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Visual inspection (VT)

 

The method relies upon the detection of surface imperfections using the eye. Normally applied without the use of any additional equipment, VT can be improved by using aids such as a magnifying glass to improve its effectiveness and scope.

 

VT is considered to be the primary NDT method. Since it relies on an evaluation made using the eye, VT is generally considered to be the primary and oldest method of NDT. With its relative simplicity and because it does not require sophisticated apparatus, it is a very inexpensive method thus provides an advantage over other NDT methods. A further advantage of VT is that it is an ongoing inspection that can be applied at various stages of construction.

 

The primary limitation of VT is it is only capable of evaluating discontinuities, which can be seen on the surface of the material or part. Sometimes there may be some visual indication of a subsurface imperfection that may need an additional NDT method to provide verification of the subsurface discontinuity. VT is most effective when it is performed at all stages of any new fabrication, and is the main method used during the inspection of pressure equipment. If applied say after welding has been completed, it is possible that subsurface flaws may not be detected. Therefore, it is important to appreciate that VT will only be fully effective if it is applied throughout any fabrication or inspection. An effective VT programme if applied at the correct time will detect most defects or discontinuities that may later be found by some other costly and time consuming NDT method. The economics of VT can be seen in welding if we consider how much easier and inexpensive a welding problem can be corrected when found at the right time, i.e. as it occurs.

 

For example, a flaw, such as incomplete fusion at the weld root, can be repaired easily and quickly right after it is produced, saving on expense and time required repairing it after the weld has been completed and inspected using some other NDT technique. VT will also give the technician instant information on the condition of pressure equipment regarding such things as corrosion, bulging, distortion, correct parts, failures, etc.

 

VT requires three basic conditions to be in place. These are:

 

• good vision, to be able to see what you are looking for
• good lighting, the correct type of light is important
• experience, to be able to recognize problems.

 

As mentioned previously, one of the advantages of VT is that there is little or no equipment required, which improves its economy or portability. Equipment, which may be employed to improve the accuracy, repeatability, reliability, and efficiency of VT, include various devices. Magnifying glasses may also be utilized for a more detailed look at some visual feature. However, care must be taken to avoid making erroneous decisions regarding the size or extent of some discontinuity when its image is magnified.

 

As mentioned before, the primary limitation of VT is that it will only detect surface discontinuities. It is also limited to the visual acuity and knowledge of the technician. A summary of advantages and limitations are shown below:

 

Limitations

 

• Restricted to surface inspection
• Good eyesight required
• Good lighting required
• Person performing the inspection must know and be able to recognize what he/she is looking for

 

Advantages

 

• Primary method of inspection
• On-going inspection
• Most economical inspection method
• Applicable at any stage of fabrication

 

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