Non-Destructive Testing (NDT) 5 March 2014
Definition Methods or Techniques employed to examine an subject and/or material without impairing their future usefulness Copyright (KC Wong) 2
Type of Test Acoustic Emission Eddy Current Leak Magnetic Particle Neutron Radiographic Penetrant Radiographic Ultrasonic Visual Symbol AET ET LT MT NRT PT RT UT VT
Any Cracks?
Cracks Revealed?
Introduction Sensitive method for locating surface discontinuities on non-porous materials (cracks, porosities) Reveals surface discontinuities by the bleedout of a penetrating medium against a contrasting colored background
Penetrant is remained on the surface for a prescribed time (dwell time) for drawing into any surface opening by Capillary Action
Subsequent removal of excess penetrant and application of a developer draws remaining penetrant from discontinuities The resultant indications are shown in high contrast and magnify the presence of the discontinuity so that it can be visually interpreted
Grouping of Penetrant Material Type of Penetrant Indications Methods of excess penetrant removal
Visible dye Usually red, produces a vivid red indication against a white developer background when viewed under white light Fluorescent penetrant Produces a greenish and fluorescent indication when observed under ultraviolet (black) light
Use of fluorescent penetrant can result in a more sensitive test (human eye can more readily perceive a fluorescent indication than a visible indication)
Water washable Contains an emulsifier which allows the oily penetrant to be rinsed off with a low pressure water spray Solvent removable Requires solvent to remove the penetrant from the test object
Post-emulsifiable Penetrants are removed by adding an emulsifier after the dwell time and permitting to remove with water in the same manner as the water washable type
By combining the characteristics of the grouping, following six different types of penetrants can be produced Visible/water washable Visible/solvent removable Visible/post-emulsifiable Fluorescent/water washable Fluorescent/solvent removable Fluorescent/post-emulsifiable
3 major type of Developers Dry powder Water-based wet developer Non-water-based wet developer
In most cases, wet developers are used and applied either by dipping or spraying Dry powder is used on rough surface and for objects with concave corners and threads (Applied by dusting)
Operational Procedural Steps 1. Pre-cleaning the test surface 2. Application of penetrant 3. Cleaning of Penetrant 4. Application of Developer
The test object should be free of oil, dirt, rust, paint, etc. Prevent the block of surface openings of a discontinuity Care must be taken if mechanical technique, an aggressive mechanical cleaning operation on soft material might tend to mask the surface metal
Dry the surface before application of penetrant Application of penetrant by dipping, brushing or spraying Suitable dwell time must be allowed for the penetrant drawing into tight crack by capillary action (normally from 5 min to 60 mins)
The exact time depends on the manufacturer s recommendations, the temperature of the part and the size of the discontinuities of concern Kept moisture during the dwell time
Thoroughly and carefully cleaned of the excess penetrant Should not be so aggressive that it washes penetrant out of shallow discontinuities
Developer can be a dry powder which readily evaporates, leaving the powder on the surface Should be applied in a thin, uniform layer and preferable to apply in several very thin layers (allowing a couple of minutes between successive developer for avoiding excessive developer buildup)
Developer can be a dry powder which readily evaporates, leaving the powder on the surface Should be applied in a thin, uniform layer and preferable to apply in several very thin layers (allowing a couple of minutes between successive developer for avoiding excessive developer buildup)
The sensitivity of the PT is dependent upon the size of the developer powder particles as well as the thickness of developer layer on the test surface Large particle size and thick layers of developer will tend to decrease the sensitivity of the PT All indications are evaluated in accordance with applicable standards (ASTM E-165)
Not limited to metallic test object Relatively inexpensive and reasonably rapid Simple, portable and little difficulty in learning Can be applied to nonmagnetic metals when other techniques are not applicable Well suited for evaluating weld or braze joints between dissimilar metals
Discontinuities must be cleaned and open to the surface Will not detect subsurface discontinuities Time-consuming test Deleterious effects on the welds and base metal (affect service life) Penetrants are difficult to remove completely from discontinuities Surface condition has a significant effect on their reliability
It is imperative to remove all testing materials including excess penetrant, cleaner and developer prior to welding Welding arc not only affects weld quality, but it can also result in formation of noxious or even hazardous fumes which can create a serious safety hazard for personnel