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A study on the compressive strength of thick carbon fibre-epoxy laminates

Lee, J. and Soutis, C. (2007) A study on the compressive strength of thick carbon fibre-epoxy laminates. Composites Science And Technology, 67 (10). pp. 2015-2026. ISSN 0266-3538

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Abstract

This paper describes an experimental study that examines the effect of specimen size on the axial compressive strength of IM7/8552 carbon fibre/epoxy unidirectional laminates (UD). Laminate gauge length, width and thickness were increased by a scaling factor of 2 and 4 from the baseline specimen size of 10 mm x 10 mm x 2 mm. In all cases, strength decreased as specimen size increased, with a maximum reduction of 45%; no significant changes were observed for the axial modulus. Optical micrographs show that the failure mechanism is fibre microbuckling accompanied by matrix cracking and splitting. The location of failure in most specimens, especially the thicker ones, is where the tabs terminate and the gauge section begins suggesting that the high local stresses developed due to geometric discontinuity contribute to premature failure and hence reduced compressive strength. Two generic quasi-isotropic multi-directional (MD) lay-ups were also tested in compression, one with blocked plies [45n/90n/-45n/0n]s and the other with distributed plies [45/90/-45/0]ns with n=2, 4 and 8. The material used and test fixture was identical to that of the unidirectional specimens with three different gauge sections (30 mm x 30 mm, 60 mm x 60 mm and 120 mm x 120 mm) to establish any size effects. Strength results showed no evidence of a size effect when the specimens are scaled up using distributed plies and compared to the 2 mm thick specimens. All blocked specimens had similar compressive strengths to the sub-laminate ones apart of the 8 mm specimens that showed a 30% reduction due to extensive matrix cracking introduced during the specimen's cutting process. The calculated unidirectional failure stress (of the 0° ply within the multidirectional laminate) of about 1710 MPa is slightly higher than the average measured value of 1570 MPa of the 2 mm thick baseline unidirectional specimen, suggesting that the reduced unidirectional strength observed for the thicker specimens is a testing artefact. It appears that the unidirectional compressive strength in thicker specimens (>2 mm) is found to be limited by the stress concentration developed at the end tabs and manufacturing induced defects.

Item Type: Article
Copyright, Publisher and Additional Information: © 2007 Elsevier Science . This is an author produced version of a paper published in Composites Science and Technology. Uploaded in accordance with the publisher's self-archiving policy.
Keywords: Composite laminates, Thick composites, Compressive strength, manufacturing defects, stress concentration, size effects
Institution: The University of Sheffield
Academic Units: The University of Sheffield > Faculty of Engineering (Sheffield) > Department of Mechanical Engineering (Sheffield) > Aerospace Engineering
Depositing User: Sherpa Assistant
Date Deposited: 04 Sep 2007 08:54
Last Modified: 08 Feb 2013 16:54
Published Version: http://dx.doi.org/10.1016/j.compscitech.2006.12.00...
Status: Published
Publisher: Elsevier Science
Refereed: Yes
Identification Number: 10.1016/j.compscitech.2006.12.001
URI: http://eprints.whiterose.ac.uk/id/eprint/2661

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