Machine design 2 selection of materials, stressing
description
Transcript of Machine design 2 selection of materials, stressing
Machine design 2
selection of materials, stressing
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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.
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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) 10+10 points
Assignments (valve, key joint, coupling) (10+15+15)
Written exam 60 point
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machine elements
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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
material
manufacturing money
DESIGN
optimum - opportunism
time, place6
aims• no failure (surface or volume)
• no big deformations
• enough service time
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loads
• mechanical forces, moments…
• thermal • chemical changing in place/ time
• …static cyclic dynamic
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materials
• metals– ferrous/ non~
• cast iron• steel
– carbon/ alloy
• plastics
• ceramics
• composite
• density• conductivity• …• elasticity• plasticity• ductility• brittleness• toughness (impact)• creep
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loads & stresses
(Mohr, H-M-H)
Stress distribution uniforn/linear
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loss of stability
Buckling
elastic/ plastic
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Tensile test
stress – strain diagrams
Load deformation
ductile – carbon steel rigid – cast iron
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simplifyed material models
elasto-plastic rigid
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testUTS Rm
YS
UTS
YS ReH
eng.
real
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strain hardening
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elastic properties
elastic (Young) modulus: E [MPa]
(Hook’s law = E)
shear modulus: G [MPa] (= G)
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main static properties
elastic limit Rp0.2
yield strength ReH
ultimate tensile strength Rm
maximal strain A
hardness HHB…
Charpy (V-notch) test Kcu
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Influencing factor – (not only) static load
shape factor
αk > 1
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Stress concentration
sudden area changes
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Influencing factor – dynamic load
Tényl= d
dynamic load factor
1.1~ 3Depending on machine type & size (handbooks, measurements, calculations)
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Periodic stresses
fatique
propagating crack – final static break21
90-95% of all failures!
Wöhler- curve
load cycles
fatique limit N > 106
Rm
many factors - stohastic phenomenon
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types of varying load
r min
max
23v m
max
cycle assymetry ratio
cycle mean stress ratio
σv σrFatique limits
constructing Smith- diagram
constructing Smith- diagram
constructing Smith- diagram
constructing Smith- diagram
constructing Smith- diagram
constructing Smith- diagram
constructing Smith- diagram
using Smith-diagram
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Influencing factors – fatique strength
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size
surface roughness
notch sensitivity ηk (0.5-1.0 )
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Stress concentration factor βk
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kk
k
1
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stress conc.
sensitivity
shape
σaver.
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
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That’all, folks!
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