Direct measurement of fiber failure in fiber bundles and in fiber reinforced composite materials

• Main Authors: Toffoli, S M, Lehman, R E, Sigel, G H, Shanefield, D J, Greenhut, V A
• Format: Dissertation
• Language: English
• Published: 01.05.1997
• ISSN: 0366-6913

The mechanical failure behavior of individual reinforcing fibers is an important performance aspect of fiber reinforced composite systems. This is particularly true of ceramic matrix fiber composites in which the majority of the work of failure occurs during fiber failure and pullout in the post-matrix cracking and post-ultimate strength region. This dissertation seeks to understand the statistical behavior of fiber failure in brittle matrix composites and to develop methods for determining the onset of fiber failure as a signal for the onset of irreversible failure of the reinforcing structure. Fiber failure in aligned fiber bundles, both free and in model composites, was tested under tensile loading to determine the onset of fiber failure and the statistical distribution of fiber failures by direct interrogation of the continuity of model fibers. Fiber interrogation monitored in real time the electrical continuity of aluminum metal coated galss fibers. A model composite consisting of the Al-coated glass fiber in an epoxy resin matrix was studied to determine the failure behavior of individual fibers in a composite environment. Glass fibers in an epoxy matrix was selected as a composite model due to the high E sub f /E sub m , high sigma sub f / sigma sub m and nearly brittle behavior of the epoxy matrix selected. An electronic high speed analog/digital interface was used to monitor the electrical, and hence mechanical, continuity of the fibers under test. These data were used to assess the onset of the individual fibers failure, the calculation of apparent fiber strengths, and the determination of Weibull distribution of the apparent fiber strength values. The procedure developed in this work for assessing fiber failure in bundles and composites has been shown to be sufficiently sensitive to resolve the onset of fiber failure in bundles and in model composites prior to measurable reductions in load carrying capacity, thus providing a means for early detection of fiber failure. The procedure is also a means by which the failure mode of fibers in composites can be compared to classical bundle theory failure, thus supporting or refuting a popular model.