2. Failure analysis
- FA is a systematic approach of investigation to establish the important causes
of the failure.
Therefore, it is worth to familiarize with
– fundamental causes of failure of mechanical
components,
– general approach to be used for the failure
analysis and prevention.
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3. When to consider a failure
• In general, an engineering component or assembly is considered to have
failed under the following three conditions when the component
is
–Inoperable,
– Operates but doesn’t perform the intended
function
– Operates but safety and reliability is very poor
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4. Failure of mechanical components
» Failure of a mechanical component can occur in many ways
» – elastic deformation is beyond acceptable limit,
» – excessive and unacceptable level of plastic deformation,
» – complete fracture and
» – loss of dimension due to variety of reasons
» Elastic deformation
» Plastic deformation
» Creep
» Fracture
» wear
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5. Fundamental causes of failure
» The failure of an engineering component in actual working conditions can
occur due to very large of factors related with
» – Design,
» – Materials,
» – Manufacturing,
» – Service conditions.
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6. Need to know causes of failure
» To have systematic understanding on various factors that can
lead to failure of mechanical components :
» – Improper design
» – Improper selection of materials
» – Defects and discontinuities in metal itself
» – Improper processing of materials
» – Poor service conditions
» – Poor assembling
» – Poor maintenance
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7. Engineering disasters
» A disaster is referred to as an engineering disaster when it’s
caused by an engineering failure:
» – design flaws or
» – materials failures
» – insufficient knowledge,
» – different underestimations,
» – even carelessness or negligence.
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8. Titanic Ship, 1912
» RMS Titanic was a British passenger ship that
sank after hitting an iceberg on her maiden
voyage from Southampton (UK) to New York
City, in April 1912. The tragedy claimed the
lives of over 1,500 people.
» Several rivets of the 3 million rivets that held
the Titanic together were recently recovered
and tested, and found to be made of low
quality iron, which on impact caused them to
fall apart.
» Another engineering fault was that the 16
watertight compartments that kept the boat
afloat, were not individually sealed, but rather
connected near the ceiling. This enabled the
water to spill from one compartment to
another and sink the boat.
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9. St. Francis Dam flooding (1928)
» The St. Francis Dam was a concrete dam built between
the years 1924 and 1926, to create a water reservoir for
Los Angeles.
» The dam was located in a canyon around 40 miles (64
km) from the city.
» On March 12, 1928, just hours after being inspected by
the Chief Engineer William Mulholland, the dam failed,
sending a massive water wave 120 ft tall, and killing as
many as 600 people in one of the worst American civil
engineering disaster
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10. Tacoma Narrows Bridge collapse (1940)
» The first Tacoma Narrows Bridge was a suspension
bridge in Washington state that opened in 1940 and
collapsed at the same year. At the time of its
construction, the bridge was the world’s third longest
suspension bridge, by main span length.
» The bridge was known to move vertically in windy
conditions, and in November 7, 1940, under 40 mph
(64 km/h) winds it collapsed. The collapse was
caught on video and made an impact on science and
engineering, especially bridge designing till today
» . The cause of failure was aero elastic flutter – a
dynamic instability of an elastic structure.
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11. Kadalundi Train Disaster
» There was a structural failure. The railway bridge was around
140 years old and it definitely needed repair.
» What happened was one of the pier had settled down causing
internal stresses to grow in rails.
» As soon as a train came near it, the track snapped and the train
derailed which eventually landed into river.
» So lesson one, never ever use a structure of high importance
without testing it. Always, keep an eye on it and test it
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13. Rafiganj rail bridge
» Blamed to poor design and construction as well as low
maintenance.
» It is said that the plate girder deteriorated over time and was
losing its strength in fatigue due to vibrations from trains.
» Nobody paid attention to it. Also the bridge was rusty to a
great extent.
» When the train was passing by, the plate girder gave up
leading to this fatal crash.
» Lesson two, make sure your design is checked against
fatigue and steel is coated with anti-corrosion paints.
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14. Industrial Engineering Tools for FA: Failure Mode &
Effect Analysis
» What Is A Failure Mode?
» A Failure Mode is:
» The way in which the component, subassembly, product, input,
or process can fail to perform its intended function
» Things that can go wrong
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15. Benefits of FMEA
» Methodology that facilitates process improvement
» Identifies and eliminates concerns early in the development of a process
or design
» Focuses on prevention
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Approach of FMEA
A structured approach for :
– Identifying the ways in which a product or process can fail
– Estimating risk associated with specific causes
– Prioritizing the actions that should be taken to reduce risk
– Evaluating design validation plan (design FMEA) or current control plan (process FMEA)
16. When to Conduct an FMEA
» Investigation for the process improvement investigation
» Design of new systems, products, and processes
» Modification of existing designs or processes
» Existing designs for new applications
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17. FMEA: A Team Tool
» A team approach is necessary.
» Team should be led by the Process Owner who is the responsible manufacturing
engineer or technical person, or other similar individual familiar with FMEA.
» The following should be considered for team members:
» – Design Engineers
» – Operators
» – Process Engineers
» – Reliability
» – Materials Suppliers
» – Suppliers
» – Customers
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18. FMEA Procedure
» For each process input (start with high value inputs), determine the ways in
which the input can go wrong (failure mode)
» For each failure mode, determine effects – Select a severity level for each effect
» Identify potential causes of each failure mode – Select an occurrence level for
each cause
» List current controls for each cause – Select a detection level for each cause
» Calculate the Risk Priority Number (RPN)
» Develop recommended actions, assign responsible persons, and take actions –
-Give priority to high RPNs – MUST look at severities rated a 10
» Assign the predicted severity, occurrence, and detection levels and compare
RPNs
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19. Severity, Occurrence, and Detection
» Severity – Importance of the effect on requirements
» Occurrence – Frequency with which a given cause occurs and
creates failure modes (obtain from past data if possible)
» Detection – The ability of the current control scheme to detect
(then prevent) a given cause (may be difficult to estimate early
in process operations).
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20. Rating Scales & RPN
» There are a wide variety of scoring “anchors”, both quantitative or
qualitative
» Two types of scales are 1-5 or 1-10 • The 1-5 scale makes it easier for the
teams to decide on scores
» The 1-10 scale may allow for better precision in estimates and a wide
variation in scores (most common)
» Severity – 1 = Not Severe, 10 = Very Severe
» Occurrence – 1 = Not Likely, 10 = Very Likely
» Detection – 1 = Easy to Detect, 10 = Not easy to Detect
» RPN is the product of the severity, occurrence, and detection scores.
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