Dye Penetrant Testing

Dye penetrant testing examines the surface of an item (non destructively) for surface-breaking flaws, such as cracks.  A liquid penetrant is applied to the surface and left to soak.  The liquid is drawn into any cracks via capillary action.  The liquid is typically brightly colored or a fluorescent (under UV light) dye.  After the soak time has expired, the excess penetrant is wiped from off and a developer is applied.  The developer is usually a dry white powder (for example chalk powder) suspension that is spayed on the component.  The developer is drawn out of the crack by reverse capillary action, resulting in a colored indication on the surface that is broader than the actual flaw, and therefore, much more visible. This technique can be used to detect surface flaws on essentially any non-porous material.  Typical applications include:

Grinding cracks
Heat zone cracks
Poor weld penetrations
Heat treatment cracks
Fatigue cracks
Inclusions

An example of the process is shown below.

Step 1.

  
A bicycle crank arm was tested because the owner noticed a crack on the square axle attachment point and suspected a crack at the pedal attachment point.

Laminations
Micro shrinkage
Hot tears
cold shuts
Stress corrosion cracks
Intergranular corrosion

Step 2.
The suspect areas were sprayed with a red dye penetrant and left to soak.

Step 3.
The dye was wiped off and the crank was sprayed with developer (chalk).  The crack at the axle attachment is clearly visible.  The suspected crack at the pedal attachment showed no red dye, indicating that it was a scratch and not a crack.

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