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New Standard Supports Design of Composite Structural Joints

It doesn’t get any better for testers of advanced composites seeking bearing fatigue data for material specifications, design allowables, R&D, and quality assurance.

Structural analysts and test-lab technicians who generate composite bolted joint information for aerospace, marine, automotive, and civil infrastructure can access procedures for generating bearing fatigue data in D 6873, Standard Practice for Bearing Fatigue Response of Polymer Matrix Composite Laminates.

ASTM International released the new standard in April. “The purpose of the standard is to document a set of procedures and guidelines for monitoring the onset and propagation of hole elongation under bearing fatigue loads, for use in the design of composite structural joints,” says Adam J. Sawicki, an associate technical fellow of The Boeing Company, Philadelphia, Pa.

D 6873 supports the study of fatigue damage in a polymer matrix composite-bearing specimen by determining the loss in strength associated with fatigue damage and discontinuing cyclic loading to obtain the static strength with ASTM Test Method D 5961/D 5961M, Standard Test Method for Bearing Response of Polymer Matrix Composite Laminates.

D 6873 modifies ASTM D 5961/D 5961M to determine the fatigue behavior of composite materials subjected to cyclic bearing forces. “Advanced composite structures are generally considered to be more resistant to repetitive (fatigue) loadings than are metallic structures,” says Sawicki, who helped to develop the standard. “However, some composite design configurations have demonstrated some fatigue sensitivity; bolted joints represent one such configuration. If bolted joint fatigue loadings are sufficiently high, the composite material will begin to exhibit permanent bearing deformation/damage at the fastener hole, characterized by an elongation of the hole. While this is an initially benign failure mode, hole elongation can result in an adverse reduction in the joint stiffness, and can ultimately result in a catastrophic rupture of the joint.”

Sawicki chaired a task group of structural analysts, technicians, academics, and representatives of government certification agencies (including the Federal Aviation Administration, NASA, and the military services) who collaborated on D 6873 under ASTM Subcommittee D30.05 on Structural Test Methods in ASTM Committee D30 on Composite Materials. In the standard, the task group references studies conducted by the Naval Air Development Center, The Boeing Company, and NASA.

The primary property resulting from D 6873 is the fatigue life of the test specimen under a specific loading and environmental condition. Replicate tests may be used to obtain a distribution of fatigue life for specific material types, laminate stacking sequences, environments, and loading conditions.

“This standard can be used in a composite structural certification program, and will provide information to set inspection intervals and define permissible component lives for structures subjected to repetitive loads,” Sawicki adds. “It can be used to generate design data at an economical level (coupon testing in lieu of component testing) and thus improve the certification process. Similarly, the structural design ‘product’ can be improved through increased knowledge of its response to fatigue loading.”

For further technical information, contact Adam Sawicki (phone: 610/ 591-2807). Committee D30 meets Oct. 28-30 in Charleston, S.C. For membership or meeting details, contact Jim Olshefsky, director, Committee Services, ASTM International (phone: 610/832-9714). //

Copyright 2003, ASTM