An enhanced,acoustic emission-based,single-fiber-composite test

In the single-fiber-composite (SFC) test, a fiber imbedded in a matrix is loaded in tension, resulting in a fragmentation of the fiber. In the conventional version of this test, the final fiber fragmentation length distribution is used with a micro-mechanical model to determine the average fiber/matrix interfacial shear stress. In the enhanced version of this test, one also determines the applied stress at each fiber fracture, and from this, one can evaluate the strength of the fiber at short gage lengths. In our measurement system, we utilize an acoustic emission (AE) technique to detect the fiber fractures and to locate the fiber breaks and so determine both the fiber failure stresses as well as the fiber fragmentation lengths while the test is in progress. Critical to the success of this test is a broadband AE system that utilizes point-like AE sensors, procedures for evaluatingin situ, the wavespeed of the first wave arrival and signal processing techniques for determining the arrival time of this signal as precisely as possible for a broad range of wave shapes. Here we describe the application of such an enhanced SFC test procedure to investigate the failure of a Nicalon fiber in an epoxy matrix.     

The single-filament-composite test: a new statistical theory for estimating the interfacial shear strength and Weibull parameters for fiber strength

For over two decades the single-filament-composite (SFC) test has been an important tool in the study of the failure of fibrous composites. The SFC test itself involves a single brittle fiber embedded along the center-line of a matrix specimen of both large cross-sectional area and strain to failure. With increasing strain, the fiber fractures progressively, breaking into an increasing number of shorter and shorter fragments. Surrounding each break a shielded or exclusion zone develops within which no further breaks typically occur. At some strain level ‘saturation’ occurs abruptly as the shielded zones finally occupy the whole fiber, thus leaving a final distribution of fiber fragments end-to-end. Two uses for the SFC test have emerged: one has been to estimate the interfacial shear stress, τ, in the exclusion zone, sometimes called the interfacial shear strength and usually idealized as a constant over this zone. The other has been to estimate the fiber strength distribution and in particular the Weibull shape and scale parameters, ρ and σ_l, for fiber strength appropriate to some characteristic ‘gage’ length, l, such as the mean fragmentation length. In the past, theoretical bases for these estimates have handled the statistics of shielding in ways that have led to quite large biases. The purpose of the present paper is to use some recent theoretical advances to develop more sophisticated estimation procedures for τ and the Weibull fiber strength parameters ‘ in situ’, and thus to eliminate various errors in previous methods. Straightforward computer programs (written in release 3 of Maple), which calculate the various quantities in the paper, will be provided by the first or second author on request.