INTRODUCTION

Penetrant testing is a highly sensitive method used for detecting discontinuities which are less likely to be detected by visual inspection. A flaw such as a fine crack open to the surface could go undetected during visual examination, but would be exposed with the proper application of the penetrant test.

Picture 1: A Capillary action of penetrant liquid

A unique characteristic of penetrant testing is its capillary action. It is the ability of a liquid to redistribute itself into or between closely fitted surfaces. This allows a flaw or discontinuity to be detected.

Liquid penetrant has certain limitations. Generally, it cannot be used on materials with very porous or highly absorbent surfaces. These materials could likely give false indications. The two basic types of liquid penetrant testing are:

• Fluorescent
• Visible dye

Picture 2: Discontinuos with visible dye

Picture 3: Discontinuos with fluorescent dye penetrant

Both types have several variations, but share the same basic procedures. They are the application of a liquid penetrant, the removal of the excessive penetrant, and the application of a developer to draw penetrant from any openings in the surface.

The main concept of both types of penetrant is to read the contrasting colors. For example, when fluorescent penetrant is applied to a surface containing a defect or discontinuity, the solid surface will appear as a deep violet. The flaw will show up as a yellow-green glow under an ultraviolet light. This is the most sensitive method.
 

Visible dye penetrant appears as a bright indication on a white background. It is easy to apply and is performed under normal lighting conditions.
 

The surface should be cleaned before application of the penetrant. There are several methods of cleaning. Precaution should be practiced when performing these cleaning operations because they involve chemicals. They may be highly flammable or toxic if inhaled.
 

Some cleaning methods such as grinding, chipping, machining, or shot blasting can prevent parts from being accurately inspected because discontinuities can be hidden by these procedures.

The penetrant is applied after the proper cleaning procedures have been completed. It can be applied by dipping, brushing, or spraying the area to be tested. The most portable method is by pressurized spray cans.
 

WATER WASHABLE PENETRANT is a dye placed in a water-based solution. For nonporous materials, it provides the lowest sensitivity because of the ease of penetrant washout of shallow openings.

POST EMULSIFIABLE PENETRANT is a dye placed in an oil-based solution. The post emulsifiable variation needs an application of an emulsifier to chemically break down the oil base of the excessive penetrant to render it water washable. For shallow surface openings, it allows for greater sensitivity because the penetrant remains in the openings as the emulsifier is washed away.

SOLVENT REMOVABLE PENETRANT remains the most popular. It is usually applied by a spray can to the inspection surface. Then, after a specified time, known as the dwell time, it is removed from the surface with a solvent on aleaning cloth, leaving the penetrant trapped below the surface intact. When performed
correctly, it can reveal ultra-fine openings only afew micro-inches in size.

APPLY VISIBLE DYE PENETRANT
Step 1

Penetrant is applied to a properly cleaned surface by brushing, dipping, or spraying. The method of application is based on the amount and size of the part being inspected

Picture 4: clean surface by brushing, dipping, or spraying

Step 2

Allow sufficient dwell time so the penetrant can enter any openings. Dwell time is based on the type of material and the degree of accuracy required. Longer time allows penetrant into smaller openings. Refer to the testing procedure or manufacturer’s recommendation for dwell time (usually 5 minutes to several hours).

Picture 5: Add penetrant liquid on surface

Step 3

Remove the excess penetrant by the appropriate means. When using water washable penetrant, rinse excess with a spray of water. Water temperature and angle of spray should be specified in a written test procedure. This avoids washing out of penetrant. When using the post emulsifiable penetrant, emulsifier dwell time should be monitored so only excess penetrant is removed.

Step 4

After the surface is properly cleaned and dried, the developer is applied. The blotter action of the developer draws out the penetrant left in any openings in the test area.

Picture 6: Add the develop on surface

Wet developers are powders suspended in liquid.  They can be applied by dipping, brushing or spraying.

Dry developer is in powder form and is dusted on the test surface.

The pressurized spray can is the most portable method of applying penetrant, cleaner, and developer.

Step 5
Examine and record test data drawn from the results of the penetrant test.

Picture 7: Cluster Porosity 

FLUORESCENT DYE PENETRANT INSPECTION

This type of penetrant testing is done in the same way as visible dye penetrant, except a fluorescent light source is needed to read the results after the dwell time on the developer has elapsed.

Picture 8: Numerous crack Indications overlapped to form patches

GENERAL INFORMATION ON PENETRANT INSPECTION

When penetrant inspection is performed on multipass welds, the penetrant must be thoroughly cleaned between every pass. Welding over test solutions could create discontinuities. Penetrant testing is an invaluable quality control technique. Like visual testing, care must be taken in drawing conclusions. The depth of a flaw cannot reasonably be measured. The inspector can only see penetrant indications that appear on the surface. The larger the flaw and the longer the time allowed for penetrant bleed out, the larger the indication will appear.

In conclusion, remember that the proper application of this test is based on the steps discussed and by
following the details of the written test procedures.