Sunday, December 18, 2011

Trinity NDT - Brief basics of Visual and Optical Testing(VT)

Visual Testing(VT) basic requirements

The tool that is used to perform VT is the human eye.  The following are the requirements for the performance of VT:
Visual   Acuity   -   Personnel   who   perform   VT   must   pass   an   annual eye examination  in accordance  with  industry  standards. The  eye  examination checks   for   conditions   such   as   visual   acuity,   color   blindness,   and   depth perception.
Distance  -  The  examiner’s  eye  should  be  located  within  24  inches  and  at  an angle  of  not less  than  30  degrees  to  the  surface  of  the  weld  that  is  being examined  to  conduct  a direct  visual  examination. Mirrors  can  be  used  to improve the angle of vision.
Access - If the area to be examined is not directly accessible, an examination aid can be used.  Examination aids are covered later in this Module.
Lighting - A flashlight or other additional lighting should be used to sufficiently illuminate the area that is to be examined.   A minimum of 35 foot candles of light   should   be   available   for   normal   visual   weld   examinations. When performing  VT  for  small  indications,  a  minimum  of  50  foot  candles  of  light should  be  available. If  required  by  procedure,  a  light  meter  can  be  used  to determine the exact amount of illumination that is available.

Examination Aids

Examination  aids  sometimes  are  used  to  facilitate  visual  examinations. The  following  are examples of commonly used visual examination aids:
1. Mirrors
2. Portable Lighting
3. Flashlights
4. Light Meters
5. Straight Edges and Rulers
6. Magnifying Lenses
7. Boroscopes
8. Microscopes
9. Video Cameras
10. Weld Gages
When  the  use  of  visual testing/examination  aids  (such  as  boroscopes)  are  necessary  to  perform  remote examinations, the  image  resolution  must  be  at  least  equal  to  the  image resolution  that  is attainable by direct visual examination.
Several types of weld inspection gages are available to simplify the measurements of complex weld  configurations  when  conducting  VTs.
The  following  are some of the gauges that are generally used:
Cambridge Gage - This gage can measure undercut, weld reinforcement, bevel angle, root opening, fillet weld size, and joint mismatch.
GAL “Hi-Lo” Gage - This gage can measure joint mismatch, root opening, and weld reinforcement.
AWS Type Gage - This gage also can measure various sizes of standard fillet welds.

Common Applications
The following are common applications for visual weld examinations
  • To determine the size and length of fillet welds on structural members
  • To inspect the weld joint fit-up including bevel angle, root opening, land, and cleanliness of piping welds.
  • To inspect in-process welds and completed welds prior to additional NDT.
  • To inspect the proper fit-up of socket weld fittings on small diameter pipe.
The   following   are   the   common   discontinuities   that   can   be   detected   during    visual examination:
  • Cracks
  • Slag
  •  Porosity
  •  Undercut
  •  Cold Lap
  •  Arc Strikes
  • Many more surface discontinuties 
Purpose of Visual Inspection method The purpose of a visual weld examination (VT) is to detect visible surface discontinuities on a weldment.   VT is the most frequently used method of examination, and welders and welding inspectors continuously use VT during welding operations to make better quality welds.   VT often will identify problems during welding that can be repaired “in process” to prevent the discovery of a discontinuity by a subsequent nondestructive test.
Advantages of Visual Inspection method VT is the quickest and most cost-effective method of NDT for use in the identification of a surface  discontinuity  on  a  weld. Because  the  VT  method  of  NDT  requires  the  fewest technical and interpretive skills, VT also is the simplest NDT method to learn.  The use of VT throughout  the  welding  process  significantly  improves  the  success  of  subsequent  NDT  and reduces the cost of repairs.

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