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Machine design 2 selection of materials, stressing 1.

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Presentation on theme: "Machine design 2 selection of materials, stressing 1."— Presentation transcript:

1 Machine design 2 selection of materials, stressing 1

2 aim to introduce the structure of machines, the function, type and load of machine elements and the main methods of designing them Main chapters of this semester: –Fits & tolerances –Basics of stressing –Joints –Drive systems 1. 2

3 Requirements 3 hours weekly (mostly 2 lecture, 1 laboratory) Attendance and participation are important! Engineering is not a spectator sport. (allowed absence four occasions) Course requirements: Tests (tolerances, stressing) points Assignments (valve, key joint, coupling) ( ) Written exam 60 point 3

4 machine elements 4

5 general purpose machine elements static elements joints frames springs (mechanical) drive systems bearings shafts couplings & clutches drives vessel systems pipes valves tanks (technical fluids) 5

6 material manufacturing money DESIGN optimum - opportunism time, place 6

7 aims no failure (surface or volume) no big deformations enough service time 7

8 loads mechanical  forces, moments… thermal  chemical changing in place/ time … static cyclic dynamic 8

9 materials metals –ferrous/ non~ cast iron steel – carbon/ alloy plastics ceramics composite density conductivity … elasticity plasticity ductility brittleness toughness (impact) creep 9

10 loads & stresses (Mohr, H-M-H) Stress distribution uniforn/linear 10

11 loss of stability Buckling elastic/ plastic 11

12 Tensile test stress – strain diagrams Load  deformation ductile – carbon steel rigid – cast iron 12

13 simplifyed material models elasto-plastic rigid 13

14 test UTS R m YS UTS YS R eH eng. real 14

15 strain hardening 15

16 elastic properties elastic (Young) modulus: E [MPa] (Hook’s law  = E  ) shear modulus: G [MPa] (  = G  ) 16

17 main static properties elastic limit R p0.2 yield strengthR eH ultimate tensile strengthR m maximal strainA hardnessH HB … Charpy (V-notch) testK cu 17

18 Influencing factor – (not only) static load shape factor α k > 1 18 Stress concentration

19 sudden area changes 19

20 Influencing factor – dynamic load  Tényl =  d   dynamic load factor 1.1~ 3 Depending on machine type & size (handbooks, measurements, calculations) 20

21 Periodic stresses fatique propagating crack – final static break % of all failures!

22 Wöhler- curve load cycles  fatique limit  N > 10 6 RmRm many factors - stohastic phenomenon 22

23 types of varying load 23 cycle assymetry ratio cycle mean stress ratio σvσv σrσr Fatique limits

24 constructing Smith- diagram

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31 using Smith-diagram 31 no failure

32 Influencing factors – fatique strength 32 size surface roughness

33 notch sensitivity η k ( ) 33

34 Stress concentration factor β k 34 stress conc. sensitivity shape σ aver.

35 Safety factor (SF; n) load, material… all bears uncertainity to be on the safe side use SF n>1 (1.15 ~ 8-10!) importance of element manufacture material testing calculations 35

36 That’all, folks! 36


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