SYMPOSIA PAPER Published: 01 January 1981

Developing Design Allowables for Composite Helicopter Structures


The overall procedures for developing composite material static and fatigue design allowables are presented. The allowables are established at the material lamina level accounting for a range of environmental conditions. Design values are derived from an interpolation method, which was confirmed by screening tests. Static test results are presented as a percentage of mean room temperature dry strength and show that the environmental effects are more pronounced for the matrix modes than for the fiber modes of fracture. Kevlar/epoxy is more sensitive to the environmental design conditions than graphite/epoxy. Fatigue test results are presented in a constant-life diagram as a percentage of room temperature wet static tensile strength. The constant-life fatigue diagram shows that the tension-compression and compression-compression range may be the governing aspect for fatigue design. Statistical evaluations are made and indicate normal distribution, and relatively small sample sizes are sufficient to establish reliable design allowables.

Author Information

Rich, MJ
Sikorsky Aircraft Division of United Technologies Corp., Stratford, Conn.
Maass, DP
Sikorsky Aircraft Division of United Technologies Corp., Stratford, Conn.
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Developed by Committee: D30
Pages: 181–194
DOI: 10.1520/STP29310S
ISBN-EB: 978-0-8031-4800-0
ISBN-13: 978-0-8031-0700-7