1. Mechanical Concept
MOLDFLOW KOREA
1995.8

2. Mechanical Concept UNIT SI kg m sec N Pa Metric kg mm sec kgf kgf/cm2
mass length time force pressure
SI kg m sec N Pa
Metric kg mm sec kgf kgf/cm2
English lb in sec lbf psi

3. Mechanical Concept Examples for UNIT SI Metric English Force Pressure
0.102 kgf
1 kgf
0.453 kgf
0.225 lbf
2.206 lbf
1 lbf
Force
1 MPa
0.1 MPa
6.9 kPa
10.2 kgf/cm2
1 kgf/cm2
0.07 kgf/cm2
145 psi
14.22 psi
1 psi
Pressure
Pa = N / m psi = lb / in2

4. Mechanical Concept Stress : S = P / A Strain : / L L L L
Stress & Strain
A ( m2 or cm2 ) L L
P ( N or kgf )
Stress : S = P / A
Strain : / L L
Unit : N/m2 = Pa (kgf/cm2 ) Unit : %

5. Mechanical Concept  Strain-Stress Curve (1) E Elastic Plastic
Ductile Fracture
Elastic
Plastic
Yield Point
Tensile Strength
= Stress at Yield 
Break Point
Brittle Fracture
Elastic Limt E
Proportional Limit
Tensile Strength
= Stress at Break

6. Mechanical Concept Stress-Strain Curve (2) E =  Strain Energy ;
Material-A
Material-B  
E = 
Strain Energy ;
Toughness
Toughness
Material A > Material B
Strength & Stiffness
Material A < Material B

7. Mechanical Concept Examples for Tensile Properties Acetal ABS Nylon
Tensile Strength (73F,lb/in2)
Modulus (73F,b/in2X106)
Acetal
ABS
Nylon
Polycarbonate
Polyethylene
Polyester
PMMA
PPO
Polypropylene
Polystyrene
Polysulphone
PVC 10
4 - 8
8 - 12
8 - 10
1 - 6
8 - 11
4 - 6
5 - 11
9 - 10
5 - 9
0.5
0.3
0.4
0.2

8. Mechanical Concept Poisson's Ratio () H / H L / L  Poisson's Ratio

9. Mechanical Concept Examples for Poisson's ratio 0.28 - 0.29 0.33 0.19
탄소강
알루미늄
콩크리트 유리
Rubber
Most Crystalline
& Glassy Polymer
Eng. Plastics
0.33
0.19
0.25

10. Mechanical Concept Principal Stress Normal Stress Principal Stress =
Max. Normal Stress
Shear Stress
Shear Stress is zero
on this Surface
Surface

11. Mechanical Concept yx xy xy Mohr Circle y y y x'y' x x x 2
+ y
x' y
yx y

+yx
xy
x'y' 
x'y'
x' x  x x
xy 2 1 y
-

12. Mechanical Concept xy xy   Principle Stress
prin-1xyxyxy
prin-2xyxyxy
[Example ]  
xy
xy

13. Mechanical Concept 1. Von Mises Stress V.MISES > Y
Criterion for Yield
1. Von Mises Stress
V.MISES

( Shell Element )
V.MISES > Y

14. Mechanical Concept  tresca = 2. Tresca Criterion max
Criterion for Yield
2. Tresca Criterion
+
max
| max min |
 tresca =   2 1
 tresca > Y
-

15. Mechanical Concept    # Shear Modulus Shear Stress l Shear Rate
= Shear force / Area l
= G
# Shear Modulus
G  


16. Mechanical Concept   @ Von-Mises ; 0.6 Tresca ; 0.5 Shear Stress P
Shear Modulus =
X Tensile Modulus 
@ Von-Mises ; 0.6
Tresca ; 0.5 
전단면
위의 결과는 Yield Point 기준

17. Mechanical Concept Bending Stiffness = E I M M Flexural Rigidity
Neutral Axis y
@ E : Tensile
Modulus
@ I : Moment
of Inertia
Compression x
Tension

18. Mechanical Concept P M = P L Smax = M / Z Ymax = PL3/3EI L Moment Ymax
Position A
M = P L
Smax = M / Z
Ymax = PL3/3EI
M : Moment
I : Moment of Inertia
Smax : Max. Stress
Ymax : Max. Deflection
Z : = I / C
( Max. Moment & Stress ) P
Ymax L C

19. Mechanical Concept P P P L L L 1 1/4 1/8
Examples for Moment with Various Boundary condition P P P L L L
M A X I M U M S T R E S S 1
1/4
1/8
M A X I M U M S T R E S S P O S I T I O N
at Center
& Support
at Support
at Center

20. Mechanical Concept I = A y2 dA 100 100 274 36.5 4600 2.17
Moment of Inertia
Neutral
Axis y
I = A y2 dA
Moment
of Inertia
Deflection w t
100
100 r
274
36.5 h
4600
2.17
w=101.6, t=3.18, r=4.76, h=22.22

21. Mechanical Concept
Examples for Moment of Inertia
Thickness = L/4 L L L L L L L
100 wt wt wt wt L L L L L L L L
78 wt wt wt wt

22. Mechanical Concept L / L CV = T2 - T1
Coefficient of Thermal Expansion T1 L T2 L L
Heating
CV =
L / L
T2 - T1
( Unit : mm/mm/degC )
Sthermal = CV X E)T2X ( T2 - T1 )

23. Mechanical Concept Examples for Coefficient of Thermal Expansion
Coefficient of Thermal Expansion (x10-5,in/in/C)
Acetal
ABS
Nylon
Polycarbonate
Polyethylene
Polyester
PMMA
PPO
Polypropylene
Polystyrene
Polysulphone
PVC
8.1
9.0
8.3
6.5
12.0
6.0
7.0
5.2
5.8
5.4
10.0

24. Mechanical Concept Creep ( under Load control ) l t = 0 t =  hour
High Temperature l
APPARENT MODULUS
TIME (HOURS)

25. Mechanical Concept l1 = l2
Stress Relaxation ( Under Displacement Control )
l1
High Temperature
t = 0
l1 = l2
APPARENT MODULUS
l2
TIME (HOURS)
t =  hour

26. Mechanical Concept Examples for Tensile Creep Modulus Acetal ABS Nylon
Applied Load
(lb/in2)
Creep Modulus (73F,b/in2X103)
1 hr
100 hr
1000 hr
Acetal
ABS
Nylon
Polycarbonate
Polyethylene
Polyester
PPO
Polypropylene
Polystyrene
Polysulphone
PVC
1500
1000
3000
4000
390
295
160
345 50
440
430
165
135
350
330
280
255
115
320 30
400 90 20
250
210
100
310 25
380
320 65 10
180

27. Mechanical Concept Fatigue Failure ( S-N Curve ) Load (kg/mm2 ) Cycle
Cyclic Loading
Load (kg/mm2 )
Load
400
300
Time
200
100
103
104
105
106
Cycle

28. Mechanical Concept Examples for Fatigue Data Acrylic Acetal PC PVC ABS
Load ( lb/in2 )
Acrylic
Acetal PC
PVC
ABS
12000
9000
6000
3000
102
103
104
105
106
107
Cycle

29. Mechanical Concept Impact Resistance ( Dynamic Failue )
Velocity =  m/sec
Weight =  kg
Plastics
Impactor
Load (kg) & Energy (kg.m)
Time (msec)
Load
Energy

30. Mechanical Concept Factor of Impact Impact (continued) 1. Stiffness
2. Mass
3. Course & Velocity
Impact Load
Static Load
Force (N)
If TL > (5-6) Tn ,
Loading Type = Static
Tn : Natural Frequency )
tL)1
tL)2
Time (sec)

31. Mechanical Concept Failure Type Impact (continued) Rapid Deformation
1. Elastic Deformation higher than Yield Strength
2. Permanent Deformation
3. Tearing
4. Fracture
Rapid Deformation
1. Fast Loading Rate
2. Rate Sensitivity of Mechanical Properties

32. Coefficient of Friction
Mechanical Concept
Coefficient of Friction
When Max. Angle
with no sliding
Friction Force)
F =  N S W
@  : Corfficient of
Friction (Static)
N : Normal Force
 : Angle of Friction S F N 

33. Mechanical Concept Examples for Coefficient of Friction   s k s k
Steel on Polymer
Polymer on Polymer s  k s  k
Acetal
Nylon
Polycarbonate
Polyethylene (LD)
Polyethylene (HD)
PET
PTFE
PVC
PVDC
0.14
0.37
0.60
0.27
0.18
0.29
0.10
0.45
0.68
0.13
0.34
0.53
0.26
0.10
0.28
0.05
0.40
0.45 -
0.42
0.33
0.12
0.27
0.04
0.50
0.90 -
0.35
0.33
0.11
0.20
0.04
0.40
0.52 
s : Static Friction Coefficient k 
: Kinetic Friction Coefficient

34. P Mechanical Concept - Indentation Test Hardness - Stiffness
- Wear & Scratch
if Hardness-A >> Hardness-B
( ex. Steel >> Polymer )
Abrasive Wear

35. Mechanical Concept Wear V V Abrasive Wear Adhesive Wear
Abrasive Wear
Adhesive Wear
Abrasive Resistance
Acetal, Nylon,
PC, PVC,PPO
ABS, PP
PE, PS