The technique has been applied to a unidirectional glass fibre reinforced epoxy. Its behaviour has been studied under uniaxial and biaxial stress fields using cantilever beam specimens and circular disc specimens under diametral compression. Fibre orientation in the specimens has been varied. In each case it has been observed that the cracks represent the direction of principal strains in the specimen material and not the direction of principal stresses. Application of brittle coating techniques has been suggested to establish the direction and magnitude of principal stresses and strains at every point in a problem with unknown stresses and strains.     

Theory of multiple fracture of fibrous composites

The theoretical stress-strain behaviour of a composite with a brittle matrix in which the fibre-matrix bond remains intact after the matrix has cracked, is described. From a consideration of the maximum shear stress at the fibre-matrix interface, the extent of fibre debonding and the crack spacing in a partially debonded composite are derived. The energetics of cracking and the conditions leading to an enhanced matrix failure strain are then discussed and, finally, the crack spacing expected in composites containing fibres isotropically arranged in two or in three dimensions is derived for the case of very thin and hence very flexible fibres.     

Longitudinal tensile failure of unidirectional fibrous composites

This paper presents a theoretical treatment of the tensile strength of a unidirectional fibrous composite, subjected to a tensile load in the fibre direction. The fibres are treated as having a statistical strength distribution which results in fibre failure prior to composite failure. The failure geometry of the model is similar to the observed geometry of fractured glass/epoxy and glass/polyester composites. Failure criterion is established and the strength is shown to decrease as the length of the specimen is increased. This size effect is very small.     

Asymmetric behaviour of fibrous metal matrix composites

Abstract

Experimental investigations have illustrated that unidirectional metal matrix composites (MMCs) show asymmetric behaviour under uniaxial tensile and compressive loading. This asymmetry occurs when the material is loaded along the fibre direction and also when loaded in the transverse direction. In this paper, results from finite element micromechanical models are presented. The models were used to study the asymmetric behaviour of unidirectional fibre reinforced MMCs subjected to longitudinal and transverse loading. The effects of the thermal residual stresses arising from the manufacturing process were included in the study. Also, the influence of the degree of bonding of fibre to matrix was examined, from perfectly bonded to completely debonded. Results reveal that thermal residual stresses are responsible for the asymmetric behaviour of the MMCs in the longitudinal direction. In transverse loading, both the degree of interface bonding and residual stresses account for the asymmetric behaviour. The predicted stress–strain response of the MMC shows good agreement with the available experimental data for both tènsile and compressive loading. Results also suggest that in order to predict accurately the yielding behaviour of MMCs, the current symmetric yield criteria require modification.     

Effective anti-plane properties of piezoelectric fibrous composites

Anti-plane shear of piezoelectric fibrous composites is theoretically investigated. The geometry of composites is described by the 2-dimensional geometry in a section perpendicular to the unidirectional fibers. The previous constructive results obtained for scalar conductivity problems are extended to piezoelectric anti-plane problems. First, the piezoelectric problem is written in the form of the vector-matrix ${\mathbb{R}}$ -linear problem in a class of double periodic functions. In particular, application of the zeroth-order solution to the ${\mathbb{R}}$ -linear problem yields a vector-matrix extension of the famous Clausius–Mossotti approximation. The vector-matrix problem is decomposed into two scalar ${\mathbb{R}}$ -linear problems. This reduction allows us to directly apply all the known exact and approximate analytical results for scalar problems to establish high-order formulae for the effective piezoelectric constants. Special attention is paid to non-overlapping disks embedded in a two-dimensional background.