Global Fastener News

Aerospace Composite Failure At Fastened Joints Examined

Jason Sandefur

MEDIA SPOTLIGHT
Editor’s Note: Articles in Media Spotlight are excerpts from publications or broadcasts that show the industry what the public is reading or hearing about fasteners and fastener companies.

Aerospace engineer P.P. Krimbalis is one of four authors of a new study called “Validated Damage Prediction Methodology for the Strength of Fiber Metal Laminates in Pin Bearing.”

The study involves fiber metal laminates or FMLs, an advanced composite material made from alternating layers of aluminum and fiberglass, the Toronto Star reports.

FMLs are used in the Airbus A380. Krimbalis examines how parts made from the material react when fastened or bolted in place.
Krimbalis found that FMLs are superior to aluminum alone because they can withstand many small cracks and not break down.

Fatigue is not an issue; fibreglass won’t corrode and FMLs are fireproof, lightning-proof and impact-resistant. “What you get out of the marriage of materials is the best of both worlds.”

There are other benefits as well. FMLs are much lighter, which makes planes more fuel efficient.

Krimbalis noted that the material isn’t indestructible. His study focused on predicting when FMLs might fail. “I focused on fasteners, the pins, bolts, rivets and the like,” he told the Star, because that is where the material is most vulnerable. “In order to try to prevent failure, we need to know how it breaks.”

Using math equations and computer simulations, along with tests in a tensile machine, Krimbalis looked at how a bolted piece of FML reacts under a heavy load. In the study, which appears in the August edition of the Journal of Composite Materials, finite element (FE) analyses are carried out on bolt bearing testing scenarios based on published data. Both layer-by-layer and smeared property FE models are created to calculate the compressive characteristic dimension (CCD) for three GLARE variants.
A re-definition of conventional CCD is proposed which is governed by the yield strength of aluminum. The new definition also incorporates the two-phase nature of GLARE, as well as the delamination/ buckling phenomenon for pin/bolt bearing, in a bearing failure mode.
“Results of the buckling analysis suggest that the prepreg contribution to bearing strength, in a bearing failure mode, is at best negligible and joint collapse is governed by the yielding and delamination of the aluminum layers,” according to an abstract from the Journal. “Calculation of a CCD, based on the new yield strength definition, produced consistent values amongst all GLARE variants considered in the layer-by-layer analysis suggesting that the CCD is a property of the material alone.”

Krimbalis hopes the model he developed will help engineers builder safer and stronger FMLs. “Now we have a model that can accurately predict it,” Krimbalis said. �2008 FastenerNews.com