Since fiber-matrix interface strength is critical to properties of carbon fiber-reinforced composites, measurement and analysis of interface strength are crucial steps in tailored design of composites. In the present work, the single fiber push-out test and the short-beam shear test were applied to measure the fiber-matrix interface strength in uni-directionally and two-directionally carbon fiber-reinforced phenolic resin matrix composites. The technical difficulties in processing the specimen and in realizing the fiber push-out were also discussed and clarified. For obtaining the successful test, typically, the thickness of the specimen should be smaller than 100 mm. During the fiber push-out, the de-bonding and fiber sliding at the interface were analyzed from the load-displacement curve features. The results indicated that both methods could be applied to determine the interface strength. The single fiber push-out and the short-beam shear tests resulted in a similar phenomenon in regard to the interface strength of uni-directionally and two-directionally carbon fiber-reinforced phenolic resin matrix composites, but expressed different values. The low interface strength measured from the short-beam shear test could be associated with multiple interlaminar shear failures. Furthermore, it was found that the interface strength of uni-directionally carbon fiber-reinforced phenolic resin matrix composites is somewhat higher than that of two-directionally carbon fiber-reinforced phenolic resin matrix composites. The difference in the interface strength could be attributed to the thermally induced residual stresses caused by the coefficient of thermal expansion mismatch of fiber and matrix. The approaches applied in the current work can be used for the evaluation of the interface strength of carbon fiber-reinforced phenolic resin matrix composites with different fiber-matrix bonding properties.

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In order to investigate the influence of carbon fiber's surface state on the interlaminar shear properties of carbon fiber-reinforced plastic (CFRP) laminate, the carbon fiber's surface state was modified by thermal treatment at elevated temperatures. The interlaminar shear strength (ILSS) of CFRP laminates reinforced with treated fibers was measured by means of short-beam shear test, and the surface state of fiber was characterized by Electron Spectroscopy for Chemical Analysis (ESCA) analysis to reveal the dominate factor for controlling the ILSS. Combining the ILSS measurement with the ESCA analysis, the results indicated that: (1) the ILSS is strongly dependent on the oxygen-containing functional groups on the surface of carbon fiber; (2) the fiber treated at 600°C has the highest oxygen-containing functional groups that lead to the highest ILSS of CFRP; and (3) at temperatures beyond 600°C, the oxygen-containing functional groups decrease with increasing the heat treatment temperature, resulting in a low ILSS of CFRP laminates. Furthermore, from the microstructure observation, it was found that the CFRP mainly failed in the mode of multi-interlaminar shear. The multi-interlaminar shear failure in the CFRP laminates with low ILSS is more severe due to a weak fiber-matrix interface.

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