material failure analysis failure and root cause analysis
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MATERIAL FAILURE ANALYSISFailure and Root Cause Analysis
Types of Analysis
Two types of analysis:1. Failure analysis
how the material failed
2. Root cause analysis how it could have been prevented
Failure Analysis
Determining:1. How did the material fail?2. Why did it fail?3. Who or which party is responsible?
Root Cause Analysis
Emphasizes managerial role in failures How managerial techniques can be improved Focus on prevention, not failure determination
Large plants Construction sites Manufacturing facilities
Steps in Failure Analysis
Assess the situation Photography and sample handling Determine product and material
standards Macroscopic examination Microscopic inspection
Steps in Failure Analysis
Material characterization Mechanical testing Hardness testing Crack testing Bend testing Strain testing
Determine how failure occurred from initial to final state
Hardness tester
Public domain image from: https://upload.wikimedia.org/wikipedia/commons/thumb/2/2f/Vickers-tester.png/373px-Vickers-tester.png
Baseball Madness
Suspect 3
Suspect 2Suspect 1
Baseball Madness Principle:
Glass is brittle and will fracture if struck by a medium to high velocity projectile.
Fact: A glass window was broken.
Fact: A baseball was found in the vicinity of the broken window.
Fact: Suspect 3 has a baseball bat.
Conclusion: Suspect 3 hit a baseball that struck the window and caused
the glass to fracture.
Failure Analysis
Just like pathologists determine cause of death, mechanical or materials engineers must determine cause of failure
Failure Analysis
Components can fail because of: Material selection Design Produce usage Method of production Defect within product
Failure modes of a book shelf
A. UnloadedB. Books in middle
causing sag in woodC. Shelf made of
thermoplastic materialD. Support place close
togetherE. Manufactured crack in
middle of shelfF. Shelf infested with
wood worms
Causes of Demise
Congenital Manufacturing defect that leads to failure of material
Disease Corrosion, degradation, wear, etc. that leads to failure
Consequence of previous operation Failure stemming from a previous repair or adjustment
Trauma Sudden failure caused by gross mechanical overstress
(e.g., collision)
Example
Car (black) pulls out of driveway and is side swiped by on coming car (white). A manufacturing fault in the white car’s brakes leads to progressive weakening of the clutch plate over a period of service.
Example
Disease: Degradation of clutch brake
due to surface defect in the in forging initiates a fatigue crack that runs to completion after many thousands or millions of stress cycles.
Clutch plate of a truck
Example
Consequence of previous operation: Failure stemming from an
earlier repair to fix the clutch plate resulted in welding that altered the microstructure. This introduced residual stress and cracks in heat affected zones that lead to eventual failure by fatigue.
Clutch plate of a truck
Answering the three main questions
How did it fail? Clutch plate failure
Why did it fail? Old repair lead to crack formation leading to
fatigue crack failure. Who or which party was responsible?
Owner of the white car.
What caused the component to fail?
Failure Mechanisms
Mechanisms of Failure
Manufacturing defects Flaw introduced during manufacturing process
Material overload Inappropriate consumer use of the material Loading material beyond specifications
Normal wear through use Fatigue, corrosion, creep, etc.
Manufacturing Defects
Congenital faults in a product Result in (1) failure the first time it comes
under load or (2) progressive weakening and eventual failure by fatigue
Material Overload
Materials have limits Products should be used according to
material and engineering specifications Materials cannot be used universally
Material Overload
The strength of a material is measured by laboratory tests
ASTM (American Society for Testing and Materials) and BSI (British Standards Institution) provide standards covering safety, performance, and reliability of products
Most common measurement is tensile strength; other strength data includes compression tests and shear tests
Sample of ASTM Standards
Example: Hardness testing
Example: ASTM E9
Hardness tester
1. Start with old clutch plate and brand new clutch plate2. Test hardness using ASTM standard protocol and
hardness tester3. Determine if hardness is same as regular clutch plate4. Theorize why the hardness varies between failed plate
and new plate
Old clutch plate New clutch plate
Material Overload
Types of materials exhibit different behaviors depending on the way their atoms and molecules are bonded and how they stack together to form crystal structures
copper diamond
Material Overload
Material Overload
Normal Wear
Ultimately, products will fail with time Wear, fatigue, corrosion, creep
Normal Wear
Wear Damaged rendered through normal period of use
Fatigue Repeated cycles of loading and unloading
Corrosion Wasting away of metal surfaces due to chemical reactions
among positively charged ions Reduces material’s capacity to support loads
Creep Gradual deformation under steady tensile load Eventually causes creep rupture failure
Summary
Engineers determine to source of material failure Deductive reasoning as well extensive laboratory
techniques are utilized Manufacturers often introduce source of failure
into product Materials will ultimately fail over period of use of
the product from manufacturing flaws, material overload, or wear
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