The effect of laminate thickness on the fracture behavior of laminated graphite/epoxy (T300/5208) composites has been studied. The predominantly experimental research program included the study of the [0/±45/90]<sub>ns</sub> and [0/90]<sub>ns</sub> laminates with thicknesses of 8, 32, 64, 96 and 120 plies and the [0/±45]<sub>ns</sub> laminate with thicknesses of 6, 30, 60, 90 and 120 plies. The research concentrated on the measurement of fracture toughness utilizing the center-cracked tension, compact tension and three point bend specimen configurations. Fracture toughness was computed using the stress intensity factor results of a finite element stress analysis of each specimen geometry which treated the composite as homogeneous but anisotropic. The development of subcritical damage at the crack tip was studied nondestructively using enhanced x-ray radiography and destructively using the laminate deply technique.
The test results showed fracture toughness to be a function of laminate thickness. The fracture toughness of the [0/±45/90]<sub>ns</sub> and [0/90]<sub>ns</sub> laminates decreased with increasing thickness and asymptotically approached lower bound values of 30 ksi√in (1043 MP a√mm) and 25 ksi√in (869 √mm) respectively. The fracture toughness of the [0/±45/90]<sub>ns</sub> laminate was independent of crack length at 8 and 120 plies. The fracture of the thin and thick [0/±45/90]<sub>ns</sub> laminates were self-similar, macroscopically. However, the [0/90]<sub>2s</sub> laminate (8 plies) exhibited fracture toughness values that increased sharply as a function of increasing crack size. This was attributed to large axial splits which formed perpendicular to the crack tip in the 0° plies and extended in the direction of applied load. The fracture toughness of the [0/90]<sub>ns</sub> laminate was independent of crack length at 90 plies. The axial splits in the 0° plies of the thicker specimens were confined to the surface and the final fracture was self-similar. For both the [0/±45/90]<sub>ns</sub> and [0/90]<sub>ns</sub> laminates, the center-cracked tension, three-point bend and compact tension specimens gave comparable results.
In contrast to the other two laminates, the fracture toughness of the [0/±45]<sub>ns</sub> laminate increased sharply with increasing thickness but reached an upper plateau value of 40 ksi√in (1390 MP a√mm) at 30 plies. Fracture toughness was independent of crack size at 6 and 90 plies. The 6 ply specimens failed by an apparent uncoupling mechanism where the two interior -45° plies delaminated from the adjacent +45° plies and failed by matrix splitting parallel to the fibers. The surface 0° plies failed by broken fibers along a +45° line in association with matrix splitting parallel to the fibers in the +45° plies. The thick [0/±45]<sub>ns</sub> laminates exhibited a surface boundary layer in which 45° fiber breaks and splits were evident along with delaminations. However, the interior of the specimens failed in a self-similar manner with fibers in the 0° plies breaking along a line collinear with the starter notch. The compact tension and three-point bend specimens defined a constant fracture toughness at about 15% below the plateau exhibited by the center-cracked tension specimens.
The general toughness parameter model, a strain criterion developed by C. C. Poe, Jr. of NASA Langley, was the only candidate thin laminate failure criterion that was successful in using thin laminate parameters to predict the fracture of thick laminates. The "universal” general toughness parameter value of 1.5 √mm quite closely predicted the fracture of the thick laminates. / Ph. D.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/101291 |
| Date | January 1983 |
| Creators | Harris, Charles E. |
| Contributors | Engineering Mechanics |
| Publisher | Virginia Polytechnic Institute and State University |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation, Text |
| Format | xi, 192 pages, 3 unnumbered leaves, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 10653535 |
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