Lighting tips for looking at Stress-Coat?

[oghk2000]

I am a mechanical engineer. Where I work we often apply a thin layer of a brittle lacquer called stress-coat over prototype metal parts. Then we apply an appropriate load to the part. If cracks or crazing appear in the stress-coat we know we have found a weak spot in the part.

But finding these cracks in the coating can be tricky. We often use whatever flashlight is in someone's tool box to shine on the surface to find cracks in the stress-coat. We find that if we hold the flashlight close to the part and shine across the surface it helps more than just shining the light perpendicular to the surface.

Has anyone else out there done this and found ways of making the stress coat cracks more visible? I see red filters used for night time, and blue filters used for tracking; would some color of filter help? I am a newbie with flashlights and don't have my own arsenal of lighting readily available to try different things with. I do have a 2 AA Mini-Maglite with red and blue filters. But it has so much artifact (see, I DID read the website) I am not very happy with it.

You would think the companies that sell the coating would be capitalizing on this and selling flashlights to use too, but I haven't seen any suggestions from them.

I appreciate any suggestions you might have.

 

[yellow]

sounds like You basically search for shadows created by those cracks.
imho thats nothing a colored light would be more useful.
Color would be effective if You woul, f.e., apply some kind of solution that enters cracks and lights up under special light.

Just get a led light (brightness while still runtime, no artifacts) and try if it works the same / better

PS: as this sounds like very short range illumination, a wide flood souds good

 

[Marduke]

You use it all the time? There are all sorts of nasty federal regulations on it's frequency of use in the US.

 

[SilverFox]

Hello Oghk,

Welcome to CPF.

I use a SureFire 8NX for that type of inspection. The SureFire beam has a large, artifact free, hot spot that does a much better job of evenly illuminating the surface.

I have tried various LED lights, and feel that the micro cracks are more visible with incandescent lighting. You can see them with LED's, but you have to look a little harder. To me, they jump out at you under incandescent lighting.

Your technique is correct, but there is a "sweet" spot in the angle that you have to find. If your parts are of a complex shape, it is difficult to maintain the "sweet" spot as you move around the part.

I believe there is also a base coat that you can spray on to help with contrast.

The industry has moved away from this and is now using photo stress plastic sheets. They respond to polarized light and give more accurate results. Plus they are easier to document.

I would recommend that you go to your local gun shop and test a SureFire incandescent light. Study the beam profile and memorize it. Now you have an idea of what you are looking for and can shop around.

Tom

 

[oghk2000]

Marduke: I am sure my company is in compliance with all the regulations. They are very proud of their safety and environmental records.

SilverFox and Yellow: Thank you for your recommendations. Regarding the photo stress plastic sheets; I bet they cost more, right? You won't see it here unless it costs less. Just like if I want to use a better flashlight I'll have to buy it myself. Again, thank you.

 

[KenAnderson]

oghk2000,

Have you tested UV yet? I'm quite sure that a bottle of UV detectable spray would help. Spray a mist over the item and hit it with ultra violet, the mist collects in the cracked areas and lights up perfectly. Don't know if this is allowed in your environment or not. The UV without mist may provide an answer but make sure to wear protective glasses.

Ken

 

[Marduke]

oghk2000,

Have you tested UV yet? I'm quite sure that a bottle of UV detectable spray would help. Spray a mist over the item and hit it with ultra violet, the mist collects in the cracked areas and lights up perfectly. Don't know if this is allowed in your environment or not. The UV without mist may provide an answer but make sure to wear protective glasses.

Ken

ZYGLO does just that, but it's meant for detecting cracks in a metallic surface, and wouldn't work well with a cracked brittle-crack painted surface.

I personally use flat white spray paint to detect fatigue cracks, and FEA to identify areas of interest for precision strain gages.

 

[oghk2000]

I spend most of my time doing the FEA part. I don't do any of the application of the stress-coat. I just get called to help look for cracks and suggest locations for strain gages.