8 Nondestructive Evaluation Techniques
There are times when we need to evaluate a product to determine is it assembled correctly or when looking for clues concerning a failure mechanism. Simple visual inspection may be sufficient and there are times when we need more information or detail.
A great first step is the use of an appropriate nondestructive evaluation (NDE) method.
Done correctly, we keep the item unchanged and available for shipment or further inspection.
Whatever the reason for the inspection, you first need to have an awareness of the range of possible inspection methods. Some techniques simply observe the material and its properties, others measure the response to a harmless stimulus. Here is a short list of 8 approaches for NDE.
Dimensions, fit, color, and many other product attributes are visible and simple observation is all that is needed. Other times we may need enhancements to our unaided vision. Microscopes, optical comparators, and light wave length detection (color detection) let us get closer or increase the accuracy of a measurement.
Holographic techniques use lasers to create interference patterns, for a trained technician the patterns may reveal defects. Fiber optic techniques let one view beyond the line of sight.
2. Eddy Current
Conductive materials respond to current flow in a measurable way that reveals wall or coating thickness, defect location, hidden defects such as pits or voids, location of seams, and pockets.
The testing is generally rapid and low cost, yet the results are comparative and the equipment may require frequent calibration.
The analysis of sound reflections lets us detect material thickness, voids, inclusions, change of material property, and cracks. You can also determine the smoothness of a surface of the parallel-ness between two surfaces.
Tools range in capability from the detection of microscopic flaws to subsurface geological formations.
4. Liquid Dye Penetrant
To reveal changes in material properties or the existence of surface cracks, apply a liquid dye. The key is to match the dye to the application and materials to produce the desired result and not harm the product.
Done correctly the defects or changes become visually apparent.
5. X-ray or Gamma Ray
Moving beyond visible light, other elements of the spectrum permit penetration and pass though of certain materials. I learned about X-ray inspection when first looking at solder joints of ball grid array package. The chip and package hide the solder joints from direct visual observation.
We would shine an X-ray beam through the material and look at the shadow created as the solder (metal) would occlude the beam, where the laminate and thin copper traces would pass x-rays though. We could find faulty joints by interpreting the shadows.
Like a dental x-ray, this technology lets us see the internal structure.
There are tools to measure temperature or temperature flow, including thermocouples, thermal waxes (melt or change color at specific temperatures), and infrared cameras (able to create an image mapping temperature differences).
Useful to detect unwanted hot spots or excessive power dissipation issues.
While not useful with metals, these tools can measure thickness in some materials. They are also useful to measure moisture content or chemical composition.
In operation, a motor or pump creates a vibration signature. Changes to that signature indicate a change in the balance, alignment or loading. Bridges, buildings, rotating equipment, engines, and devices receiving an impact or cyclic load has some form of vibration response. Measuring the vibration response may reveal material changes, cracks, hidden flaws, alignment, or balance issues.
The field of nondestructive inspection continues to evolve and improve.
This brief overview just touches on the possibilities and general approaches, so please comment and provide your favorite examples of NDE.
4 Electronics Nondestructive Evaluations (article)
Part Selection Process and Reliability (article)
8 Factors of Design for Maintainability (article)