Visual Testing (VT)

Visual Testing (VT) is the oldest and most widely used non-destructive testing (NDT) method. It involves the direct observation of a component’s surface to detect visible defects. VT is simple, cost-effective, and forms the foundation of all inspection activities.

Overview

VT is performed by trained inspectors using the naked eye or with the aid of optical instruments. It is often the first step in any inspection process, ensuring that obvious surface flaws, dimensional inaccuracies, or surface conditions are identified before applying more advanced NDT methods.

Apparatus and Working

Apparatus

  • Lighting: Adequate natural or artificial light sources.
  • Magnifiers: Hand lenses or microscopes for enhanced viewing.
  • Borescopes / Endoscopes: For internal or inaccessible areas.
  • Measuring tools: Rulers, gauges, calipers, weld profile gauges.
  • Surface preparation tools: Wire brushes, cleaning agents, and cloths.

Working Steps

  1. Surface preparation: Clean the surface to remove dirt, oil, or scale.
  2. Lighting setup: Ensure sufficient and uniform illumination.
  3. Observation: Inspect the surface directly or with optical aids.
  4. Measurement: Use gauges or tools to check dimensions and tolerances.
  5. Recording: Document findings with notes, sketches, or photographs.

Principle

VT is based on the principle of direct human observation. Defects that are open to the surface or cause visible irregularities can be detected by the inspector’s eye, often enhanced with magnification and proper lighting. The effectiveness of VT depends on inspector skill, eyesight, and inspection conditions.

Advantages and Disadvantages

Advantages

  • Simple, quick, and inexpensive.
  • Requires minimal equipment.
  • Can be applied to almost all materials and components.
  • Immediate results with no processing required.
  • Useful for detecting obvious surface flaws and dimensional issues.

Disadvantages

  • Limited to surface-visible defects only.
  • Dependent on inspector’s eyesight, skill, and experience.
  • Requires good lighting and surface accessibility.
  • Not effective for subsurface or very fine cracks without magnification.
  • Subjective results if not standardized with procedures.

Applications

  • Welding: Checking weld profiles, surface cracks, undercut, overlap, and spatter.
  • Castings: Detecting surface porosity, shrinkage cavities, and misruns.
  • Forgings: Identifying laps, seams, and surface cracks.
  • Machined parts: Verifying dimensions, surface finish, and tool marks.
  • General inspection: Corrosion, erosion, wear, and surface damage in service components.

Welding Defects Detectable by VT

  • Surface cracks: Visible cracks on weld face or toe.
  • Undercut: Groove-like defect along the weld toe.
  • Overlap: Excess weld metal lying on the surface without fusion.
  • Porosity (open to surface): Gas holes visible on the weld surface.
  • Incomplete fusion (surface-reaching): Lack of bonding visible at weld edges.
  • Excessive reinforcement: Overfilled weld bead beyond acceptable limits.
  • Misalignment: Improper joint fit-up or weld placement.
  • Spatter: Small globules of weld metal adhering to the surface.