Metallurgical Failure Analysis
Metallurgical failure analysis is the determination of the cause and nature of the failure of metallic parts. A failure is defined as any situation where a part fails to function as intended by its design, fabrication and application. Metallurgical failure includes occurrences that vary from the catastrophic breaking of a structural component such as a construction beam in a bridge to the premature corrosion of a contact in an integrated circuit. The goal of a complete failure analysis is to establish the cause and mechanism of the failure and recommend corrective action to prevent recurrence of the problem.
The causes of metallurgical failures generally fall into the following categories:
1. Improper design.
a. Flawed design due to incorrect stress analysis or geometric constraints.
b. Improper material selection that does not meet the requirements (strength, corrosion resistance, etc.) of the application.
c. Improper material treatment such as heat treatment, coatings, etc.
2. Faulty fabrication.
a. Use of the wrong material.
b. Pre-existing material defects such as casting defects.
c. Defects induced during processing (working, forging, heat treating, machining, welding, etc.).
d. Assembly errors that can range from an untightened bolt to incorrect lubricant.
3. Application-related failures.
b. Poor maintenance.
c. Accidental overload.
d. Unforeseen operating conditions.
The cause of failure results in a related mode of failure. The most common modes of failure are corrosion, erosion (including wear and abrasion), fatigue (the slow propagation of a crack during regular use), brittle fracture (instantaneous cracking), ductile fracture and creep (both of which involve plastic deformation). The actual mechanism of failure can be a combination of these modes such as stress corrosion (corrosion accelerated by tensile stress) or erosion-corrosion (erosion promoted by corrosive attack).
A failure investigation is performed in the following stages:
1. Background. A complete history of the failure is obtained along with any part
specifications (including any drawings and material or properties requirements), the fabrication process and a description of the conditions of the failure. This description should include how the part was being used, any deviations from regular use, any affect due to or on adjacent parts, and any alteration (such as cleaning) to the part after the failure. The part is photographed in the as-received condition to determine any corrosive attack, physical damage or other changes in the condition of the failed part that may have occurred between the time of the failure and the failure investigation.
2. Macroscopic (visual) examination. The part is visually examined and its condition
evaluated. Any fractures (primary fracture surfaces as well as any secondary fractures), visible distortion, corrosive attack, wear or other damage to the part is thoroughly described and documented. It is then determined if any corrosion or wear is normal to the application or may be associated with the failure. No sectioning or other altering of the part is done before a complete visual examination.
3. Nondestructive testing. If the investigation warrants, nondestructive testing (dye penetrant, magnetic particle, ultrasonic, radiographic or x-ray inspection) is performed to detect cracks or material discontinuities.
4. Fractographic examination. Any fracture surface on the failed part is examined
visually and by optical and scanning electron microscope for topographic features that can indicate the mechanism, and occasionally the cause, of the failure. A replica of the fracture surface may be prepared for transmission electron microscope examination. Other features such as corrosive attack, material defects, erosion or wear can also be examined at this stage for clues to the cause or mode of failure. Secondary cracks may require opening for examination.
5. Metallographic examination. The locations for cross-sectional microscopic
examination are selected and the samples are metallographically prepared and examined by optical microscope. Evaluation of the metallographic samples can identify the class of material and subsequent treatments along with defects and many types of material problems such as grain boundary attack or decarburization. Metallographic examination can give clues to and sometimes determine the cause of failure. This type of examination requires sectioning of the part, is often not repeatable and can alter or destroy evidence.
6. Chemical analysis. Chemical composition analysis of the material (along with any
coatings, etc.) will determine whether the specified materials were used during fabrication. Chemical analysis can also be performed on corrosive products or certain defects such as inclusions to determine their source. The usual instrumentation we use for chemical analysis in a failure analysis includes an optical emission spectrograph possibly along with combustion analysis or an inductively coupled plasma spectroscope to determine the composition of base materials and an energy dispersive x-ray (EDX) spectrometer for analysis of coating materials, inclusions, corrosive products, etc. EDX analysis can be performed on a wide variety of samples such as a fracture surface or a metallographically-prepared cross section.
7. Mechanical properties. Failure analysis typically includes testing for hardness along
with other mechanical properties such tensile strength, yield strength, elongation and impact energy. The results can determine if the part meets the specified material requirements. Some investigations require additional testing for properties such as fatigue strength, notch sensitivity, corrosion resistance, coefficient of thermal expansion, etc.
8. Compiling and evaluation of all the evidence and test results, formulation of conclusions and writing of the report along with recommendations.
A optical photomicrrograph of a galvanized coating with numerous defects which contributed to the failure of the coating.
One of the most important steps of any failure analysis investigation is not included on this list. That step is communication with the people involved in the failure. It is important to know the questions to ask and be able to correctly interpret answers in order to obtain information that may have been deemed irrelevant, unintentionally omitted or intentionally omitted to avoid potential liability.
Occasionally no clear cause of the failure can be initially determined and it is necessary to work with the client to evaluate and minimize the affects of various possible contributing factors until a solution is found.
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