Fatigue can be defined as progressive localized damage due to fluctuating stress and strains on materials. Failure of metal or components occurs for reasons like irregularities in loading, defects in the material, inadequacies in design, deficiencies in maintenance, deficiencies in construction, and due to environmental conditions. It is very important to know how to investigate the failure of metal in order to be able to identify the reason for the failure. By doing this, we can take necessary corrective action in design, maintenance, and operation to avoid a repeated failure. Most metal component failures are due to fatigue that is the result of cyclic loading.
Typical Stress vs. Strain Diagram for a Ductile Material (e.g. Steel) where Fracture Occurs
Steps Involved in Investigating Metal or Component Failure
The steps involved in investigating a failure are the following:
- Initial observation of the failure includes making a detailed visual study and analysis of the actual component or parts that have failed during operation.
- Gather and record all details of the failure by photographs so that it can be used for analysis of failed parts in future.
- Analysis must be made of deformation markings, fracture appearance, deterioration, etc.
- Collect all data concerned with specification and drawings, design of the component, fabrication of the component, any repair done before the failure, any important work carried out prior to the failure, and the service period of the component, as well as any problems noticed during the operation of that particular component or machine.
When all this data is accurately collected, you should be able to analyze the reason for the failure of the component.
Laboratory studies in the investigation of metal failure include verification that the chemical composition of the material that failed is within the specified limits. The studies also include the checking of dimensions and physical properties of the failed component.
Supplementary tests must be made if required, to confirm the properties of the component or parts. For example, carrying out the hardness test to determine the hardness of the material, determining the microstructure of the material to check the heat treatment, checking the ductility of the material by the free bend test, carrying out nondestructive test to check for any defects or cracks on the component, and also checking for the composition of corrosion products on the metal, etc.
Laboratory Analysis of Metal Failure
Synthesis of Failure
Before concluding the investigation, study all the facts and evidence of the failure, both positive and negative, in order to provide the answers to the typical questions given below.
Was the part properly installed?
Was the part properly designed?
Was the part maintained properly?
How long was the part in service?
What was the nature of the stress at the time of failure?
Is there is any vibration noted on the part before failure?
Was the part subjected to overload?
Was it subjected to service abuse?
Where there are any changes in the environment before failure?
Was the part properly maintained during operation?
Was the failure is due to ductility, brittleness, or a combination of both?
Did the crack started recently or had it been growing for a long time?
Did the failure start at one point, or did it originate at several points?
Did the failure start at or below the surface?
This combined with the theoretical analysis should indicate the problem that caused the failure.
Note: nearly 50% of all the failures can be attributed to faulty design. The rest of the failures are attributed to production and service problems, bending fatigue, and surface problems such as pitting, crushing, cracks and scoring.
Wikipedia Commons: Stress Strain Ductile Material (image)