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Failure Analysis

 

Components fail for one of four reasons: abuse, improper design, improper material selection, improper processing. Usually two reasons can be quickly eliminated, which helps to define the optimal testing sequence to properly answer the failure question. A metallurgist and a mechanical engineer represent a strong team to assess mode of failure. Causes of failure include: over stressed, excessive brittleness, corrosion, stress corrosion cracking (SCC), fatigue, design error, material quality, cold temperatures, hot temperatures, quality of water contacting the material, etc. 

Determining cause of failure is routinely used in accidents (assessing product liability, cause of personal injury), insurance claims, and industry to keep America running as efficiently as possible. 

Reitz, W., “Failure Analysis – Theory and Engineering,” NACE, 46, (12), 54-59, 2007.
Reitz, W., “Failure Analysis – Case Histories,” NACE, 47, (1), 72-75, 2008.

Failure due to abuse

Many times components fail because they were used in a manner they were not designed for; have you ever broken the tip of your blade on your pocket knife while trying to pry something?

Failure due to inadequate design

Sometimes components fail because the engineer did not take into account all the forces present or had to down-size the component, which increases the stresses present even though the forces remained the same.

Reitz, W., “Powerline Tower Arms Failure Analysis,” Practical Failure Analysis, 2, (6), 80-84, 2002.

Failure due to poor material selection

The proper materials need to be selected to achieve a desired performance; mousetraps cannot be made entirely from balsa wood.

Failure due to improper processing

Heat treating steel can make it strong, but cooling too fast or too slow can alter the desired performance. Phase Diagrams are the road maps for processing.

Reitz, W., “SO2 Heat Exchanger Failure,” Practical Failure Analysis, 2, (3), 45-49, 2002.

Metallurgy

This is the study of metals. Microstructure reveals the type of metal and processing history.

Reitz, W., “The Methodology of Metallurgy,” Gear Solutions, 24-33, May 2006.

Reitz, W., “Long Term Benefits of Interacting with Metallurgy Consultants,” Soc. of Manufacturing Engineers, Technical Paper TP05PUB235, http://www.sme.org/ProductDetail.aspx?id=47991&terms=tp05pub235 , 30 pp., 11/9/05.

Mechanical characterization

Tensile testing, hardness testing, and fatigue testing are probably the most frequently encountered types of testing. These tests characterize the strength of the material.

Fatigue

Fatigue is the phenomenon leading to fracture under repeated fluctuating stresses having a maximum value less than the tensile strength of the material. Fatigue characterizes the life of the component under alternating stresses; how many times can you bend a paperclip before it breaks? There are three stages: 1) crack initiation, 2) crack propagation, and 3) failure.

Reitz, W., “Laser Shock Peening Solves many Performance Issues, “Surface Engineering, 18, (1), 1-3, 2002.

Stress Corrosion Cracking (SCC)

Three attributes need to be present for SCC to occur: 1) stress present, 2) corrosive environment, and 3) a material susceptible to SCC. The microstructure reveals a root-like structure crack.

Reitz, W., “Failure Analysis of Brass Bolt from Mausoleum,” http://www.reitzmetallurgy.com/downloads/Aug05CaseHistoryReitz.pdf , J. of Failure Analysis & Prevention, 5, (4), 22-27, 2005.

Dezincification

Dezincification, a.k.a., leaching, is the selective removal of an element during corrosion, which generally, leaves a weak, spongy-like material.

Tensile Strength

This is the ratio of maximum load to original cross-sectional area.

Ductility

Ductility characterizes a material’s ability to deform prior to failure. Strength and ductility describe opposite attributes.

Wear

Wear is the undesired removal of material from contacting surfaces by mechanical action. There are numerous types of wear that include: abrasive, erosive, corrosive, 3-body, fretting, adhesive, cavitation, etc.

Corrosion

Corrosion is the deterioration of a metal by chemical or electrochemical reaction with the environment. There are numerous types of corrosion that include: galvanic, uniform, crevice, SCC, concentration cells, etc.

Embrittlement

The material reacts with the environment and loses its ductility.

 

 

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