1. Mechanical Failure(파괴)
Fatigue Failure of crankshafts
Associate Professor Su-Jin Kim
School of Mechanical Engineering
Gyeongsang National University

2. Ductile, Brittle Failure (연성, 취성 파괴)
Ductile fracture (Al)
Brittle fracture (Cast Iron)
Plastic deformation warning before fracture
Little or no deformation warning
• Ductile fracture is usually more desirable than brittle fracture!

3. Fracture Toughness (파괴인성)
The ability (of material containing a crack) to resist fracture. Kc (MPa√m)
Fracture toughness is a property which describes the ability of a material containing a crack to resist fracture
Fracture toughness is a quantitative way of expressing a material's resistance to brittle fracture when a crack is present. If a material has much fracture toughness it will probably undergo ductile fracture. Brittle fracture is very characteristic of materials with less fracture toughness.
The subscript Ic denotes mode I crack opening under a normal tensile stress perpendicular to the crack, since the material can be made deep enough to stand shear (mode II) or tear (mode III). a
Crack

4. Fracture Toughness
Graphite/
Ceramics/
Semicond
Metals/
Alloys
Composites/
fibers
Polymers 5 K Ic
(MPa · m
0.5 ) 1
Mg alloys
Al alloys
Ti alloys
Steels
Si crystal
Glass -
soda
Concrete
Si carbide PC 6
0.7 2 4 3 10
<100>
<111>
Diamond
PVC PP
Polyester PS
PET
C-C
(|| fibers)
0.6 7
100
Al oxide
Si nitride
C/C
( fibers)
Al/Al oxide(sf)
Al oxid/SiC(w)
Al oxid/ZrO
(p)
Si nitr/SiC(w)
Glass/SiC(w) Y O
/ZrO
Material: Kc S40C : 54 Al7075: 24 Nylon : 2.2 Glass : 0.7

5. Stress Concentration (응력집중)
Stress at crack, sm
t = radius of curvature
so = applied stress
Kt = stress concentration factor
Crack propagates if crack-tip stress (sm) exceeds a critical stress (sc) t
Stress concentrated at crack tip

6. Fatigue(피로) Failure Fatigue is failure under repeated loading
Crack initiation
Fatigue crack propagation
Catastrophic rupture
1. Crack initiation
2. Crack growth
3. Rapid fracture
Fatigue is the weakening of a material caused by repeatedly applied loads. It is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values that cause such damage may be much less than the strength of the material typically quoted as the ultimate tensile stress limit, or the yield stress limit.

7. Fatigue Life (피로수명)
Fatigue life : the number of stress cycles before failure
S-N Curve : a graph of a stress amplitude S - against the number of cycles to failure N
Effected by material, surface defects, mean stress
No fatigue if stress < Fatigue limit (Endurance limit)
Stress amplitude, S
unsafe
safe
Number of cycles, N
Al2014
S45C
Fatigue limit
300
100
500
MPa
103
106
109
time
Stress σ
amplitude
ASTM defines fatigue life, Nf, as the number of stress cycles of a specified character that a specimen sustains before failure of a specified nature occurs.
S-N Curve is a graph of the magnitude of a cyclic stress (S) against the logarithmic scale of cycles to failure (N).

8. Improving Fatigue Life
Eliminate surface defects (high surface finish)
Impose compressive surface stresses (to suppress surface cracks from growing) : short pining, carburizing
Remove stress concentrators : fillet concave corner
Decrease mean and amplitude stress
C-rich gas
Compression stress C
Fillet

9. Creep (크리프)
Creep is elongation under a static load for a long time though stress is smaller than yield stress σ < σy
Occurs at elevated temperature, T > 0.4 Tm (in K)
time t
strain ε
T3 > T2 > T1
ε/t=
creep rupture