Volume 12, Issue 3, 2003

Bending Failure CharacteriSation of Laminated Beams with Braided Fabric Reinforcement

Zheng-Ming Huang1*, K. Fujihara and S. Ramakrishna2

1Department of Engineering Mechanics, Tongji University, 1239 Siping Road, Shanghai, P. R. China
2Biomaterials Laboratory, Division of Bioengineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260

*(Author to whom correspondence should be addressed)
e-mail: huangzm@mail.tongji.edu.cn

(Received 4/02; accepted 4/03)

ABSTRACT

An experimental procedure has been performed in this work to characterise failure behaviour of braided fabric reinforced composite laminates subjected to flexural load. The material system used was carbon/epoxy. The laminated beams were made using eight layers of diamond braid reinforcement. Two braiding angles, i.e., 50 and 150, were applied. Six beams with laminate arrangements of [5/5/5/5/5/5/5/5], [15/15/15/15/15/15/15/15], [5/5/15/ 15/15/15/5/5], [15/15/5/5/5/5/15/15], [5/5/15/15/5/5/15/15], and [15/15/5/5/15/15/5/5] were loaded under 4-point bending to their ultimate failure, where the braiding angle is measured with respect to the beam axis. Bending stiffnesses, ultimate bending strengths, and load-deflection curves of the tested beams were reported. It was found that all the bending failures of the laminated beams were initiated from their top surface inwards, as a consequence of an excessive compressive stress in the beam cross-section. Further evidence in failure mechanism was recognised through a matrix combustion and fabric peeling out technique. It was shown that after the bending test the only un-failed lamina in some of the laminates was its bottom surface ply. The evidence clearly indicates that the neutral plane in a laminated beam keeps shifting once a lamina in the laminate has failed. The failed lamina in the laminate can hardly sustain any additional load. Thus the present experiment confirms that a total stiffness discount should be adopted in the layer-by-layer progressive failure analysis of a laminated composite. Using this discount procedure, a simulation method is highlighted in the paper. The predicted load-deflection curves up to the third-ply failure agreed reasonably with the measured ones.

Keywords: Laminated composite, braided fabric composite, bending test, load-deflection, progressive failure, failure mechanism, stiffness discount

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