|Metallurgical Failure Analysis Sample Removal|
The decision to remove a sample specimen is an important decision that does not receive enough caution or consideration. Samples selected should be characteristic of the material and contain a representation of the failure or corrosive attack.
For Example: if a structural member has failed in service and the cause of the failure is determined by metallographic examination, the sample should be removed from the particular region of the fracture where it will reveal the maximum amount of information. For comparative purposes, a sample should be taken from a sound and normal section as well.
There are various methods for sample removal, such as; acetylene torch, air arc, saw, trepan, or drill. However, two things must be kept in mind. First, all cuts made by air-arc should be at least four inches from the defect, providing that the splatter is not on the surface area to be examined. If a cut were made closer to the area than stated, the heat generated could alter the microstructure, obscure the type of corrosive attack or be cumulative to the failure, all of which would render the sample useless.
If the available distance for the removal of a sample is less than four inches, saw, trepan, or drill out the sample. Prime importance must be placed on removing the sample without any physical damage to the area examined. Beating on the sample with a hammer may destroy surface condition information. Physical abuse of the sample may also destroy the corrosion product. Analysis of the corrosion products is important in determining the corrosive agent. Normally, the surveyor will not physically cut and remove the sample from the piece of equipment. Therefore, the specific sample area must be marked or the surveyor should outline the exact area to be removed from the equipment
By the surveyor being present when the sample is removed, improper removal procedures are less likely to occur. A photograph of the piece of equipment including the sample to be removed should be taken. This will suffice to show the relationship of the examined area to the remainder of the piece of equipment.
If more than one sample is to be taken, proper designation of the samples and their location relative to the piece of equipment should be noted. The dimensions of the sample should be recorded, as well as the date the failure occurred.
To perform a complete metallurgical examination of a failure, samples from, adjacent to, and away from the failure are all necessary elements for a comprehensive failure analysis.