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Fracture face analysis
7

Fracture face analysis

Fracture face analysis

(OP)
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.
 

RE: Fracture face analysis

coreman73;
Based on what little information we have regarding prior service conditions of these T-slot bolts (dynamic loading, environment, etc.,) the fracture surfaces as shown appear to be subjected to undirectional, high stress conditions. The remainder of the fracture beyond the thumnail-shaped pre-cracked region could be brittle fracture induced very rapid (immpact) load conditions.

Just a guess until more information can be provided.

RE: Fracture face analysis

Are there any cracks in the other thread roots?

RE: Fracture face analysis

Really, you shoud get these bolts to the lab for analysis.  Anything else is just a reasoned guesstimate.

Adam Potter MEng CEng MIMechE
www.ax-ea.co.uk

RE: Fracture face analysis

This is a general observation regarding alloys 1045,4140,etc,parts are quite often made and used in unheattreated condition. What benefit is achieved by specifying such alloys,which are supposed to perform best in heat treated condition. The stock reply I encounter is the drawing specifies it and then a blank stare.

RE: Fracture face analysis

(OP)
Thanks metengr.  These bolts were said to be used for fixturing during machining applications.  Unfortunately, that's all the info I have on their use.  I do have some "competitor" bolts that were used in identical fashion but did not fail.

Swall, yes there is a crack in nearly every single thread root of the failed bolts.  The competitor bolt was completely free of root cracks.

Microhardness results of the failed bolts show a surface hardness of ~27-30 HRC with core ~20-25 HRC.  For comparison, the competitor bolt I tested has surface hardness ~35 HRC with core hardness ~30 HRC.

Chemical analysis showed the same steel grade was used for both the failed bolts and competitor bolts.

I've attached some photos illustrating the thread profiles for both the failed and competitor bolts.  The failed bolts typically show inconsistent crest widths along with visibly different circular thickness between threads.  The competitor bolt demonstrates consistent profiles throughout.

For the failed bolts, it appears that the damage pattern is different between threads.  Some threads show crest deformation while others show deformation of the flank.  

Could it be that the failed bolts did so simply due to poor quality of thread rolling?

RE: Fracture face analysis

The bolt looks overloaded as you can see the crest of the tooth as moved to the left of the photo and the thread profile in cross section so the cracking and root deformation may be symptomatic of that. I.e. the bolt overloaded, cracked the root then subsequent vibration caused it to fatigue.  The bottom (competitor) bolt has not seen deformation so either was not loaded as highly or is stronger.

Is one bolt machined thread and the other rolled? (etching the micro will see the reveal the microstructure and this can be easily seen).  Rolled threads tend to be stronger than machined in most cases which my be why the 1st bolt failed prematurely.

Since it seems that you have met lab then are you a Metallugist?

Adam Potter MEng CEng MIMechE
www.ax-ea.co.uk

RE: Fracture face analysis

why do you think the theads were rolled?

RE: Fracture face analysis

coreman73;
Obviously, the failed bolts were without adequate heat treatment in comparison to the competitors bolts, in addition to the observed thread profiles.  Poor thread profile will result in local stress concentration.
I would presume the competitors bolts are quenched and tempered, which is more desirable from the strength and notch toughness standpoint. Make sure that the bolts are adequately preloaded for the intended service, otherwise fatigue failures will occur in service.
 

RE: Fracture face analysis

The uneven deformation would be due to the fact that bolt/nut contacts tend to be uneven.  The highest stresses are int he first 2 or 3 threads then drops off dramatically after that.  Also the machining varies hence the reason why the distortion'll occur in different areas.

The higher hardness in the competitor bolts'll indicative of higher yield and UTS so might be a stronger bolt.

Adam Potter MEng CEng MIMechE
www.ax-ea.co.uk

RE: Fracture face analysis

(OP)
Adamuk,
It was provided that the failed bolt MAY have been roll threaded. Otherwise, it's pure speculation as to how the threads were made for either of these bolts.  I'm not really a metallurgist but have been trained to do some failure analysis.  Otherwise, I do mostly material audits/evaluations.  The lab I work in is set up to do both.

I've attached some photos illustrating the microstructure of the failed and competitor bolts.  Both bolts show the same basic core microstructural constituents (ferrite and pearlite), which are also seen near the threaded surface of the failed specimen.  In contrast, the competitor bolt transitions to a microstructure of what appears to be mostly tempered martensite throughout all threaded surface areas. Also, as expected the deformation is clearly more severe in the failed bolt versus the competitor.

So from these photos how could I tell how the threads from each sample were formed (rolled versus machined)?

Metengr,
Is it safe to say that the poorly formed threads could have contributed to failure?

RE: Fracture face analysis

Coreman73---in the last set of micros you posted, the second to last set has two micros. The bottom is labled "failed bolt", but what about the top? It is captioned "near surface of thread" but it is not clear if this was the competitor bolt or the failed bolt.

RE: Fracture face analysis

coreman73;
Normally, evidence of thread rolling is indicated by local subsurface deformation of the microstructure, whereas, machined threads will reveal a cut surface with a very minor amount of subsurface deformation.

Yes, poorly formed thread profiles can result in local stress concentration and failure.
 

RE: Fracture face analysis

Macroetching a sample should reveal grain flow if you have a deep etch tank available. A strong HCl solution will work at ambient temps if you do not have a heated tank.

If you can take a photomicrograph at a lower mag (50x) that may help with the polished sample

RE: Fracture face analysis

Normally, evidence of thread rolling is indicated by local subsurface deformation of the microstructure, whereas, machined threads will reveal a cut surface with a very minor amount of subsurface deformation.

Looks like I was beaten to it by metengr

Adam Potter MEng CEng MIMechE
www.ax-ea.co.uk

RE: Fracture face analysis

(OP)
Swall,
Sorry about that.  The photo "near surface of threads" is related to the competitor bolt.

Metengr and Adamuk,
From what I can tell, it looks like there is little if any subsurface deformation so I'll go with machined threads for the failed bolts.  I see much more subsurface deformation for the competitor bolt so will say the threads were formed by rolling.

RE: Fracture face analysis

coreman73,

Based on the information that you have provided so far, my assessment is as follows:

1. Bolt failure mode is by fatigue fracture, with relatively high nominal stress (as noted previously by metengr).

2. Failed bolts are overtightened which leads to thread deformation, root cracks, etc.

3. T-slot bolts should be quenched and tempered to at least Property Class 8.8 (ISO 898-1, ASTM F568, etc.) as required by product standards such as DIN 787.  PC 8.8 requirement is for core hardness of 22-32 HRC (diameter < 16, otherwise 23-34 HRC).

4. T-slot bolts should also feature rolled threads, not cut.

FWIW, depending on the size of the bolts, 1045 may not provide sufficient hardenability to guarantee core hardness of 22 HRC minimum.
 

RE: Fracture face analysis

(OP)
TVP,
Thanks for the complete assessment.  I just received confirmation from the manufacturer that the failed bolts feature cut threads.  You stated that T-slot bolts should have rolled threads.  Is there a standard that states this or what is the reason for this?

RE: Fracture face analysis

coreman73,
Considering that the bolts were reported to be used for fixturing during machining operations, repetitive tightening and loosening would be expected. Tightening tension would be expected to vary considerably, from over tight to under tight. The failure mode should be expected, especially when manufacturing (machine cutting) threads from common, as rolled, 1045 bar stock.  

RE: Fracture face analysis

The near surface microstructure in the last image set appears to be martensitic. Either that, or it was only lightly etched. If the former, this suggests that the threads were hardened. This might make sense for a threaded item designed to be assembled/disassembled multiple times during its lifespan. Following up on TVP's comments on thread root cracks--these could also be additional fatigue cracks.

RE: Fracture face analysis

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.).

RE: Fracture face analysis

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.  
 

RE: Fracture face analysis

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&amp;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

RE: Fracture face analysis

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
www.ax-ea.co.uk

RE: Fracture face analysis

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.      

RE: Fracture face analysis

(OP)
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.  

RE: Fracture face analysis

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
www.ax-ea.co.uk

RE: Fracture face analysis

(OP)
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.

RE: Fracture face analysis

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  ;)

 

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