February 2000

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Tech News

Newly Approved Standard Determines Allowable Strength of Composite Aircraft Structures

NEW Standard D 6484, Test Method for Open-Hole Compressive Strength of Polymer Matrix Composite Laminates, will be available next month. A comprehensive test that derives properties of open hole (notched) compressive strength (OHC), it was created with broad industry involvement by ASTM Subcommittee D30.05 on Structural Test Methods in Committee D-30 on Composite Materials.

“The OHC test is used by every company which designs composite airframe structures: military and commercial, fixed wing and rotorcraft, large transport and fighter, down to corporate and light general aviation aircraft,” says Carl Rousseau, principal engineer, Bell Helicopter Textron, Inc., Ft. Worth, Texas, and subcommittee chair. “The resulting data are used to determine allowable structural compressive strengths. Hundreds of OHC tests are performed per material in order to develop a statistical strength database at all relevant environmental conditions. The test can be run anywhere in a -65 degrees F to +700 degrees F temperature range.”

ASTM D 6484 is designed to produce notched compressive strength data for structural design-allowable material specifications, research and development, and quality assurance. “There are a variety of company test methods for OHC, as well as a SACMA [Suppliers of Advanced Composite Materials Association] OHC test method, SRM 3R-94,” Rousseau explains. “In 1998, SACMA asked ASTM D-30 to attempt to incorporate several of their more widely used test methods, including OHC, into ASTM test methods.”

“Also in 1998, a European OHC test method was unsuccessfully proposed as an ISO new work item. In order to more effectively share and compare OHC data industry-wide, a single, widely-accepted OHC method was needed. Due to SACMA's request, and the need to have a viable national standard to submit if necessary in the ISO arena, a standards development effort was initiated within D30.05 in November of 1998.”

Test method D 6484 determines the OHC strength of multi-directional polymer matrix composite laminates reinforced by high-modulus fibers. The composite material forms are limited to continuous-fiber or discontinuous-fiber (tape and/or fabric) reinforced composites in which the laminate is balanced and symmetric with respect to the test direction. The range of acceptable test laminates and thicknesses are noted in the standard.
Rousseau describes widescale input: “Bell Helicopter, Boeing (Commercial Transport), and SACMA actively contributed to the preparation of the document. Representatives from five different countries provided editorial comments.” Also participating, according to Rousseau, were “OEM users Lockheed/Martin, and Northrop-Grumman; fabricator Wyoming Test Fixtures; contract test lab users Delsen, and SwRI, material suppliers Hexcel, Dupont, and Gougeon Bro.; U.S. government users NASA Glenn, National Institute of Standards and Technology, Lawrence Livermore National Lab, and U.S. Air Force/Air Force Research Lab; airworthiness agencies U.S. Federal Aviation Agency, and Civil Aviation Administration of Israel; research users Aeronautical Research Institute of Sweden, University of Delaware, University of Maryland, MIT, Penn State, Imperial College, Florida Atlantic University, and two others.”

For further technical information, contact Carl Rousseau, Bell Helicopter Textron, Inc., P.O. Box 482, Ft. Worth, TX (817/280-4965; fax: 817/280-4933). Committee D30 meets March 13-15 during ASTM Committee Week, Toronto, Ontario, Canada. For meeting or membership information, contact D30 Staff Manager Jim Olshefsky, ASTM (610/832-9714).