Several fracture models used for prediction of notched residual strength of composite laminates are reviewed. Emphasis has been placed on analytical fracture models which are simple to utilise. Published experimental results on the notched strength of composite laminates containing a circular hole or centre crack and subjected to quasi-static axial tensile loading are also reviewed. Characteristic parameters associated with fracture models have been determined and theoretical predictions of residual strength are compared with all notched strength data sets. Applicability of the different fracture models in predicting the residual strength of composite laminates is also discussed.
The paper provides an overview over a 4 years research work embedded in the framework of a BRITEEURAM Project. In this study, PEEK and PA12 as matrix systems intermingled with the carbon fiber yarns, either continuous or staple spun yarns, were investigated. Structure-properties relations of knitted fabric reinforced composites were elucidated varying systematically, fiber content, number of knit layers, yarn and matrix properties. The 3-dimensional fiber orientation in the composite was assessed by reconstruction of the bundle geometry from stacked cross sections. Static tensile and bending properties were determined at various angles between wale and course direction indicating a strong correlation between these properties and the respective knit orientation. Fatigue tests revealed a considerable influence of the yarn properties indicating a higher fatigue resistance for staple yarns than for continuous yarns. Fracture mechanical as well as impact failure studies supported the importance of the fiber bundle for the understanding of the properties of the composite and they proved the insensitivity of these materials against delamination and notch effects.
Fiber reinforced plastics are essentially heterogeneous materials where plastic matrix is reinforced by fibers. Therefore, it is important to know the properties of fiber/matrix interaction in addition to the properties of fiber and matrix themselves. To measure the fiber/matrix adhesive strength, various kinds of tests such as fiber pull-out, fragmentation, microbond, etc. have so far been tried. However the correlation of the strength among these test methods is not necessarily clear although the argument of Herrera-Franco and Drzal  has a point. In the present study, we tried two kinds of tests, microbond test and fragmentation test, to get reliable interfacial shear strength. According to the present study, the interfacial shear strength by means of the microbond test was a little smaller than that of the fragmentation test, which coincided with the other researchers' result. The interfacial shear strength of a sample where siging agent was removed was smaller than that of as-received specimen. It was also demonstrated from two-fiber fragmentation tests that the interfacial property affects the failure process of unidirectional composites.
This paper investigates the strength and failure mechanisms of interphase in a carbon fibre/polyetheretherketone (PEEK) matrix composite which are affected by the cooling rate from the moulding temperature. The interphase properties are characterised using the single fibre pull-out test and short-beam shear test. The pull-out processes and fracture surface of composites are in situ evaluated under an optical microscope and a scanning electron microscope (SEM), respectively. A slow cooling rate results in a high degree of crystallinity and high modules of the matrix, leading to a high interfacial shear strength (IFSS) along with prevailing brittle interface fracture. An amorphous dominant matrix is developed at high cooling rates, resulting in relatively low IFSS due to the interphase shear deformation. The interlaminar shear strength (ILSS) is roughly proportional to the IFSS with respect to cooling rate, although the absolute values are 20-30% lower for the former than the latter. Both the IFSS and ILSS become insensitive once the cooling rate is higher than about 500°C/min, while the degree of crystallinity continues to decrease with cooling rate. The implications of fracture surfaces are presented with regard to the interface failure mechanisms due to the change of the cooling rate.
Refractory silicon-based barrier coatings are a promising approach for improving the environmental durability of C/C composites in severe environment such as high temperature oxidation atmosphere. The oxidation resistance of C/C composites coated with a newly developed multi-layer C-Si (composite with SiC fiber)-SiC (CVI) was investigated. The C-Si interleaving layer between C/C composite substrate and SiC coating is expected to act as a buffer layer that reduced the thermal stress caused by the mismatching of the coefficient of thermal expansions (CTEs) between the substrate and the coating. The multi-layer coated 3-D C/C sample exhibited significantly improved oxidation resistance owing to change the crack propagation (such as the crack termination and deflection) of the CVD-SiC coating when compared to that without C-Si interleaf. The coated specimens were examined by oxidation test at 1700°C in the methane-fired exhaust gas.
Polycrystalline α-alumina matrix composite fiber dispersed with YAG (yttrium-aluminum garnet, Al5Y3O12) particles have been prepared by sol-gel process using α-alumina fine seed particles. The α-alumina seed particles accelerated the crystallization of alumina matrix from θ to α phase resulting in composite fibers with dense and homogeneous microstructure. The YAG particles were evenly dispersed within the matrix grain as well as at grain boundaries. The effect of α-alumina seed particles and YAG on crystallization and microstructure of the composite fiber is discussed.
The authors have proposed a simple model that predicts dental composites' (small glass particle composites) elastic moduli very well. The fit of this model to other small particle composite materials is demonstrated in this paper to be the best of the current models. With volume filler fractions varying from 0 to 1, and Ef/Em ratios from 670, 000 to approximately 1.7, this paper compares results from the literature to the various models. The models analysed are, iso-strain, iso-stress, Hashin-Shtrikman's upper and lower, Ravichandran's upper and lower, Kerner's, Einstein's, Chantler's and the law of mixtures for particulate composites equations. The results show that the models that best predict the experimental data are Chantler's models. These models provide a guide to the expected modules for a two-phase reinforced particulate composite. At the extremes lie the Hashin-Shtrikman upper and lower boundaries which will include nearly all generated data but provide poor predictability.
The effects of fiber waviness on the nonlinear behavior of unidirectional composites under pure bending were studied theoretically and experimentally. Constitutive models were proposed for the predictions of the flexural properties and nonlinear behavior of composites with fiber waviness. Three types of wavy patterns (uniform, graded and localized fiber waviness) were considered. The material nonlinearity and geometrical nonlinearity due to fiber waviness were incorporated into the analyses. Specimens with various degrees of fiber waviness were fabricated. Bending tests were conducted to compare the experimental results with the predictions for the uniform fiber waviness model. It was found that the experimental results were in good agreement with the predictions.
Elastomer-based fibrous composites are characterized by a significantly nonlinear whilst usable deformation range under external loads, which provides a great challenge to the development of a constitutive description for the composites. This paper presents a micromechanical modeling approach to the entire stress-strain response of such composites based on knowledge of constituent fiber and matrix properties only. A new and accurate rubber-elasticity theory is applied to describe stress-strain behavior of the elastomer matrix material. A bridging model is used to determine internal stresses generated in the constituent materials, and the overall compliance matrix of the composite at each load level follows easily. The proposed model has been applied to an interlock weft knitted polyester fiber fabric reinforced polyurethane matrix composite. Reasonably good correlation has been found between the theoretical and experimental results.
The phenomena of crack propagation and interface debonding can be regarded as formation of new surface. Thus, it is quite natural to model these problems by introducing the mechanism of surface formation. The authors proposed a method in which the formation of new surface is represented by interface element based on the interface potential energy. The general idea of the interface element and its application to peeling test of bonded plates, push-out test of fiber in matrix, dynamic crack propagation and ductile tearing of steel plate are presented.
Impact compressive tests of the carbon fiber reinforced unidirectional vinyl ester composites were carried out using the split Hopkinson pressure bars. Composites of six different fiber volume fractions were tested. The dynamic stress-strain curves of CFRP were obtained at a strain rate of 103s-1. Quasi-static compressive tests of the same specimen were conducted for comparison. Failed specimens under impact loading were examined by SEM (scanning electron microscope). The damage evolution under static loading was recorded by video camera. Test results showed that the compressive failure mechanism changed from shear failure in the unidirectional composites of low fiber volume fraction to kinking and splitting of high fiber volume fraction. The shear failure or kinking occurred on a plane, and the direction of the failure plane was found to depend upon the fiber volume fraction. Experiments also showed that the strain rate has strong effect on the compressive strength.
Plastic optical fiber is a multi-mode optical fiber and used for short distance communication, illumination, etc. Plastic optical fibers can be also used as a damage sensor by measuring optical power loss. Plastic optical fibers are much cheaper and easier to connect than silica fibers, and suitable to be embedded in FRP because of the similarity of its elasticity and coefficients of thermal expansion. In this work, plastic optical fibers were embedded in three types of GFRP specimens including unidirectional and cross-ply laminates. The relations among the optical power, the strain and the number of cracks were studied. Cracks in GFRP lamintes were observed by a video microscope under tensile load. As the strain increased, the optical power decreased linearly before the initiation of cracks and nonlinearly after that. The optical power loss by a single crack was simulated by using a three-dimensional ray tracing model and the results were compared with the experimental results. Then, it was concluded that the nonlinearity of curve of the optical power loss was affected by the cracks and the plastic optical fiber has much potential to be used as a crack-detecting sensor for smart composites.
It has long been known that the inclusion of fibreoptic strain sensors into composite materials produces adverse effects on both, (i) the composite microstructure and (ii) sensor measurements that become ambiguous. The microstructural disadvantages are probably inevitable whatever the type of the fibreoptic sensor used. However, measurement ambiguities can be eliminated by replacing strain as the traditionally preferred measurand of choice with deformation-curvature as a measurand particularly suitable for evaluation of the loading states of composites. Performance examination of recently developed fibreoptic ‘curvature gauges’ embedded between composite laminate is reported in this paper. Not only that unambiguous measurements were achieved, but environmental noise effects were reduced and an additional design flexibility for reducing the degradation of the overall structural properties has been achieved.
The research works related to the use of fibre reinforced plastics (FRP) in civil concrete engineering application have been found increasingly only in recent years. Most of the studies were concentrated on the use of bonding FRP onto the plain rectangular beams and columnar concrete specimens under three point bending and uni-axial compression tests, respectively. The results gave a compromising solution for both strengthening and retrofitting of plain concrete samples. Unfortunately, the defection of the reinforced sample cannot be measured visibly due to the coverage of the bonding patch. To increase the efficient and precision in monitoring of the strengthened structure, Fibre-optic Bragg Grating (FBG) sensor is being introduced in embedding it into the structure to measure the internal strain under applied load. This paper presents the experimental results of cylindrical concrete specimens with and without strengthening by wrapping glass fibre composites under uni-axial compression loading. FBG sensors were pre-embedded into concrete specimens and at the interface between the reinforced laminate and concrete surface. The test results show a considerable increase in the ultimate compressive strength and confine the lateral expansion of the strengthened specimens. The strains extracted from the embedded FBG sensor compare well with that measured by the surface bonded electrical strain gauges.