A wide group of analysis techniques used in science andindustry to evaluate the properties of amaterial, component or system without causing damage. Nondestructive examination (NDE) Nondestructive inspection (NDI) Nondestructive evaluation (NDE)
Does not permanently alter the article being inspected Save both money and time in product evaluation, troubleshooting and research Can be used to detect flaws in an in-process machine part
NDT methods rely upon use of electromagneticradiation, sound, and inherent properties of materials(such as thermal, chemical, magnetic etc.) to examinesamples.
Very short ultrasonic pulse-waves are launched into materials to detect internal flaws. Used for steel and other metals and alloys, can also be used on concrete, wood and composites (with less resolution). Used in many industries including aerospace, automotive and other transportation sectors.
Two methods of receiving the ultrasound waveform: Reflection Through Transmission
Principle:LEFT: A probe sends a sound wave into a test material. There are two indications, one fromthe initial pulse of the probe, and the second due to the back wall echo.RIGHT: A defect creates a third indication and simultaneously reduces the amplitude of theback wall indication. The depth of the defect is determined by the ratio D/Ep.
Part is magnetized. Presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak, since air cannot support as much magnetic field per unit volume as metals. Ferrous iron particles are then applied to the part. Particles will build up at the area of leakage and form what is known as an indication.
Penetrant may be applied to the test component by dipping, spraying, or brushing After adequate penetration time, the excess penetrant is removed, a developer is applied. Developer helps to draw penetrant out of the flaw where an invisible indication becomes visible to the inspector
1. Section of material with a surface-breaking crack that is not visible to the naked eye.2. Penetrant is applied to the surface.3. Excess penetrant is removed.4. Developer is applied, rendering the crack visible.
Short wavelength electromagnetic radiation (high energy photons) to penetrate various materials. The amount of radiation emerging from the opposite side of the material can be detected and measured
Tube exhibiting no cracking Tube exhibiting light crackingTube exhibiting moderate cracking Tube exhibiting severe cracking
Uses electromagnetic induction to detect flaws inconductive materials.
Variations in the phase and magnitude of these eddy currents can be monitored using a second receiver coil, or by measuring changes to the current flowing in the primary excitation coil. Variations in the electrical conductivity or magnetic permeability of the test object, or the presence of any flaws, will cause a change in eddy current and a corresponding change in the phase and amplitude of the measured current.
NDT Technique Nature of defectUltrasonic Testing Sub-surface, interstitialMagnetic Particle Inspection Surface and slightly subsurface discontinuities in ferroelectric materialsDye Penetrant Inspection Surface-breaking defects in all non-porous materialsRadiographic Testing Surface, Sub-surface defectsEddy-Current Testing Surface, Sub-surface defects (depending on conductivity)
A process plant contained two stainless steel vessels which had been operating for 21 years. The contents of the vessels were flammable, mildly toxic and contained 500 ppm of chlorides. The vessels were operated from full vacuum up to 15 psi for 20 cycles per day. They contained an agitator which was used in part of the process. Both vessels had been hydraulically tested to 70 psi when new but had not been subjected to a test since
Company thought that stainless steel would not break. Hence, no leak detection equipment had been installed and reliance was placed on plant operators noticing the smell or observing drips. A competent Person from a large insurance company prescribed external visual examination supplemented by a hammer test every 2 years. Was this suitable?
The combination of stainless steel and chlorides immediately raises concerns regarding the possibility of stress corrosion cracking. While the cracks were likely to initiate on the inner surface, an external examination could detect the presence of through wall cracks only. However, stress corrosion cracks can be very tight and difficult to see with the naked eye. The hammer test offers no benefit - who knows what a good vessel should sound like!
During a thorough examination, it was found that vessel developed leaks at 40 psi. Further investigation of the vessel found thousands of through wall cracks. The vessel had not leaked in service because the contents were too viscous to pass through the tight stress corrosion cracks. Finally it was decided to go for NDT with Dye Penetrant Inspection using fluorescent dyes, which give a higher sensitivity and much better results in the confined, dark space of the vessel.
NDT techniques provide cost-effective and reliable analysis under realistic conditions. Each NDT technique has certain capabilities and limitations and often more than one technique is used to cover various parts. Increasing availability of robotic scanners improve the speed of testing large surfaces, hence minimizing the testing time.
THE IMPORTANCE OF WELDING QUALITY CONTROL,K.M. WONG & Scarlett YEUNG,A.E.S. Destructive & Non-Destructive Testing Ltd http://www.hse.gov.uk/comah/sragtech/techmeasndt.htm#CaseStudie s http://en.wikipedia.org/wiki/Nondestructive_testing#Methods_and_te chniques http://testex-ndt.com/from-the-field/corrosion-detected-in-pipelines- using-lfet/