November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Benjamin Leonard Post-Doctoral Research Associate Third Body Modeling Using a Combined Finite Discrete Element Approach

2 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Outline Motivation Objectives Combined Finite-Discrete Element Model Sliding Plates Fretting Contacts Summary and Conclusions

3 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Motivation Third body particles play an important role in many industrial applications –Wear debris –External objects The fretting phenomenon is caused by small scale reciprocating motion leading to failure from fatigue or wear –Due to the small scale motions the third body effect is large in fretting In Situ Photograph of a Fretting Contact Diagram of Third Body Wear

4 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Objectives Develop a numerical model for fretting wear which includes third body effects Study the effects of various parameters –Loading –Surface roughness –Coatings Develop a stress based approach for modeling fretting wear

5 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Modeling of the Third Body The “third body” is composed of loose wear particles or external debris inside a contact In the FDEM the third body is modeled using loose spherical particles –Third body particles interact with first bodies –Third body particles interact with each other Motion of Third Body Particles in the FDEM

6 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Compression of the Third Body Shifting particles cause discontinuities in the force- deflection curve Third body contact stiffness controls its effective elastic modulus Compression of a Mass of Third Body Particles Reaction Force from Third Body

7 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Friction and the Velocity Gradient The velocity gradient between two surfaces depends on their coefficients of friction By varying the coefficient of friction no slip conditions can be achieved on each surface The effect of lower surface coefficient of friction on the velocity gradient for μ of (a) 0.2, (b) 0.3 and (c) 0.4. Velocity Gradient Disposition of Platelets (a) (b) (c)

8 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Velocity Gradient Disposition of Platelets With unlinked particles, the third body behaves as a Newtonian fluid Regions of the third body clump together when platelets interlock –This effect grows larger as platelets become longer –The velocity gradient is not constant with time Effect of Platelet Length on the Velocity Gradient

9 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) The Third Body in a Fretting Contact Third body particles can be introduced into worn fretting contacts Wear particles (individual and platelets) have been placed into the worn slip zones at the edge of the contact Loading of a Fretting Contact Finite Element Domain Variation in Platelet Length

10 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) The Effect of Particle Size in a Fretting Contact The maximum pressure and force carried by a single particle increases with diameter The pressure in the stick zone does not vary significantly from a single particle The effect of particle size on the contact pressure for diameters of (a) 0.1 μm, (b) 0.2 μm, (c) 0.4 μm and (d) 0.6 μm. (a) (b) (c) (d)

11 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Effect of A Small Number of Particles on a Fretting Contact As the number of particles increase, the maximum pressure decreases The outermost (4th) particle does not come into contact due to curvature of the surface The effect of (a) 2, (b) 4, (c) 6, and (d) 8 of particles with diameters of 0.6 μm on contact pressure. (a) (b) (c) (d)

12 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) The Effect of Increasing Numbers of Particles on the Pressure Profile Increasing the number of particles has several effects: –The total force carried by the slip zone increases –The pressure in the slip zone decreases Frictional shear stress in the slip zones is not uniform on each side of the contact 120 particles220 particles320 particles 420 particles

13 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Wear Particles at the Stick Zone-Slip Zone Interface The normal force (red arrows) from the first bodies result in a net lateral force on the third bodies (blue arrow) pushing them away from the edge of the stick zone (green circle) Initial disposition of wear particles in the Hertzian fretting contact (120 particles). The stick zone-slip zone interface in a fretting contact

14 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Effect of Platelet Length on Partial Slip Fretting Contacts Longer platelets lead to formation of a thicker third body mass Thicker third body masses are pushed further from the stick-slip zone interface 2 particles 5 particles10 particles14 particles Particle Location After Loading Frictional Shear Stress Pressure Profile

15 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Wear Particles During Fretting Evolution The wear particles group together due to the pressure and surface profile shape Pressure is not longer uniform in the slip zone 40k80k 120k 160k Subsurface Stress (σy) Pressure Groups of Clustered Wear Particles

16 November 14, 2013 Mechanical Engineering Tribology Laboratory (METL) Summary and Conclusions A model of the third body has been created using the combined finite discrete element method Third body properties can be controlled using size, spring stiffness and platelet length Longer platelets interlock forming thicker third body masses The third body supports load and takes the stress off the edge of the stick zone in fretting contacts Loose third body particles tend to clump together in fretting contacts which may lead to platelet formation