NASA/Army Gear Crack Propagation Research

Mechanical Components Branch, NASA Glenn Research Center
Vehicle Technology Center, U.S. Army Research Laboratory
Cleveland, Ohio USA

Project Contact: Dr. David G. Lewicki
U.S. Army Research Laboratory
NASA Glenn Research Center
21000 Brookpark Road, Mail stop 23-3
Cleveland, Ohio, USA 44135
Email: David.G.Lewicki_AT_nasa.gov

BACKGROUND

Reduced weight is a major design goal in aircraft power transmissions. Some gear designs incorporate thin rims to help meet this goal. Thin rims, however, may lead to bending fatigue cracks. These cracks may propagate through a gear tooth or into the gear rim. A crack that propagates through a tooth would probably not be catastrophic, and ample warning of a failure could be possible. On the other hand, a crack that propagates through the rim would be catastrophic. Such cracks could lead to disengagement of a rotor or propeller from an engine, loss of an aircraft, and fatalities. Gears w/cracks

TWO-DIMENSIONAL GEAR CRACK PROPAGATION

In initial two-dimensional studies, analytical and experimental studies were performed to investigate the effect of rim thickness on gear tooth crack propagation The goal was to determine whether cracks grew through gear teeth or through gear rims for various rim thicknesses A finite element based computer program (FRANC2D - FRacture ANalysis Code 2-Dimensional), developed at Cornell University simulated gear tooth crack propagation The analysis used principles of linear elastic fracture mechanics Quarter-point, triangular elements were used at the crack tip to represent the stress singularity The program had an automated crack propagation option in which cracks were grown numerically using an automated re-meshing scheme Crack tip stress intensity factors were estimated to determine crack propagation direction Gears with various backup ratios (rim thickness divided by tooth height) were tested to validate crack path predictions Gear bending fatigue tests were performed in the GRC Spur Gear Fatigue Rig From both predictions and tests, gears with backup ratios of 3.3 and 1.0 produced tooth fractures while a backup ratio of 0.3 produced rim fractures For a backup ratio of 0.5, the experiments produced rim fractures and the predictions produced both rim and tooth fractures, depending on the initial geometry of the crack. Recently, the effect of centrifugal force on crack propagation path was analytically investigated. By including these effects, the correlation of predicted crack propagation paths to previously performed experiments was rather good. Comparison experiment vs predicted crack path in 4 gears

Comparison experiment vs predicted crack path in 4 gears

THREE-DIMENSIONAL GEAR CRACK PROPAGATION

Three-dimensional crack growth simulation was performed on a split-tooth gear design using boundary element modeling and linear elastic fracture mechanics using the FRANC3D computer code. Initial cracks in the fillet of the teeth produced stress intensity factors of greater magnitude (and thus, greater crack growth rates) than those in the root or groove areas of the teeth. Crack growth simulation was performed on a case study to evaluate crack propagation paths. Tooth fracture was predicted from the crack growth simulation for an initial crack in the tooth fillet region. Tooth loads on the uncracked mesh of the split-tooth design were up to five times greater than those on the cracked mesh if equal deflections of the cracked and uncracked teeth were considered. Predicted crack shapes as well as crack propagation life are presented based on calculated stress intensity factors, mixed-mode crack propagation trajectory theories, and fatigue crack growth theories. 3D view of crack propigation path

PUBLICATIONS

	Lewicki, D.G., "Crack Propagation Studies to Determine Benign or Catastrophic Failure Modes for Aerospace Thin-Rim Gears", PhD Dissertation, Case Western Reserve University, May 1995, also NASA TM-107170, Army Research Laboratory Report ARL-TR-971, May 1996. Click here for information on report (pdf file not available).
	Lewicki, D.G., and Ballarini, R., "Gear Crack Propagation Investigations", Proceedings of the Integrated Monitoring, Diagnostics, and Failure Prevention Technology Showcase, Apr. 1996, pp. 693-702, also NASA TM-107147, Army Research Laboratory Report ARL-TR-957. Click here for information on report (pdf file not available).
	Zakrajsek, J.J., and Lewicki, D.G., "Detecting Gear Tooth Fatigue Cracks in Advance of Complete Fracture", Proceedings of the Integrated Monitoring, Diagnostics, and Failure Prevention Technology Showcase, Apr. 1996, pp. 703-714, also NASA TM-107145, Army Research Laboratory Report ARL-TR-970 (pdf format).
	Lewicki, D.G., and Ballarini, R., "Effect of Rim Thickness on Gear Crack Propagation Path", Proceedings of the 7th International Power Transmission and Gearing Conference, Oct. 1996, pp. 53-64, also NASA TM-107229, Army Research Laboratory Report ARL-TR-1110 (pdf format).
	Lewicki, D.G., and Ballarini, R., "Effect of Rim Thickness on Gear Crack Propagation Path", ASME Journal of Mechanical Design, Vol. 119, No. 1, Mar. 1997, pp. 88-95.
	Lewicki, D.G., and Ballarini, R., "Gear Crack Propagation Life Studies", Proceedings of the PPM and Other Propulsion R&T Conference, Vol. II, NASA CP-10193, May 1997, pp. 22-1 - 22-10.
	Lewicki, D.G., and Ballarini, R., "Gear Crack Propagation Investigations", Gear Technology, The Journal of Gear Manufacturing, Randall Publishing Co., Nov.-Dec. 1997, pp. 18-24.
	Lewicki, D.G., and Ballarini, R., "Rim Thickness Effects on Gear Crack Propagation Life", International Journal of Fracture, Vol. 87, Issue 1, Jan. 1998, pp. 59-86.
	Lewicki, D.G., Sane, A.D., Drago, R.J., and Wawrzynek, P.A., "Three-Dimensional Gear Crack Propagation Studies", Presented at the 4th World Congress on Gearing and Power Transmission, Paris, France, Mar. 1999, also NASA TM-1998-208827, Army Research Laboratory ARL-TR-1833 (pdf format)

OTHER SITES

Cornell Fracture Mechanics Group | U.S. Army Research Laboratory Vehicle Tech
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