Chapter 41. Microcracking and Changes in Elastic Moduli of Random, Discontinuous Celion Graphite-Borosilicate Glass Composite
- John B. Wachtman Jr
Published Online: 28 MAR 2008
Copyright © 1991 The American Ceramic Society, Inc.
Proceedings of the 15th Annual Conference on Composites and Advanced Ceramic Materials, Part 1 of 2: Ceramic Engineering and Science Proceedings, Volume 12, Issue 7/8
How to Cite
Karandikar, P. G., Chou, T.-W. I., Chen, O., Takeda, N. and Kishi, T. (1991) Microcracking and Changes in Elastic Moduli of Random, Discontinuous Celion Graphite-Borosilicate Glass Composite, in Proceedings of the 15th Annual Conference on Composites and Advanced Ceramic Materials, Part 1 of 2: Ceramic Engineering and Science Proceedings, Volume 12, Issue 7/8 (ed J. B. Wachtman), John Wiley & Sons, Inc., Hoboken, NJ, USA. doi: 10.1002/9780470313831.ch41
- Published Online: 28 MAR 2008
- Published Print: 1 JAN 1991
Print ISBN: 9780470375099
Online ISBN: 9780470313831
Two types of random, discontinuous graphite fiber-reinforced borosilicate glass composites with identical constituents but different microstructures and stress-strain behavior were produced. The first type shows three distinct stages in the tensile stress-strain curve, with the middle stage exhibiting an increase in longitudinal strain with little increase in stress. In the second type of composite, three distinct stages in the stress-strain curve cannot be identified and the initial Young's modulus is lower. These differences may be attributed to the presence of randomly oriented microcracks in the second type of composite in the as-fabricated conditions, different fiber-matrix interface strengths, and different fiber strengths, produced due to different manufacturing conditions. This provides a good opportunity to study the fracture modes in the two materials and understand how the different stress-strain responses evolve. Hence, monotonic and quasi-static-cyclic (load-unload-reload) tensile and four-point bend tests are conducted on these materials. The microcrack density, Young's modulus, and the Poisson's ratio are obtained systematically as a function of the applied strain in the tensile tests. In the bend tests, evolution of microcracking on the tensile surface and through the thickness has been studied. Transverse microcracking is produced in both types of composites when unidirectional stress is applied. The experimentally determined crack spacing can be used to estimate the interface shear strength of the composite. The estimated low interface shear strength explains why the moduli of these composites are much lower than the rule of mixtures predictions. The fracture mechanisms in the two composites differ significantly. A careful analysis of evolution of crack densities, the final densities, and microscopy provided insight into different fracture modes. The Young's modulus can be used as a damage parameter to estimate the density of processing-induced microcracking. During the flexural tests, evolution of the microcracking on the tensile face is similar to that in the tensile test. But, microcracking which begins on the tensile surface progresses only gradually toward the neutral axis. As a result the stress-strain response under flexural loading is quite different from that under tensile loading.