1. Lecture 6 MECHANICAL FAILURE
6.1 Fracture
6.2 Fatigue
6.3 Creep

2. 6.1 FRACTURE MECHANISMS Brittle fracture Ductile fracture
Characterize by extensive plastic deformation in the vicinity of an advancing crack.
The process proceeds relatively slow as the crack length is extended.
Brittle fracture
Little or no plastic deformation
Occurs suddenly and catastrophically without any warning
Consequence of spontaneous and rapid crack propagation

3. Ductile vs Brittle Fracture
• Classification:
Very
Ductile
Moderately
Brittle
Fracture
behavior:
Large
Moderate
%AR or %EL
Small
Adapted from Fig. 8.1, Callister 7e.
• Ductile
fracture is usually
desirable!
Ductile:
warning before fracture
Brittle: No warning

4. Example: Failure of a Pipe
• Ductile failure:
--one piece
--large deformation
• Brittle failure:
--many pieces
--small deformation
Figures from V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 4.1(a) and (b), p. 66 John Wiley and Sons, Inc., Used with permission.

5. Moderately Ductile Fracture
• Evolution to failure:
void
nucleation
void growth
and linkage
shearing
at surface
Initial necking s
fracture
• Resulting
fracture
surfaces
(steel)
50 mm
particles
serve as void
nucleation
sites.
From V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig , p. 294, John Wiley and Sons, Inc., (Orig. source: P. Thornton, J. Mater. Sci., Vol. 6, 1971, pp )
100 mm
Fracture surface of tire cord wire loaded in tension. Courtesy of F. Roehrig, CC Technologies, Dublin, OH. Used with permission.
Void = lompang

6. Ductile vs. Brittle Fracture
cup-and-cone fracture
brittle fracture
Adapted from Fig. 8.3, Callister 7e.

7. Brittle Fracture
Direction of crack motion is very nearly perpendicular to the direction of the applied tensile stress and yield a relatively flat fracture surface.

8. Brittle Fracture Surface
A series of V-shaped “chevron” markings
Arrows indicate part at which failure originated
Adapted from Fig. 8.5(a), Callister 7e.

9. Brittle Fracture Surface
Lines or ridges that radiate from the origin of the cracks in a fanlike pattern.

10. Brittle Fracture Surfaces
• Intergranular
(between grains)
• Transgranulat
(within grains)
304 S. Steel (metal)
Reprinted w/permission from "Metals Handbook", 9th ed, Fig. 633, p Copyright 1985, ASM International, Materials Park, OH. (Micrograph by J.R. Keiser and A.R. Olsen, Oak Ridge National Lab.)
316 S. Steel (metal)
Reprinted w/ permission from "Metals Handbook", 9th ed, Fig. 650, p Copyright 1985, ASM International, Materials Park, OH. (Micrograph by D.R. Diercks, Argonne National Lab.)
160 mm
4 mm
Polypropylene
(polymer)
Reprinted w/ permission from R.W. Hertzberg, "Defor-mation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.35(d), p. 303, John Wiley and Sons, Inc., 1996.
Al Oxide
(ceramic)
Reprinted w/ permission from "Failure Analysis of Brittle Materials", p. 78. Copyright 1990, The American Ceramic Society, Westerville, OH. (Micrograph by R.M. Gruver and H. Kirchner.)
3 mm
1 mm
(Orig. source: K. Friedrick, Fracture 1977, Vol. 3, ICF4, Waterloo, CA, 1977, p )

11. Flaws are Stress Concentrators!
Results from crack propagation
Griffith Crack
where t = radius of curvature
so = applied stress
sm = max. stress at crack tip t
Flaw= rekah
Crack=retak
Stress concentrated at crack tip
Adapted from Fig. 8.8(a), Callister 7e.

12. Concentration of Stress at Crack Tip
Adapted from Fig. 8.8(b), Callister 7e.

13. Engineering Fracture Design
• Avoid sharp corners! s
r/h
sharper fillet radius
increasing
w/h
0.5
1.0
1.5
2.0
2.5
Stress Conc. Factor, K t s
max o =
r ,
fillet
radius w h o s
max
Adapted from Fig. 8.2W(c), Callister 6e.
(Fig. 8.2W(c) is from G.H. Neugebauer, Prod. Eng. (NY), Vol. 14, pp )

14. Crack Propagation Cracks propagate due to sharpness of crack tip
A plastic material deforms at the tip, “blunting” the crack.
deformed region
brittle
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
plastic

15. When Does a Crack Propagate?
Crack propagates if above critical stress
where
E = modulus of elasticity
s = specific surface energy
a = one half length of internal crack
Kc = sc/s0
For ductile => replace gs by gs + gp
where gp is plastic deformation energy
i.e., sm > sc
or Kt > Kc

16. Brittle Fracture of Ceramics
Characteristic Fracture behavior in ceramics
Origin point
Initial region (mirror) is flat and smooth
After reaches critical velocity crack branches
mist
hackle
Adapted from Figs & 9.15, Callister & Rethwisch 3e. 16 16

17. Crazing During Fracture of Thermoplastic Polymers
Craze formation prior to cracking
– during crazing, plastic deformation of spherulites – and formation of microvoids and fibrillar bridges
fibrillar bridges
microvoids
crack
aligned chains
Note: crack spreads by breaking C-C bonds
Ductile plastics have large craze zones that absorb large amounts of energy as it spreads
Adapted from Fig. 9.16, Callister & Rethwisch 3e. 17 17 17 17

18. Impact Fracture Testing
The impact test quantifies the response of materials to violent loads by measuring the absorbed energy upon fracture.
(Charpy)
final height
initial height
Adapted from Fig. 8.12(b), Callister 7e. (Fig. 8.12(b) is adapted from H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc. (1965) p. 13.)

19. Impact Test Techniques
Two standardized tests,
Charpy
Izod,
Designed to measure the impact energy, and sometimes also termed as notch toughness.
For both Charpy and Izod, the specimen is in the shape of a bar of square cross section, into which a V-notch is machined

20. Differences Between Charpy & Izod Impact Test
The primary difference between the Charpy and Izod techniques lies in the manner of specimen support.
For Charpy Impact Test, requires the specimen to be placed horizontally on the specimen holder.
For Izod Impact Test, the specimen will be placed vertically on the specimen holder.

21. Temperature • Increasing temperature...
--increases %EL and Kc
• Ductile-to-Brittle Transition Temperature (DBTT)...
FCC metals (e.g., Cu, Ni)
BCC metals (e.g., iron at T < 914°C)
polymers
Impact Energy
Brittle
More Ductile
High strength materials ( s y
> E/150)
Kc=critical stress concentration factor
Adapted from Fig. 8.15, Callister 7e.
Temperature
Ductile-to-brittle
transition temperature

22. Impact energy vs Temperature graph for metal.
or HCP metal
Low strength

23. Function of Impact Test
To determine whether a material experiences a ductile-to-brittle transition with decreasing temperature
The ductile-to-brittle transition is related to the temperature dependence of the measured impact energy absorption.
At higher temperature the Charpy V-notch energy is relatively large,
a ductile mode of fracture.
As the temperature is lowered, the impact energy drops suddenly over a relatively narrow temperature
mode of fracture is brittle.

24. Macroscopic Aspect of the Fractures

25. Impact Fracture Testing
SEM Micrographs for a SAE 1045 tested at:
200oC
0oC
Ductile fracture surface appears fibrous or dull (or of shear character)
Brittle fracture surface have a granular texture (or cleavage character)

26. Effect of carbon content on Impact Test
Increase the strength of the steel
Raise the Charpy V-notch transition of the steel

27. Impact Test of the Titanic Steel
Six panels were ripped off on the starboard side of the forward hull
Artistic reconstruction of the
stern port side middle section

28. Example 3 Temperature (oC) Impact Energy (J) 50 76 40 30 71 20 58 10
Following is tabulated data that were gathered from a series of Charphy impact tests on a commercial low-carbon steel alloy.
Plot the data as impact energy vs temperature
(b) Determine a ductile-to-brittle transition temperature as that temperature corresponding to the average of the maximum & minimum impact energies
(c) Determine a ductile-to-brittle transition temperature as that temperature at which the impact energy is 20 Joule
Temperature (oC)
Impact Energy (J) 50 76 40 30 71 20 58 10 38 23
-10 14
-20 9
-30 5
-40 2

29. Example 3 … cont 1 Solution:
The plot of impact energy versus temperature is shown below
(b) The average of the maximum and minimum impact energies from the data is
As indicated on the plot by the one set of dashed lines, the ductile-to-brittle transition temperature according to this criterion is about 10°C.
(c) Also, as noted on the plot by the other set of dashed lines, the ductile-to-brittle transition temperature for an impact energy of 20 J is about -2°C.

30. 6.2 FATIGUE
• Fatigue = is a form of failure that occur in structures subjected to dynamic and fluctuating stresses.
tension on bottom
compression on top
counter
motor
flex coupling
specimen
bearing
Adapted from Fig. 8.18, Callister 7e. (Fig is from Materials Science in Engineering, 4/E by Carl. A. Keyser, Pearson Education, Inc., Upper Saddle River, NJ.) s
max
min
time m S
• Stress varies with time.
-- key parameters are S, sm, and frequency
• Key points: Fatigue...
--can cause part failure, even though smax < sc.
--causes ~ 90% of mechanical engineering failures.

31. Fatigue Testing Machine
Fatigue testing apparatus for making rotating-bending tests.

32. Cyclic Stresses
Variation of stress with the time that account for fatigue failure:
Reversed stress cycle, in which the stress alternates from max tensile stress to a maximum compressive stress of equal magnitude.

33. Cyclic Stresses
Variation of stress with the time that account for fatigue failure:
Repeated stress cycle, in which max and min stresses are asymmetrical relative to the zero stress level

34. Cyclic Stresses
Variation of stress with the time that account for fatigue failure:
Random stress cycle

35. Cyclic Stresses Parameters to characterize fluctuating cycle:
Mean stress, σm
Range of stress, σr

36. Cyclic Stresses Parameters to characterize fluctuating cycle:
Stress amplitude, σa
Stress ratio, R

37. Fatigue Design Parameters
• Fatigue limit, Sfat:
--no fatigue if S < Sfat
Sfat
case for
steel
(typ.)
N = Cycles to failure 10 3 5 7 9
unsafe
safe
S = stress amplitude
Adapted from Fig. 8.19(a), Callister 7e.
• Sometimes, the
fatigue limit is zero!
Adapted from Fig. 8.19(b), Callister 7e.
case for Al
(typ.)
N = Cycles to failure 10 3 5 7 9
unsafe
safe
S = stress amplitude

38. Fatigue Mechanism • Crack grows incrementally • Failed rotating shaft
typ. 1 to 6
increase in crack length per loading cycle
crack origin
• Failed rotating shaft
--crack grew even though
Kmax < Kc
--crack grows faster as
• Ds increases
• crack gets longer
• loading freq. increases.
Adapted from
Fig. 8.21, Callister 7e. (Fig is from D.J. Wulpi, Understanding How Components Fail, American Society for Metals, Materials Park, OH, 1985.)
K=stress concentration factor

39. Improving Fatigue Life
1. Impose a compressive
surface stress
(to suppress surface
cracks from growing)
--Method 2: Case hardening
C-rich gas
--Method 1: shot peening
put
surface
into
compression
shot
Carburizing or nitriding process
A component is exposed to a carbonaceous or nitrogenous atmosphere at an elevated temperature.
A carbon or nitrogen rich outer surface layer (case) is introduced by atomic diffusion from the gaseous phase.
The improvement of fatigue propertis results from increased hardness within the case.
Small hard particles are projected at high velocities onto the surface to be treated.
The resulting deformation induces compressive stresses to a depth of between one quarter and one-half of the shoot diameter.

40. Improving Fatigue Life
2. Remove stress
concentrators.
Adapted from
Fig. 8.25, Callister 7e.
bad
better
Probability of fatigue failure may be reduced by;
avoiding the structural irregularities
making design modification whereby sudden contour changes leading to sharp corners are eliminated

41. 6.3 CREEP
Creep can be defined as the slow and progressive deformation of a material with time under a constant stress
Creep becomes important only at elevated temperatures.
The creep behavior of a metal is of primary importance in determining its usefulness under the extreme conditions of temperature and stress developed, for example, in gas-turbine and jet-engine components.
Elevated temperature = suhu yang tinggi

42. Creep Tester

43. Creep Sample deformation at a constant stress (s) vs. time s s,et
Primary Creep: slope (creep rate) decreases with time.
Secondary Creep: steady-state creep rate i.e., constant slope.
Tertiary Creep: slope (creep rate) increases with time, i.e. acceleration of rate.
Rupture lifetime
Eng. Design parameter that is considered for long-life applications. E.g. nuclear power plant component
Adapted from
Fig. 8.28, Callister 7e.

44. Creep • Occurs at elevated temperature, T > 0.4 Tm tertiary primary
secondary
elastic
Adapted from Figs. 8.29, Callister 7e.

45. Thank you