Fiber Pull-out Mechanism Research Articles

Micromechanical Model of Crack Growth in Fiber Reinforced Brittle Materials

AbstractA model based on the micromechanical mechanism of crack growth resistance in fiber reinforced brittle matrix composites is presented. The formulation of the model is based on a small scale geometry of a macrocrack with a bridging zone, in this case the process zone, which governs the resistance mechanism. The effect of high toughness of the fibers in retardation of the crack advance, and the significance of the fiber pullout mechanism on the crack growth resistance, are reflected in this model. The model allows one to address issues such as influence of the fiber spacing and fiber-matrix friction.Two specific cases are analyzed. One represents the fracture initiation and concentrates on the development of the first microcrack between fibers. The second case deals with the development of an array of microcracks forming the bridging zone. In both cases a discrete fiber distribution is assumed. The analysis is based on an exact solution of the corresponding. boundary value problem.

Peritectic solidification and precipitation of α-Re in β-NiAl Alloys containing Re additions

Recently it has been shown that solidification of β-NiAl alloys which contain additions of Re will produce eutectic microstructures. The microstructure of the eutectic was determined to be composed of a matrix of β-NiAl with fibers of α-Re. It was thus concluded that a eutectic quasi-binary isopleth between β-NiAl and α-Re was present. Electron microprobe analysis of the eutectic areas showed a eutectic composition (in atomic percent) of 47.5 Ni, 51 Al and 1.5 Re.The fracture of these eutectic structures has been the primary interest in studying these materials. It appears that the α-Re may improve the fracture toughness of β-NiAl by a fiber pull-out mechanism, see Fig. 1. Careful examination of the α-Re fibers will reveal that each fiber exhibits plastic deformation. Indeed if the eutectic structure could be produced by directional solidification, the fracture toughness of this composite structure could be determined. The purpose of the present study is to further clarify the nature of this quasi-binary phase diagram.

Some considerations of evaluation of interfacial frictional stress from the indentation technique for fibre-reinforced ceramic composites

Recent studies of fibre-reinforced (or whisker-reinforced) ceramic composites have revealed that a substantial improvement in toughness is achieved via the fibre pullout or bridging mechanism. Optimal conditions for toughening of these composites require an unbonded or a weakly bonded fibre-matrix interface that allows frictional sliding between the fibre and the matrix [1-3]. These requirements have prompted recent studies of interfacial properties of composites with frictional interfaces [4-11]. The fibre pullout test is not widely used, due to problems of sample preparation and inconsistent experimental results [4]. Conversely, the indentation technique has been used extensively to evaluate the interfacial frictional (shear) stress (IFS) [4-7]. Using this technique, an indentor is used to push on the end of an embedded fibre. Thick and thin samples* have been adopted such that partial sliding and push-out of the fibre occur, respectively, when the fibre is subjected to the indentation force (i.e. compressive loading). For the thick sample the average IFS is evaluated from the load-displacement relationship and the sliding length of the fibre based on force equilibrium considerations [5, 6]. For the thin sample the uncertainty in determining the sliding length is avoided and the average IFS is calculated from the thickness of the sample and the minimum force required to push out 'the fibre [7]. Both analyses ignore the variation of the IFS along the sliding length. However, when the fibre is subjected to compression in the axial direction, transverse expansion (i.e. the Poisson effect) of the fibre occurs which induces compressive stresses at the fibre-matrix interface and causes the IFS to vary due to Coulomb friction [8-10]. Consequently, the following questions must be raised. (1) Is the IFS constant along the entire sliding length? (2) If not, does the average value of the IFS bear any physical significance? (3) Can the IFS measured by compressive loading represent the value appropriate to fibre pullout in tension during cracking? The first question has been addressed previously. Theoretical analyses in recent studies show that when the end of the embedded fibre is subjected to a compressive load, the IFS is not constant, decreasing along the fibre length underneath the loaded surface due to the Poisson effect of the fibre [8-10]. The third question has also been analysed. The results show that when Poisson's ratio of the fibre is zero (i.e. no transverse strains due to axially applied stresses) the IFS is constant and has the same value along the sliding length for both fibre pullout and push-down. For Poisson's ratios greater than zero, compared with fibre push-down, fibre pullout has a lower IFS and a longer sliding length for the same magnitude of the load [11]. The purpose of the present study is to answer the second question: i.e. is the average IFS an intrinsic property of the composite when the fibre is subjected to compression (or tension)? In other words, does the average IFS depend on the load for the thick sample and on the thickness of the sample for the thin sample? The geometry used in the present study is a composite cylinder model [12]. A fibre of radius a is embedded in a coaxial cylindrical shell of matrix with

Strength and fracture properties of asbestos-cement mortar composites

The strength and fracture properties of random asbestos fibre-reinforced cement mortar composites are reported in this paper. The fibre content varies between 5% and 20% by weight. Both the ultimate tensile strength (σ t) and the modulus of rupture (σ b) increase with increasing fibre-volume fraction. These results are shown to agree satisfactorily with the law of mixtures modified for randomly oriented short fibre-reinforced composites. The critical stress intensity factor (K c) and the specific work of fracture (R) have been determined using three-point bend edge-notched beams and grooved double-cantilever-beam (DCB) specimens. There is generally good agreement between these two physical quantities estimated from the two testpiece geometries. It is shown that the fibre pull-out mechanism is dominant in the fracture of asbestos cements and that the specific work of fracture can be reasonably well predicted by considering the energies absorbed in both the pull-out and the fibre/matrix interfacial debonding processes.