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