Forgot your password?
New user?
New Window Opens on the Secret Life of Microbes: Scientists Develop First Microbial Profiles of Ecosystems
CAS announces the release of SciFinder Scholar for Mac OS X
Press Releases
Press Releases
| Check out our New Publishers' Select for Free Articles |
Applied Composite Materials: An International Journal for the Science and Application of Composite Materials (v.6, #4)
Experimental Verification of Rose's Constant K Solution in Bonded Crack Patching by Richard Müller; Robert Fredell; Cornelis Guijt; James Dally (pp. 205-216).
Bonded composite patches have been used for two decades to extend the lives of fatigue-damaged F-16, F-111, B-1B, C-141B, and many other aircraft. One of the key features of the technology is extremely slow crack growth under the bonded repair. Researchers have performed hundreds of experiments on repaired cracked panels, and have reported near-constant crack growth rates for a variety of relatively thin sheet (t < 3 mm or 0.125 inch) configurations and constant amplitude load cases. Constant crack growth rates rely on the existence of a constant crack tip cyclic stress intensity factor, Δ K, underneath the patch.The paper describes the results of experimental stress analyses carried out on cracked aluminum panels with bonded composite patch repairs. Experimental strain gage and photoelastic measurements of K underneath a bonded repair validated Westergaard's analytical stress field description. These measurements, combined with fatigue crack growth studies, have verified that a constant K condition (predicted by L. R. F. Rose) indeed exists for cracks under bonded repairs. For the configuration tested, this held true while the crack size was less than roughly 80% of the repair width. These results are key to providing accurate predictions of crack growth rates and subsequent nondestructive inspection intervals in service.
Keywords: aging aircraft; bonded repair; crack patching; damage tolerance; Rose model; stress intensity factor K
Analysis of Multiple Bonded Patch Interaction Simple Design Guidelines for Multiple Bonded Repairs in Close Proximity by Richard Müller; Robert Fredell (pp. 217-237).
The use of bonded composite patches to extend the life of aging metal airframes continues to grow as air forces and airline companies operate larger fleets beyond their original design lives. The crack-slowing power of bonded composite patches is well-known; often the process can be accomplished at a few percent of the cost of replacing the damaged component. However, care must be taken in the design of repairs to multiple site fatigue and corrosion damage so the repairs in close proximity do not create additional structural problems by inducing high stresses into the area of the repair.The current paper discusses recent modeling efforts to quantify the effects of multiple bonded composite patches in close proximity. It is well-established that a single structural repair attracts load into the locally stiffened area around the patch; further, single-sided repairs induce significant bending in the skin. The paper addresses the situation when two or more structural repairs occur near each other, and illustrates with simple design rules the acceptable practices for this case.
Keywords: fatigue; bonded repair; bending; load attraction; multiple repairs
Development of a Bonded Repair for the F-16 FS479 Bulkhead Vertical Tail Attach Bosses by Alan Kerr; Joe Ruddy (pp. 239-249).
A bonded repair has been developed to repair cracked FS479 bulkheads on the F-16 aircraft. The cracking occurs at the vertical tail to aft fuselage interface in a sharp fillet between the vertical tail attach boss and the top flange of the bulkhead. The repair approach entails machining off the attach bosses along with any existing cracks, and bonding on a replacement 'saddle' with an optimized contour that eliminates the stress concentration caused by the fillet. As a final step the saddle is reinforced with boron doublers to optimize the load transfer. This repair is a cost effective alternative to bulkhead replacement because it can be applied in the field and does not require the removal of the bulkhead from the aircraft. To date, the repair has successfully completed proof of concept static testing over a temperature range of − 65○F to 200○F and durability testing at room temperature to 1.5 lifetimes. This paper details the repair design and summarizes the results obtained in the proof of concept program.
Keywords: bonded repair; fatigue; bulkhead cracking F16; surface; preparation; primary; structure
Repair Substantiation for a Bonded Composite Repair to F111 Lower Wing Skin by A. A. Baker; L. R. F. Rose; K. F. Walker; E. S. Wilson (pp. 251-267).
This paper presents the results obtained to date as part of a comprehensive repair-substantiation program currently in progress for a safety-critical repair to an F-111 lower wing skin. From the viewpoint of demonstrating compliance with certification requirements, this F-111 repair represents the most technically challenging bonded repair undertaken to date by the RAAF. It is particularly significant that the repair was designed and implemented entirely by RAAF personnel, on the basis of design principles and application procedures developed by DSTO. This constitutes an important milestone in technology transfer from DSTO to the RAAF.The repair substantiation involves both detailed finite element (FE) stress analyses and structural testing at three levels, ranging from coupon-size specimens (representative bonded joints) to quasi- full-scale specimens representing a spar-stiffened wing-box structure. The close coupling of analysis and testing is shown to lead to a time- and cost-efficient certification package, with a confidence level comparable with full-scale structural testing.
Keywords: composite; fatigue; repair; stress analysis; structural testing; validation
An Integrated Bonded Repair System: A Reliable Means of Giving New Life to Aging Airframes by R. Fredell; C. Guijt; J. Mazza (pp. 269-277).
The application of bonded composite repairs to restore cracked or corroded metallic airframes has seen rapid expansion recently, following twenty years of technology development. However, the lack of standardized bonded repair methods and certification concepts impedes wider application of the technique. Existing bonded repair technologies are maturing rapidly, but several specialities (design and analysis, surface preparation, installation, inspection, training and certification) must be integrated into a seamless package before civil and military airworthiness authorities will embrace bonded 'crack patching.' This paper outlines efforts by the United States Air Force to develop an integrated system to allow aircraft maintenance engineers to cover the complete life cycle of a bonded repair. The successful construction of this integrated system will speed the widespread acceptance of the technology and help to assure the continued airworthiness of the aging aircraft fleet.
Keywords: aging aircraft; bonded repair; fatigue