Knowledge

Penetrant Testing (PT)

This involves applying a liquid dye to the surface of a material and leaving the dye to “dwell” on the surface for a pre-determined period of time. The liquid can be either a color that is easily visible under normal lighting conditions or a yellow/green fluorescent color that requires special lighting conditions to be seen effectively.

This liquid dye enters into discontinuities that are open to the surface of the material through a phenomenon called “capillary action”. This capillary action takes place throughout the dwell time and the discontinuity retains this dye when the excess dye is cleaned from the surface. A type of developer is then applied to the surface of the material and the dye that is trapped inside the surfacediscontinuities is blotted back out on to the surface and forms an indication. This indication is then interpreted by a qualified interpreter.


Types of Penetrant Materials:

  • Type 1 - Fluorescent Penetrants: High sensitive, comes usually green in color and fluoresce brilliantly under ultraviolet light
  • Type 2 - Visible Penetrants: Less sensitive, usually red in color, viewed under adequate white light
  • Type 3 – Dual mode penetrants: Viewed under black light or white light

Advantages:

  • High sensitivity to small surface discontinuities
  • Large areas and large volumes of parts/materials can be inspected rapidly and at low cost
  • Parts with complex geometric shapes are routinely inspected
  • Aerosol spray cans make penetrant materials very portable

Disadvantages:

  • Only surface breaking defects can be detected
  • Only materials with a relatively nonporous surface can be inspected
  • Pre-cleaning is critical since contaminants can mask defects
  • Metal smearing from machining, grinding, and grit or vapor blasting must be removed prior to LPI
  • The inspector must have direct access to the surface being inspected
  • Surface finish and roughness can affect inspection sensitivity
  • Post cleaning of acceptable parts or materials is required
  • Chemical handling and proper disposal is required

Probability of detection:

In general, penetrant inspections are more effective at finding

  • Small round defects than small linear defects
  • Deeper flaws than shallow flaws
  • Flaws with a narrow opening at the surface than wide open flaws
  • Flaws on smooth surfaces than on rough surfaces

Application:

  • Plates, weld, cast, wrought, and forge products.
  • In- service boilers and pressure vessel weld joint and high stress areas in refinery and processing industry.
  • Propellers, drive shafts, engine crank shafts, hull structure, gears, crane hooks, in Ship Building industry.
  • Wings, landing gears and wheel hubs, fuselage, links, hydraulic cylinders, turbine blades and shafts, leading and trailing edge flaps in aerospace industry.
  • Turbine and generator parts, heat exchangers, headers, and pumps and valves etc. in power industry.
  • Automotive industry