Professor Stephen D. Downing

Transcription

1 Professor Stephen D. Downing Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign Stephen Downing, All Rights Reserved

2 1. Comparison to Wrought Metals 2. Conceptual Models 3. Stress-Strain Behavior 4. Fatigue Behavior Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 1 of 45

3 Cast iron is a composite material Steel matrix Graphite particles of different shapes Graphite makes cast iron More prone to surface cracking Stiffer in compression than tension New methods need to account for differences Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 2 of 45

4 Nodular Iron Spheroidal graphite Fairly consistent size Behavior similar to steel Gray Iron Graphite flakes Behavior very different from steel Compacted Flake Iron Intermediate behavior Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 3 of 45

5 Major Assumptions: Local stresses and strains control fatigue behavior Accurate determination of K f Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 4 of 45

6 S Nominal stress, Plastic Zone, Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 5 of 45

7 Material Data N curve curve Component Geometry K f Analysis Fatigue Life Estimate Service Loading S, S m Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 6 of 45

8 Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 7 of 45

9 Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 8 of 45

10 Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 9 of 45

11 Strain Amplitude During cyclic deformation, the material deforms on a path described by the cyclic stress strain curve Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 10 of 45

12 Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 11 of 45

13 Hysteresis loop Cyclic Masing behavior Symmetrical p e Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 12 of 45

14 f c ' f 2 E ' f b ' c (2N f ) ) f(2nf 0.01 b ' f E 2N t Reversals, 2N f = 1 Cycle, N f Reversals, 2N f Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 13 of 45

15 A C G I A, I G C H H E E B D F Loading history B F D Stress-strain response Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 14 of 45

16 K T K K K T S K f 2 S e K T e Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 15 of 45

17 Smith Watson Topper max max 2 For R = -1 loading only, max 2 ' f 2N f b leads to a formulation in terms of the standard strain-life curve ' 2 f 2b ' ' bc max (2Nf ) ff (2Nf ) 2 E Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 16 of 45

18 Elastic-plastic behavior in stress concentrations control fatigue life Rainflow counting is used to determine damaging events corresponding to closed elastic-plastic hysteresis loops. Mean stresses are tracked according to input loading sequences Smith-Watson-Topper parameter accounts for mean stress Neuber' Rule is used to determine notch root stress and strains A new model for stress-strain response is needed Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 17 of 45

19 Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 18 of 45

20 Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 19 of 45

21 Much stiffer in compression No linear region Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 20 of 45

22 1. Curvature in the tensile stress/strain curve is not only associated with elastic and plastic deformation of the matrix, but is also due to volume increase in the spaces occupied by the graphite. 2. This volume increase is most pronounced on the specimen surface where graphite flakes, oriented perpendicularly to the load can actually crack or debond from the matrix. 3. Gray iron is stiffer in compression than tension because the spaces occupied by the graphite do not see corresponding decreases in volume. Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 21 of 45

23 2 Fully Plastic 1 P Elastic Elastic - Plastic ys Displacement, Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 22 of 45

24 Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 23 of 45

25 Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 24 of 45

26 Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 25 of 45

27 Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 26 of 45

28 (E S) linear E0 m S /(E ) S linear / (E m ) 0 S R Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 27 of 45

29 E m K 0 1n New stress-strain equation R K 1n Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 28 of 45

30 1n C E m K 0 C C 1n T E m K 0 T T Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 29 of 45

31 F A A m m g g 1A A m g g g Elastic F E 1A E A m g g g Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 30 of 45

32 Dominated by steel matrix Symmetric? Masing Behavior? Material Memory? B B E0 mbb KB 1 n B B 2 E0 mb 2 B 2KB 1n E E, A 0.25 m g g B i B i1 B Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 31 of 45

33 Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 32 of 45

34 Greater stiffness in compression Graphite approaches incompressibility Compressive stress is transferred to to the inherently stiffer matrix G M B if 0 C C 0 if 0 Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 33 of 45

35 Debonded graphite in tension Unloading modulus provides evidence E u Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 34 of 45

36 Eu E0 mu E Aeff 1 E m u u max E 0 0 ( M) T A eff( B) T ( ) B T E( ) 0 M T E m ( ) 0 u M T Easier way to get bulk stress Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 35 of 45

37 A eff( B G) (1A eff) cc Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 36 of 45

38 Q( ) q cc max q (B 2 /B 1)( max ) Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 37 of 45

39 1.82(N ) max a f 0.25 Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 38 of 45

40 Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 39 of 45

41 Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 40 of 45

42 Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 41 of 45

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44 Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 43 of 45

45 John Deere Caterpillar efatigue.com Safe Technology ncode International Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 44 of 45

46 Fatigue Seminar Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 45 of 45

47 Cast Iron Fatigue