Metallurgical failure analysis
Metallurgical failure analysis is the process to determine the mechanism that has caused a metal component to
Failure can be broadly divided into functional failure and expected performance failure. Functional failure occurs when a component or process fails and its entire parent system stops functioning entirely. This category includes the common idea of a component fracturing rapidly. Expected performance failures are when a component causes the system to perform below a certain performance criterion, such as life expectancy, operating limits, or shape and color. Some performance criteria are documented by the supplier, such as maximum load allowed on a tractor, while others are implied or expected by the customer, such gas consumption (miles per gallon for automobiles).[1]
Often a combination of both environmental conditions and stress will cause failure. Metal components are designed to withstand the environment and stresses that they will be subjected to. The design of a metal component involves not only a specific elemental composition but also specific manufacturing process such as heat treatments, machining processes, etc. The huge arrays of different metals that result all have unique physical properties. Specific properties are designed into metal components to make them more robust to various environmental conditions. These differences in physical properties will exhibit unique failure modes. A metallurgical failure analysis takes into account as much of this information as possible during analysis. The ultimate goal of failure analysis is to provide a determination of the root cause and a solution to any underlying problems to prevent future failures.[2]
Failure investigation
The first step in failure analysis is investigating the failure to collect information. The sequence of steps for information gathering in a failure investigation are:[1][3]
- Collection information about the circumstances surrounding the failure and selection of specimens
- Preliminary examination of the failed part (visual examination) and comparison with parts that have not failed
- Macroscopic examination and analysis and photographic documentation of specimens (fracture surfaces, secondary cracks, and other surface phenomena)
- Microscopic examination and analysis of specimens (fracture surfaces)
- Selection and preparation of metallographic sections
- Microscopic examination and analysis of prepared metallographic specimens
- Nondestructive testing
- Destructive/mechanical testing
- Determination of failure mechanism
- Chemical analysis (bulk, local, surface corrosion products, deposits or coatings)
- Identify all possible root causes
- Testing most likely possible root causes under simulated service conditions
- Analysis of all the evidence, formulation of conclusions, and writing the report including recommendations
Techniques used
Various techniques are used in the investigative process of metallurgical failure analysis.[1][3]
- Macroscopic examination: camera, stereoscope
- Microscopic examination: light microscopy, electron microscopy, x-ray microscopy, metallographic etching
- Mechanical testing: hardness testing, tensile testing, Charpy impact testing
- Chemical testing: microprobe analysis, energy dispersive spectroscopy
Non-destructive testing: Non-destructive testing is a test method that allows certain physical properties of metal to be examined without taking the samples completely out of service. NDT is generally used to detect failures in components before the component fails catastrophically.
Destructive testing: Destructive testing involves removing a metal component from service and sectioning the component for analysis. Destructive testing gives the failure analyst the ability to conduct the analysis in a laboratory setting and perform tests on the material that will ultimately destroy the component.
Metallurgical failure modes
There is no standardized list of metallurgical
Caused by corrosion and stress
- Stress corrosion cracking[7] Stress corrosion (NACE term)
- Corrosion fatigue
- Caustic cracking (ASTM term)
- Caustic embrittlement (ASM term)
- Sulfide stress cracking (ASM, NACE term)
- Stress-accelerated Corrosion (NACE term)
- Hydrogen stress cracking (ASM term)
- Hydrogen-assisted stress corrosion cracking (ASM term)
Caused by stress
Caused by corrosion
- Erosion corrosion
- Pitting corrosion Oxygen pitting
- Hydrogen embrittlement
- Hydrogen-induced cracking (ASM term)
- Corrosion embrittlement (ASM term)
- Hydrogen disintegration (NACE term)
- Hydrogen-assisted cracking (ASM term)
- Hydrogen blistering
- Corrosion
Potential root causes
Potential root causes of metallurgical failures are vast, spanning the lifecycle of component from design to manufacturing to usage. The most common reasons for failures can be classified into the following categories:[1]
Service or operation conditions
Failures due to service or operation conditions includes using a component outside of its intended conditions, such as an impact force or a high load. It can also include failures due to unexpected conditions in usage, such as an unexpected contact point that causes wear and abrasion or an unexpected humidity level or chemical presence that causes corrosion. These factors result in the component failing at an earlier time than expected.
Improper maintenance
Improper
Improper testing or inspection
Testing and/or inspection are typically included in component manufacturing lines to verify the product meets some set of standards to ensure the desired performance in the field. Improper testing or inspection would circumvent these quality checks and could allow a part with a defect that would normally disqualify the component from field use to be sold to a customer, potentially leading to a failure.
Fabrication or manufacturing errors
Manufacturing or fabrication errors occur during the
Design errors
Use of computational methods for failure analysis
Computational methods have been increasing in popularity as a method to test possible root because they do not need to sacrifice a component to prove a root cause. Common cases where computational methods are used are for failures due to erosion,[8][9] failures of components under complex stress states,[10][11] and for predictive analyses.[12][13][14][15] Computational fluid dynamics is used to determine the flow pattern and shear stresses on a component that had failed due to erosive wear.[8][9] Finite element analysis is used to model components under complex stress states.[10][11] Finite element analysis as well as phase field models can be used for predicting crack propagation and failure,[12][13][14][15] which are then used to prevent failure by influencing component design.
See also
References
- ^ S2CID 241618812.
- ^ http://www.g2mtlabs.com/failure-analysis/what-is-failure-analysis/ G2MT Labs - "What is Failure Analysis?"
- ^ ISBN 978-1-62708-270-9.
- ^ “Standard Terms Relating to Corrosion and Corrosion Testing” (G 15), Annual Book of ASTM Standards, ASTM, Philadelphia, PA.
- ^ ASM-International Metals Handbook, Ninth Edition, Corrosion, ASM-International, Metals Park, OH
- ^ NACE-International NACE Basic Corrosion Course, NACE-International, Houston, TX
- ^ M&M Engineering Conduit Fall 2007 “Chloride Pitting and Stress Corrosion Cracking of Stainless Steel Alloys,”
"Archived copy" (PDF). Archived from the original (PDF) on 2011-07-14. Retrieved 2010-08-20.
{{cite web}}
: CS1 maint: archived copy as title (link) - ^ ISSN 1350-6307.
- ^ ISSN 1350-6307.
- ^ ISSN 1350-6307.
- ^ ISSN 1350-6307.
- ^ S2CID 122765562.
- ^ S2CID 240317543.
- ^ S2CID 20139734.
- ^ ISSN 0045-7825.