Liquid Penetrant Testing (PT)

Liquid Penetrant Testing (PT) is a simple and widely used non-destructive testing (NDT) method for detecting surface-breaking defects in non-porous materials. It is valued for its ease of use, low cost, and ability to reveal very fine cracks that may not be visible to the naked eye.

Overview

PT works by applying a liquid with high surface wetting characteristics to the surface of a component. The liquid penetrates into surface-breaking flaws through capillary action. After excess penetrant is removed, a developer is applied, which draws the penetrant back out of the flaws to form visible indications. This makes defects such as cracks, porosity, and seams clearly visible under normal or ultraviolet light.

Apparatus and Working

Apparatus

  • Penetrant: Colored (visible red) or fluorescent liquid with high wetting ability.
  • Cleaner/Remover: Solvent or water-based cleaner to remove excess penetrant.
  • Developer: Powder or suspension that draws penetrant out of flaws and provides contrast.
  • Light Source: White light for visible penetrants; UV (black light) for fluorescent penetrants.
  • Accessories: Spray cans, brushes, lint-free cloths, and inspection booths.

Working Steps

  1. Pre-cleaning: Surface is cleaned to remove dirt, grease, or paint.
  2. Application of penetrant: Penetrant is applied and allowed to dwell for a set time.
  3. Excess removal: Excess penetrant is carefully removed from the surface without flushing flaws.
  4. Developer application: Developer is applied to draw penetrant out of flaws.
  5. Inspection: Surface is examined under proper lighting; defects appear as bright or colored indications.
  6. Post-cleaning: Component is cleaned if required for service.

Principle

The principle of PT is based on capillary action. When a liquid penetrant is applied to a clean surface, it seeps into any surface-breaking discontinuities due to surface tension and capillary forces. After removal of excess penetrant, the developer acts like a blotter, pulling the penetrant out of the flaws and creating a visible indication that highlights the defect.

Advantages and Disadvantages

Advantages

  • Simple, inexpensive, and portable method.
  • Can detect very fine and tight surface cracks.
  • Applicable to a wide range of materials (metals, ceramics, plastics) as long as they are non-porous.
  • Provides immediate results with minimal equipment.

Disadvantages

  • Only detects surface-breaking defects (not subsurface).
  • Surface must be properly cleaned; contaminants can mask defects.
  • Not suitable for porous materials (penetrant will soak in everywhere).
  • Post-cleaning is required to remove chemicals from the component.

Applications

  • Aerospace: Inspection of turbine blades, landing gear, and structural components.
  • Automotive: Checking engine blocks, crankshafts, and suspension parts for cracks.
  • Welding: Detecting surface cracks, porosity, and lack of fusion in welds.
  • Power generation: Inspection of boiler tubes, turbine rotors, and pressure parts.
  • General engineering: Quality control of machined parts, castings, and forgings.

Welding Defects Detectable by PT

  • Surface cracks: Hot cracks, cold cracks, and stress-induced cracks open to the surface.
  • Lack of fusion (surface-breaking): Areas where weld metal has not fused with base metal.
  • Incomplete penetration (surface-reaching): Root defects that are open to the surface.
  • Porosity (open to surface): Gas pores that break through the weld surface.
  • Seams and laps: Rolling or forging defects that extend to the surface.
  • Undercut (if surface-breaking): Groove-like defects along the weld toe.
  • Overlap: Excess weld metal lying on the surface without fusion.