Leaky Guided Wave Propagation Along Imperfectly Bonded Fibers in Composite Materials

Leaky guided modes propagating along embedded fibers in a composite material can be used for characterizing the fiber-matrix interface. This principle can be applied to real composites containing small-diameter fibers by using laser interferometric detection of very fine lateral resolution on the order of a few microns. The main purpose of this paper is to develop the analytical tools needed to assess the sensitivity of guided wave inspection to interface properties in composite materials. Typically, the sound velocity is much lower in the matrix than in the fiber and the guided modes are strongly attenuated by leaking their energy into the matrix as they propagate. As a result, the velocity of the lowest-order axisymmetric longitudinal mode decreases while its attenuation increases with increasing interfacial stiffness between the fiber and the matrix. It is shown that loose fibers can be readily identified from early signals produced by fast guided modes. In the case of a well-bonded fiber-matrix interface, these guided modes are slowed down and strongly attenuated by the loading of the matrix depending on the fiber diameter and the interracial stiffness of the interface. -Interestingly, the relative difference between the well-bonded and free fibers is greater at low frequencies. Therefore, good sensitivity to the sought interracial stiffness can be achieved at a few MHz, i.e., when the fiber diameter is still much smaller than the acoustic wavelength. Our analytical results show that the leaky guided mode technique is mainly sensitive to the transverse interfacial stiffness of the fiber-matrix interface. At typical ultrasonic frequencies between 1 and 20 MHz, the technique works best in the 101~-1013 N/m 3 interfacial stiffness range which is one or two orders of magnitude lower than the optimal sensitivity range of the more conventional bulk velocity and reflection methods.