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Is there a relatively inexpensive (let's say under $500US) crack-detection device and related mobile app that would be easy enough for a non-engineer/non-metallurgist to use to determine whether a bicycle frame was forming cracks that would be undetectable to the human eye?

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    $\begingroup$ There are aerosols with a “crack detecting” fluid and an indicator powder but more than that is probably outside your budget... $\endgroup$ – Solar Mike Jan 27 at 18:06
  • $\begingroup$ Thanks. A little outside the budget or way outside the budget? A few hundred more or thousands more? $\endgroup$ – Tᴚoɯɐuo Jan 27 at 18:08
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Solar Mike is exactly right, the method he cites is called dye penetrant and it works for ferrous metals, aluminum alloys, and composites as well. all you need is a bottle of the fluorescent dye, the spray-on developer powder, and a UV light. This will cost less than $100.

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There are three dye-pene types. The cheapest one is likely good enough ;1- A dye is applied ( usually red), 2- allow to soak ,3 - clean off surface dye( May use a cleaner for this ), 4- spray on a white developer: Depending on experience ,very small cracks can be found in any material. The second type of dye-pene uses a fluorescent dye and UV light ; faster than the red dye but requires a dark room and UV light. The third type uses magnetic fluorescent dye , a magnet and UV light but only works with steel . Cracks revealed by the red dye viewed days later. Also , I think red dye is the best method to reveal porosity or other non-linear imperfections . Also best if results need to be viewed by several people to make a judgement. Not likely to be your problem , but I remember well several people tying to squeeze under the curtain to view the UV light findings. The red dye only requires three aerosol cans ( cleaner, dye, and developer) but I have no idea what the price is today....PS - The method I call "magnetic fluorescent dye" is commonly called "wet mag".

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  • $\begingroup$ Are microscopic cracks in the metal transmitted through a paint layer? $\endgroup$ – Tᴚoɯɐuo Jan 28 at 8:57
  • $\begingroup$ Depends on what size "microscopic" is. Generally yes ; it depends on the amount of local strain. There is an analysis technique called "brittle lacquer" where a paint is put onto a model and then it is loaded. The paint cracks show the strain pattern. $\endgroup$ – blacksmith37 Jan 28 at 16:08
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Dye penetrant is a good solution but it's finite, once gone you'll need to buy more. A long term solution is an Ultrasonic Flaw Detection device. Unfortunately your looking at around 2k for a cheap one.

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  • $\begingroup$ So, not really a relevant answer as the budget was "under $500"... $\endgroup$ – Solar Mike Feb 2 at 9:15
  • $\begingroup$ I was hoping the answer would be "Sure, you can spend 250 dollars on a transducer and 50 dollars on app X which is geared to the layman" but I gather that's pie in the sky. What I had in mind was something bike owners would have done at least annually, so the per-inspection cost would have to be an amount the average bike owner would be willing to fork out :) $\endgroup$ – Tᴚoɯɐuo Feb 3 at 10:35

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