4. Material Properties
There a 5 properties typically used to describe a
materials behavior and capabilities:
1. Strength
2. Hardness
3. Ductility
4. Brittleness
5. Toughness
6. Fracture Mechanic
Engineering Materials Chapter 3 Mechanical Properties 4
5. 1.ความแข็งแรง (Strength)
The ability to resist deformation and maintain its shape
คอคอด(Necking)
Engineering Materials Chapter 3 Mechanical Properties 5
10. Stress and Strain
In order to compare materials, we must have measures.
• Average Stress : load per unit Area
F dA F dA A σ
F
A
F : load applied in Newton(N)
A : cross sectional area in mm2
: stress in N/mm2
A
F F
Engineering Materials Chapter 3 Mechanical Properties 10
11. Stress and Strain
l0
F F l f l0
F
l0
F
lf
l0 เป็ นความยาวเร่ิ มต้น,
lf - l0 เป็ นความยาวที่เปลี่ยนไป
Engineering Materials Chapter 3 Mechanical Properties 11
13. Stress and Strain
Engineering strain:
l l0
l0
Geometric quantity (units: 1 = m/m)
l0 = ความยาวเดิมทีกาหนดขนาดความยาวเกจ(mm.)
่
l = ความยาวสุดท้ายเมือผ่านการดึง(mm.)
่
Hooke’s Law:
E is the modulus of elasticity E
Engineering Materials Chapter 3 Mechanical Properties 13
14. • Percent elongation at fracture, % เปนค่าความ
็
สามารถในการยืดได้ของว ัสดุภายใต้แรงดึง คานวณได้จาก
l f l0
% 100
l0
• Percent reduction in area at fracture, %RA เปนการว ัด
็
ี ี ่ ้ั ี้ ึ
ค่าความเหนียวของว ัสดุอกวิธหนึง รวมทงสามารถชถง
ั้ ิ้
คุณภาพของว ัสดุนนๆด้วย กล่าวคือ ถ้าชนงานมีจดบกพร่อง
ุ
่ ิ่
เชน รูโพรง, สงปลอมปน จะทาให้คา %RA น้อยลง
่
A0 A f
% RA 100
A0
Engineering Materials Chapter 3 Mechanical Properties 14
15. Modulus of Elasticity
Metal Forming&
Metal Working
Engineering Design
Engineering Materials Chapter 3 Mechanical Properties 15
16. 0.2% Yield Strength
1. plastic deformation of 0.2%
2. Permanent when the force is
released
3. Material (or part) has a defined
elastic-plastic transition
4. Renewed application of a load
less than the yield stress
creates no further plastic
deformation
Engineering Materials Chapter 3 Mechanical Properties 16
17. Stress and Strain: Tensile Test
• Tensile test measures the resistance of a material to a static of slowly
applied load
• A strain gage measures the amount of stretching of a part
Engineering Materials Chapter 3 Mechanical Properties 17
18. Exercises
1. วัสดุชนิดหนึ่ งมีค่า yield stress เท่ากับ 200 MPa เรา
ต้องให้แรงดึงอย่างน้ อยเท่าใดจึงจะสามารถดึงชิ้นงาน
ชนิดนี้ ที่มีพืนที่หน้ าตัด 100 mm2 ให้เกิดการเสียรูปแบบ
้
ถาวรได้
yield force = yield stress x area
= (200 x 106 N/ m2) x (100 x 10-6 m2)
= 20,000 N
Engineering Materials Chapter 3 Mechanical Properties 18
19. 2. วัสดุชนิดหนึ่ งมีค่าเปอร์เซ็นต์การยืด 10% ถ้าวัสดุนี้เริ่มต้น
ยาว 200 mm เราจะต้องดึงวัสดุนี้ให้มีความยาวเพิ่มอีก
เท่าใดจึงจะขาด
l f l0
% 100
l0
% lo
l f lo
100
10 200
l f lo 20mm
100
Engineering Materials Chapter 3 Mechanical Properties 19
20. Mode of Fracture
Moderately Brittle
Ductility Cast Iron ,Ceramic
Mild steel ,Cu
Au ,Ag ,Al
Engineering Materials Chapter 3 Mechanical Properties 20
21. Ductile fracture
• The material exhibits substantial plastic deformation
in the vicinity of an advancing crack with high energy
absorption before fracture. There is evidence of
appreciable gross deformation at fracture surfaces (e.g.,
twining and tearing).
• It proceeds relatively slowly as the crack
length is extended
•Crack is stable, i.e., resists any further extension
unless there is an increase in applied stress.
•Cup-and-cone facture type.
Engineering Materials Chapter 3 Mechanical Properties 21
22. Ductile fracture (Cons.)
• Ductile fracture preferred due to:
– Ductile fracture gives warning
(due to associated plastic deformation).
This allows preventive measures to be taken.
–More strain energy is required to induce
ductile facture (Ductile materials are tougher).
Engineering Materials Chapter 3 Mechanical Properties 22
23. Ductile fracture (Moderately)
• Evolution to failure:
void void growth shearing
necking fracture
nucleation and linkage at surface
• Resulting 50 mm
50 mm
fracture
surfaces
(steel)
particles 100 mm
serve as void
nucleation
sites.
Engineering Materials Chapter 3 Mechanical Properties 23
24. Brittle fracture
•The material experiences little or no plastic deformation
with low energy absorption.
• Cracks may spread extremely rapidly.
• Crack is unstable, i.e., crack propagation,
once started, will continue spontaneously without
an increase in applied stress.
• Direction of crack propagation is nearly
perpendicular to direction of applied tensile
stress and yields relatively flat fracture surface.
Engineering Materials Chapter 3 Mechanical Properties 24
26. Brittle fracture (Cont.)
• Cleavage: In brittle fracture, crack propagation corresponds
to successive and repeated breaking of atomic bonds
along specific crystallographic planes.
• Cleavage is Trangranular since cracks pass
through the grains.
• Crack surface may have grainy or faceted texture due to
changes in orientation of cleavage planes from one
grain to another.
Engineering Materials Chapter 3 Mechanical Properties 26
28. (3)
(3) Brittle fracture
(2) with no elongation
(1)
(2) Ductile fracture
(1) Ductile fracture with with necking
uniform elongation
Engineering Materials Chapter 3 Mechanical Properties 28
29. Effect of Temperature
• Yield strength, tensile strength, modulus of elasticity, and fracture
point decrease or relocate with higher temperature
• Vibrational energy of individual atoms increases with temperature
– slip becomes easier
– ductility increases
Engineering Materials Chapter 3 Mechanical Properties 29
30. Anisotropies
• Highest strength parallel to
the rolling direction
• Ductility is highest at a 45o
angle to rolling direction.
• Compressive stress
• Shot peening
Engineering Materials Chapter 3 Mechanical Properties 30
31. Microstructure change
• Cold deformation
• Anisotropic properties
depend on direction
• Highest strength is achieved
in axial direction
• Sheet texture is produced
with cold rolling
Engineering Materials Chapter 3 Mechanical Properties 31
33. 2.ความแข็ง (Hardness)
Performance of the material property to resist
indentation ,abrasion and wear.
• This property is tested by subjecting the metal to
an instrument that measures depth of penetration
by a penetrator.
• Common instruments include ;
– Rockwell Hardness test
– Brinell Hardness test
– Vickers Hardness test
– Micro Hardness test
Engineering Materials Chapter 3 Mechanical Properties 33
43. การทดสอบความแข็งแบบจุลภาค นูป(Knoop)
หล ักการทดสอบ ิ้
ค่าความแข็งของชนงานจะ
้
ขึนอยูก ับขนาดของรอยกด
่
Knoop test methods are
defined in ASTM E384
450HK0.5
Where 450 is the calculated hardness
and 0.5 is the test force in kg.
Engineering Materials Chapter 3 Mechanical Properties 43
44. การทดสอบความแข็งแบบจุลภาค นูป(Knoop)
• แรงกด 10-1000 กร ัม
• เวลากด 10 - 15 วินาที.
้ ้
• ว ัดรอยกดเสนทแยงมุมเสนยาว(l)
• กาหนดค่าความแข็งโดยการคานวณค่าจากสูตร
F
HK 14.230 2
l l
450HK0.5
Where 450 is the calculated hardness and 0.5 is the test force in kg.
็ ี ่ ุ่ ่
• เปนวิธการทดสอบทีมงเน้นการว ัดความแข็งของโครงสร้างจุลภาค เชน เกรน
ี
ขอบเกรน รวมทงเฟสต่าง ๆ ได้แก่เฟอร์ไรท์ ซเมนไตต์ คาร์ไบด์ หรือ
ั้
ื่ ่
โครงสร้างของ งานเชอม เชน HAZ , WM หรือโครงสร้างเกรนยาว Columnar
, Fine Grain เปนต้น
็
่ ึ
• ต้องเตรียมผิวหน้างานให้เรียบเหมือนการเตรียมเพือศกษาโครงสร้างจุลภาค
•การให้แรงกดต้องให้อย่างสมาเสมอ ่
Engineering Materials Chapter 3 Mechanical Properties 44
54. หล ักการทดสอบด้วยแรงกระแทก
• Charpy V-Notch Test (continued)
- The potential energy of the pendulum before and after
impact can be calculated form the initial and final location
of the pendulum.
- The potential energy difference is the energy it took to
break the material. absorbed during the impact.
- Charpy test is an impact toughness measurement test
because the energy is absorbed by the specimen very
rapidly.
- Purpose : to evaluate the impact toughness as a function of
temperature
Engineering Materials Chapter 3 Mechanical Properties 54
55. ผลการทดสอบด้วยแรงกระแทก
• Charpy V-Notch Test (continued)
Brittle Ductile Transition Temperature
Ductile Facture
Charpy Toughness(lb·in)
Ductile High impact Energy
Brittle
Behavior Behavior
Brittle Facture
Transition
Temperature
Low impact Energy
Temperature (°F)
Engineering Materials Chapter 3 Mechanical Properties 55
57. ผลการทดสอบด้วยแรงกระแทก
• Charpy V-Notch Test (continued)
- At low temperature, where the material is brittle and
not strong, little energy is required to fracture the material.
- At high temperature, where the material is more ductile
and stronger, greater energy is required to fracture the
material
-The transition temperature is the boundary between brittle
and ductile behavior.
The transition temperature is an extremely important
parameter in selection of construction material.
Engineering Materials Chapter 3 Mechanical Properties 57
59. Facture
1. Brittle Facture
A
D Ductile Facture C
C % Brittle 100
Brittle Facture A
2. Ductile Facture
D Ductile Facture D
C % Ductile 100
Brittle Facture A
A
Engineering Materials Chapter 3 Mechanical Properties 59
60. Transition temperature
• Transition temperature average impact strength of a fully brittle
and fully ductile specimen.
Engineering Materials Chapter 3 Mechanical Properties 60
61. Charpy Test
High Carbon Steel Stainless Steel
Engineering Materials Chapter 3 Mechanical Properties 61
62. Brittle Facture
Liberty ships
Problem: Used a type of steel with a DBTT ~ Room temp.
Engineering Materials Chapter 3 Mechanical Properties 62
65. Fatigue
• A form of failure that occurs in structures
subject to dynamic and fluctuating
stresses.
• Failure occurs at stress levels lower than
yield or tensile stresses for static loads.
• It occurs after a lengthy period of
repeated stress of strain cycling.
• Comprise approximately 90% of metallic
failures.
Engineering Materials Chapter 3 Mechanical Properties 65
66. Fatigue Rotating Beam Test
• The repeated application of stress typically produced by
an oscillating load such as vibration.
• Sources of ship vibration are engine, propeller and waves.
Engineering Materials Chapter 3 Mechanical Properties 66
79. อิทธิพลของแรงเค้นก ับการคืบ
High Temp or High Stress
Strain Medium Temp or
Medium Stress
Low Temp or Low Stress, <0.4Tm, Metals show
primary creep but negligible secondary creep
เวลา
อัตราการเกิดการคืบจะสูงขึนเมื่อปริมาณแรงเค้นเพิ่มขึน
้ ้
Engineering Materials Chapter 3 Mechanical Properties 79