Experimental determination of the interface residual stresses of carbon-fiber-reinforced polymers

Combining the matrix crack method with the geometric phase analysis (GPA) method, the interface residual stresses in carbon-fiber-reinforced polymers were measured at nanoscale. A cross-grating with a pitch of 138 nm was fabricated on the surface of composite material by electron beam lithography. The residual stresses around fibers located in the 90 degrees, 45 degrees, and 0 degrees fiber layers were released through the matrix crack method using a Berkovich nanoindenter. The released residual stresses led to a change in the pitch of the grating and the corresponding residual strains were obtained by the GPA method. The Hooke's law was employed to calculate the interfacial residual stress in the 90 degrees fiber layers (-6.7 +/- 1.3 MPa) in a range of 13.8 nm around the fiber, which agreed well with the result calculated by the Bright's equation, and then, the interfacial residual stresses in the 45 degrees fiber layers and residual stress along the fiber axial direction were obtained. The effect range of the radial residual stresses was equal to about one fiber radius.