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Metallurgists: I need advice on material selection to avoid stress-fracture

Gordon Heaton

Stainless
Joined
Feb 19, 2007
Location
St. George, Utah
Good Morning.

At my place of employment we fabricate a product that is exposed to all manner of climatic stress. This particular query relates to wind-induced cyclic stress on a solid mild steel/stainless steel shaft. I cannot post a photo or drawing of the actual product, but I can give you a simple and very similar analogy: Think of it as a common weather-station sensor array. In our case, there is a base section made from ordinary sch-40 steel pipe which varies in diameter by application. One end of the pipe is firmly attached to a ground-mount set in concrete, and the other end has a female thread into which the solid steel/stainless steel 'stinger' is screwed. Atop the stinger is the wind-affected package.

Let's say the lower pipe size is 1 1/4" X 72" long, and the solid stinger is 5/8" X 48" long. (All production combinations were contract-engineered for wind load, but not for repetitive 'sway') In a proper installation, the sway is absorbed by the lower and upper sections together. The problem we face now was caused by the municipality installing the array, when they disregarded the instructions and cut the lower section from the specified length to very, very short. Around 12"-18". The stingers are work-hardening and fracturing, not at, but just above the thread where they are mated to the pipe. The wind load is not applied at the tip of the stinger. It is applied along the entire length, more like a distributed load.

The current stingers are 304L solid rounds. These were formerly made from 1018 and powder-coated. We switched to SS because of the added labor involved in fitting bearings where needed over powder coat. In the thread about forming an aluminum mast, DiggerDoug posted this link:
What is Fatigue Limit - Fatigue Strength - Definition | Material Properties

On that web page is a graph (2nd one down) showing relative benefits of various materials. Titanium is not feasible, 4340 would be fine. I've been trying to find a similar comparison showing 304L and 1018 but haven't yet been successful. Like every thing else, there are a hundred variables, most of which are way outside my ability to evaluate, so I'm looking for general tips and suggestions.

Yes, the problem was cause by the installer not following directions but they want it to work the way it is, and we're hoping to find a way to help make it happen. We like to keep our customers happy and our reputation excellent, so... what do you think? -Forgot something: we cannot increase the diameter of the stinger.
 

BugRobotics

Stainless
Joined
Jun 22, 2015
Location
Denver, CO
What is the transition look like from the bottom pipe to the solid stinger? Any way to support the solid bar with a bolt on collar to move the stresses away from the threaded area?
 

Gordon Heaton

Stainless
Joined
Feb 19, 2007
Location
St. George, Utah
No special transition. The threaded end of the pipe is either crimped and threaded, or has a welded steel adapter/reducer for the female threads.Because of the way the top part fits over the stinger there are multiple internal parts which do not lend themselves to re-design. The stinger must be of uniform, specified diameter from top to bottom. The bottom of the stinger is a male thread, and just above the male threads are shallow wrench flats for installation. It's been working fine for a very long time now. We're just hoping for a material change that will negate the lack of flex in the lower section for this one installation.
 

MrWhoopee

Hot Rolled
Joined
Nov 8, 2017
I'm not metallurgist, studied just enough to be dangerous. Just as a SWAG, replace the 304 with 17-4? I can't make a recommendation about heat treat. I'm sure others with more expertise will chime in shortly.
 

cmccull166

Hot Rolled
Joined
Mar 29, 2013
Location
NW Pa
About a month ago there was a conversation here on bolt failures leading to bridge
collapses.you may find some helpful info
 

Gordon Heaton

Stainless
Joined
Feb 19, 2007
Location
St. George, Utah
Million dollar Q....how many have failed and is it DEFINATELY due to what you're saying? . . .ie...it hasn't been broken off by "something"???

There are hundreds of installations around this country and some overseas. None have failed in this manner. This particular location is the median between busy streets. There is likely a component of gusty turbulence caused by vehicular, especially truck, traffic. However, there are 'normal' installations in very windy locations and none have experienced this failure.
 

Gordon Heaton

Stainless
Joined
Feb 19, 2007
Location
St. George, Utah
This may be too simplistic, but can you change the dimensions of the bottom pipe to duplicate the amount of flex you get from a full length one?

Unfortunately, the only thing I can think of in that regard would be to use a spring instead of the pipe. The top section cannot be allowed to sway or swing very far at all due to the proximity to the street and the fact that there are multiple elements in the installation.
 
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BoxcarPete

Stainless
Joined
Nov 30, 2018
Location
Michigan, USA
Yeah, about the only thing you can do is use an overkill material which can sustain the higher stress without fatiguing. Do you have a good way to recalculate the load for the deviant shortened installation? The good news is that you have lots of room to improve from 1018/304 in terms of strength. If it's just for this one time, I'd go with something seriously strong (according to Engineering Toolbox, 18-8 stainless gets you up over 70 ksi fatigue, and also mentions a rule of thumb for most steels that the fatigue limit is near half of its ultimate tensile strength).

Another bit of good news is that going with a stronger material shouldn't have much impact on the stiffness, if that's relevant to the way the assembly works.
 

Gordon Heaton

Stainless
Joined
Feb 19, 2007
Location
St. George, Utah
Thanks, Pete. Extra stiffness wouldn't be an issue as long as it didn't increase the concentration of stress at the union. Very flexible materials like small-diameter fiberglass or carbon fiber won't work, because the stinger is a 'shaft within a shaft', the outer tube is quite rigid, and the two can't come in contact with each other. As we speak, the boss is considering 4340 for the fix so far.
 

BoxcarPete

Stainless
Joined
Nov 30, 2018
Location
Michigan, USA
Thanks, Pete. Extra stiffness wouldn't be an issue as long as it didn't increase the concentration of stress at the union. Very flexible materials like small-diameter fiberglass or carbon fiber won't work, because the stinger is a 'shaft within a shaft', the outer tube is quite rigid, and the two can't come in contact with each other. As we speak, the boss is considering 4340 for the fix so far.

Well, the thing is that a stronger steel won't be extra stiff. Funny thing about heat treating and alloying is that it generally only makes a significant difference in maximum stress, not stress per unit deflection. If you do go with a stronger material, it will deflect more before failure than a weaker one before reaching its strength limit. But if this was not a point of concern for the deviant installation before the fatigue failure, it wouldn't be a problem moving forward with another stainless alloy.
 

Orange Vise

Titanium
Joined
Feb 10, 2012
Location
California
The stinger must be of uniform, specified diameter from top to bottom. The bottom of the stinger is a male thread, and just above the male threads are shallow wrench flats for installation.

The wrench flats and the male threads are stress risers.

Take a look at how pull studs are designed. The high stress concentration is distributed over a relatively large radius. The threads and the wrench flats are isolated from the high stress areas.

If there's nothing you can do about the design, 4340 or 300M might be your best bet.

A jamnut on top might help as well.

What's the size/pitch of the thread?
 

Modelman

Titanium
Joined
Sep 12, 2007
Location
Northern Illinois
You haven't said how many have broken, but have said that they are adjacent to a traffic lane used by large trucks, so I would investigate if they were really just snagged by a rope, broken band iron or loose tarp on a passing truck.

Dennis
 

Gordon Heaton

Stainless
Joined
Feb 19, 2007
Location
St. George, Utah
The wrench flats and the male threads are stress risers.

Take a look at how pull studs are designed. The high stress concentration is distributed over a relatively large radius. The threads and the wrench flats are isolated from the high stress areas.

If there's nothing you can do about the design, 4340 or 300M might be your best bet. A jamnut on top might help as well. What's the size/pitch of the thread?

The failure doesn't seem to be influenced at all by the wrench flats, and nearly all of them (failures) have been above the thread. Thread size ranges from 3/8-16 to 3/4-10 in the stainless versions. Those 1" and larger are still made of 1018 and none are involved in this installation.
 

Gordon Heaton

Stainless
Joined
Feb 19, 2007
Location
St. George, Utah
You haven't said how many have broken, but have said that they are adjacent to a traffic lane used by large trucks, so I would investigate if they were really just snagged by a rope, broken band iron or loose tarp on a passing truck.

Dennis


Dennis, I've not personally been to the site and I don't know exactly how close they are to the vehicles. I doubt they are close enough for physical contact, and they are fragile enough that even a minor impact with a truck or appendage would be obvious.
We cold-bend large quantities of 304L rounds with a pretty tight radius. If these were snagged or hit in this manner they'd simply bend over. The fractures are typical of work hardening with the resulting crack formation and propagation. There is no evidence of bending.
 

Milland

Diamond
Joined
Jul 6, 2006
Location
Hillsboro, New Hampshire
You'd mentioned bearings on the shaft, so it's active torsionally? And is the failure a torsional one?

If so, somewhere near the typical failure area try cutting in a specific torsion zone, basically a slightly reduced diameter with radius transitions to the OD. The actual reduction and length of this undercut might require some modeling by an engineer with proper FEA experience and an understanding of the oscillatory loads on the stinger (wind speed, natural frequency, area of the system that catches the wind, etc.).

The surface finish in the torsion zone needs to be as smooth and uniform as possible to give it the best fatigue life, and perhaps lightly stainless shot blasted to give a compressed skin.

Another approach for a quick and dirty fix might be to make a clamp on metal bar with a mass at the end. The bar/mass would change the natural frequency of the structure and with a little tuning take it out of the range of excitement by the wind loads.
 

Rickyb

Cast Iron
Joined
Jan 21, 2011
Location
Troy mi
Gordon, you must fix the problem without changing anything but the material. Tall order.

What I gather from your post is you have a stress concentration induced failure as a result of a shorter stand. Taking this at face value, the I see that the section modulus of the stand pipe is 12x higher than your stinger. The difference between the 2 is what determines your stress concentration factor (Kt). This will not change with a shorter or longer stand pipe. Switching to 4340 will substantially increase your allowable stress and your fatigue life. By how much can only be determined by knowing the actual stress level and how it varies in each cycle.

Some things would be nice to know at this point like, is this a pure bending failure, combined with torsion, low cycle or high cycle fatigue, origin site(s) and/or stress corrosion related.

Sans that info 4140 at no more than Rc35 would be my choice. Any higher hardness won’t like your Kt or road salt(stress corrosion). Not much benefit to 4340 for a section that small. I’m leaning toward a stress-corrosion initiator as your issue. Road salt, bird droppings, whatever.
 

Gordon Heaton

Stainless
Joined
Feb 19, 2007
Location
St. George, Utah
Milland, no torsion at all. The outside assembly rotates freely on dual ball bearings or a ball-bearing/bronze bushing pair, and on larger parts an additional thrust bearing. On the sizes involved you could (except for the weight) resist the torsion with your finger stuck in the bearing center. They rotate as freely as the anemometer on the weather station.

Rickyb, pure bending. The amount of bend would in most cases be invisible to someone driving by, except in a very strong wind. The stress is repetitive, the direction from which it comes varies and the cycle count is unknown since it is determined by the whims of mother nature. I think we're ready to try the 4340 shafts and see what happens. The stainless failed after about one year in operation. We have installations over 20 years old still functioning as intended, but as mentioned above the very oldest parts are of 1018. Some areas are exposed to salt (oceanside) and that's one of the driving factors in switching to stainless. If we try 4340 we intend to have it nickel plated although this particular location is in the desert.

At present there is no plan to modify production with a different material, just trying to fix this one because it is so different.

Gotta run for a bit... its 70F outside and my bicycle is calling loudly!
 

Milland

Diamond
Joined
Jul 6, 2006
Location
Hillsboro, New Hampshire
Milland, no torsion at all. The outside assembly rotates freely on dual ball bearings or a ball-bearing/bronze bushing pair, and on larger parts an additional thrust bearing. On the sizes involved you could (except for the weight) resist the torsion with your finger stuck in the bearing center. They rotate as freely as the anemometer on the weather station.


Whoops, sorry for my misunderstanding.


Gotta run for a bit... its 70F outside and my bicycle is calling loudly!

It's still in the single digits at night here... :(
 

boslab

Titanium
Joined
Jan 6, 2007
Location
wales.uk
You’d be looking calcium treated niobium stabilised steel, which is what most large diameter pipes are, the inclusions in the steel get modified from pointy (accular) to spherical to reduce the tendency for cracking, most of the feedstock for the tube mills up to the 42” line in work were niobium stabilised
Wrong stuff and some nice cracks would ensue.
Ductile irons use the same trick
Mark
 








 
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