SYMPOSIA PAPER Published: 01 January 1979
STP36932S

Fatigue Effects on Delaminations and Strength Degradation in Graphite/Epoxy Laminates

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The utilization of advanced composites for aerospace structures portends significant economic savings in system life-cycle costs. That utilization depends, in part, on the durability of those materials throughout the broad spectrum of service environments. Concern for the effects of flaws manufactured into the structure has been one of the inhibitors to the widespread use of advanced composites. An investigation into the effects of delamination flaws found in manufacture was conducted. This paper describes the philosophy behind the specimen configuration with its controlled, reproducible, internal delaminations.

The effects of both static and fatigue loading on flawed specimens was the focus of the investigation. Large magnitude static load causes propagation of the delamination concurrent with a bulging out of the delamination. The bulge-out and hysteresis loop are reported for the unconstrained loading, support-interface, and unloading phases. The in-phase growth of the bulged delamination is presented for various levels of load and is shown to cause a residual permanent bulge-out. The magnitude of the bulge-out is a linear function of the applied static load. The retention of the compressive strength of the specimen relates to the initial size of the delamination, test temperature, and moisture level of the material.

The propagation of inherent defects and delaminations resulting from the application of fatigue loading was traced using ultrasonic technology periodically throughout the spectrum loading and thermal cycling. The test spectrum chosen for the test program was a truncated, accelerated version of the B-1 anticipated loads for a 1300 flight life together with a thermal excursion spectrum two orders of magnitude less than the anticipated. The delamination growth pattern under R = -1 loading consists of the extension from the built-in circular shape to an elliptical one spreading most rapidly to the specimen edges.

The baseline for comparison of the effects of flaws was the residual strength of unflawed undelaminated specimens. These results encompassed the residual strengths of specimens subjected to testing at ambient, dry, and moisturized elevated temperature conditions. Strength retention of flawed specimens tested in fatigue, both dry and moisturized, and at room and elevated temperatures exhibited large amounts of scatter. Behavior of the flawed specimens as delamination progressed between one and two lives displayed what appears to be two mechanisms of delamination propagation.

Author Information

Konishi, DY
Rockwell International, Los Angeles, Calif.
Johnston, WR
Advanced Metallic Structures, Air Force Flight Dynamics Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio
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Details
Developed by Committee: D30
Pages: 597–619
DOI: 10.1520/STP36932S
ISBN-EB: 978-0-8031-4743-0
ISBN-13: 978-0-8031-4495-8