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DIY crack testing

Author: John Hopwood

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At the October Meeting of West Lancs Group, member John Rigby, who is an Engineering Inspector for an international insurance company, gave an explanatory talk and demonstration of crack-testing, using the 'magnetic particle method'. This is far more 'searching' than 'dye penetrant' and is really a rather simple process.

After removing all paint, rust and grease (Fig.1), John first sprayed the part under test with a fast-drying white paint followed by a ferrous powder in a liquid suspension. A horse-shoe magnet was then applied to the part, in the region of suspected cracks. The magnetic flux is concentrated at discontinuities (cracks) which acts to attract the ferrous powder and shows up clearly against the white background. My steering-arm is shown, as tested, (Fig.2) and cracks are evidenced by the two transverse black lines in the area we all know to be highly stressed.

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I was rather miffed at this as I regularly test it using 'dye penetrant', which has shown nothing in the past. Have I been driving round in blissful ignorance, or is this something recent?

Not wishing a failure whilst driving along one of the narrow Polder Dyke roads on the Dutch Windmill Run (with a 5 metre drop either side into water) something had to be done!

Before investing in a new (£75) arm and having plenty of 'play-time' at this time of year, I decided to investigate further to see if the cracks were just shrinkage-cracks at the surface on cooling after forging - as has been suggested – or were potentially rather more sinister. I therefore needed to be able to 'see' these cracks, in order to know when I had reached the bottom.

A further test using my 'dye penetrant' again showed nothing (Fig.3), so this was no use for my purpose. What I needed was a kit like John had used – could I reproduce his result using materials readily available in most of our garages? I already had a good horse-shoe magnet. The white paint was easy, a spray can of white primer worked well.

The ferrous powder proved a little more elusive – I tried digging out the debris from under my bench-grinder, but this was far too coarse to stay in suspension. Diligent rubbing of a piece of steel on a sheet of 600 wet & dry eventually produced enough powder of the right consistency, which was then suspended in a small amount of paraffin to enable it to be dribbled over the part.

Applying my magnet resulted in a most satisfactory conclusion, I could see the cracks appearing before my eyes (Fig.4) very clearly.

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I measured the section of the arm then set about grinding in this area, a little at a time (don't want to go any deeper than necessary) with re-tests along the way. I 'hit bottom' at 40 thou. so I blended-in and polished the ground area (Fig.5) before my final test (Fig.6) – which shows a perfect result.

DIY crack testing< DIY crack testing<

Conclusion:

  1. Yes, they were surface cracks, but could have acted as stress-raisers if left.
  2. I consider that the reduction in section of 6% to remove them is perfectly acceptable.
  3. My DIY method of magnetic particle crack-testing is very effective and so easy that you should all try it over the winter – you might surprise yourself!
  4. Results using 'dye penetrant' are suspect when dealing with small cracks.

Finally, I thank John Rigby for steering me along the way to a satisfying conclusion

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