Mechanical Properties Of Fiber-reinforced Composites Research Articles

Modelling of interfacial effects on the mechanical properties of fibre-reinforced composites

Abstract The implementation of an imperfect bond between the fibre and the matrix in the micromechanical model of cell assembly is achieved by two approaches. In the first one, a displacement jump at the fibre–matrix interface is introduced. The second approach replaces solely the fibre and the interphase by an equivalent medium. Our results show that the two approaches give the same estimation of effective elastic properties which describe the behaviour of the composite in its transverse plane. The combination of the tangential and normal interphase stiffness effect has been observed by loading a composite at a 45° off-axis. When the displacement discontinuity approach is used, the fibre–matrix interface has an important effect on failure modes generated in the composite. We have noticed that the loss of material strength is much greater as the fibre–matrix bond becomes poor.

Influence of Residual Stresses, Interface Roughness, and Fiber Coatings on Interfacial Properties in Ceramic Composites

Fiber pushout tests are performed on zircon‐matrix composites especially fabricated with a variety of silicon carbide reinforcing fibers and fiber coatings in order to create samples with different interfacial properties, surface roughness, and possibly in different states of residual stress to demonstrate their role on the interfacial and mechanical properties of fiber‐reinforced composites. The data obtained from fiber pushout tests are analyzed using linear, shear‐lag, and progressive debonding models to extract important interfacial properties, residual stresses, and surface roughness. The nature and magnitude of residual stresses in composites are independently characterized by measuring the coefficient of thermal expansion of the fiber, the matrix, and the composite for comparison with similar values measured using the fiber pushout tests. These results are then compared for self‐consistency among different ways of analyzing data and with independently measured and calculated values. The results have shown that independent and complementary methods of data acquisition and analysis are required to fully understand interfacial properties in ceramic composites. In particular, independent measures of the coefficient of thermal expansion, residual stresses, and surface roughness are required to confidently interpret interfacial properties obtained by different analytical approaches and then relate them to the overall mechanical response of composites. It is also shown that composites with optimum mechanical response can be created by suitably engineering the interface using multiple fiber coatings.

Adhesion and Hydrolytic Stability of the Interface Between High-Modulus Polyethylene Fibers and Acrylic Resins

Abstract The effects of oxyen plasma treatment on the surface chemistry of Spectra 1000® high modulus polyethylene fibers and on the mechanical properties of fiber-reinforced composites of the fibers in a Bis-GMA based acrylic resin have been studied. X-ray photoelectron spectroscopy and diffuse reflectance FTIR spectroscopy have been used to show that the majority of oxygen on the fiber surface exists mostly in the form of ether and/or epoxy linkages, with carbonyl-, carboxylic- and ester-containing compounds accounting for less than 10 percent of the total. While the untreated and plasma-treated fibers have similar chemical compositions, the surfaces of the plasma-treated fibers are more polar and the oxygen is chemically bonded instead of being merely physisorbed. The interfacial shear strength between the fibers and the acrylic resin is increased by a factor of 2.3 by the plasma treatment indicating the presence of a weak boundary layer on the surface of the untreated fibers. The hydrolytic stability of the composite interfaces was investigated for fibers sized with several Bis-GMA-based adhesives. Maximum stability was attained by sizing with Bis-GMA containing a peroxide catalyst or an amine accelerator. The flexural properties of composites utilizing plasma-treated and untreated fibers were compared in three-point bending. The ultimate bending loads for composites using treated fibers were much higher than those for composites with untreated fibers, but only a fraction of that for glass or Kevlar®-reinforced materials.

The influence of thermal treatments on interfacial properties and fatigue crack growth resistance in SCS-6/Ti β21s composites

The mechanical properties of fibre-reinforced composites are dependent on the properties of the fibres, the matrix and the fibre/matrix interface. The latter two were investigated in a metastable β Ti matrix (Ti β21s) reinforced with silicon carbide (SCS-6) fibres by heat treatment for 2 and 6 h at 910°C (solution treatment) and 8 h at 540°C (ageing). Characterization of the microstructure using scanning and transmission electron microscopies revealed that the interfacial reaction layer had become thicker in the solution-treated specimens. This variation did not result in any significant change in interfacial shear strength, determined by a fibre push-out technique, nor in the fatigue crack growth resistance in three-point bending. Conversely, ageing at 540°C resulted in the precipitation of 34% volume fraction of α plates with negligible change to the fibre/matrix reaction layer thickness. This resulted in a large increase in both the interfacial debonding and frictional shear strength, which was consistent with local volume changes in the matrix on precipitation of α. The fatigue crack growth resistance of the peak-aged composite was also reduced under some testing conditions employed. In particular, the number of cycles to failure was decreased markedly after ageing.

Load-dependent constitutive response of fiber composites with compliant interphases

I n this paper, the influence of applied load on the overall transverse mechanical properties of fiberreinforced composites with compliant interphases is examined from a micromechanical perspective. The composite is modeled by a regular hexagonal array of circular fibers in an infinite matrix. It is assumed that a thin reaction zone (intermolecular bonding at the fiber/matrix interface) establishes the bond between the fiber and matrix phases. The model of the present paper allows us to derive expressions for the overall elastic constants in the transverse plane as a function of applied load. The finite element method is used to evaluate these expressions, and the results are discussed.

7683. Characterization of epitaxial (111) MgO thin films: C R Henry and H Poppa,Thin Solid Film, 189, 1990, 303–312

The mechanical properties of fibre-reinforced composites are dependent on the properties of the fibres, the matrix and the fibre/matrix interface. The latter two were investigated in a metastable β Ti matrix (Ti β21s) reinforced with silicon carbide (SCS-6) fibres by heat treatment for 2 and 6 h at 910°C (solution treatment) and 8 h at 540°C (ageing). Characterization of the microstructure using scanning and transmission electron microscopies revealed that the interfacial reaction layer had become thicker in the solution-treated specimens. This variation did not result in any significant change in interfacial shear strength, determined by a fibre push-out technique, nor in the fatigue crack growth resistance in three-point bending. Conversely, ageing at 540°C resulted in the precipitation of 34% volume fraction of α plates with negligible change to the fibre/matrix reaction layer thickness. This resulted in a large increase in both the interfacial debonding and frictional shear strength, which was consistent with local volume changes in the matrix on precipitation of α. The fatigue crack growth resistance of the peak-aged composite was also reduced under some testing conditions employed. In particular, the number of cycles to failure was decreased markedly after ageing.

Characterization of interfaces in CVD-coated nicalon fiber-reinforced SiC

The mechanical properties of fiber-reinforced composites are directly related to the nature of the fiber-matrix bond. Fracture toughness is improved when debonding, crack deflection, and fiber pull-out occur which in turn depend on a weak interfacial bond. The interfacial characteristics of fiber-reinforced ceramics can be altered by applying thin coatings to the fibers prior to composite fabrication. In a previous study, Lowden and co-workers coated Nicalon fibers (Nippon Carbon Company) with silicon and carbon prior to chemical vapor infiltration with SiC and determined the influence of interfacial frictional stress on fracture phenomena. They found that the silicon-coated Nicalon fiber-reinforced SiC had low flexure strengths and brittle fracture whereas the composites containing carbon coated fibers exhibited improved strength and fracture toughness. In this study, coatings of boron or BN were applied to Nicalon fibers via chemical vapor deposition (CVD) and the fibers were subsequently incorporated in a SiC matrix. The fiber-matrix interfaces were characterized using transmission and scanning electron microscopy (TEM and SEM). Mechanical properties were determined and compared to those obtained for uncoated Nicalon fiber-reinforced SiC.