Non-Destructive Evaluation Methods
How To Test Products and Systems without Inflicting Damage
Apr 22, 2008
Non-destructive evaluation (NDE) is the inspection or qualification of an object or systems that is performed in such a way that the object or system is not adversely affected by the inspection process. NDE is useful for the evaluation of items that are in service, such as aircraft fuselages, highway bridges, and nuclear vessels.
NDE techniques are used to characterize or detect flaws in objects or systems. There are a wide variety of NDE techniques, and each excels in identifying specific types of flaws or characterizing a system in a certain way.
Types of NDE Techniques
There are two basic categories of NDE methods: active and passive.
- Active NDE methods impart energy in some form into the object being evaluated, and the difference between the input energy and the output energy provides some information about the system. Examples of active NDE methods include ultrasonics and radiography.
- Passive NDE methods rely on interpreting the response of the object to an external input. Examples of passive NDE methods include acoustic emission and dye penetrants.
NDE techniques can also be classified by the type of qualification they provide: surface, near-surface, or volumetric.
- Surface-only techniques can provide information on only the outside surface of the object. Surface-only methods include visual inspections and dye penetrants.
- Surface/Near-Surface techniques provide information on the surface of the object or just below the surface of the object. The depth of the evaluation depends on the method itself and the material of the inspected object. Surface/near-surface methods include electromagnetic techniques, such as eddy currents.
- Volumetric methods allow for the inspection of the complete depth of a part, rather than just the surface or near-surface as in the other methods. Examples of volumetric methods include radiography and ultrasonics.
Choosing an NDE Technique
NDE methods allow operators to find flaws before they become critical, reducing the potential losses due to a catastrophic failure and resulting unscheduled maintenance. Choosing the right NDE method for a specific object or system depends on:
- The size and orientation of potential flaws
- The location of potential flaws (surface or sub-surface)
- Material type
While NDE methods can be used to reduce the incidence of failures, it is up to the operator to review and interpret the inspection data and properly identify flaws in the object or system. NDE operators often undergo training and certification with the American Society for Nondestructive Testing (ASNT) to ensure competence in their evaluations.
NDE methods allow engineers and operators to use sensors and other methods to determine the current state of their systems without damaging or destroying parts within the system. NDE methods also allow operators to identify and repair potentially critical flaws before they turn into failures, saving their companies time and expenses in downtime.
Sources
American Society for Nondestructive Testing website
Bray, D., Stanley, R., Nondestructive Evaluation: A Tool in Design, Manufacturing, and Service, CRC Press, 1997.
