MS&E Colloquium: Dr. Jeffrey O. Bunch, Boeing Integrated Defense Systems
Date: March 13, 2009 from 2:00 pm to 3:00 pm EDT
Location: Columbia University
Morningside Campus
Interschool Lab - 750 CEPSR
Contact: For further information regarding this event, please contact Chad Gurley by sending email to cg2029@columbia.edu .
Info: Click Here to Visit Website.
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Test Demonstrated Damage Tolerance Capability of
Lugs with ForceMate Bushings

 

Jeffrey Bunch

The Boeing Company

 

It is well known that cold expansion processes improve the fatigue life of metallic components, but in order to take "design credit", the life improvement must be validated by tests.  To insure that there is no influence of scale-up factors, the final verification preferably is performed on full scale components.  The F-22 wing is attached to each of six fuselage frames by a pair of upper and lower lugs.  The lugs are titanium and are fitted with ForceMate bushings for increased durability and damage tolerance.  During installation the bushings plastically yield the titanium resulting in residual compressive stresses at the surface of the lug bore which leads to improved fatigue performance. 

 

To demonstrate the damage tolerance capability of this process, full-scale lug test specimens were damage tolerance tested with 0.05 inch corner pre-cracks.  Pre-cracked lugs were tested under fatigue spectrum loading with ForceMate bushings installed, and to provide reference data with which to judge the life improvement, lugs with net-fit bushings were also tested. The test data generated demonstrates the benefit and the capability of the ForceMate bushings to improve fatigue crack growth performance; however, contact fatigue mechanisms (i.e. fretting and galling) ultimately contributed to failure.  The extent of fretting was sufficient to dramatically reduce the fatigue life improvement expected from the ForceMate process. 

 

Additional experimental and analytical work was performed to understand the important design parameters that could be adjusted in order to eliminate fretting failure.  Both analytical finite element modeling results and failure analysis supporting the conclusion that fretting reduced the expected life improvement will be presented. The results presented include finite element prediction and experimentally measured residual stresses.  The role of bushing expansion level in mitigating fretting as a failure mechanism will also be presented.  Once the failure mode was understood, a proposal to increase the expansion of the ForceMate bushing was tested and verified to resolve the fretting fatigue issue. This solution to install a bushing producing higher levels of cold work and interference will be discussed.


Jeffrey Bunch is an Associate Technical Fellow for The Boeing Company where he is currently assigned to the F-22 program as the Senior Lead Engineer for Durability and Damage Tolerance. Dr. Bunch's career focus is the structural integrity of airframe structures, particularly those issues affecting the crack initiation life and crack growth life of metallic structures. He has directed programs to improve structural life prediction methods and test programs to evaluate the effect of engineered residual stresses on fatigue life. In addition to the F-22, he gained valuable experience working on the B-2 program while employed by Northrop Grumman, and also on the AH-64 at Boeing Rotorcraft (then known as McDonnell Douglas Helicopters).

 

Dr. Bunch received his Ph.D. from The University of Connecticut where he studied fatigue crack nucleation and crack growth processes under the direction of Columbia alumnus (and New Your City native) Professor Arthur McEvily. In addition to his professional assignments, Dr. Bunch has led task groups and subcommittees within the American Society for Testing and Materials (ASTM) Committee E08 on Fatigue and Fracture.

Hosted by Prof. I. C. Noyan.