Stress Corrosion/Hydrogen Embrittlement/low Clampload help

[preload]

This is the new problem I am working on, on one of the mounting bolts. This time I will try my level best to give you guys as much of info as possible with pictures. This is our major issue (Customer satisfaction)

Problem where find: In the field, somewhere between 5hr to 440hr of engine run in water (no problem in assembly). Problem is seen only on high horsepower engines.

Joint Description: the whole engine will be supported by a structure, the engine is mounted on to the structure with 2 upper mount bolts and 4 lower mount bolts. So in service the whole load is carried by those 6 bolts and the problem is with the 2 upper mount bolts which are breaking underhead. Warranty issue. Some of them breaking at underhead of bolt and some of them come loose.

Fasteners used: ½-13 UNRC-2A,17.4 strength bolt (grade 8 high strength stainless steel) cadmium coated, fastener driven into blind hole (no nut), split lock washer under head and loctite 272 on the threads.
Depending on the heat treatement of the bolt, the proof load of the bolt can vary from 14,687lbs min to 17500lbs max. our supplier can get a mixed fasteners so we really want to keep the 14,687lbfs as proof load for margin of safety.But our tensile lab did some tests on 4 production bolts and they found the proofload to be 18000lbs.

Clampload we are shooting to: 9000 lbs average on each bolt.

Service loads on the joint : don’t know, design engineer dosent know either,but he said he gonna try to get the number.

Engineering dept thought: our eng dept thinks that , the failure/fracture of bolt head is due to hydrogen embrittlement or stress corrosion failure. But we see some of the bolts failed in 5 hrs of the engine run, so how can it be a corrosion failure if u have the engine in water for just 5 hrs? some of them failed in 440 hr (this I can agree it may be of corrosion failure)

What my guess is 9000 lbs of clamp load is like 50-60% of the bolt proof load. I think the bolts are failing because of the low clampload as cyclic loading/vibrations.services loads acting on the joint.

What do u guys think?

 

[unclesyd]

I take it to be the ration of torque that is strictly applied to the bolt. The problem is that that this is called out to be 10% to 30% depending on a bushel of other variables. Your two problems are prime examples in that in one you are turning just the nut and the other your are engaging the threads.

I'm still after an article that I ran across that developed a relationship between the two, K and mu.

 

[preload]

Unclesyd,

are u looking for formula that relates k and mu? If that is what u r looking, I have the formula and some values of mu to the corresponding K

 

[unclesyd]

Thanks for the reply. Here is a excerpt from a book that has some of the information that I spoke about. There was an article that also covered the topic to the point of determining which value to use. Though not a participant in the testing aspect we did a lot of work in this area when we started using H11 SHCS and Allen Nuts. As I recall there was a lot conflict when it came to the torque values and the thread deformation. We also had to go to hardened washers to achieve some of bolt stresses we required to meet our process requirements. I was looking at the values of mu versus the K that might govern the bolt stress in these two cases.
Start on page 156

http://books.google.com/books?id=TA...ctor&sig=SyQ2VQ1U4gTierWyBpgT3TllqAQ#PPA22,M1

 

[rob768]

Preload,

sorry I was away for a couple days and haven't checked forums. What i meant was that there may be resonance conditions during which excessive bolts stresses occurr. Apart from broken bolts, you also mention bolts running loose and in my book that means vibrations.
And don't know anything on the installation or oprating conditions, but if it is a diesel, misfiring is a condition whhich is often overlooked when designing an installation.

 

[preload]

Rob768,

Thanks for the explanation.

Thanks a lot guys for your ideas on this issue. Our engineering dept has identified the root cause and they are pretty confident solving this problem. So for now I dont need to focus on this issue.

 

[bob330]

Preload,

What were the final conclusions from the engineering department? Looks like a very interesting problem with multiple potential causal issues.

bob