Determining the Fatigue Life of Composite Aircraft Structures Using Life and Load-Enhancement Factors

摘要

Current regulations require airframes to demonstrate fatigue service life by testing and/or analysis. This requirement is identical for both metals and composites. As composites exhibit higher scatter than metals, extensive scatter analysis was conducted by Northrup/Grumman using several material databases. These data represented several structural details and variables such as laminate lay-up, loading mode, load transfer, specimen geometry, and environment. The U.S. Navy program included only autoclaved 350DGF-cure graphite-epoxy materials, and the analysis was conducted primarily on fiber-dominated failures of laminated construction and did not include sandwich construction or bonded joints. The data analysis of this program developed a load-enhancement factor (LEF) that shortened the fatigue testing time while maintaining equivalent reliability to that of metals. The research performed in this study extended the developed methodology to new material systems and construction techniques. This study included data for materials commonly used in aircraft applications, including adhesives and sandwich construction. Testing consisted of various element-type tests and concentrated on tests that were generic in nature and were representative of various loading modes and construction techniques.