mag particle minimum detectable crack length? 7

[triumph406]

We have made a part that requires us to analyze the part using AFGROW fracure prediction software.

The customer specifies that we need to conduct the analysis using an initial flaw size of .03" long by .015" deep. With the part we have made we are getting marginal cycles to failure.

Can mag particle inspection (or dye penatrant) detect cracks shorter than .03" long? The customer specifies .03" as he thinks it's the shortest crack that can be reliably detected.

If mag particle can't detect a crack shorter than .030" then I can't deliver a part that I can gaurantee is crack free, if it could detect a .020" long crack, then I can run AFGROW witth a .020" crack, and then we would have an acceptable part.

The area in question is around the edge of a hole in a plate. We have radiused the entry into the hole to reduce the stress concentration, and will polish the area as well. Unfortuantly the part is already made.

Thanks in advance for any help

 

[desertfox]

Hi truimph406

flaws 1mm high by 5mm long according to this site:-

http://www.metserve.co.za/magnetic-paricle.htm

desertfox

 

[triumph406]

desertfox, thanks, I looked at that site as well, that's larger than I would have expected.

 

[desertfox]

Hi again

Well why not ring up an NDT company as an inquiry and ask them what a typical minimum crack size is for detection.


desertfox

 

[triumph406]

I did call the local NDI house, their mag particle operator said that ANY crack detectable was cause for rejection, they never had cause to measure the crack length, that wasn't required either by his customers or by MIL-STD 1944.

He did think that they could find crackes less than .030" long, but he wouldn't commit that to paper

 

[desertfox]

Hi again

Look at this link and go to page 4:-

http://mmptdpublic.jsc.nasa.gov/prc/7050c.pdf

desertfox

 

[Ron]

0.03" long is a difficult proposition for MPT. Dye penetrant would probably be better but it is difficult to measure actual length of an indication with DP due to the spread of penetrant past the deliniation of the crack. I would suggest a DP inspection followed by in-place microscopy to measure the actual crack size.

All of this requires more than your typical technician doing NDT/NDE. I would require an ASNT Level III and/or metallurgist.

 

[tbuelna]

triumph406,

The highest acceptance class (class A) for wrought materials in MIL-STD-1907, table I gives limits for mag particle surface discontinuity indications at .031 inch, and subsurface indications at .047 inch.

I doubt any inspectors would be willing to commit to anything more stringent than class A.

Good luck.
Terry

 

[JR97]

Triumph406,

I will give my two cents worth. I have been an ASNT Level III in MT and PT for 20 years. What others in this thread have posted are specification acceptance criteria; which in many cases, is driven more by the design community than the NDE community. I agree the literature does not put a minimum length of a crack to be reliably detectable. That is because there are so many different variables involved such as crack orientation relative to the magnetic field, crack depth, strength of the magnetic field and surface opening distance. In my experience, the smallest linear indication I have even seen with wet fluorescent MT is 1/32 inch; with dry visible MT is 1/16 inch. As your local NDE house said, that is based on inspection results, not a study of detectability for cracks of different orientations and depths with different strength magnetic fields.

My recommendation (again based only on my experiences) is 75% detectability of a 1/32 inch crack with a wet fluorescent inspection and 1/16 inch crack with a dry visible inspection. If you want to get up to 2 sigma reliability maybe triple those numbers.

JR97

 

[racookpe1978]

Sounds like you're trying to verify a crack detection method in a specific configuration on a specific part. Also, you r client appears to have some very, very specific (expensive) interest in getting you to accept/reject based on a theoretical length less than 0.030 - 0.020 in length. If so, your client should be willing to accept some test expense.

Dye pen can't detect anything not actually touching (penetrating) clear to the surface, but it can detect pinholes smaller than 0.010.

My recommendation? Make a second part - maybe make it out of a different/cheaper material, or skip the elaaborate machining that causes the expense. You probably cannot induce a specific flaw of a specific length inside the second/third/fourth casting, but check each of that run to see what is being detected. Then, carefully section whatever indications you do get, and section the test castings at four-five other spots where no indications were found. Measure what is present when/if any internal pits or cracks are found.

If 5 parts are sectioned at the critical point and you have found no cracks, and no cracks were found with MT at that area either on all 5 test parts, then you've verified your production methods and material. If your MT finds any flaws, you've checked those flaws and can determine what the actual lower limit is of the cracks you are finding.

 

[triumph406]

RON,

We have recommended that our customer use dye penetrant inspection, and the use of a microscope to verify the absence of cracks.

Apparently their technicians are well versed in the use of dye pen. We’ve recommended a vigourous system, where the customer will check the part every xxx minutes of testing.

JR97,

I think this is why our customer dictates a fracture analysis with the assumption that there is a pre-existing crack .030 long. This agrees with your statement that the smallest crack you have seen with wet fluorescent MT is 1/32inch.

racookpe1978,

Correct, if I could tell the client that I could detect a crack less than .030, then I could modify the fracture analysis accordingly and get increased fracture life. If I could use a crack length of .010 in the fracture analysis it would make a significant increase in cycles to failure.

The part we make are always made from forged, rolled or extruded materials. It’s extremely rare that we use castings, and if we did they would likely not be used for a structural part.

Most of the important parts are UT inspected to MIL-STD 2154 class AA before we do any machining, this is to ensure there are no large flaws before machining. Sometimes if flawas are found by UT, we can orientate the part in the material to avoid the flaw. Sometimes we need new material.

The machining processes we use on these parts are all milling operations on a 3-axis mill. So my assumption is that in general if the flaw is not in the original material, then assuming we do not drop the material, run the tool into the material, run the tool dry and burn up the tool, etc, then we should be free from cracks.

We do wire and plunge EDM parts, but we don’t use these processes on critical parts, at least not inb critical areas. Where do use the EDM process, we will machine/polish off the recast layer.

We’ve had failures due to parts cracking, but so far the the cracks have either been a failure due to a design error, or manufacturing error (sharp corner for instance)

 

[Tmoose]

Back when doing a lot of wet method mag particle (Magnaglow) and fluorescent dye penetrant inspection in a non-certified automotive shop I did some back-to-back tests that left me pretty well satisfied that Magnaglow was much more sensitive finding "indications" in crankshaft fillets, etc. where fatigue failures were likely to grow. Minimum length was not investigated. The dye pen completely missed indications that caused Magnaglow to bay like a hound.

 

[tbuelna]

triumph406,

racookpe1978 pointed you in the right direction.

If your AFGROW analysis is part of a typical aerospace fracture control plan, and you want to use an NDT procedure as part of your QA validation process that has accept/reject criteria outside the scope of existing practice, then you would likely need to develop and document your own qualified process using that criteria.

For a fracture control plan to be valid, it must demonstrate that the NDT procedure proposed in the FMEA will consistently detect the min. flaw size used in the fracture analysis. Developing, qualifying, and implementing a NDT QA process like the one you envision is no small task.

QA in aerospace is predicated upon using tightly controlled process and procedure.

Good luck.
Terry

 

[triumph406]

The initial question is from a project I did a few months ago, we had a support bar that we had to widen due to a stress concentration caused by a hole in the support bar. It wasn't until after we had made the part that we realised that we had a problem with the fracture analysis.

I had a question posted at the time about 15-5 vs 13-8 in the thread

http://www.eng-tips.com/viewthread.cfm?qid=279275&page=1

The same problem arose again when somebody I used to work with was completing a stress report and had the same fracture analysis issue.

Unfortuantly there isn't budget provided by the customer, or company funds availible to do any comprehensive testing or validation. The only time we have ever had budget was to do some fatigue etsting on Aermet 100.

We have recommended to customers that they monitor for any crack growth during testing. So far I've never seen any problems with crack growth, which suggets that there wasn't a crack in the critical region to begin with. The only time I've ever had a part crack was due to design error/brittle material/manufacturing error.

We had a mobile service come in that did onsite mag particle inspection, I had to be very specific about the orientation of cracks we were looking for, without specific guidance I'm not sure we would be getting the best inspection possible.

It's all moot anyway, I was laid off from my job this last Monday, so somebody else can worry about it.

 

[triumph406]

Tmoose,

If you were doing Dye pen inspection on a used crankshaft (I assume it was used) maybe the crack(s) were full of oil, which might prevent proper absortion of the dye.

Just wondering

 

[Tmoose]

Hi Triump406,

It's possible I guess. Back then We used a hot 111 trichlor vapor degreaser for initial cleaning before MX or Zyglo.

Here's a picture of a sad very used gear I was looking at yesterday. I think the shop did prep cleaning just with manual wiping and maybe light manual flush with lacquer thinner prior to (non-fluorescent) spot check. The penetrant got at least part-way into the tooth cracks fairly well.