A systematic study was conducted to examine the influence of fiber surface treatment and sizing on the formation of fiber-matrix interphase and its effects n the mechanical properties of composite laminates. Three material systems having the same Apollo graphite fibers and HC 9106-3 toughened epoxy matrix, but with different fiber surface treatments and sizings were used in this study. The fibers used in the 810A and 820 A systems received 100% and 200% industry standard surface treatments respectively and were sized with Bisphenol-A unreacted epoxy material. The 810 O system was manufactured with 100% surface treated fibers that were sized with pvp (polyvinylpyrrolidone), a thermoplastic material.
The presence of different interphase in these materials was confirmed using a permanganic etching technique. Results indicate that the interphase is discontinuous and made of linear chain polymeric material in the 810 A system. The interphase in the 810 O system has a gradient morphology while the 820 A system does not possess a well defined interphase.
Mechanical test results indicate that the 810 O system significantly greater longitudinal tensile strength and failure strain compared to the 810 A system. The 810 A and 820 A systems have similar longitudinal tensile properties. Transverse tensile test results indicate that the 820 A system has the highest strength while the 810 O system has the lowest strength. The (0,90₃), cross-ply laminates from the three material systems exhibit different damage mechanisms and failure modes under monotonic tensile loading.
Fatigue test results indicate that the 810 O laminates have longer fatigue lives at higher load levels and shorter fatigue lives at lower load levels compared to the 810 A laminates. The 820 A laminates have longer life compared to the other two materials systems, at all three load levels. The 810 O material exhibits greater damage and stiffness reduction than the other two materials. The 810 A and 820 A systems exhibit a brittle stress concentration controlled failure, while the pvp sized 810 O system exhibits a global strain conuolled failure.
A micromechanics model was developed to investigate the role of the interphase on the tensile strength of unidirectional laminates. A new parameter called the ‘efficiency of the interface’, is introduced in the model. A simple scheme that uses the experimentally determined tensile modulus of unidirectional laminates in a concentric cylinders model, is described to estimate the interfacial efficiency. The tensile fatigue performance of cross-ply laminates is predicted using this micromechanics model in a cumulative damage scheme. The predicted fatigue lives and failure modes agree well with experimental results. / Ph. D.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/37216 |
| Date | 25 January 2008 |
| Creators | Subramanian, Suresh |
| Contributors | Engineering Science and Mechanics, Stinchcomb, Wayne W., Kander, Ronald C., Kriz, Ronald D., Loos, Alfred C., Reifsnider, Kenneth L. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation, Text |
| Format | xv, 231 leaves, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 31228851, LD5655.V856_1994.S837.pdf |
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