Fiber Pull-out Length Research Articles

Processing and properties of metal matrix composites reinforced with continuous fibres for the control of thermal expansion, creep resistance and fracture toughness

Al-, Zn-, and Cu-based matrix composites reinforced with continuous fibres of carbon, SiC, Al2O3, or steel have been processed by squeeze casting or powder metallurgy. Interface reactions can be controlled by alloying additions in the matrix. Interface adhesion has been characterized from the distributions of fibre pull-out lengths on fracture surfaces. Thermal expansion curves reveal the magnitude of the stress transfer at interfaces. In the case of low melting point matrices, ductile steel fibres offer the best combination of fracture toughness and creep resistance.

Fracture of CFRP containing impregnated fibre bundles

The fracture behaviour of carbon-fibre-reinforced plastic composites (CFRP) has been studied by using specimens containing fibre bundles which were impregnated with polymer before being incorporated in an epoxy matrix. The results on CFRP with impregnated bundles show that the impact fracture toughness, the crack growth resistance in terms of stress intensity factor, and the specific work of fracture are enhanced by more than 100%, depending on the impregnant material and fibre volume fraction, as compared to the control composites without bundle impregnation. These beneficial effects are attributed to the increased stability of cracking associated with the bridging of fracture surfaces by the impregnated bundles which act as single fibres of huge diameter effectively increasing the fibre pull-out length. Statistical analysis also demonstrates that bundle impregnation can improve the tensile strength of the bundle as well as the uniformity of strength distribution.

The mechanics of failure of silicon carbide fiber-reinforced glass-matrix composites

Tensile and bending tests have been performed on a high-strength silicon carbide fiber-reinforced glass-matrix (LAS) composite. The experimental results indicate that the strength of the composite component is strongly dependent on the geometry and loading. A model of failure is developed which allows a uniform interpretation of the experiments when due account is taken of the strength of the matrix, the statistical nature of the properties of the fibers, and the dimension of the fiber pull-out length.

Effects of interfacial coating and temperature on the fracture behaviours of unidirectional Kevlar and carbon fibre reinforced epoxy resin composites

The enhancement of transverse fracture toughness of unidirectional Kevlar and carbon fibre reinforced epoxy resin composites (KFRP and CFRP) has been studied using polymer coatings on the fibres. The results obtained show a substantial improvement in the impact fracture toughness of both KFRP and CFRP with polyvinyl alcohol (PVAL) coating without any loss of flexural strength; but there is only a moderate increase in impact toughness with other types of coating (i.e. carboxyl-terminated butadiene acrylonitrile (CTBN) copolymer and polyvinyl acetate (PVA)) with some reduction in flexural strength. The dependence of impact fracture toughness of these composites (with and without PVAL coating) on temperature was analysed on the basis of existing theories of toughening mechanisms from measurements of fibre-matrix interfacial properties, debond and fibre pull-out lengths and microscopic observations. The beneficial effect of fibre coating with PVAL on transverse fracture toughness is shown to sacrifice little damage tolerance of the composites against delamination fracture.

Fatigue crack propagation in short and long glass fiber reinforced injection-molded polypropylene composites

Fatigue crack propagation (FCP) in unfilled and short (SGF) and long glass fiber (LGF) reinforced injection-molded polypropylene (PP) composites was studied on notched compact tension (CT) specimens in tension-tension mode. In the FCP response, a fatigue crack deceleration stage (range I) and an acceleration stage (range II) could be distinguished. The former was explained by the development and 'stabilization' of the damage zone. The latter range could be adequately described by the Paris-Erdogan power law. Increasing fiber loading resulted in improved resistance against FCP. Incorporation of longer fibers yielded an even higher FCP resistance. The use of LGF reinforcement also resulted in a quasi-isotropic FCP behavior, whereas a clear dependance of the propagation rate on crack direction could be observed for SGF-filled composites. All these differences could be interpreted by differences in microstructural parameters of the LGF in comparison to the SGF systems. Failure processes were studied by light and and scanning electron microscopy, and are discussed. Increased matrix ductility at higher FCP rates and corresponding changes in the fiber-related events, especially in fiber pull-out length, were attributed to crack tip heating effects.

Weibull statistics applied to fiber failure in ceramic composites and work of fracture

Fiber failure locations with respect to a given matrix crack are determined for single and multiple matrix cracking in ceramic composites by defining a pointwise failure probability in terms of the Weibull strength theory. The composite is assumed to be reinforced with uniaxially aligned continuous fibers with frictionally constrained interfaces. The average fiber pullout lengths after complete and partial fiber failure are given for both single and multiple matrix cracking. Previous results reported by Sutcu [( J. Mater. Sci. 23, 928 (1988)] for the work of fracture are refined, and the composite strength beyond matrix cracking is estimated for multiple matrix cracking, assuming that all matrix cracks behave in a similar manner throughout the composite volume. The frictional stress caused by bridging fibers is considered in the strength computation in order to improve the strength prediction, especially beyond the ultimate load. The predicted results are compared to experimental results on LAS-Glass/Nicalon fiber composite.

The toughness of a composite containing short brittle fibres

A method is presented whereby various potential contributions to the toughness of a polymer containing short brittle fibres can be quantified. It relies on a model for predicting the cumulative probability distribution of fibre pull-out lengths. The method reveals that toughness increases to a maximum value with increasing fibre length. Good agreement between theory and experiment supports the validity of our approach.

The Influence of Heat Treatment upon Fiber Pull-Out in a Ceramic Composite

AbstractThe toughness of ceramic-matrix composites is strongly influenced by fiber pull-out. The extent of the pull-out depends upon the properties of the fiber and the fiber/matrix interface. Samples of a SiC/LAS composite were subjected to different heat treatments in order to systematically vary these properties. The predicted distribution of the fiber pull-out lengths was calculated by combining a shear lag analysis with Weibull statistics for the fiber strengths. Comparison of the analysis with experiments and microstructural observations contribute to an understanding of the role of the fiber/matrix interface upon the mechanical properties.

Hygrothermal ageing and fracture of glass fibre-epoxy composites

The effects of hygrothermal ageing upon the failure characteristics and work of fracture of glass fibres in epoxy are described. A collection of data based on the direct observation and measurement of fibre debond length and fibre pull-out length after fracture is displayed in empirical failure diagrams. Similarly, a vast number of experimental measurements of work of fracture is displayed in a three-dimensional diagram where the axes are work of fracture, humidity and ageing time. This information is combined with models of fracture in the construction of a fracture map which is used to interpret hygrothermal ageing phenomena.

Time and temperature dependence of fracture in a unidirectional glass-reinforced epoxy

The results of a study on the time- and temperature-dependent behaviour of unidirectional glass fibre-reinforced epoxy are described and analysed. The fracture parameters examined are the fracture strength, the work of fracture and the apparent fracture toughness. It is shown that the fracture strength decreases with increasing temperature and decreasing loading rate; the work of fracture exhibits a sharp minimum in the vicinity of room temperature, and the fibre pull-out length increases by a factor of 4 at 76K as compared with the room temperature length; the fracture toughness is found to be independent of the crack length and only dependent on the fracture strength; thus its trend with loading rate and temperature follow those of the fracture strength.

The fracture energy of hybrid carbon and glass fibre composites

The fracture energy of a model carbon fibre/glass fibre/epoxy resin hybrid composite system has been evaluated as a function of the carbon fibre/glass fibre ratio. Work of fracture measurements were less than a rule of mixtures prediction and a pronounced negative synergistic effect was observed at high carbon fibre and high glass fibre contents. Fibre debonded lengths and fibre pull-out lengths for the carbon and glass fibres were accurately measured using a projection microscope technique. Models of microscopic fracture behaviour, together with these measurements, were successful in quantitatively describing the observed fracture behaviour of the hybrid fibrous composites. It was found that post-debond friction energy provided a major contribution to the fracture energy of the glass fibres. The post debond sliding mechanism was also shown to be primarily responsible for the non-linear behaviour of the work of fracture of the hybrid composite.

Experimental data for the longitudinal tensile strength of unidirectional fibrous composites—Part 2: Composites

Experimental results are presented for the longitudinal tensile strength of continuous unidirectional carbon fibre-epoxy resin matrix composite materials. The composite specimens contained a single fibre tow and were prepared from three types of carbon fibre and two epoxy resin formulations. The composite strength results and observations made on the fracture surfaces suggest that fibre pull-out length may be used as a guide to the length of debonding between fibre and matrix for the purpose of predicting composite strength.

Height measurements of surface features with the scanning electron microscope

A technique has been developed whereby the vertical dimension of a topological feature may be measured in situ in the scanning electron microscope. The technique, which consists of relating objective lens current to focal point location, is reproducible over the range 0–5 mm to an accuracy of ±0.08 mm. The utility of the technique is demonstrated by measuring fiber pull-out lengths from a boron-aluminum composite fracture surface. Results are compared to those obtained by light optical microscopy.