University of Kentucky

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Failure Analysis in Material Science

Of all the news being reported in the world, the most televised always deals with events depicting catastrophic disasters. These disasters range anywhere from airline crashes, buildings collapsing, and any other type of failures that lead to the loss of life. Generally, what the general public doesn't understand is what factors cause these terrible things to happen. Scientists and Materials Engineers use a process called failure analysis to answer these mysteries.

Failure is defined as any change in size, shape, or material properties of a structure, machine or machine part. Or in other words, a part becomes unable to satisfactorily perform its intended function.

The failure of most materials can be accredited to one of four specific reasons.

1) Design:

Often in failure analysis it is determined that the engineering design is a major factor leading to catastrophe. Sometimes a material is modified to such an extent that it no longer serves its original designed purpose. The material can be subjected to overly high stresses or overly high stress concentrations very easily if it is altered from its “on paper” design.

2) Manufacturing:

The way certain materials are processed and pieced together can also generate failure modes. For example, weldments are a trouble prone area for metals, as are weld lines or knit lines in molded plastics. High residual stresses can result from metal forming, heat treatment, welding and machining.  

http://www.uky.edu/~tzhai0/web%20pages/home_files/image013.jpg http://www.uky.edu/~tzhai0/web%20pages/home_files/image015.jpg                            www.engr.sjsu.edu                                www.rwfei.com                       

(Left) The Heverill Fire Department aerial ladder failure. A faulty weld and higher than calculated stresses led to this ladder collapse.(Right) Typical weld failure due to overly high stress concentrations.

Similarly, high frozen-in stresses in injection molded plastic parts often contribute to failure. Porosities and voids are common to metal castings and plastic molded parts. These serve as stress raisers and reduce load carrying capability.

3) Service:

Environmental degradation is one of the most important service-related causes of failure for metals and plastics. One major environmental factor is the oxidation of iron, commonly called rust. This type of service related failure causes many problems especially in older components.

Below: an example of oxidized metal. This type of failure is caused due to the fact that oxidization significantly weakens the normal mechanical properties of the material.

http://www.uky.edu/~tzhai0/web%20pages/home_files/image016.jpg

www.metengr.com

Other factors include excessive wear, damage upon impact, overloading, and electrical discharge. Environmental factors were very prevalent in the Space Shuttle Challenger disaster. It was found that unusually cold weather caused an o-ring to not expand to its designed specifications. This led to a liquid fuel leak which caused the explosion that killed all seven astronauts on board.

 http://www.uky.edu/~tzhai0/web%20pages/home_files/image018.jpg 

www.space.com

This photo displays the Challenger Shuttle shortly after lift off. It was determined by failure analysts that a leak in the liquid fueled rockets sparked the explosion.

4) Material:

Other manufacturing and material-related problems that may lead to failure include; adverse thermo-mechanical history, poor microstructure, material defects and contamination from foreign particles.

Some of these types of failure will be more closely analyzed in the case study page of this site.

(This part of the webpage is the result of an independent study by Mr. Jared Lefaivre in Dec. 2005)

(An Introduction)

(Failure Analysis, Forensic Engineering, Expert Witness, Testimony)

1.      Surgical Rod (stainless steel) deposition (3/12/2003)

2.      Refilling stainless steel hose, outside-court settlement (2/13/2004)

3.      Fan shafts (re-designed)

4.      Fan rods (re-designed)

5.      Fan hub

6.      A brass water tap

7.      An engine fastening rod in a racing car

8.      A tank shaft

9.      Telescope ladder