define the term failure. types of failures and mechanisms, failure modes. failure mode and criticality analysis. Failure mechanisms are physical, chemical, or other processes which lead or have led to failure.

1. ‫الرحيم‬ ‫الرحمن‬ ‫هللا‬ ‫بسم‬
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LEC3: Failure Theory

2. System Degradation and Failure
 Definition 3.1
 Failure mechanisms are physical, chemical, or other processes
which lead or have led to failure.
 Failure mechanisms depend on;
 the type of component (electrical, mechanical, pneumatic, etc.),
 material (wood, metal, composite, plastic, glass, etc.),
 manufacturing processes (annealing, casting, machining, etc.) and
the operating environment – load (electrical, mechanical, thermal,
etc.),
 chemical properties (pH level of gas or fluid in a pipe network), and
so on.

3. Effect of Materials and Manufacturing
 Materials differ in mechanical, physical, chemical, and fabrication
properties.
 Mechanical properties :- Elastic deformation, plastic deformation,
and fracture.
 Physical properties such as densities and thermal properties are
important in many engineering applications.
 Chemical properties describe the interaction between materials
and various substances in the environment, such as oxygen, sulfur,
and chlorine, to form other substances, leading, for example, to
corrosion that degrades the performance of the material.
 Fabrication properties determine the response of materials to
various fabrication methods.

4. Effect of Materials and Manufacturing
 Manufacturing processes may introduce some
flaws in a component such as voids, pores, and
small cracks that may be the starting points of
some failure mechanism.

5. Stress and Strength of a Component
 Definition 3.2
 Stress is the intensity of the internally distributed forces or components of
forces that resist a change in the volume or shape of a material that is or has
been subjected to external forces. Stress can be either direct (tension or
compression) or shear.
 Definition 3.3
 Strength is the property of a metal part that resists the stresses imposed upon
the part.

6. Stress and Strength of a Component
 There are many forms of stress: –
 Mechanical (force on component),
 Electrical (voltage across or current flowing
through),
 Hydraulic (pressure – fluid, gas, etc.), and
 Thermal (heat).
 Many of these stresses may be acting
simultaneously on a particular component.

7. Stress and Strength of a Component
 atomic structures of a components can fails due
to: –
 Stress causes the atomic bonds to separate or
 The atomic bonds are attacked and removed.
The stress on the component and the strength of a
new component are both variables having
distributions defined by a probability density
functions.

8. Stress and Strength of a Component
 The two possible scenarios are as follows: –
the component can withstand the
load and is 100% reliable
there is a non‐zero probability of
the component failing, reliability
is less than one.

9. Classification of Failure Mechanisms
 Two categories:
 (i) overstress mechanisms: Failure occurs when the
stress exceeds the strength
 (ii) wear‐out mechanisms: the stress causes damage
that usually accumulates irreversibly. The cumulative
damage does not cause any performance degradation
as long as it is below the endurance limit.

10. Examples

12. Overstress Failure Mechanisms
1. Large Elastic Deformation: occurs in slender items. Being elastic,
the deformations are reversible and therefore do not cause any
permanent change in the material. Failures occur in structures such
as long antennas and solar panels, where large deformations can
trigger unstable vibration modes and thereby affect the
performance.
2. Yield plastic strain causes a permanent deformation . Associated
with ductile and are crystalline materials. Permanent deformation
may or may not constitute a failure, depending on the context.
3. Buckling Deformation in the direction of the compressive load can
suddenly change at a critical point, resulting in an instantaneous and
catastrophic deformation in a direction perpendicular to the loading
direction.

13. Overstress Failure Mechanisms
4. Brittle Fracture In brittle materials (such as glass and ceramics),
high stress concentration can occur at local microscopic flaws under
overstress. Cause brittle fracture or cracking resulting in failure.
5. Ductile Fracture In ductile fracture, the failure is due to sudden
propagation of a pre‐existing crack in the material under external
stress.
Example 3.1 Gas Pipeline2
A natural gas transmission pipeline ruptured in San Bruno, CA in 2010.
Eight people were killed, many injured, and 38 homes were destroyed.
The examination of the seam by optical and electron microscopy
concluded that the rupture was initiated at a pre‐existing crack in the
weld that had grown first by ductile fracture and then by fatigue.

14. Wear‐Out Failure Mechanisms
 Fatigue Crack Initiation and Propagation When a component is subjected
to a cyclic stress, failure occurs at stresses below the ultimate tensile
strength and is due to the accumulation of damage. Examples are fatigue
cracking in airframes (the cyclic loading being the result of landings and
take‐offs), rotating shafts, reciprocating components, and large structures
such as buildings and bridges.
 Corrosion and Stress Corrosion Cracking Corrosion is the process of chemical or
electrochemical degradation of materials :
 The uniform reactions occurring at the metal–electrolyte interface are uniform over
the surface of the item.
 Galvanic corrosion occurs when two different metals are in contact. In this case, one
acts as a cathode and the other as an anode
 Pitting corrosion occurs at localized areas and results in the formation of pits. The
corrosive conditions inside the pit accelerate the corrosion process.

15.  Example 3.2 Chemical Plant3
 A Methyl Methacrylate (MMA) process plant was shut down after
four months of operation. Several types of stainless steel are
utilized in process plants for their high resistance to corrosion, good
weldability, and superior material properties at high temperatures.
For this plant, failure analysis revealed that the failure was due to
SCC caused by the chloride that remained in the pipe. Corrosion
pitting occurred on the inside surface of the pipe. The stress
corrosion cracking started from the pits and grew out through the
thickness of the pipe. Concentrated chloride was found in the
deposit stuck to the pipe in addition to the pre‐process MMA
materials. Many work‐hardened grains were observed in the area of
the SCC, providing evidence of high residual stress due to welding,
which could have served as the driving force for SCC.

16. Wear
 Wear is the erosion of material resulting from the sliding
motion of two surfaces under the action of a contact force.
Erosion can be due to physical and chemical interactions
between the two surfaces.

17.  Adhesive wear: The molecular attractions existing between two
relatively moving surfaces create adhesion between the
touching asperities. If the adhesive strength is greater than the
internal cohesive strength of the material, there is a tendency
to create a wear particle after several cycles of contact.
 •• Abrasive wear: When a hard material is sliding against a soft
material.
 •• Surface‐fatigue wear.
 •• Corrosive (chemical) wear: Sliding surfaces may wear by
chemically reacting with the partner surface or the
environment or both. The oxide layers resulting from reactions
with the environment may have a protective role unless the
thickness tends to grow during the cycle contact process.