By Talbott Associates, Inc.
Metallurgy & Failure Analysis Expert Witness
Metallurgy & Failure Analysis Expert Witness
Failure analysis determines the root cause of failure. The information presented in the failed component allows a company to reduce the possibility of re-occurrence. This paper will discuss failure analysis in general terms.
Failure analysis is a broad discipline that includes metallurgy and mechanical engineering. There are numerous steps in completing a failure analysis study and they should be performed in the proper sequence.
This paper introduces the above concepts.
Failure analysis provides insight into failure mechanisms. This paper discusses the use of a few of the tools, but they are all inter-related. There are several references the reader can obtain to become familiar with all the possible tools available. (1, 2)
The overall condition of the component is quite important, beyond just looking at the fracture surface. It is important to determine the exposure of the entire component to the environment, which includes temperature, acid, tensile or compressive stresses, impact forces, corrosion, and wear. (3)
The initial view of the fractured surface provides many clues that will aid the failure analyst in determining the responsible failure mechanism. The presence of oxide, a smooth surface, ratchet marks and beach marks provide clues as to the possible failure mechanism. (1, 4)
This examination technique is essential for determining processing history, temperature during use, and presence of internal defects. (5)
Scanning electron microscopy (SEM)
This inspection tool is able to provide details of the fractured surface! The depth of field makes this technique invaluable to the failure analyst.
STEPS IN CONDUCTING A FAILURE ANALYSIS
Collect and preserve failed components - preservation of the fractured surfaces is paramount in performing a comprehensive examination. Do not even remove oxides until it is necessary.
Obtain pertinent background information regarding operation and environment
Conduct chemistry analysis - it is important to know which alloy is being examined. Then handbook values can be used for comparison in determining expected level of performance.
Develop hypotheses for possible failure mechanisms
Perform loading analysis - determine order of magnitude of forces on the component.
Conduct mechanical tests - tensile, compressive, hardness, microhardness, and impact tests can readily be performed from representative material.
TYPICAL FAILURE MECHANISMS
There are numerous failure mechanisms that might occur. There are many publications that cover these mechanisms and the reader is referred to them for additional detail.(1, 2)
Fatigue - indications include ratchet marks and beach marks that are visible at 1X. SEM inspection can show striations at high magnification. Fatigue is influenced by notches, scratches, and transition areas where diameter changes occur.
Tensile overload - signs of plastic deformation or fractured planes perpendicular to the applied tensile forces are indicative of this type failure.
Torsion - shafts usually fail via this mechanism. Shear occurs and is somewhat opposite in appearance from tensile overloads; brittle fractures are at an angle, while ductile fractures are across the diameter of the shaft.
Bending - beams usually encounter this type failure. One outer surface experiences tensile overload, while the opposite outer surface experiences compressive overload.
Corrosion - this type failure has too many aspects to be covered in this paper. There are numerous references to consult. (6, 7)
Wear - just like corrosion, there are too many aspects to cover in this paper. Please refer to the following references. Common types of wear that exist are abrasive, adhesive, fretting, galling. (8)
Mausoleum bolt – SCC
Fracture surfaces contain many features that help the failure analyst determine the root cause of failure.(3, 9) The bolts were exposed to ammonium via fertilizer (10) If one of the three criteria for SCC had been removed ( presence of stress, corrosive environment, or susceptible material), then these failures could have been prevented.
Weld parameters – HAZ cracking
A manufacturing company was having difficulty with cracking in their welds on stainless steel. Knowledge of microstructures is essential to understanding the relationship between processing, alloy, performance, and structure. (9)
1. Preserving failed components for future evaluation is paramount in conducting a successful failure analysis.
2. Developing hypotheses and using the proper tools validates or eliminates the possible failure mechanisms.
3. Visual, microscopic and SEM results along with chemistry and mechanical data allow the metallurgist to formulate a reasonable failure scenario.
4. The metallurgist can make recommendations regarding design, material selection, material processing, or presence of abuse to minimize future failures.
5. Manufacturing companies can schedule preventive maintenance, insurance companies can pay valid claims, and lawyers can be justifiable.
1. C.R. Brooks and A. Choudhury, Metallurgical Failure Analysis, McGraw-Hill, NY, NY, 1993.
2. Metals Handbook, Desk Edition, 2nd ed., American Society for Metals, Materials Park, OH, 1988.
3. W. Reitz, “Power Line Tower Arm Failure Analysis,” Practical Failure Analysis 2, (6), 80-84, 2002.
4. Metals Handbook, 9th ed., vol. 12: Fractography, American Society for Metals, Metals Park, OH, 1987.
5. ASM Metals Handbook, 8th ed., vol. 8, Metallography, Structures, and Phase Diagrams, Metals Park, OH, 1973.
6. W. Reitz and J. Rawers, “Materials Basics for the Corrosionist,” ASM Metals Handbook, 10th edition, vol. 13A, Corrosion, Materials Park, OH, 980-991, 2003.
7. W. Reitz, “SO2 Heat Exchanger Failure,” Practical Failure Analysis, 2, (3), 45-49, 2002.
8. M. Phol, “Material Failure through Wear,” Systematic Analysis of Technical Failures, G.A. Lange (ed.) DGM Informationsgesellschaft-Verlag, Braunschweig, Germany, 1986.
9. R.C. Rice, (ed), Fatigue Design Handbook, 2nd ed., Society of Automotive Engineers, Warrendale, PA, 1988.
10. W. Reitz, “Failure Analysis of Brass Bolt from Mausoleum,” J. Failure Analysis and Prevention, 5, (4), 22-27, 2005.
ABOUT THE AUTHOR: Wayne Reitz, PhD, PE
CNSE, temp. Sr. Research Scientist.
9/97 – 5/04 Assistant Professor, NDSU, Mech. Eng.
6/80 - 9/97 Principal Engineer, Babcock and Wilcox, VA.
“Surface Engineering: Successes and Failures,” i4th Symp. for Surface Eng., Puerto Ordaz, Venezuela, 9 Nov 2006
“Metallurgy of Stainless Steel Alloys – Welding and Corrosion,” SME, TP06PUB19, 3/8/06.
“Long-term Benefits of Interacting with Metallurgy Consultants,” SME, TP05PUB235, 11/9/05.
“Failure Analysis of Brass Bolt from Mausoleum,” J. Failure Analysis and Prevention, 5, (4), 22-27, 2005.
W. Reitz and J. Rawers, “Materials Basics for the Corrosionist,” ASM Handbook, vol. 13A, Corrosion, “Materials Basics for the Corrosionist,” pp. 980-991, 2003.
“Power Line Tower Arm Failure Analysis,” Practical Failure Analysis 2, (6), 80-84, 2002.
“SO2 Heat Exchanger Failure,” Practical Failure Analysis, 2, (3), 45-49, 2002.
“Laser Shock Peening Solves Many Performance Issues,” Surf. Eng., 18, (1), 1-3, 2002.
Copyright Talbott Associates, Inc.
Disclaimer: While every effort has been made to ensure the accuracy of this publication, it is not intended to provide legal advice as individual situations will differ and should be discussed with an expert and/or lawyer.For specific technical or legal advice on the information provided and related topics, please contact the author.