Fibre matrix interface

In order to understand the effect of discontinuous fibres in a polymer matrix it is important to understand the reinforcing mechanism of fibres. Fibres exert their effect by restraining the deformation of the matrix as shown in Fig. 3.28. The external loading applied through the matrix is transferred to the fibres by shear at the fibre/matrix interface. The resultant stress distributions in the fibre and matrix are complex. In short fibres the tensile stress increases from zero at the ends to a value ([Pg.226]

With optimum selection of fibres and matrices, favourable residual stress conditions can be established in the matrix, which lead to increased A Tc. Above A Tc, matrix cracks appear but the presence of crack-deflecting fibre-matrix interfaces ensures minimal effect on mechanical properties as the fibres remain largely unaffected. As damage is also confined mostly to the surface of the materials, changes in mechanical and thermal properties are more readily identified by means other than mechanical testing. 

Only a slight drop in the flexural strength of a woven Nicalon /Al203 composite was observed by Fareed et al. (1990) after quenching through A T = 1000°C and 1200°C. This was attributed to the effectively engineered weak fibre/matrix interface. 

Degradation of the fibre-matrix interface and removal of fibres. This type of damage appeared at A T= 600°C but was attributed to both thermal shock and/or oxidation effects. 

A recent analysis by Kastritseas etal. (2004c) suggested that in both cases the magnitude of the thermal shock-induced stresses was overestimated as the anisotropic character of the materials was not taken into account. If material anisotropy is accounted for, then both (15.36) and (15.37) cannot predict A Tc accurately even for the largest possible value of the thermal shock-induced stresses (corresponding to a maximum value of the stress reduction factor, A = 0.66). To explain the discrepancy, it was proposed that the interfacial properties may be affected by the shock due to the biaxial nature of the induced stress field, which dictates that a tensile thermal stress component that acts perpendicular to the fibre-matrix interface is present for the duration of the shock. 

R. F. Cooper and K. Chyung, Structure and Chemistry of Fibre-Matrix Interfaces in Silicon Carbide Fiber-Reinforced Glass-Ceramic Composites An Electron Microscopy Study, J. Mater. Sci., 22, 3148-3160 (1987). 

Short fibre polymer composites are being increasingly used as engineering materials because they provide mechanical properties superior to neat polymers and can be processed easily by the same fabrication methods, e.g. injection moulding. The mechanical properties of these materials are dependent on a complex combination of several internal variables, such as type of matrix, fibre-matrix interface, fibre content, fibre dimensions, fibre orientation, and external... 

With reference to the au=l case (see the intercepts on the right-hand side vertical axis in Figs. 5 to 7) the increase in fracture toughness with increasing loading rate (Fig. 5c) and decreasing temperature (Fig. 6b) can be explained as an effect of the increase in pull-out force, which may occur with either weak or strong fibre-matrix interface. 

If the fibre-matrix interface is weak, the pull-out force increases, as the loading rate increases. 

In the former case (weak fibre-matrix interface) fracture surfaces can be expected to show clean looking pulled-out fibres, as found in POM G15 specimens (Fig. 10a), while in the latter case (strong fibre-matrix interface) fracture surfaces can be expected to show dirty looking pulled-out fibres, bearing scraps of the matrix polymer on them, as found in PA 6.6 G30 specimens (Fig. 10b). 

Also the increase in fracture toughness with increasing water content in PA6.6 G30 (Fig. 7) can be explained as due to an increase in pull-out force, accompanied by a weak fibre-matrix interface. If this is the case the pull-out force increases because of the swelling of the matrix which may increase the compression exerted on the fibre and hence fiiction. 

Abstract The effects of the amount of rubber, the concentration of fibres and the state of the fibre/matrix interface upon the mechanical behaviour of short glass fibre-reinforced rubber-toughened nylon 6 ternary blends are described. First, tensile tests were carried out on different intermediate materials and then on the ternary blends to derive the stress-strain curves and document the damage mechanisms. Fracture toughness tests were implemented on compact tension specimens and the results were correlated to fractographic observations and acoustic emission analysis to assess the role of the different constituents. 

The purpose of this investigation is to determine the effects of the rubber, the fibre concentration and the state of fibre/matrix interface upon the damage and fracture behaviour of short glass fibre-reinforced nylon-based materials. 

In this paper, all the blends and composites are designated by the type of matrix (G for the neat nylon, D for the 8 wt % rubber-modified nylon and N for the 20 wt % rubber-modified nylon), the concentration of fibres and the type of fibre/matrix interface (A or B). As an example, a material designed DlOB is a ternary blend made of DZ matrix and 10 wt% of type B fibres. After drying the specimens for 24 hours at 100°C, they were stored in plastic bags inside a desiccator. In comparison with freshly injection moulded samples, the moisture content in the specimens ready for mechanical testing is about 2 wt%. All the mechanical tests were conducted in an environmental chamber in controlled conditions a temperature of 20°C under a continuous argon flow. 

Fig. 6 Influence of the concentration of fibres and state of fibre/matrix interface on the uniaxial tensile stress-strain curves of fibre-reinforced nylon composites (a) type A interface (b) type B interface.

Whatever the concentration of fibres and the state of fibre/matrix interface, stable crack propagation is obtained up to total failure. With type A interface, increasing the fibres concentration results in a loss of resilience by simultaneous reduction of the peak load and the ultimate displacement. 

In figure 14a are plotted the loading curves associated to the DZ and NZ matrices reinforced using 30 wt% of fibres. In the case of the DZ matrix, the enhancement of the crack growth resistance when using type B fibres is due to the strongly bonded fibre/matrix interface. For the NZ matrix, the compatibility of the type B fibres results in an appreciable rise of the peak load the displacement at break being controlled by the ductility of the matrix. 

Addition of brittle fibres to a rubber-toughened nylon 6 provides a higher stiffness but brittleness increases with the concentration of fibres whatever the state of the fibre/matrix interface. 

Extending work done previously (1 - 2), the purpose of this paper is to examine how these characteristics could be determined using inverse gas chromatography (IGC) and to what extent these acid/base interactions are relevant to the description of the fibre-matrix interface. 

Determination of Fibre-Matrix Adhesion. The average shear strength x and maximum shear strength Xmax at the fibre-matrix interface CZ, 22) are given by... 

In previous work (30-321. it has been suggested that the adhesion between a carbon fibre and an epoxy matrix is essentially the result of physical bonds, either dispersive or polar. It is clear from the results in the last column of Table VI that there is no correlation between x and the reversible energy of adhesion WA, calculated as the sum of the dispersive and polar interactions at the fibre-matrix interface. 

Dispersive and specific interactions are considered to contribute independently to the adsorption of probe molecules at the adsorbent surface. It was presented that the adhesion of the fibre-matrix interface depends clearly on the measured strength of acid/base interactions of both fibre and polymer-matrix. Fowkes [2,3] indicated also that the surface of fillers can be chemically modified to enhance acid-base interaction and increase adsorption. 

Tillie et al. (1998) examined the effect of the fibre/matrix interface on the cure of glass-fibre-filled epoxy-resin systems. They found that the introduction of a lower-Tg interphase based on hydroxylated PDMS oligomers allowed an increase in toughness without a reduction in modulus or Tg. This was due to a modification of the stress field under load due to the elastomeric interphase. 

The structure and properties of the fibre-matrix interface play a mayor role in the mechanical properties of composite materials too. The fibre can be used as reinforcement in composite materials since it is indicate that the length of fibre more length than critical length of the fibre. 

Rush, R., Eichhorn, S.J. Determination of the stiffness of cellulose nanowhiskers and the fibre-matrix interface in a nanocomposite using Raman spectroscopy. Appl. Phys. Lett. 93, 033111 (2008)... 

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