Failure Analysis: Cause & Prevention
Failure analysis makes use of macrostructural and microstructural evaluation
methods, as well as other investigative tools such as nondestructive
testing, mechanical property measurements, and chemical analysis.
The failure investigations performed at LTI are limited to metal components and concentrate on the metallurgical aspects of the failure. They generally do not emphasize factors such as stress or load distributions. The steps followed and the order of the steps performed in a failure investigation may vary depending on the type of failure being investigated.
Collecting and Assembling Background Information
The failure investigation should begin by obtaining background information on the failure. This includes collecting available information about the manufacturing, processing, fabricating method, and service history of the failed component. Processing and service records, pertinent codes, specifications and related standards, as well as specifications, drawings, and design criteria should be included. Data should be provided about the material used, mechanical properties, heat treatment, and any surface preparation such as cleaning, grinding, plating or painting. The service history should include the date, time, temperature, and environmental conditions of the component.
In addition to collecting the background information, photographs of the failed component and the failure site should be taken. These photographs will document the location and type of failure and will become part of the permanent record of the failure.
Nondestructive Testing
Nondestructive testing can be useful in failure investigations, particularly magnetic particle inspection for ferrous metals, liquid penetrant inspection, and ultrasonic inspection. These inspection methods are used to detect surface cracks and discontinuities. Radiography is used to examine components for internal discontinuities, such as voids and porosity.
Visual Examination
Until the investigator can examine the failed component, it should be isolated and protected from further damage. The component must be handled carefully and must be sheltered from any corrosive environment to preserve the failure.
The investigation should begin with a visual examination of the failure site and part. All observations should be recorded or photographed and information must be gathered before the part is cut for destructive testing. The visual examination should include inspection of the fracture faces and crack paths, as well as an assessment of abnormal conditions or abuse that the part endured during its service life.
Macroscopic Examination
Macroscopic examination is an extension of the visual examination and evaluates quality and homogeneity of the part. It is used to determine the origin of the failure and the type of fracture such as ductile, brittle, torsion or fatigue. Macrostructural features can be used to assess internal quality, presence of hydrogen flakes, chemical segregation, hard cases, flow lines and welds.
Mechanical Properties
Mechanical property tests were discussed in a previous newsletter, therefore, only a list of typical mechanical properties are included here. These tests include hardness, tensile and Charpy Impact.
Metallographic Examination
Metallographic examination is performed to determine microstructural features such as inclusion content, grain size and rolling direction. Metallographic examination involves the use of high magnification microscopy. These examinations may include optical as well as scanning electron microscopy. Optical microscopic examination is used to determine grain size, microstructure and inclusion type and content. On the other hand, scanning electron microscopy is used to determine abnormalities, such as inclusions, segregation, and surface layers, as well as fracture features and, with the use of energy dispersive analysis, can identify inclusion type and corrodents on the fracture face. Microhardness testing is performed to measure case depths, evaluate cold work, determine weld quality and determine mechanical properties of small parts.
Chemical Analysis
In a failure investigation, chemical analysis is performed routinely to ensure that the material is what was specified. In addition to obtaining the bulk composition of the material to confirm the grade used, it may be necessary to analyze surface contaminants. Chemical analyses are performed on the base material as well as corrosion products, deposits or coating layers.
Simulation Tests
Sometimes it may be helpful to determine the cause of the failure by means of testing that simulates the conditions under which the failure occurred. The simulation may provide information as to the suitability of the material to the environment and can also confirm the heat treatment or performance of the part in a controlled environment such as salt spray. Certain types of simulation tests require accelerated testing in order to obtain information in a reasonable time. Interpretation of accelerated tests must be done with care because the limitations of the tests have to be recognized.
Data Analysis, Conclusions and Report
After the completion of the outlined steps, the investigator should be ready to interpret and summarize the data that has been collected. Some of the work performed may not contribute in determining the cause of the failure, yet it may be helpful in eliminating some possible causes. In combination, the steps that have been outlined will, in most cases, enable the investigator to conclude the cause of the failure. The report should provide the following:
- Description of the failed component
- Conditions at the time of failure
- Background service history
- Mechanical and metallurgical data about the failed part
- Evaluation of the material quality
- Discussion of mechanisms that explain the cause of the failure
- Recommendations for prevention of future failures or for action to be taken with similar parts