摘要:

Incertain engineering applications, graphite fiber polymer matrix composites may be exposed to both moisture and electrochemical polarization. The combined effect to these environmental conditions can cause fiber/matrix interfacial breakdown and matrix degradation. In previous research, electrochemical impedance spectroscopy (EIS) was used to examine the effects of electrochemical polarization on bismaleimide (BMI)/graphite fiber (GF) composites, and revealed changes in the impedance spectra that suggested porous behavior. This 'porosity' was attributed to moisture ingress at the fiber/matrix interface. At present, no techniques are available to determine the depth of moisture penetration in these materials in a nondestructive manner. This paper introduces a new method for the determination of moisture penetration depth at the fiber/matrix interface in graphite fiber polymer matrix composites as a function of time and degree of polarization. The method is a nondestructive,in situtechnique which has the added advantage of utilizing the phase angle of the impedance. The phase angle is typically the most sensitive indicator of interfacial changes and is also an intrinsic parameter that is not encumbered by the surface area fraction or number of fibers exposed at the surface. A 0°, 8-ply BMI/GF composite was investigated due to its susceptibility to damage by cathodic polarization. The calculated penetration rates follow the trends predicted for increases in time and overpotential. More importantly, the penetration predicted by extrapolation of 24 hour EIS data is reflective of moisture penetration determined by 12 month seawater exposures of epoxy/graphite fiber composites coupled to magnesium. Although additional verification is needed, EIS could provide a very powerful tool for examining the fiber/matrix interfacial stability as a function of fiber and matrix chemistries, processing, and exposure conditions.

DOI:10.1163/156855494X00300

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The applicability of the microbond test to evaluate the interfacial properties between cellulosic fibers and thermoplastics was studied. Acetylation and heat treatment were applied to modify the surface of cellulosic fibers (rayon, cotton, and wood). The apparent diameters and surface free energies of the fibers were estimated by dynamic contact angle (DCA) analysis. Interfacial shear strengths between the cellulosic fibers and the polystyrene matrix were determined using the microbond test method. The test results indicate that acetylation increases the total surface free energy of the wood fibers, whereas heat treatment dramatically decreases the surface free energy of all cellulosic fibers tested. For heat treated and acetylated fibers, the greater the surface free energy, the greater the interfacial shear strength (ISS) regardless of fiber types. For control group fibers, a low ISS exists even though the fibers have high surface free energies because of the formation of a weak boundary layer. The high ISS between the acetylated wood fiber and the polystyrene matrix is attributed to the improved wetting and spreading of the melting polystyrene on the acetylated wood fiber surfaces. As such, the interfacial properties between the cellulosic fibers and polystyrene matrix system can be successfully characterized by dynamic contact angle and microbond test.

DOI:10.1163/156855494X00319

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Bymonitoring the peak position of strain-sensitive Raman bands, Raman spectroscopy can be used to study the distribution of stress or strain along discontinuous aramid fibres fully embedded in an epoxy resin matrix from which the interfacial shear stress can be calculated. The technique has been used to study the effect of fibre tensile properties and applied surface treatments upon the efficiency of stress transfer from the matrix to the fibre. At low levels of matrix strain the distribution of stress or strain along the fibre can be defined using classical shear lag theory from which the effect of fibre modulus can be distinguished. Values of interfacial shear stress were found to be higher for those fibres pretreated with an epoxy solution. This may be attributed to changes in the matrix properties in the vicinity of the fibre/matrix region. In the case of the untreated fibre samples interfacial failure was initiated by yielding of the fibre/matrix interface followed by debonding at the fibre ends. The behaviour was modelled using a modified shear lag analysis.

DOI:10.1163/156855494X00328

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Pull-out experiments have been carried out with Kevlar fibres embedded in epoxy resin. Friction accompanied debonding, and had to be allowed for in the analysis. The debonding stress was about equal to the matrix strength for 80°C cured epoxies. However, debonding appears to be a brittle fracture process, and the works of fracture corresponding to the apparent interface strengths are very low, ranging from ca. 20-40 Jm-2depending on the surface treatment and degree of cure of the resin. Water immersion for 2300 h at room temperature reduced the apparent strengths and works of fracture with some of the surface treated fibres, but not with the untreated fibres. Interface pressures during debonding were 10-15 MPa for the 20°C cured specimens and 20-30 MPa for the 80°C cure. Water soaking markedly reduced the friction coefficients. Post-debonding friction was high, but estimates of the parameters was probably unreliable due to the fibre having a somewhat thick end due to fibrillation when being cut.

DOI:10.1163/156855494X00337

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

We report on the development of a novel computational method for molecular dynamics simulations which explicitly includes variable charge transfer between anions and cations. This method is found to be capable of describing the elastic properties, surface energies, and surface relaxation of crystalline metal-oxides accurately. We present results for a simulation of adhesive failure at a model metal/oxide heterophase interface between an aluminum (111) face and an α-alumina (0001) face. Our results indicate that this approach can provide physically realistic empirical potentials for future simulations on mixed metal/metal-oxide systems.

DOI:10.1163/156855494X00346

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor