Quantitative Acoustic Emission from a Fiber Pullout Test.

In this thesis the failure of fiber/matrix interfaces are experimentally examined using quantitative acoustic emission (QAE) measurements. A single glass fiber with one end embedded in a droplet of epoxy is pulled until the interface fractures and the fiber pulls out. Failure on the fiber diameter size scale is difficult to measure because friction and fracture processes occur almost simultaneously. The energy stored in the loading system continues to pull the fiber after the initial fracture. Static measurements of the initial and final states of stress do not separate these components. In the standard version of this test, a model is employed to apportion the relative contribution of friction and fracture to the strength of the interface. In contrast, QAE is a dynamic measurement technique that is capable of resolving fracture events on very short time scales. A comparison of the observed AE signals compared to a known fracture source, shows the interface failure is initially a fracture process. Acoustic waves emanating from the interface are used to characterize the mechanism of failure. Theoretical responses of the system to different source types were calculated in the form of dynamic Green's functions. Acoustic radiation pattern data are fit to linear combinations of theoretical sources by a least squares algorithm. The source of the failure is molded by combinations of forces and double forces which can be represented as a moment tensor. The eigenvectors and values of this moment tensor correspond to crack orientation and type respectively. The work of fracture for the failure is determined from the amplitudes of the longitudinal and shear wave arrivals. The source-time function is determined from the detected signals using a singular value decomposition deconvolution algorithm and the waves reconstructed from the fit moment tensor. Three surface conditions of the glass fibers were studied: Bare, carbon-coated and aluminum-coated, all with the fiber embedded in an epoxy matrix. From the higher -order source terms the principal directions of the source were found to be aligned with the fiber, but there was no preferred direction in the plane normal to the fiber axis. The results of the decomposition suggest a competition between failure modes depending on the material system. The majority of failures events exhibited mixed mode I and II crack opening during interface failure.