Dye penetrant inspection

Introduction

Liquid penetrant inspection is a method that is used to reveal surface breaking flaws by bleedout of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action. After a period of time called the "dwell," excess surface penetrant is removed and a developer applied. This acts as a blotter. It draws the penetrant from the flaw to reveal its presence. Colored (contrast) penetrants require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet "black light".

Penetrants are classified into sensitivity levels. Visible penetrants are typically red in color, and represent the lowest sensitivity. Fluorescent penetrants contain two or more dyes that fluoresce when excited by ultraviolet (UV-A) radiation (also known as black light). Since Fluorescent penetrant inspection is performed in a darkened environment, and the excited dyes emit brilliant yellow-green light that contrasts strongly against the dark background, this material is more sensitive to defects.

When selecting a sensitivity level one must consider many factors, including the environment under which the test will be performed, the surface finish of the specimen, and the size of defects sought. One must also assure that the test chemicals are compatible with the sample so that the examination will not cause permanent staining, or degradation. This technique can be quite portable, because in its simplest form the inspection requires only 3 aerosol spray cans, some lint free cloths, and adequate visible light. Stationary systems with dedicated application, wash, and development stations, are more costly and complicated, but result in better sensitivity and higher samples through-put.

Application

Materials that are commonly inspected using LPI include the following:

• Metals (aluminum, copper, steel, titanium, etc.)
• Glass
• Many ceramic materials
• Rubber
• Plastics

Liquid penetrant inspection can only be used to inspect for flaws that break the surface of the sample. Some of these flaws are listed below:

• Fatigue cracks
• Quench cracks
• Grinding cracks
• Overload and impact fractures
• Porosity
• Laps
• Seams
• Pin holes in welds
• Lack of fusion or braising along the edge of the bond line

Benefits

• High sensitivity (small discontinuities can be detected).
• Few material limitations (metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive materials may be inspected).
• Rapid inspection of large areas and volumes.
• Suitable for parts with complex shapes.
• Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.
• Portable (materials are available in aerosol spray cans)
• Low cost (materials and associated equipment are relatively inexpensive)