Shear strength interfacial

Imposing a shear stress parallel to the fiber axis of a unidirectional composite creates an interfacial shear stress. Because of the disparity in material properties between fiber and matrix, a stress concentration factor can develop at the fiber-matrix interface. Linder longitudinal shear stress as shown by the diagram in Fig. 13, the stress concentration factor is interfacial. The analysis shows that the stress concentration factor can be increased with the constituent shear modulus ratio and volume fraction of fibers in the composite. Under shear loading conditions at the interface, the stress concentration factor can range up to 11. This is a value that is much greater than any of the other loadings have produced at the fiber-matrix interface. 

Interfacial shear strength is a critical property for composites. This previous analysis shows that the interfacial stresses under shear loading can be very large. Therefore as in the case for transverse strength, the interphase itself can be the controlling factor in the level of interfacial shear strength attainable for a given epoxy composite. 

Their results illustrate the necessity for including the interphase as a component of the composite and, therefore, as a factor in any materials behavior model of the composite. 

Two different polyacrylonitrile precursor carbon fibers, an A fiber of low tensile modulus and an HM fiber of intermediate tensile modulus were characterized both as to their surface chemical and morphological composition as well as to their behavior in an epoxy matrix under interfacial shear loading conditions. The fiber surfaces were in two conditions. Untreated fibers were used as they were obtained from the reactors and surface treated fibers had a surface oxidative treatment applied to them. Quantitative differences in surface chemistry as well as interfacial shear strength were measur-ed. 

The results are plotted in Fig. 14. The upper two lines refer to the A fiber and the lower two lines to the HM fiber. For both fibers, the addition and removal of surface chemical groups did not produce reversible interfacial behavior. The untreated fiber surfaces produced results that could not be duplicated when the surface groups were removed. Microtoming of single fiber specimens pinpointed changes in the locus of interfacial fracture that were relatable to the interphase conditions caused by the surface treatment. 

---

Wear. Ceramics generally exhibit excellent wear properties. Wear is deterrnined by a ceramic s friction and adhesion behavior, and occurs by two mechanisms adhesive wear and abrasive wear (43). Adhesive wear occurs when interfacial adhesion produces a localized Kj when the body on one side of the interface is moved relative to the other. If the strength of either of the materials is lower than the interfacial shear strength, fracture occurs. Lubricants (see Lubricants and lubrication) minimize adhesion between adj acent surfaces by providing an interlayer that shears easily. Abrasive wear occurs when one material is softer than the other. Particles originating in the harder material are introduced into the interface between the two materials and plow into and remove material from the softer material (52). Hard particles from extrinsic sources can also cause abrasive wear, and wear may occur in both of the materials depending on the hardness of the particle. 

Fig. 2. Results of interfacial shear strength measurements of the same fiber/matrix systems using four different micro-mechanical tests during a round-robin program involving 12 different laboratories, (a) Results for untreated, unsized carbon fibers, (b) Results for carbon fibers with the standard level of surface treatment. Redrawn from ref. [13].

Assuming the work of adhesion to be measurable, one must next ask if it can be related to practical adhesion. If so, it may be a useful predictor of adhesion. The prospect at first looks bleak. The perfect disjoining of phases contemplated by Eq. 1 almost never occurs, and it takes no account of the existence of an interphase , as discussed earlier. Nonetheless, modeling the complex real interphase as a true mathematical interface has led to quantitative relationships between mechanical quantities and the work of adhesion. For example, Cox [22] suggested a linear relationship between Wa and the interfacial shear strength, r, in a fiber-matrix composite as follows ... 

The interfacial shear strength for carbon/nylon may be taken as 4 MN/m Solution... 

Example 3.18 Calculate the maximum and average fibre stresses for glass fibres which have a diameter of 15 /xm and a length of 2.5 mm. The interfacial shear strength is 4 MN/m and i,/ = 0.3. 

Sf = tensile stress at the fiber, and T = fiber-matrix interfacial shear strength. 

For this, failure interfacial shear strength (t) is obtained by dividing the maximum load P, by interfacial area A. 

The Bowyer and Bader [96] methodology can be used to predict stress-strain response of short fiber-rein-forced plastics. The stress on the composite (cT( ) at a given strain can be computed by fitting the response to a form of Eq. (4) with two parameters, the fiber orientation factor (Cfl) and interfacial shear strength (t,). 

The interfacial shear strength can be further defined as follows ... 

All of the variables except x, can be readily determined experimentally and thus the interfacial shear strength can be calculated. This was calculated for seven different PET formulations and the optically measured interfacial shear strength... 

Chen, F. and Jones, F. R Injection moulding of glass fibre reinforced phenolic composites 1. Study of the critical fibre length and the interfacial shear strength, Plast.. Rubber Composites Proc. Appl., 23, 241 (1995). 

Kharrat, M., Chateauminois, A., Carpentier, L. and Kapsa, P., On the interfacial behavior of a glass/epoxy composite during a micro-indentation test assessment of interfacial shear strength using reduced indentation curves, Composites, A, 28, 39 (1997). 

Std., standard surface treatment ISS, interfacial shear strength SD, one standard deviation CV, coefficient of variation. 

Chuang, S.L. and Chu, N.J. (1990). Effect of polyamic acids on interfacial shear strength in carbon fiber/ aromatic thermoplastics. J. Appl. Polym. Sci. 41, 373-382. 

Desaeger, M. and Verpoest, I. (1993). On the use of the microindentation test technique to measure the interfacial shear strength of fiber reinforced polymer composites. Composites Sci. Technol. 48, 215-226. 

Drzal, L.T., Rich, M.J., Camping, J.D. and Park, W.J. (1980). Interfacial shear strength and failure mechanisms in graphite fiber composites. In 35th Annual Tech. Conf., Reinforced Plast. Compo.sites Inst., SPI, Paper 20C. 

Gaur, U. and Miller, B. (1989). Effects of Environmental exposure on fiber/epoxy interfacial shear strength. Polyni. Composites II, 217-222. 

Miller, B., Gaur, U- and Hirt, D.E. (1991). Measurement of mechanical aspects of the microdebond pullout technique for obtaining fiber/resin interfacial shear strength. Composites Sci. Technol. 42,207-219. 

Morscher, G, Pirouz, P. and Hener, A.H. (1990). Temperature dependence of interfacial shear strength in SiC-fiher-reinforced RBSN. J. Am. Ceram. Soc. 73, 713-720. 

Netravali, A.N., Hcnstenburg, R.B., Phoenix, S.L. and Schwartz, P. (1989a). Interfacial shear strength studies using the single filament composite test, part I Experiments on graphite fibers in epoxy. Polym. Composites 10, 226-241. 

---