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A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.
Issue 62: June/July 2015
Non-Destructive Testing and Product Liability
By John L. Ryan, P.E.
© 2015 M.A.S.E. LLC
Non-destructive testing is a method that allows engineers and other safety professionals to assess the qualities of a material without significantly altering the physical characteristics of a material. This has many uses in industrial and manufacturing settings. Non-destructive testing (NDT) is used for quality control in manufacturing settings without having to damage the final product. NDT can be used to identify a variety of flaws including internal cracking, fatigue cracking, porosity, voids, weld discontinuities, thickness measurement, chemical composition defects or alterations of a material, corrosion, and other properties. Defects can exist in material prior to manufacturing or assembly. Flaws also can develop in the manufacturing process through heat treatment, welding, overtightening of fasteners, and other methods. Flaws can also develop once a product, structure, or machine is in use due to vibration or fatigue stresses, or the structure being inadequately designed resulting in deformation or failure of components.
Radiology, Fluoroscopy, CT Scans
Radiology involves using electromagnetic waves to gain information that is unavailable to the five senses. X-rays, and radioactive isotopes are used to image solids. Defects can be detected using these techniques such as weld discontinuities, internal cracking, voids, and other defects. Radiation is absorbed to a lesser degree in defects which allows them to be identified visually due to a difference in optical density on the film which absorbs the radioactive waves. These methods can all be used in real-time with video instead of in the past where photographs were often used, such as one would find with a traditional medical X-ray.
Laser inspection is used to detect surface defects and measure thickness, particularly in applications involving continuous sheets of material. The laser is reflected off of the inspected surface, and the reflection can be recorded photographically. Holographic testing involves using lasers three dimensionally to image the part. Laser inspection is used to detect unbonded areas of laminates, and stress or corrosion cracking in metal components.
Similar to other wave types of testing, ultrasonic testing involves using ultrasonic vibrations to identify flaws in a material. Sound waves are applied to a part, and the sound signal is analyzed after passing through a material. Ultrasonic waves travel in a straight line unless they are reflected by a material discontinuity. The reflection is detected by the transducer and displayed visually. This method is useful in identifying very small cracks or voids deep in a material. Nuclear power plants use this technology to identify defects. This is also used to determine the thickness of vessels at different points, as only one side of the surface needs to be accessed.
Eddy Current Testing
This is another wave type of testing where a small electrical current is passed through a part. These currents are detected by a test coil. Changes in the current are caused by discontinuities such as cracks, voids, increased porosity, etc. This is used to identify a variety of defects, and can be used to measure thickness of platings on materials. Eddy currents can also be used to help identify the composition of some materials.
Hardness testing is a useful method of determining the material properties of something. Most methods of hardness testing involve applying a load to an indentation device, and the depth of indentation determines the hardness of the material. There are many different methods and scales for hardness testing but the theory is the same, and the different scales can be correlated to one another. Hardness testing provides an accurate estimation of the strength of a material. It can be used to determine if there are global material defects in a structure or product, or if a single component contains material defects.
Magnetic testing also uses waves to identify discontinuities in ferrous materials such as steel, iron, cobalt, nickel. A magnetic field is applied to a part, and if there is a defect, the defect will interrupt the magnetic field and result in distortions to the magnetic field. This method is useful in identifying defects at or near the surface of a material.
Penetrant testing is a method where a liquid chemical substance is sprayed on an object, or submerged. The basic concept is that the penetrant increases the visible contrast between the defect and the surrounding material. This penetrant can find its way into cracks that are not visible to the eye. The excess penetrant is removed or washed from the part, and then a developer is applied which will make any remaining penetrant visible that is captured in cracks or other flaws. Fluorescent penetrants are used as they are more easily detected visually.
This method can differentiate between materials that have significantly different thermal conductivity. Heat is applied to an unknown material, and the conductivity and rate of cooling are compared to known materials.
Electron microscopes allow extremely high magnification of objects, which can allow specialists to identify defects in a material and to identify failure modes as fatigue, shear, or tensile failure.
Electron microscope image of stainless steel corrosion
Electron microscopes can also be used to apply an electron energy beam to a material. Electrons are lost depending on the properties of the material, and the amount of loss can identify material composition, thickness, and pressure. This is useful to identify material defects.
How We Can Help
At MASE, we can determine the cause of an accident, whether it is due to a design defect, manufacturing error, material defect, or human error. We can help you determine if nondestructive testing would be useful in a product or structural failure. Call us at (855) 627-6273 or email us at email@example.com
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© 2015 Mechanical and Safety Engineering