Page 41 - Engineered Interfaces in Fiber Reinforced Composites

24 Engineered interfaces in jiber reinforced composites

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Fig. 2.9. Variation of the position of the 1580 cm-’ peak with fiber strain (a) for a polyacrylonitrile
(PAN)-based HMS4 carbon fiber, and (b) for Thornel 50 carbon fiber. After Robinson et al. (1987).

stress transfer mechanisms across the fiber-matrix interface in the fiber fragmen-
tation test geometry (Galiotis, 1993a). The variation of fiber axial strain and
interface shear stress (IFSS) measured along the length of Kevlar 49 fiber embedded
in an epoxy matrix is shown in Fig. 2.10 for different levels of applied strain. The
IFSS is calculated based on the force balance between fiber axial direction and
interface shear.

2.3.4. X-ray photoelectron spectroscopy

XPS, also known as electron spectroscopy for chemical analysis (ESCA), is a
unique, non-destructive analytical technique that provides information regarding
the chemical nature of the top 2-10 nm of the solid surface with outstanding
sensitivity and resolution. In XPS, the solid surfaces are subjected to a beam of
almost monochromatic X-ray radiation of known energy in a high vacuum
environment (4 x 10-9-1 x lop8 Torr). Electrons are emitted from the inner orbital
with kinetic energies characteristic of the parent atoms. The intensities of the kinetic
energy are analyzed and the characteristic binding energies are used to determine the
chemical composition. The total absorbed X-ray photon energy, hv, is given by the
sum of the kinetic energy, EK, and the electron binding energy, EB

hv=EK+EB. (2.12)
Once the kinetic energy is measured with an electron spectrometer for a given X-ray
photon energy, the binding energy characteristic of the parent atoms can be directly
determined. The electron binding energy represents the work expended to remove an
electron from a core level of the inner orbital to the Fermi level in its removal from
the atom. Peaks in the plots of electron intensity versus binding energy correspond
to the core energy levels that are characteristic of a given element.