Fracture face analysis 7

[coreman73]

I have a few fractured T-slot bolts that failed prematurely while in service. Simply based on what is visible from the attached fracture surface photos, what would be the general opinion on type of failure mode? I believe it's obviously fatigue but am unsure of anything else beyond that. Maybe by unidirectional bending? Any input would be appreciated.

Background information is that these bolts were formed from AISI 1045, not heat treated and surface finished with black oxide. The failures shown occurred in the threaded portion of shank. It is unknown how long these bolts were used before failure happened.

 

[TVP]

coreman73,

I am not aware of any standard that requires rolled threads for T-slot bolts. It is just good practice, based on what stanweld and the others already mentioned (repeated assembly/disassembly, variation in clamping force, cyclic stresses due to vibrations, etc.).

 

[Guest102023]

I'll offer my opinions, but I won't offer any more until I renew my professional liability insurance:

(1) The threads are clearly rolled; the crests are truncated. The roots are free of obvious forming defects (the competitor's root in fact shows a minor fold or lap defect).
(2) The smooth portions of the fracture surfaces are consistent with fatigue, which is caused by cyclic loading with a significant tensile component.
(3) The rough portions of the fracture surfaces are the result of sudden, final fracture. This occurs once the diminished load-bearing surface is insufficient to withstand the bolt preload.

We can't conclude much more than the above without more information. The deformation of the threads is not necessarily a consequence of overloading; it might be post-fracture damage.
__________________________________________

You need to haul these parts off to a qualified professional METALLURGICAL failure analyst if you want better answers. This is obligatory if there are even remote safety implications to a repeat failure.

As I read through some of the posts I grew increasingly annoyed by the uninformed conjecturing. For example, we can NOT conclude from the evidence that the bolts were overloaded. Worse, it is in general impossible (without X-ray vision) to read anything about the structure when the section has not been etched(!) Failure analysis is not something a person learns by being peripherally involved in a few incidents; it is not like learning how to do a stress calc or a mass balance. It especially should not be done just because the pointy-haired boss assigns it to you because you have an engineering degree. End of rant.

 

[desertfox]

Hi coreman73

Looking at the photo in your first post then I agree with the others its definitely fatigue as it has the classic signs of fatigue failure. It would be interesting to see a picture of the fixture or sketch showing the position of the bolts that failed in the fixture if thats possible.
I did notice that the instanteous area of both the failed samples is quite large, this indicates the bolts in these positions are quite heavily loaded.

http://www.maintenanceworld.com/Articles/reliabilitycenter/Failure-analysis-mechanical.htm

http://books.google.co.uk/books?id=G8pQm3B9g3cC&pg=PA449&lpg=PA449&dq=how+to+spot+a+

rolled+screw+thread&source=bl&ots=IpxkXXEUSx&sig=DLwxr_
WfwFJS3neEoFl10u4Riqs&hl=en&ei=q1MpTYP1A86GhQeopsGjAg&s
a=X&oi=book_result&ct=result&resnum=7&sqi=2&ved=
0CEYQ6AEwBg#v=onepage&q&f=false

The first link explains about observing bolt fracture surfaces and the second link shows the difference between cut and rolled threads, note on the latter that the stock diameter for a cut thread is roughly the major diameter of the screw and for the latter its the pitch diameter.

desertfox

 

[adamuk]

You need to haul these parts off to a qualified professional METALLURGICAL failure analyst if you want better answers. This is obligatory if there are even remote safety implications to a repeat failure.

As I read through some of the posts I grew increasingly annoyed by the uninformed conjecturing. For example, we can NOT conclude from the evidence that the bolts were overloaded. Worse, it is in general impossible (without X-ray vision) to read anything about the structure when the section has not been etched(!) Failure analysis is not something a person learns by being peripherally involved in a few incidents; it is not like learning how to do a stress calc or a mass balance.

I couldn't agree more. This is a great forum but shouldn't be a substitute for professional advice. Most of the forums I lurk on seem to be about chemical, petrochemical or oil and gas problems and an internet forum's not the place to get advice, especially if the consequences of failure could be severe.

Adam Potter MEng CEng MIMechE

http://www.ax-ea.co.uk

 

[cloa]

Internet forum is fine place to gather advice and guidance which you must assess yourself against the standards, textbooks and other authoritative sources but it is a poor place for engineering judgement of a problem (which stands alone and carries a liability) is more correct. No liability is possible on an internet forum.

 

[coreman73]

brimstoner,
Thank you for your post and opinions. I agree with your rant as well. I began this thread realizing that there would be lots of conjecture. Actually, that's what I was looking for. I wanted ideas and opinions based on the information I had available to me at the time. I'm trying to continue my failure analysis education after having been mentored by a failure analyst for the last couple of years (he's retired now). How else am I supposed to do this without trying to learn from other individuals much more experienced than I am?

Considering this, I would never take on a job where my findings would be considered as part of a liability case or where someone's safety was at risk. I realize I'm not a pro by any means and am certainly not qualified for that.

What I have learned is that there are very few definites in failure analysis and that's what makes it frustrating and challenging at the same time. There are lots of likely, maybe, possibly, could be. I consider this very strongly every time I'm given advice on this site. At my job I'm part of the process, and the information I provide simply gives manufacturing more options to consider before making any final decisions.

adamuk,
You said "Most of the forums I lurk on seem to be about chemical, petrochemical or oil and gas problems and an internet forum's not the place to get advice, especially if the consequences of failure could be severe." So what is the purpose of this site? Why do you offer advice (which I appreciated very much) then if you say an internet forum isn't the place to get it?

cloa,
Those are my thoughts exactly. I use the advice/opinions I'm given along with my ASM failure analysis handbooks. Short of paying for consulting fees with professionals for their advice (filled with more maybe, likely and possibly), it's my only option.

 

[adamuk]

Hi coreman73,

IMHO, the site's useful for fellow engineers to obtain opinions from other engineers, to get maybe a bit of guidance on where they can get help or look more broadly at a problem, or even information on career development, or just some moral support.

If you look on the pressure vessels section quite a few engineers offer advice such as which part of the ASME codes to look into or where to read up on a particular issue or problem which is really helpful as it saves time.

I am happy to give my opinions on what your problems might be to allow you to investigate further (and indeed get advice too) and more than willing to help but there's a difference in trying to point people in the directions where to look (i.e. the bolt may be overloaded) vs. making a definite statement as to what the problem is and what is needed to solve it. As always: Caveat Emptor!!

I hope I didn't sound negative but the adage about not believing all that you read on the internet's kinda true here too.

Adam Potter MEng CEng MIMechE

http://www.ax-ea.co.uk

 

[coreman73]

adamuk,
I agree completely. I joined this site hoping to have people point me in the right direction through their advice based on their previous experience. Believe me, I consider everything seriously before just accepting what is said here. Thanks again for your help.

 

[Guest102023]

My apologies for perhaps coming on a little strong.
I find this site extremely useful,and appreciate the comments by those who are clearly top experts in individual fields, whereas I am a more of a generalist in failure analysis. I am a sole operator and don't have a budget for every up-to-date ASME Code book and ASM handbook. I do give back, but I try to restrict my comments to subjects where I feel competent and can actually contribute positive information. I would not give an opinion here that I would not give to a client (although I can not use sarcasm with a client!)

Coreman73, I would not have been successful or even gotten started in the failure analysis business had I not benefitted from an outstanding mentor.

My feelings are possibly also reflective of the larger issue of professional competence in engineering, and the distinct lack of direction provided by the association in my particular juristiction. To use welding engineering as an example, we have a lot of structural engineers marketing themselves as 'welding' engineers because they have attended a couple of welding seminars (nothing wrong with structural engineering). In my last company (the name of which would be recognized by many of you), every metallurgical engineer had no compunction in calling themselves 'welding engineers' when some have never walked a shop floor in their lives.

I welcome any feedback, although I realize I have broken one of the house rules by straying far off topic