What is Visual Testing (VT) in NDT?
Visual testing (VT) is also known as visual testing examination, nondestructive inspection, or nondestructive evaluation or examination. Visual testing is the most common nondestructive testing method. Visual testing is a popular NDT method, because it is so easy to perform, it is a low-cost method, and it requires minimal equipment. VT involves observing a component with the naked eye to evaluate the presence of surface discontinuities. VT can be assisted with optical instruments such as magnifying glasses, boroscopes, mirrors, and other computer equipment for remote viewing. If a component can be viewed, visual testing is the first method of testing in an NDT examination. Visual testing can be performed on components that show visible corrosion or degradation such as welds, storage tanks, piping, boilers, and pressure vessels.
Visual Inspection involves looking at a weld with the eyes or indirectly using optical instruments with some level of magnification. Typically, visual inspectors are checking for surface anomalies like cracks, pits, surface pores, undercut, underfill, missed joints, and other aspects of the weld.
Visual Inspection is limited to the surface area of the weld that is visible to the inspector, which means something like depth of penetration cannot be determined unless the weld is a full penetration weld and you can view it from inside the assembly.
Application of Visual Examination
- Examining the surface condition of a component
- Examining alignment of mating surfaces
- Checking presence of leaks
Tools for Visual Testing (VT)
- Magnifying glasses
- Fillet weld gauge
- Computer equipment (remote viewing)
- Illuminated magnifier
- Inspection glass
- The eyes! *The Most Important VT Tool*
Visual Testing Techniques:
1. Direct Visual Testing: The direct technique is to place the eye within 600 mm (24 in.) and not less than 30 degrees from the test surface. Mirrors may be used to improve the angle of vision, and aids such as magnifying lenses may be used to assist examinations.
2. Indirect (Remote) Visual Testing: The remote, or indirect, technique may include accessories such as mirrors, borescopes, video probes or cameras to correct for the distance or angles of view.
Stages of Visual Testing Examination
1. Cleaning of obstructions from the surface: Pre-cleaning is necessary because dirty surface weakens an adequate view of the test surface. The need for cleaning largely depends on the size and type of discontinuities specified by acceptance criteria.
2. Illumination -: to provide adequate contrast so that relevant objects or discontinuities are detected. Illumination (natural or supplemental white light) of the examination surface is required for the specific part, component, vessel, or section thereof being examined. The minimum light intensity shall be 100 fc (1076 lx).
3. Observation: observation and evaluation of test object as per Visual Test Specifications.
Advantages & Limitations of Visual Testing
Advantages of Visual Testing:
- Economy- basic visual testing can be simply be carried by naked eyes in presence of sufficient lighting.
2. Speed – most quick method compared to other NDT methods.
3. Sensitivity – most effective to locate surface anomalies.
4. Versatility – can be applied to every object.
5. Field mobility- portable equipments are available for both direct and indirect visual inspection.
6. Minimal training requirements – less training hours required to other NDT methods
7. Minimal equipment requirements
Limitations of Visual Testing:
- Inspector training necessary.
- Good eyesight required or eyesight corrected to 20/40.
- Can miss internal defects.
- The report must be recorded by the inspector.
- Open to human errors.