NDT Method Overview: Visual Testing

Definition of Visual Testing

Visual testing might be called the foundation of NDT. Looking at a technical component during and after manufacturing to check if it serves the intended purpose must have been the first attempt to perform quality control. This was combined with measurement when tolerances were specified and became tighter.

Subsequently, the search for discontinuities became part of visual quality control. Later on, many companies separated the visual search for discontinuities and named it in line with other NDT methods: visual testing (VT) or visual examination.

There was skepticism when VT was introduced in SNT-TC-1A in 1988 at an ASNT conference. The US nuclear industry pushed for it, whereas most other industries hesitated because of the additional costs.

As an NDT method with certification, VT stayed a US specialty until the European Union Standards Committee, the CEN, incorporated VT in their certification standard, EN 473 (2001). VT was finally accepted by the industry only after this standard clearly stated that a simple visual test with the naked eye would not be regarded as visual testing. Industry started to demand certification of all types of VT technicians once the standard was publicly available.

There is one aspect in which a direct visual test differs from any other NDT method. The discontinuities that the inspectors look for are visible. That makes VT unique because all other NDT methods look at indications of the discontinuities and never show the discontinuities directly. The inspector discerns the liquid penetrant (PT) and magnetic particle (MT) indications with the naked eye, but it is not the discontinuity itself.

Other methods such as ultrasonic and electromagnetic testing show acoustic or electronic signals only. Or put another way, PT and MT start where VT ends, once the discontinuities become too small to be seen directly.

The use of optoelectronic devices and charge-coupled device cameras in visual testing may blur categorization with image enhancement techniques so that we look at a screen instead of the object itself.

History of Visual Testing

The history of NDT is part of the history of industry. Boiler inspection in the late 1800s and early 1900s was an early application of VT in the United States. Inspectors would check the interiors of boilers that their company would ensure for corrosion and cracking. The first edition of the ASME Boiler & Pressure Vessel Code (1915) clearly states visual acceptance criteria for a casting: “free from blemishes, scale or shrinkage cracks.”

The history of borescopy started, however, outside the industry, in medicine. Endoscopes were used in medicine to look in the interior of the human body. George S. Crampton, a physician with the ambitions of an engineer, transferred his experiences from medical applications into industrial applications. In 1921, he developed the first borescope for Westinghouse Company to check for discontinuities inside a steam turbine (Figure 1). A similar patent was filed separately in 1927 for a borescope to inspect the inside of gun bores (Figure 2). During World War II, Crampton actively supported the US Army by checking the bores of anti-aircraft guns.

Overview of VT Applications: Capabilities and Limitations

VT is performed in all stages of the “life” of a component or machine—during and after manufacture as well as at regular intervals during service. Very often, testing is performed in two steps:

1. Overview or general visual testing
2. Local or detailed visual testing

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Step 1 is screening for anomalies at a lower sensitivity (illumination and magnification). When anomalies are suspected, then in a second step, the anomalies are verified and characterized at a higher sensitivity (illumination and magnification) to determine if these are acceptable or have to be rejected.

Details become more visible when a contrast is established by

• differences in reflectivity;
• color differences or discoloration; or
• casting shadows.

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The different types of contrast may occur separately or jointly/superimposed. Cracks normally become visible by a black/white contrast established by a difference in the reflection of light between the crack and its surroundings. General corrosion might be indicated by a difference in colors, as corrosion products are colored, salty substances. Stress-corrosion cracking might be shown as black/white contrast enhanced by colored corrosion products.

The types of details that can be detected by VT are

• discontinuities such as cracks and voids,
• shape deviations such as excessive reinforcement of welds, and
• surface roughness or surface patterns.

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VT can be a very simple (just look!) but powerful application. However, very tight cracks cannot be seen with the unaided eye. In order to find them, other NDT methods have to be applied such as MT or PT. Discontinuities that are not surface-breaking cannot be found by VT or PT and may be detected in only a very limited way by MT. Volumetric NDT methods, such as ultrasonic testing or radiographic testing, although able to detect surface discontinuities, are effective at indicating discontinuities in the interior of material.

Typically, VT must be performed by a Level II rather than a Level I technician because performance of VT inspection and evaluation often cannot be separated.

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Excerpted from ASNT’s Classroom Training Book: Visual Testing.

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For more information on visual testing:

• ASNT Level II Study Guide: Visual Testing
• ASNT Level III Study Guide: Visual Testing
• Relevant Discontinuities: Visual Testing
• Nondestructive Testing Handbook, 3rd edition, Vol. 9, Visual Testing Method

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From the NDT Library:

• “Remote Visual Testing: NDT Initial Inspection of Turbines and Boilers,” Materials Evaluation, Volume 74, Issue 12, 1 December 2016
• “Comparison of Visual, Eddy Current, Ultrasonic and Magnetic Particle Testing Techniques for Boiler Tube Inspections,” Materials Evaluation, Volume 70, Issue 5, 1 May 2012
  

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Visit the ASNT website for information on ASNT Refresher Courses and online learning opportunities.

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