Nondestructive Inspection (NDI) is the examination of an object or material with technology that does not affect its future usefulness. NDI can be used without destroying or damaging a product or material. Because it allows inspection without interfering with a product's final use, NDI provides an excellent balance between quality control and cost-effectiveness. The term "NDI" includes many methods that can:NDI can and should be used in any phase of a product's design and manufacturing process, including materials selection, research and development, assembly, quality control and maintenance.
- detect internal or external imperfections
- determine structure, compostion, or material properties
- measure geometric characteristics
Taken from the web pages of the American Society for Nondestructive Testing
Nondestructive Inspection (NDI) is a method of materials characterization very important to the materials engineer. Problems and defects of all kinds arise in the development and use of mechanical devices, electrical equipment, hydraulic systems, transportation mechanisms and the like. However, an extremely wide range of nondestructive testing methods are available to help the engineer to examine these different problems and various defects in an assortment of materials and under varying circumstances.
Commonly used non-destructive inspection methods include liquid penetrant, magnetic particle, eddy current and radiographic inspection, ultrasonic inspection, tomography, and real-time radiography. State-of-the-art developments in digital image enhancement [including color-enhanced images such as those included in this web page] have broadly expanded the applicaiton and utility of NDI methods in the industry.
Taken from Metals Handbook Ninth Edition, Volume 17: Nondestructive Evaluation and Quality Control, ASM Handbook Committee, ASM-International, Metals Park, Ohio.
We are all familiar with x-ray images such as those used in the medical field to locate and characterize, for example, the nature of fracture of a broken arm. In this instance, the "light source" is an x-ray source. The arm is penetrated by the x-rays, each component of the arm (bone, skin, muscle tissue) absorbing some of the incident radiation. The film, positioned on the other side of the arm from the x-ray source, is exposed to varing degrees of radiation, depending on how much 'gets through' the arm. The greater the x-ray exposure, the darker the film (ie, the negative). If a positive print is made from the film, contrast is reversed: direct radiation exposure appears bright and the 'darker' regions represent materials which more effectively absorb the incident radiation.
X-ray radiography can be used to examine engineering structures, components and devices. The image you see on the left is a radiographic image of an auto spark plug. Compare this 'view' with one taken using energetic particles, not x-rays. A neutron image of a spark plug (and turbine blade) is available on these ISIS . ISIS is a neutron source located near the River Thames in the UK [named after the principal goddess of ancient Egypt, ISIS who was able to bring the dead back to life :-) ]. A cut-away section through a commercial spark plug is shown in the upper right, thanks to the Allied/Autolite web pages (click on 'Technology'). For an idea of what materials are used to make a spark plug, you are encouraged to visit the Allied/Autolite site. An "off-the shelf" NGK spark plug (inverted with respect to the x-ray radiograph) is also illustrated.
Is it not clear from the images above that the spark plug is an excellent example of a "composite" engineering device? Metallic, ceramic and semiconductor materials work in concert to optimize the required performance of the plug. In the opening NDI descriptions at the top of the page, the following was stated:
Apply these methods to the useful information that may be extracted from x-ray radiography of the spark plug. Do one or more of these methods apply? Do all three? How so?
Let us consider another aspect of information that can be obtained using nondestructive examination methods. Consider two pieces of a metal or alloy welded together. During the welding operation, "residual stresses" can build-up in and around the weld region. Residual stress are internal stress you cannot 'see' in the metal, but they are there nevertheless. You may think you are loading some structure to a medium stress level; but, the actual stress acting on the metal could be much higher due to the residual stress component. Would it not be useful to quantify the "beginning" level of stress? Certainly! And would it not be best to be able to do this in a "non-destructive" fashion? Certainly! Neutron stress scanning is one way to accomplish this.
Neutron stress scanning is a specialized method of imaging, offering information as to the stress-state of engineering materials. This method too relies on a penetrating light source. The penetrating light is not in this instance x-rays. But then any portion of the electro-magnetic radiation spectrum that is not fully absorbed by, or reflected from, the subject is fair game. Even energetic particles like "hot" electrons and neutrons can be used. For a brief introduction to the neutron stress scanning method, visit the web pages of ISIS, the UK spallation neutron source.
The image on the left is from a research study of the thermal stress relief in welded Inconel 718 plates. Inconel 718 is a widely used nickel-based superalloy and is often joined using advanced techniques such as electron beam or laser welding due to its poor inherent weldability. These high energy welding processes produce small fusion and heat affected zones [this is good], but their high cooling rates often produce high residual stresses [bad]! The accompanying figure shows the residual stress distribution perpendicular to the weld in an Inconel 718 plate both before and after post-weld heat treatment. The thermal stress relief process can be seen to have substantially lowered the stresses at the center of the weld.
Real time radiography allows one to examine the internal structure of an object in real time without disturbing or destroying it. It uses highly penetrating x-rays to record the internal structure of an object. This technique can be applied to on-line inspection and quality control of engineering components.
An image of a necked-down gold wire in a plastic microelectronics package is shown in the example on the right. For a similar x-ray image of a failed wire visit this phoenix-xray web site. Essentially the internal components are not visible to the human eye because of the black, opaque (epoxy) plastic case that encapsulates the internal electronics. However, inspection using real-time x-ray clearly reveals wire failure. The source of the image was IBM; but for internal wire break and other revealing radiographs, visit the North Star Imaging, INC. web pages.
Process engineers determined that tensile wire breaks were the cause of reported electrical continuity failures. In addition, necked-down wires were detected which passed continuity tests. Upon subsequent inspection after package entry (package entry means that a chemical agent was used to dissolve the epoxy, thus revealing the internal components), it was found that these wires were essentially at the fail point, but held together at the time by the epoxy matrix. This defect, if present in electrically good product, would clearly jeopardize reliability.
This example clearly shows the difference between destructive and nondestructive inspection, does it not? Now that you know that the wires are failing by tensile-pull, what is your best guess as to how (or where) this may happen? At what step in the production does this occur? It is easy to see the importance of nondestructive inspection to the microelectronics industry. For specific information about this, and other materials science issues addressed by the microelectronics industry, visit this IBM link .
Here is an interesting application of x-ray radiography to the field of geology. Knowledge of how water flows through unsaturated, fractured rock is important for understanding and predicting fluid and contaminant transport, and has many applications. At the Lawrence Livermore National Laboratory, researchers have examined fracture flow and imbibition (a fancy word for absorption) in tuff (fragmented rock, volcanic in origin) by ponding water on top of a 14 x 10 x 2.5 cm sample containing a vertical fracture (fracture aperture ~25 microns) and periodically taking x-ray radiographs. The water contains 10% KI (by weight). Why: for what purpose? This study was performed in support of the Yucca Mountain Project.
Now all of the examples above represent only one tool available to the NDI engineer: x-ray radiography. Most assuredly, there are others. A few examples will be shown below, but the interested surfer is encouraged to visit the following sites to learn more about NDT:
Let us return to the subject of microelectronics and consider acoustic tomography. This non-destructive technique monitors the reflections of ultrasound caused by internal features in the device. Echoes produced by defects are distinct from echoes produced by well-bonded, homogeneous materials. In microelectronic packages, assembly-induced damage can be evaluated using this method. Acoustic imaging has been successfully applied to components ranging from capacitors to integrated circuits, both before and after board mounting; and has application to oceanographic research! Download this SONIX tutorial (Power Point presentation)" to learn more about acoustic tomography and its many applications.
The image on the left is a color-enhanced acoustic micrograph. The yellow area between the arrows indicates die delamination. This means that the Silicon chip has delaminated from the chip carrier. Further failure analysis would be required to determine if the delamination has occurred between the chip and the bonding solder (or adhesive); or between the solder and the carrier.
Another acoustic method of NDI is ultrasonic inspection. Ultrasound is used to test engineering materials for cracks and defects, and to measure material thickness either as part of a manufacturing cycle or during the life of a product or component. An instrument such as that shown in the image on the right is mainly used in the aerospace, construction, transportation and power industries to detect flaws. Krautkramer (now part of GE Inspection Technologies) is a worldwide designer, manufacturer, and marketer of high technology ultrasonic instruments and transducers.
Ultrasonic inspection instruments utilize ultrasound to examine the internal integrity of metals, plastics, and composite materials. The sound waves essentially "bounce-off" internal defects. In the image, the engineer is maneuvering an ultrasound transducer/receiver probe with his fingers. This probe is placed in contact with the part to be inspected with an oil or gel-layer acting as a carrier of the acoustic signal. The probe is moved and the acoustic response (see the display image) changes. The size and position of internal flaws can be determined by the height and position of the "peaks" observed on the display. For an "animated .gif" to help illustrate the technique, link HERE [From the web pages of the New Zealand Non-Destructive Testing Association (NDTA)].
What if the flaw is not an internal flaw but a flaw or crack at the surface? One can still use ultrasound to detect it. However, a cheap and reliable NDT alternative method is available: dye penetrant inspection. The image shows the "kit" that is required and, in the background, one sees a cracked component. The crack is evident because red dye, on a white background, confirms the existance of the crack. This is how dye penetrant inspection works:
The materials engineer is often involved in a function called failure analysis. Nondestructive testing is an important component of any failure analysis. For further information about failure analysis, please visit the Failure Analysis web pages at this site. If you are particularly interested in failure analysis of microelectronic devices, I recommend you link to the FIB International web pages on this subject.
One last thing! What type of NDI test method is indicated by the image at the top of this page? You should know from the information on this page. To test your response, click on the subject image and chase-down the answer.
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