UNIVERSITAS PERTAHANAN
Mekanika Kekuatan Material
(Material Failure)
PERTEMUAN Ke- 1
PROGRAM STUDI S1 TEKNIK MESIN
31 AGUSTUS 2021
FAKTOR PENYEBAB KEGAGALAN MATERIAL
Faktor Cacat
Product
Faktor
Lingkungan/Proses
Kimia
Faktor
Mekanik/Fisik
Patahan
Sel Satuan
• Atom-atom pada logam
membentuk sel satuan
• Masing-masing sel
satuan memiliki bidang
geser/slip
• Bidang geser, adalah
bidang yang memiliki
kerapatan atom yang
tinggi
Analisa Kegagalan 2011
Kristal Face Center Cubic (FCC)
Konsep Pembebanan
Model Patahan
Intergranulasi
Retak/Cleavage
Patah Geser
Retak Semu
Patahan
Fatik
Fracture mechanisms
• Ductile fracture
– Terdeformasi plastis
• Brittle fracture
– Tidak atau sedikit terdeformasi plastis
– Catastrophic
Ductile vs Brittle Failure
Fracture
behavior:
Very
Ductile
Moderately
Ductile
Brittle
Large
Moderate
Small
• Classification:
%AR or %EL
• Ductile
fracture is usually
desirable!
Ductile:
warning before
fracture
Brittle:
No
warning
Example: Failure of a Pipe
• Ductile failure:
--one piece
--large deformation
• Brittle failure:
--many pieces
--small deformation
Moderately Ductile Failure
• Evolution to failure:
Small cavity formation
Initial necking
Coalescence of
cavities to form
a crack
Crack propagation
Final shear
fracture
Moderately Ductile Failure
• Resulting fracture surfaces (steel)
50
50mm
mm
100 mm
Fracture surface of tire cord
wire loaded in tension.
Particles serve as void
nucleation sites.
Ductile vs. Brittle Failure
brittle fracture
cup-and-cone fracture
Brittle Failure
Arrows indicate pt at which failure originated
Brittle Fracture Surfaces
• Intragranular or Transgranular (within grains);
most brittle materials
Brittle Fracture Surfaces
• Intergranular (between grains)
Flaws are Stress Concentrators!
Results from crack propagation
• Griffith Crack
a
s m  2so 
 t
t
1/ 2



 K t so
where
t = radius of curvature
so = applied stress
sm = stress at crack tip
Kt = Stress concentration factor
Concentration of Stress at Crack Tip
Engineering Fracture Design
• Avoid sharp corners!
s
so
max
Stress Conc. Factor, K t = s
w
smax
r,
fillet
radius
o
2.5
h
2.0
increasing w/h
1.5
1.0
0
0.5
1.0
sharper fillet radius
r/h
Crack Propagation
Cracks propagate due to sharpness of crack tip
• A plastic material deforms at the tip, “blunting” the
crack.
deformed
region
brittle
plastic
Energy balance on the crack
• Elastic strain energy• energy stored in material as it is elastically deformed
• this energy is released when the crack propagates
• creation of new surfaces requires energy
When Does a Crack Propagate?
Crack propagates if above critical stress, σc
 2 E s 
sc  

 a 
1/ 2
i.e., sm > sc
or
where
–
–
–
–
–
Kt > Kc
 K cs 0
K c  Ys c a  Y 2 E s
E = modulus of elasticity
s = specific surface energy
a = one half length of internal crack
Y = Dimensionless parameter
Kc = Fracture Toughness = sc/s0
For ductile => replace s by s + p
where p is plastic deformation energy
Three Mode of Crack Displacement
Mode I
Opening or
Tensile mode
Mode II
Sliding mode
Mode III
Tearing mode
Plain Strain Fracture Toughness
Metals/
Alloys
Graphite/
Ceramics/
Semicond
Polymers
Composites/
fibers
100
C-C (|| fibers)
K Ic (MPa · m0.5 )
70
60
50
40
30
Steels
Ti alloys
Al alloys
Mg alloys
20
2
Al/Al oxide(sf)
Y 2 O 3 /ZrO 2 (p) 4
C/C ( fibers) 1
3
Al oxid/SiC(w)
5
Si nitr/SiC(w)
Al oxid/ZrO 2 (p) 4
Glass/SiC(w) 6
10
7
6
5
4
Diamond
Si carbide
Al oxide
Si nitride
PET
PP
3
PVC
2
1
0.7
0.6
0.5
1
PC
<100>
Si crystal
<111>
Glass -soda
Concrete
PS
Polyester
Glass 6
Plane Strain Fracture Toughness data
Design Against Crack Growth
• Crack growth condition:
K ≥ Kc = Ys a
• Largest, most stressed cracks grow first!
--Result 1: Max. flaw size
dictates design stress.
sdesign
Kc

Y amax
--Result 2: Design stress
dictates max. flaw size.
amax
1  K c

  Ysdesign




amax
s
fracture
no
fracture
fracture
amax
no
fracture
s
2
Loading Rate
• Increased loading rate...
-- increases sy and TS
-- decreases %EL
s
sy
TS
• Why? An increased rate
gives less time for
dislocations to move past
obstacles.
e
Larger loading rate
TS
e
Smaller loading rate
sy
e
Impact Testing
(Charpy)
(Izod)
final height
initial height
Temperature
• Increasing temperature...
--increases %EL and Kc
• Ductile-to-Brittle Transition Temperature (DBTT)...
Impact Energy
FCC metals (e.g., Cu, Ni)
BCC metals (e.g., iron at T < 914°C)
polymers
Brittle
More Ductile
High strength materials (
s y > E/150)
Temperature
Ductile-to-brittle
transition temperature
Fracture Surface of Steel
Influence of C in Iron
Fatigue
• Fatigue = failure under cyclic stress.
specimen
compression on top
bearing
motor
bearing
counter
flex coupling
tension on bottom
• key parameters
-- S, σm, σmax and frequency
smax
s
sm
smin
• Key points:
--can cause part failure, even though smax < sc.
--causes ~ 90% of mechanical engineering failures.
S
time
Fatigue Design Parameters
• Fatigue limit:
--no fatigue if
S < fatigue limit
S = stress amplitude
case for
steel (typ.)
unsafe
Fatigue limit
safe
10
• Sometimes, the
fatigue limit is zero!
3
5
7
10
10
10
N = Cycles to failure
9
S = stress amplitude
case for
Al (typ.)
unsafe
safe
10
3
5
7
10
10
10
N = Cycles to failure
9
Fatigue Mechanism
• Crack grows incrementally
da
m
 K 
dN
typ. 1 to 6
~ s  a
crack origin
increase in crack
length per loading
cycle
• Failed rotating shaft
--crack grew even though
Kmax < Kc
--crack grows faster as
• s increases
• crack gets longer
• loading freq. increases.
Final rupture
Beachmarks and Striations
Beachmarks
• Macroscopic dimension
• Found in component
that interrupted during
crack propagation stage
Single beachmark may contain
thousands of striations
Striations
• Microscopic dimension
• Represent advance
distance of crack front
during single load cycle
Improving Fatigue Life
1. Mean stress
S = stress amplitude
Increasing
near zero or compressive s m
moderate tensile s m
Larger tensile s m
sm
N = Cycles to failure
2. Remove stress
concentrators.
bad
better
bad
better
Improving Fatigue Life
• Surface Treatment (imposing residual compressive stress
within thin film outer surface layer)
--Method 1: shot peening
shot
Improving Fatigue Life
• Surface Treatment (imposing residual compressive stress
within thin film outer surface layer)
--Method 2: carburizing
or nitriding
C-rich gas
Other Fatigue due to Environment
• Thermal fatigue
σ = αEΔT
Normally induced at elevated temperature
• Corrosion fatigue
deleterious influence and produce shorter
fatigue life
Creep
Sample deformation at a constant stress (s) vs. time
s,e
s
0
t
Primary Creep: slope (creep
rate) decreases with time.
(Strain Hardening)
Secondary Creep: steadystate i.e., constant slope.
(Recovery)
Tertiary Creep: slope (creep
rate) increases with time, i.e.
acceleration of rate.
Creep
• Occurs at elevated temperature, T > 0.4 Tm
tertiary
primary
secondary
elastic
Secondary Creep
• Strain rate is constant at a given T, s
-- strain hardening is balanced by recovery
stress exponent (material parameter)
Qc 

e s  K 2s exp  

 RT 
n
• Strain rate
increases
for higher T, s
applied stress
Stress (MPa)
strain rate
material const.
activation energy for creep
(material parameter)
200
100
427°C
538 °C
40
20
10
10 -2
10 -1
Steady state creep rate
649 °C
1
es (%/1000hr)
Creep Failure
• Failure:
• Estimate rupture time
along grain boundaries.
S-590 Iron, T = 800°C, s = 20 ksi
g.b. cavities
applied
stress
20
10
Stress, ksi
100
data for
S-590 Iron
• Time to rupture, tr
T ( 20  logtr )  L
function of
applied stress
time to failure (rupture)
temperature
1
12 16 20 24 28
L(10 3 K-log hr)
24x103 K-log hr
T ( 20  logtr )  L
1073K
Ans: tr = 233 hr
Examples of Material Failure
Tugas Mandiri
1.
2.
3.
4.
Apa yang dimaksut dengan kegagalan material?
Klasifikasikan dan uraikan penyebab kegagalan material?
Uraikan dan jelaskan jenis-jenis patahan?
Jelaskan zona kurva deformasi plastis dan elastis pada
grafik uji tarik?
5. Jelaskan apa yang di maksud dengan necking?
6. Jelaskan perbedaan patahan ductile dan brittle?
7. Jelaskan dan uraikan patahan Transgranular dan
Intergranular?
8. Sebutkan dan jelaskan pengaruh kegagalan material baja?
9. Sebutkan tahapan menganalisa kegagalan material?
10. Jelaskan yang dimaksud dengan tegangan Yield Strength?
Terima Kasih