SYMPOSIA PAPER Published: 01 January 1989

A Through-the-Thickness Strength Specimen for Composites


Ongoing research into interlaminar stresses and delamination failure in laminated composites has pointed out the lack of a practical and reliable method for measuring the through-the-thickness strength of composite materials, a necessary parameter in any strength-of-materials approach for prediction of delamination initiation and growth. In this work, a specimen has been developed that allows 13.4-mm-thick laminates to be tested to failure in pure σzz (out-of-plane tension, or “flatwise loading”). This is achieved by bonding aluminum loading bars to a composite specimen tapered through its thickness. This specimen allows direct measurement of the through-the-thickness strength of such laminates. This specimen was used to measure both the transverse and through-the-thickness strength of AS4/3501-6 graphite/epoxy. The transverse strength of 57.6 MPa obtained showed good correlation with that measured with standard coupons, giving confidence in the specimen design. The measured through-the-thickness strength was 43.0 MPa. The through-the-thickness strength appeared to be independent of layup, as unidirectional and cross-ply laminates yielded only negligibly different results. Suggestions are also given as to how the specimen can be improved to facilitate manufacture and testing.

Author Information

Lagace, PA
Massachusetts Institute of Technology, Cambridge, MA
Weems, DB
Boeing Helicopter Company, Philadelphia, PA
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Developed by Committee: D30
Pages: 197–207
DOI: 10.1520/STP10027S
ISBN-EB: 978-0-8031-5071-3
ISBN-13: 978-0-8031-1196-7