Visual Testing (VT) means observing a part with the naked eye or with optical aids to find surface anomalies and confirm it meets its specification. VT is almost always the first NDT method applied, and often the most important. For welds, it is the most widely used method.
Direct and remote VT
- Direct VT: an unbroken line of sight from the eye to the surface, with or without simple aids such as a magnifier, mirror or torch. The usual rule is the eye within 600 mm of the surface at an angle of at least 30°.
- Remote VT: the line of sight is interrupted, so a borescope, fiberscope, video borescope, camera or robot is used. Its resolution must be at least equal to direct VT.
Light: the requirement people underestimate
The main VT instrument is still the human eye, and the eye needs enough light. Room lighting may feel bright, but without measuring it with a light meter you cannot prove the code minimum is met. Recent editions of ASME Section V require at least 100 fc (1076 lux) at the examination surface. Too much light is also bad because it causes glare. To reveal texture, dents and pitting, oblique (raking) light works best because it casts shadows.
The brain plays a part too: we tend to see what we expect. An inspector hunting only for porosity can miss lack of fusion at the weld toe. That is why large areas are inspected in small, overlapping sections.
VT tools for welds
- Magnifier (loupe), e.g. 10×; choose the lowest power that shows the detail.
- Inspection mirror for the inside of pipes or behind objects.
- Steel rule, calipers and micrometer for dimensions.
- Weld gauges: fillet weld gauge (leg size and throat), bridge cam gauge (bevel angle, misalignment, reinforcement, undercut depth), hi-lo gauge (pipe misalignment) and pit gauge (corrosion and undercut depth).
- Borescope, fiberscope and video borescope for areas you cannot see directly.
Basic visual weld inspection steps
- Find the governing code or written procedure, including its acceptance criteria.
- Decide on direct or remote VT, cleaning needs, lighting (measure it!) and measuring tools.
- Clean the surface of scale, slag or paint as the procedure requires.
- Inspect at the right stages: joint preparation and fit-up, root pass, between passes and the final weld. Problems caught at fit-up are far cheaper to fix.
- Measure the weld profile with gauges, record indications and accept or reject against the criteria.
- Write the report: code, procedure, location, results, examiner name and level, plus photos or sketches when required.
Weld flaws found by VT
VT finds surface porosity, cracks (including crater cracks at weld stops), undercut, overlap, arc strikes, incomplete penetration on an accessible root side, and undersized or oversized welds. Flaws inside the weld, such as slag, buried porosity, lack of fusion and root cracks inside a fillet joint, need RT or UT. That is why VT normally comes first, with other methods following when required.
Advantages and limitations
| Advantages | Limitations |
|---|---|
| Economical and fast | Needs a line of sight to the surface |
| Sensitive to many surface anomalies | Needs adequate, measured lighting |
| Works on parts of almost any size and shape | Finds surface flaws only |
| Easy to document with photos and video | Depends heavily on the inspector’s skill and experience |
Learn VT from scratch
In the Visual Testing (VT) Level II course you learn light, optics, VT tools and codes through interactive animations, then practise on a weld inspection bench simulator with a bridge cam gauge and fillet gauges. Start with free Lesson 1: VT Overview & the Human Eye.
Fact source: VT Level II workbook, pp. 7–16, 28–35, 39–51.
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