1. Mechanical Failure ISSUES TO ADDRESS...
• How do flaws in a material initiate failure?
• How is fracture resistance quantified; how do different
material classes compare?
• How do we estimate the stress to fracture?
• How do loading rate, loading history, and temperature
affect the failure stress?
Ship-cyclic loading
from waves.
Computer chip-cyclic
thermal loading.
Hip implant-cyclic
loading from walking.

2. Failure mechanisms Fracture Fatigue Creep Corrosion Buckling Melting
Thermal shock
Wear

3. 1. Fracture mechanisms Ductile fracture
Fracture is the separation of a body into two or more pieces in response to an imposed stress that is static (i.e., constant or slowly changing with time) and at temperatures that are low relative to the melting temperature of the material.
The applied stress may be tensile, compressive, shear, or torsional;
Ductile fracture
Occurs with plastic deformation
Brittle fracture
Little or no plastic deformation
Suddenly and catastrophic
Any fracture process involves two steps—crack formation and propagation—in response to an imposed stress

4. Ductile vs Brittle Failure
• Classification:
Very
Ductile
Moderately
Brittle
Fracture
behavior:
Large
Moderate
%AR or %EL
Small
Ductility is a function of temperature of the material, the strain rate, and the stress state. May be quantified in term of %AR or %EL
• Ductile
fracture is usually
desirable!
Ductile:
warning before fracture
More strain energy is required
Brittle: No warning

5. Example: Failure of a Pipe
• Ductile failure:
--one piece
--large deformation
• Brittle failure:
--many pieces
--small deformation

6. Ductile vs. Brittle Failure
cup-and-cone fracture in aluminum
brittle fracture in a mild steel
Metal alloys are ductile
Ceramics are notably brittle
Polymers may exhibit both types of fracture.

7. Impact Testing • Impact loading:
-- determine the fracture properties of
materials
-- determine DBTT or not for materials
(Charpy)
final height
initial height
(Izod)

8. Temperature
DBTT – is the material a ductile-to-brittle transition with decreasing temperature and, if so, the range of temperatures over which it occur
• 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)
Temperature
Ductile-to-brittle
transition temperature
For steel

9. Structures constructed from alloys that exhibit this ductile-to-brittle behavior should be used only at temperatures above the transition temperature, to avoid brittle and catastrophic failure. Classic examples of this type of failure occurred, with disastrous consequences, during World War II when a number of welded transport ships, away from combat, suddenly and precipitously split in half. The vessels were constructed of a steel alloy that possessed adequate ductility according to room-temperature tensile tests. The brittle fractures occurred at relatively low ambient temperatures, at about 4C (40F), in the vicinity of the transition temperature of the alloy. Each fracture crack originated at some point of stress concentration, probably a sharp corner or fabrication defect, and then propagated around the entire girth of the ship

10. Design Strategy: Stay Above The DBTT!
• Pre-WWII: The Titanic
• WWII: Liberty ships
• Problem: Used a type of steel with a DBTT ~ Room temp.

11. 2. Fatigue
• Fatigue = failure under dynamics and fluctuating stress(cyclic).
tension on bottom
compression on top
counter
motor
flex coupling
specimen
bearing 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.

12. Mean stress
Range of stress
Stress amplitude
Stress ratio

13. Fatigue Design Parameters S-N curve
• 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
• Sometimes, the
fatigue limit is zero!
case for Al
(typ.)
N = Cycles to failure 10 3 5 7 9
unsafe
safe
S = stress amplitude

14. The important parameters that characterize a material’s fatigue behavior are
fatigue limit: This fatigue limit represents the largest value of fluctuating stress that will not cause failure for essentially an infinite number of cycles(below which fatigue failure will not occur.( most nonferrous alloys (Al,Cu,…)don’t have FL
fatigue life Nf . It is the number of cycles to cause failure at a specified stress level, as taken from the S–N plot

15. Fatigue Mechanism
The process of fatigue failure is characterized by three distinct steps: (1) crack initiation, wherein a small crack forms at some point of high stress concentration;
(2) crack propagation, during which this crack advances incrementally with each stress cycle; and (3) final failure, which occurs very rapidly once the advancing crack has reached a critical size.
Cracks associated with fatigue failure almost always initiate (or nucleate) on the surface of a component at some point of stress concentration. Crack nucleation sites include surface scratches, sharp fillets, keyways, threads, dents, and the like. In addition, cyclic loading can produce microscopic surface discontinuities resulting from dislocation slip steps which may also act as stress raisers, and therefore as crack initiation sites.

16. Factors that affect fatigue life
Mean stress
Surface effects
Environmental effects

17. 1. Mean stress
The dependence of fatigue life on stress amplitude is represented on the S–N plot. increasing the mean stress level leads to a decrease in fatigue life

18. 2. Surface effect
For many common loading situations, the maximum stress within a component or structure occurs at its surface. consequently, most cracks leading to fatigue failure originate at surface positions, specifically at stress amplification sites
Design factor: Any notch or geometrical discontinuity can act as a stress raiser and fatigue crack initiation site; these design features include grooves, holes, keyways, threads and so on.
Surface treatment: 1. improving the surface finish by polishing will enhance fatigue life significantly 2. imposing residual compressive stresses within a thin outer surface layer
Case (layer)hardening: is a technique whereby both surface hardness and fatigue life are enhanced for steel alloys. This is accomplished by a carburizing or nitriding process whereby a component is exposed to a carbonaceous or nitrogenous atmosphere at an elevated temperature.

19. Environmental effects
thermal fatigue : is normally induced at elevated
temperatures by fluctuating thermal stresses; mechanical
stresses from an external source need not be present. The origin of these thermal stresses is the restraint to the dimensional expansion and/or contraction that would normally occur in a structural member with variations in temperature. The magnitude of a thermal stress developed by a temperature change T is dependent on the coefficient of thermal expansion l and the modulus of elasticity E according to
Influence of stress and temperature T on creep behavior.

20. Corrosion fatigue: Failure that occurs by the simultaneous action of a cyclic stress and chemical attack
Small pits may form as a result of chemical reactions between the environment and material, which serve as points of stress concentration, and therefore as crack nucleation sites.
Several approaches to corrosion fatigue prevention
exist:
- apply protective surface coatings,
- select a more corrosion resistant material
- reduce the corrosiveness of the environment.

21. Improving Fatigue Life
1. Impose a compressive
surface stress
(to suppress surface
cracks from growing)
N = Cycles to failure
moderate tensile s m
Larger tensile
S = stress amplitude
near zero or compressive
Increasing m
--Method 1: shot peening
put
surface
into
compression
shot
--Method 2: carburizing
C-rich gas
2. Remove stress
concentrators.
bad
better

22. 3. Creep
The time-dependent permanent deformation that occurs when material are subjected to a constant load or stress; for most materials it is important only at elevated temperatures.
For metals it becomes important only for temperatures greater than about 0.4Tm (Tm absolute melting temperature).

23. 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 i.e., constant slope.
Tertiary Creep: slope (creep rate) increases with time, i.e. acceleration of rate.

24. Creep • Occurs at elevated temperature, T > 0.4 Tm tertiary primary
secondary
elastic

26. 4. Corrosion
Corrosion is breaking down! of essential properties in a material due to reactions with its surroundings. In the most common use of the word, this means a loss of an electron of metals reacting with water and oxygen
Weakening of iron due to oxidation of the iron atoms is a well-known example of electrochemical corrosion. This is commonly known as rust This type of damage usually affects metallic materials, and typically produces oxide(s) and/or salt(s) of the original metal

27. Rust, the most familiar example of corrosion
-- Most structural alloys corrode merely from exposure to moisture in the air, but the process can be strongly affected by exposure to certain substances. Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area to produce general deterioration

28. Resistant to corrosion
Intrinsic chemistry:
The materials most resistant to corrosion are those for which corrosion is thermodynamically unfavorable. Any corrosion products of gold or platinum tend to decompose spontaneously into pure metal, which is why these elements can be found in metallic form on Earth, and is a large part of their intrinsic value
GOLD nuggets do not corrode, even on a geological time scale.

29. Passivation:
Given the right conditions, a thin film of corrosion products can form on a metal's surface spontaneously, acting as a barrier to further oxidation. When this layer stops growing at less than a micrometre thick under the conditions that a material will be used in, the phenomenon is known as passivation
Passivation in air and water is seen in such materials as aluminum, stainless steel, titanium, and silicon

30. surface treatment ( coating ):
Plating, painting, and the application of enamel are the most common anti-corrosion treatments. They work by providing a barrier of corrosion-resistant material between the damaging environment and the (often cheaper, tougher, and/or easier-to-process) structural material
Example: chromium on steel

31. 5. Buckling
In engineering, buckling is a failure mode characterized by a sudden failure of a structural member subjected to high compressive stresses, where the actual compressive stresses at failure are smaller than the ultimate compressive stresses that the material is capable of withstanding. This mode of failure is also described as failure due to elastic instability

32. Buckling in columns
A column under a centric axial load exhibiting the characteristic deformation of buckling
The eccentricity of the axial force results in a bending moment acting on the beam element

33. Euler formula that gives the maximum axial load ( critical load) that column can carry without buckling
F = maximum or critical force (vertical load on column),
E = modulus of elasticity,
I = area moment of inertia,
l = unsupported length of column,
K = column effective length factor, whose value depends on the conditions of end support of the column, as follows.
For both ends pinned (hinged, free to rotate), K = 1.0.
For both ends fixed, K = 0.50.
For one end fixed and the other end pinned, K = 0.70.
For one end fixed and the other end free to move laterally, K = 2.0.

34. 6. Melting
Melting is a process that results in the phase change of a substance from a solid to a liquid. The internal energy of a solid substance is increased (typically by the application of heat) to a specific temperature (called the melting point) at which it changes to the liquid phase. An object that has melted completely is molten
The melting point of a substance is equal to its freezing point

35. Constant temperature Molecular vibrations
When the internal energy of a solid is increased by the application of an external energy source, the molecular vibrations of the substance increases. As these vibrations increase, the substance becomes more and more disordered
Constant temperature
Substances melt at a constant temperature, the melting point. Further increases in temperature (even with continued application of energy) do not occur until the substance is molten

36. 7. Thermal chock
Thermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, and high thermal expansion coefficients
Thermal shock occurs when a thermal gradient causes different parts of an object to expand by different amounts. This differential expansion can be understood in terms of stress or of strain, equivalently. At some point, this stress overcomes the strength of the material, causing a crack to form. If nothing stops this crack from propagating through the material, it will cause the object's structure to fail

37. Thermal shock can be prevented by:
Reducing the thermal gradient seen by the object, by
changing its temperature more slowly
increasing the material's thermal conductivity
Reducing the material's coefficient of thermal expansion
Increasing its strength
Increasing its toughness, by
crack tip blunting, i.e., plasticity or phase transformation
crack deflection

38. 8. wear
Wear is the erosion of material from a solid surface by the action of another solid, or
it is a process in which interaction of surface(s) or bounding face(s) of a solid with the working environment results in the dimensional loss of the solid, with or without loss of material
Wear environment includes loads(types include unidirectional sliding, reciprocating, rolling, impact),speed, temperatures, counter-bodies(solid, liquid, gas), types of contact (single phase or multiphase in which phases involved can be liquid plus solid particles plus gas bubbles)