Chapter 4 and 5 Wear Mechanisms

Transcription

1 Chapter 4 and 5 Wear Mechanisms 1

2 Delamination Wear Mechanisms Four mechanisms of delamination wear 1. Plastic deformation of the surface 2. Crack nucleation at the sub-surface due to plastic deformation 3. Crack propagation from these nucleated cracks due to plastic deformation 4. Creation of loose wear sheets 2

3 Plastic Deformation of a Semi-Infinite Elastoplastic Solid Governing equations 1. Equilibrium condition 2. Yield condition 3. Constitutive relationships for elastic and plastic deformation 4. Geometric compatibility 3

4 Plastic Deformation of a Semi-Infinite Elastoplastic Solid S tat io na ry ( ri g i d ) p o q o x U z El as tic- P erfe ctl y Pl as tic 4

5 Plastic Deformation of a Semi-Infinite Elastoplastic Solid Mises Flow Rule (Yield Criterion) 3 3 J ' 2 = 1 (σ ' ijσ ' ij )= 1 σ 2 = k 2 2 i =1 j=1 3 5

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8 Plastic Deformation of a Semi-Infinite Elastoplastic Solid Conversion of the time derivative to gradients: d ' ' ' ' ( σij; εij;w)=u ( σ ij; ε ij;w) dt x 8

9 Plastic Deformation of a Semi-Infinite Elastoplastic Solid Residual Stress Calculation ( σ xx ) = f 1 (z ) r ( σ ) = f 2 ( z ) zz r ( σ ) yy = ν [ f 1 (z ) + f 2 ( z )] r ( σ ) = f 3 ( z ) xz r 9

10 Plastic Deformation of a Semi-Infinite Elastoplastic Solid Residual Stress Calculation -- Equilibrium Equations ( σ xx ) r ( σ xz ) + r x z = 0 ( σ xz ) r ( σ zz ) + r x z = 0 f 3 f 2 = c 1 = c 2 ( σ xx ) r = f ( z ) ( σ yy ) r = ν f ( z ) ( σ zz ) r = 0 ( σ xz ) r = 0 10

11 Plastic Deformation of a Semi-Infinite Elastoplastic Solid Residual Stress Calculation -- Equilibrium Equations 1 2ν ( ε zz ) = (σ zz )' r 2(1 ν)g r ( ) = (σ xz )' r γ xz r G σ xx r σ xx ν ( ) ' r ( ) = ( ) ' r 1 ν σ zz ( σ ) = ( ' ν ' yy σ yy )r ( σ ) r r 1 ν zz 11

12 Crack Nucleation Crack nucleation criteria 1. Energy criterion 2. Strength criterion Because of the energy criterion, there is a critical flaw size for crack nucleation, which is satisfied by most flaws in engineering materials. 12

13 Crack Nucleation Crack nucleation occurs rather readily in multi-phase metals. It takes only 100 to 1,000 cycles. Therefore, it is not the ratecontrolling mechanism in these multi-phase metals. 13

14 Crack Nucleation In single phase metals, crack nucleation at the sub-surface is very difficult. Therefore, in these metals, delamination wear may not be the primary wear mechanism. Wear by plowing of the surface by wear particles may be the primary wear-rate controlling mechanism. 14

15 Crack Nucleation Imaginary sphere A B (a) (b) Deformed sphere of the Imaginary sphere Figure

16 Criteria for Crack Nucleation Energy Criterion and Strength Criterion Energy criterion Strain, e Strength criterion Figure 4.37 d * Particle size, d 16

17 Rigid Cylinder under a General State of Stress σ r r x φ θ z τ max Figure

18 Crack Nucleation Strength criterion ( ) σ kk max σ i Energy criterion E s before E s after A γ 18

19 Crack Nucleation St rain Ene rgy C rit er i o n St re n g th Cr it e ri o n d c P articl e si ze 19

20 Contour of σ rr under a sliding contact (normalized with respect to k) Graphs removed for copyright reasons. See Figures in [Suh 1986]: Suh, N. P. Tribophysics. Englewood Cliffs NJ: Prentice-Hall, ISBN:

21 Depth of Void Nucleation Region as a function of µ and p 0 Graph removed for copyright reasons. See Figure 4.42 in [Suh 1986]. 21

22 Number of passes required for crack nucleation and the depth of crack nucleation at p 0 =4k Graph removed for copyright reasons. See Figure 4.43 in [Suh 1986]. 22

23 Number of passes required for crack nucleation and the depth of crack nucleation at p 0 =6k Graph removed for copyright reasons. See Figure 4.44 in [Suh 1986]. 23

24 Crack Propagation In fracture mechanics, crack propagation is classified in terms of three modes. 1. The load is applied perpendicular to the crack. 2. The load is applied parallel to the crack direction. 3. The load is applied transversely to the crack direction. 24

25 Crack Propagation However, crack propagation in sliding wear is due to a combined loading of compression and shear on the crack - compressive stress on the crack and shear deformation at the crack tip. 25

26 Crack Propagation The rate of crack propagation under sliding wear conditions is very slow, being about 30 microns per cycle of crack-tip sliding. Therefore, the delamination wear of many multi-phase metals is controlled by the crack propagation rate. 26

27 Sliding Wear at Low Speeds Crack propagation rate controls the delamination wear rate. (CTSD) ~ µm 27

28 α F Crack θ Constant maximum shear stress Constant maximum shear stress r Compression Tension 28

29 Variation of plastically deformed zone around crack under a moving asperity (a=10µm, d=0.5 a, µ=0.25, p o =4k=980 MPa) Plots removed for copyright reasons. See Figure 4.48 in [Suh 1986]. 29

30 Shear strain vs. distance from the left crack tip when plastic elements are used. Source: Sin, H-C. Surface Fraction and Crack Propagation in Delamination Wear. Ph.D. Thesis, MIT,

31 Shear strain vs. distance from the right cracktip when plastic elements are used. Source: Sin, H-C. Surface Fraction and Crack Propagation in Delamination Wear. Ph.D. Thesis, MIT,

32 Shear strain as a function of distance from the left tip for different depths of crack location Source: Sin, H-C. Surface Fraction and Crack Propagation in Delamination Wear. Ph.D. Thesis, MIT,

33 Void and crack formation in annealed OFHC copper Figure

34 Subsurface deformation and crack formation in Fe solid solution Figure

35 Typical plot of the crack extension per cycle versus the logarithm of the change in the stress intensity factor 10-2 Final failure K c da/dn (mm/cycle) 10-4 B 1 1 m C da/dn = C( K) m 10-6 Threshold K 0 A Figure 4.47 log K 35

36 Void formation around inclusions and crack propagation from these voids near the surface in annealed Fe-1.3% Mo Photos removed for copyright reasons. See Figure 4.35 in [Suh 1986]. 36

37 Subsurface crack under a moving asperity -θ L c L d R C Figure 5.8 Subsurface crack under a moving asperity 37

38 Model of wearing specimen Slider L λ l C d D Specimen Figure 5.9 Model of wearing specimen and slider 38

39 Worn surface of pure iron (a) sheet formation, (b) shear dimples beneath the wear sheet in (a), dimpled appearance of wear crater Photo removed for copyright reasons. See Figure 5.7 in [Suh 1986]. 39

40 Fretting Wear Graph removed for copyright reasons. See Figure 5.10 in [Suh 1986]. 40

41 Source: Sin, H-C. Surface Fraction and Crack Propagation in Delamination Wear. Ph.D. Thesis, MIT,

42 removed for copyright reasons. Sequence of graphs and photos See Figures in [Suh 1986]. 42