November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Sina Mobasher Moghaddam Ph.D. Research Assistant Effect of Mean Stress on Rolling Contact Fatigue

2 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Outlines Butterfly-wing formation in bearing steel – Background and Motivation –Stress Analysis –METL suggested theory –Results comparison and validation Effect of compressive stress on torsion fatigue –Instrument Design –Fatigue life reduction –Failure mode change –FEM simulation

3 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Butterfly Wings Detrimental Effect on RCF [1] Vincent A., Lormand G., Lamagnere P., Gosset L., Girodin D., “From White Etching Areas Formed Around Inclusions To Crack Nucleation In Bearing Steels Under Rolling Contact Fatigue”, ASTM International, 1998 [2] A. Grabulov, R. Petrov, H.W. Zandbergen, 2009, “EBSD investigation of the crack initiation and TEM/FIB analyses of the microstructural changes around the cracks formed under Rolling Contact Fatigue (RCF)” International Journal of Fatigue 32 (2010) 576– 583 Butterflies Observed by Vincent [1](top) and Grabulov [2](Bottom)

4 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Wing Span Debonded Region Coarse Grains nm Crack Fine Grains 5-10 nm ORD Butterfly Wing Characteristics Schematic of a pair of butterfly wings

5 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Stress Analysis Inclusion presence induces stress concentrations in the surrounding matrix When dealing with fatigue problems, it is important to consider stress history

6 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Butterfly Wing Evolution Butterfly wing orientation, direction, and size are consistent with the experimental observations Color spectrum of butterfly wing formation Butterfly formation according to Grabulov[1] Butterfly formation according to METL model prediction [1] A. Grabulov, R. Petrov, H.W. Zandbergen, 2009, “EBSD investigation of the crack initiation and TEM/FIB analyses of the microstructural changes around the cracks formed under Rolling Contact Fatigue (RCF)” International Journal of Fatigue 32 (2010) 576– 583

7 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) 0.7 b 0.8 b 0.38 b 0.42 b 1.1 b 0.4 b 0.6 b 1.1b [1] M.-H.Evans,etal.,”Effect of Hydrogen on Butterfly and White Etching Crack (WEC) Formation under Rolling Contact Fatigue (RCF),Wear(2013), Effect of Depth on Butterfly Growth

8 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) S-N Curve for Butterfly Formation Damage equation is calibrated by curve fitting to Torsion Fatigue data S-N curve for butterfly formation [1] Takemura H, et al., “ Development of New Life Equation for Ball and Roller Bearings”, NSK Motion & Control No. 11 (October 2001)

9 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Effect of Inclusion Size on Butterfly Wing Span [1] Lewis, Tomkins, ” A fracture mechanics interpretation of rolling bearing fatigue“, Proc IMechE Part J: J Engineering Tribology,(2012) For comparison, the wingspan to inclusion diameter ratio is compared The model results lie within the bounds of the experimental results and show the same trend

10 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Schematic showing the reversal of shear in presence of compressive stress along the inclusion- matrix interface Debonding on Inclusion/ Matrix Interface [1] A. Grabulov, R. Petrov, H.W. Zandbergen, 2009, “EBSD investigation of the crack initiation and TEM/FIB analyses of the microstructural changes around the cracks formed under Rolling Contact Fatigue (RCF)” International Journal of Fatigue 32 (2010) 576– 583 Areas of debonding (A & B) and deformation (C) observed by (Grabulov[1]) METL Model prediction (bold, black arches show the debonding areas) To find the debonding regions, stresses should be resolved along the inclusion/ matrix interface Stress transformation formulas in 2D are employed for this purpose

11 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Prediction of Crack Initiation Locations Cracks are commonly observed on top of the upper wing and bottom of the lower wing Mode I loading is suggested as the main factor for crack development in vicinity of the inclusion FEM results show maximum tensile stress during loading history is higher on top of the upper wing and bottom of the lower wing [1] Lewis, Tomkins, ” A fracture mechanics interpretation of rolling bearing fatigue“, Proc IMechE Part J: J Engineering Tribology,(2012) Maximum tensile stress resolved along the butterfly edges

12 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Effect of Compressive Stress on Torsion Fatigue RCF is a shear dominated phenomena There is a large compressive stress present in the contact zone A custom made set of clamps are designed to apply high compressive stress (up to 2.5 GPa) on torsion specimens to better simulate RCF failure Stress history at 0.5b Custom made clamps: a) exploded view b) as they appear after assembly Schematic of Hertzian contact zone in clamp/ specimen interface

13 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Effect of Compressive Stress on Torsion Fatigue Life Application of compressive clamps reduced the torsion fatigue life The reduction is up to in one order of magnitude in high cycle fatigue Steel B Steel C Steel E

14 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Effect of Compressive Stress on Fracture Mode Initiation cracks Propagation cracks As opposed to helical fracture surfaces for pure torsion tests, broken specimens form cup & cone pairs Initiation cracks are due to torsion while multiple cracks grow in the propagation stage Initiation and propagation cracks in sample failed specimens Sample failed specimens at different load levels

15 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) FEM Model Life Prediction and Failure Simulation Without compressive stress With compressive stress A user defined subroutine is developed to apply a Hertzian pressure profile at the center of the specimen FEM results show similar crack patterns to experiments Life prediction is successful implementing the damage mechanics S-N Curve: Experiment vs. FEM

16 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Summary and Future Work Summary –Damage mechanics is used to model butterfly wing formation in bearing steel –The model predicts butterfly shape and size with respect to inclusion diameter and depth successfully –S-N curve for wing development is in corroboration with experiments –Effect of compressive stress on torsion fatigue life and fracture mode is studied Future Work –Explore capabilities of damage mechanics to model DERs, WEBs, and WECs in bearings –Conduct RCF tests to expand a data base for different types of microstructural changes in bearings –Experimental and analytical investigation of effect of steel cleanliness on torsion fatigue and RCF