# small relatively inexpensive crack detection apparatus for bike shops

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? • There are aerosols with a “crack detecting” fluid and an indicator powder but more than that is probably outside your budget... Jan 27 '19 at 18:06 • Thanks. A little outside the budget or way outside the budget? A few hundred more or thousands more? Jan 27 '19 at 18:08 ## 4 Answers 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.

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".

• Are microscopic cracks in the metal transmitted through a paint layer? Jan 28 '19 at 8:57
• 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. Jan 28 '19 at 16:08

Eddy current analysis, we paid $17k per unit, but they're for manufacturing lines. I'm not sure how much a smaller hand-held unit with a stylus-type probe is. We buy our equipment from Magnetic Analysis Corp. but I don't know if they have the smaller equipment. 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. • So, not really a relevant answer as the budget was "under$500"... Feb 2 '19 at 9:15
• 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 :) Feb 3 '19 at 10:35
• UT requires a trained experiences operator , especially for thin ( < 3mm ) material . Guess \$ 500 / hour to evaluate the first job. Dec 23 '20 at 1:10