1. Mechanical Property 기계적 성질
Associate Professor Su-Jin Kim
School of Mechanical Engineering
Gyeongsang National University 1

2. Tension test
Tension test (인장 실험) is the most common test for determining the stress-strain characteristics of materials.

3. Stress strain curve
Stress strain curve (응력 변형율 곡선)

4. Elastic (탄성) Stress (응력, MPa) = applied load / original area
Strain (변형율)
= extended l. / original length
Stiffness (강성, GPa)
E: Modulus of elasticity, Young’s modulus
Hook’s law F Ao l0 d

5. Strength(강도)
Yield Strength(항복강도): Stress at which noticeable plastic deformation has occurred.
Tensile, Ultimate Strength(인장,극한강도): Maximum stress on engineering stress-strain curve. Noticeable necking of metal starts. Polymer backbone chains are aligned and about to break.
strain
stress
Yield
strength ep
= 0.002 sy su
Ultimate strength

6. Toughness (인성)
• Toughness(인성,韌性) is the ability of material to absorb energy and plastically deform without fracturing.
• Approximate by the area under the stress-strain curve.
Ductility
strain
stress
ceramic
plastic
metal
Toughness
Brittle fracture: elastic energy Ductile fracture: elastic + plastic energy

7. Ductility (연성)
Ductility(연성, 延性) is ability to deform under tensile stress; to be stretched into a wire.
Ductility is measured by elongation or reduction of area.
Malleability(가단성,可塑性) is ability to deform under compressive stress; to form a thin sheet by hammering or rolling. ㅣf Ao Af ㅣo

8. True stress and true strain
True stress is defined as
where A is the actual area supporting the load
True strain is defined as
A true stress–true strain curve is approximated as
True strain
True stress

9. Effects of temperature (온도)
Increasing temperature:
- Decrease the modulus of elasticity, yield strength, and ultimate tensile strength
- increases ductility and toughness

10. Hardness test (경도)
The most common standardized hardness tests are as follows.
HRC hardness is suggested for steels.

11. Hardness
Diamond
HRC