WOW this would help with making headers or cages....

[Drax240z]

A properly fitted cage transfers all the loads from tube to tube, and should do so without any welding. The welds are in place merely to keep everything in alignment, and rigid, not to sustain compressive loading in place of tubing. Tubing is a strictly regulated thickness and strength, while weld filling an airspace between 2 pieces of tubing has varying thicknesses, hardnesses and strength. On top of that welds in this type of configuration will be very prone to having stress risors, and will not endure fatigue nearly as well.

 

All of these possible problems come to a head right when you don't want them to. In the case of a rollover where you are relying on your cage to keep you alive, a poorly fitted/welded cage can become more dangerous than no cage at all. As the cage fails you introduce further hazards inside the cockpit, such as broken sections of tubing, or free floating tubing ends, which will make short work of the human body.

[johnc]

I'm pretty sure Pop is kidding us, but I'll play along.

 

Proper fitup reduces the chance of Hydrogen cracking in welded mild steel structures.

 

IDENTIFICATION

 

Hydrogen cracking also known as cold cracking or delayed cracking. The main feature of this type of crack is that it occurs in ferritic weldable steels, and generally occurs immediately on welding or after a short time after welding, but usually within 48hrs.

 

Hydrogen cracks can be usually have the following characteristics:

 

1. The crack will normally originate in the heat-affected zone (HAZ) but may also extend into the weld metal.

2. Cracks may also occur in the weld bead, normally transverse to the welding direction at an angle of 45° to the weld surface.

3. They are near straight, follow a jagged path.

4. In low alloy steels, the cracks can be transverse to the weld, perpendicular to the surface of the weld, but do not branch and are planar (Planar Defect).

 

On breaking open the weld, the surface of the cracks will normally not be oxidised, even if they are surface breaking, indicating they were formed when the weld was at or near ambient temperature. A slight blue tinge may be seen from the effects of preheating or welding heat.

 

POSSIBLE CAUSES

 

There are three factors, which can cause hydrogen cracking:

 

1. Hydrogen generated by the welding process, or by contamination of the weld area (paint?).

2. A hard brittle structure, which is susceptible to cracking.

3. Residual tensile stresses acting on the welded joint (restraint).

 

Cracking is caused by the diffusion of hydrogen to the highly stressed, hardened part of the weldment.

 

In C-Mn steels, because there is a greater risk of forming a brittle microstructure in the HAZ, most of the hydrogen cracks are likely to be found in the parent metal. Using the correct choice of electrodes, the weld metal will have a lower carbon content than the parent metal and, hence, a lower carbon equivalent (CE). However, transverse weld metal cracks can occur especially when welding thick sections.

 

In low alloy steels, as the weld metal structure is more susceptible than the HAZ, cracking may be found in the weld bead.

 

The effects of specific factors on the risk of cracking are::

 

Weld metal hydrogen

Parent material composition

Parent material thickness

Fitup

Stresses acting on the weld

Heat input

Weld metal hydrogen content

 

One of the principal source of hydrogen is the moisture contained in the flux ie the coating of MMA electrodes, the flux in cored wires and the flux used in submerged arc welding. Mainly the electrode type determines the amount of hydrogen generated. Basic electrodes normally generate less hydrogen than rutile and cellulosic electrodes.

 

It is important to note that there can be other significant sources of hydrogen eg moisture from the atmosphere or from the material where processing or service history has left the steel with a significant level of hydrogen. Hydrogen may also be derived from the surface of the material from oil and paint, condensation, rust, etc,.

 

Sources of hydrogen include:

 

Oil, grease and dirt

Rust

Paint and coatings

Cleaning fluids

Parent metal composition

Condensation

 

This has a major influence on hardenability and, with high cooling rates, the risk of forming a hard brittle structure in the HAZ. The hardenability of a material is usually expressed in terms of its carbon content or, when other elements are taken into account, its carbon equivalent (CE) value.

 

The higher the CE value, the greater the risk of hydrogen cracking. Generally, steels with a CE value of

 

Material thickness will influence the cooling rate and therefore the hardness level, microstructure produced in the HAZ and the level of hydrogen retained in the weld. The 'combined thickness' of the joint, i.e. the sum of the thicknesses of material meeting at the joint line and the root gap, will determine, together with the joint geometry, the cooling rate of the HAZ and its hardness. Consequently, a fillet weld will have a greater risk than a butt weld in the same material thickness.

 

The stresses generated across the welded joint as it contracts will be greatly influenced by external restraint, material thickness, joint geometry and fit-up. Areas of stress concentration are more likely to initiate a crack at the toe and root of the weld.

 

Poor fit-up in fillet welds markedly increases the risk of cracking due to excessive heat input, excessive joint thickness (combined thickness), and contamination. The degree of restraint acting on a joint will generally increase as welding progresses due to the increase in stiffness of the fabrication.

 

[Pop N Wood]

Actually I am not kidding. I see people going through all the work to fit pipes like this and have wondered just how necessary it is. I was hoping someone would come back with something less anecdotal and more quantitative. Just challenging people’s assumptions to see what I can learn.

 

There is no doubt you risk quality control issues when a weld is used to fill in a gap and I know it is always a good idea to do things in a workman like manner. But I don’t think it is intuitively obvious that a butt weld is less strong, let alone so much weaker as to be unsafe. Start throwing in variables like using the next larger size tubing or adding cross braces at critical joints and one might end up with a cage that is stronger, albeit heavier, than the 100 hour cage.

 

To pass on a story, when in the Navy I watched them cut a 10’ by 20’ hole in the side of a nuclear powered cruiser sitting in drydock. Something like 1†thick HY80 steel. 2 years later, after the ship was floating next to the pier, I watched them try to weld the original piece of steel back in place. The ship had flexed so much that there were 4 to 8 inch gaps and overlaps where the steel plate no longer lined up with the original hole. I then watched a crew of welders spend 2 weeks filling in the 8†gaps with stick welders and electric heaters. They then used these carbon sticks to “air arc†the welds to look like big, fat 3†beads. I took a closer look at the hull and that was not the first time they had done that to this 30 year old ship.

 

If that was good enough for a warship, hard to imagine a cage with some 1†fillet welds becoming a liability rather than a asset.

[johnc]

If that was good enough for a warship, hard to imagine a cage with some 1” fillet welds becoming a liability rather than a asset.

 

It all depends on the loads imparted to the welded structure. If you butt two tubes perpendicular to each other, fill the gaps with weld, and the tubes are only loaded in compression, then everything will work out fine in a wreck. But add shear loads to that joint and things will not be so fine.

 

In a rollover at speed, there are shear and compression loads on the roll cage stricture. In a side impact there are tension, shear, and compression loads placed into the roll cage structure.

[JMortensen]

Anybody else think it would be HARDER to just butt the tubes together and try to fill the 1/2" gaps than to notch the tubes with a $40 notcher?

[tube80z]

Actually I am not kidding. I see people going through all the work to fit pipes like this and have wondered just how necessary it is. I was hoping someone would come back with something less anecdotal and more quantitative. Just challenging people’s assumptions to see what I can learn.

 

I don't know about ultimate strength. I've seen fit up with gaps filled on a number of cars and some of those have survived big wrecks.

 

But I can tell you that I did a few basic tests before I started putting my car together. I fit a couple of tubes very well and did a basic torsion test. I fitted some poorly and did the same test. The results for me were that the poor fitup had different torsional strength in one direction versus the other. My guess is that this may have had something to do with the weld wire being stronger/stiffer. I'm not sure. But for my car I tried to get the fitup as good as I could without spending too much time on each tube.

 

Cary

[80LS1T]

Pop, you know johnc does this stuff for a living right? Why would the SCCA say you have to notch your tubing for a cage if butt welding it would be just as safe? Sure it may get you by but why half a$$ something like a rollcage?

 

Guy

[Pop N Wood]

Pop' date=' you know johnc does this stuff for a living right? Why would the SCCA say you have to notch your tubing for a cage if butt welding it would be just as safe? Sure it may get you by but why half a$$ something like a rollcage?

 

Guy[/quote']

 

Exactly why I didn't argue with him. Just wanted to learn what he, and others like him, know about this issue.

 

But, I am still not convinced the assumption that not notching the tubing makes the cage unsafe is a good one. More prone to failue? Probably makes sense. Unsafe? I still don't have enough information to make that leap of logic.

 

Not disrespecting anyone here, but I have noticed people get very adamant in their opinions about safety equipment. I always wonder if they really know something that make their way the only safe way to do things or if they are just making things up.

 

Good to know about the SCCA rule. Didn't know that. Guess if you build a cage then better follow the rules of whatever class you intend to race.

 

As for the SCCA, my impression of them is they have to deal with people with a wide range of varying skills. Their rules are applied after the the cars are built. They can't spec the construction process, so the only way they can assure any quality control is by basic rules that can easily be verified just prior to a race. As such one has to wonder how many of their rules are written for the lowest common denominator, or as a reactive response to some past problem. Who is to say a cage built with investment cast fittings and some exotic alloy wouldn't be safer than an SCCA spec cage? Certainly not a trackside SCCA inspector, because all he rightfully has to go by is the published SCCA rules that exist.

 

I know a few things about weld quality control. I have seen welds performed that were so cruicial, that the required welding certification tests expired after only a few hours. If the set up time took longer than that, then you had to wait for the next shift for the welder to be recertified.

 

Does that mean I know anything about notching cage tubing? Obviously not. But just blindly regurgitating internet chatter won't make me an expert either.

 

BTW, when I built my workbenches out of 2 1/2 inch scrap yard fire sprinkler pipe, I butt welded the joints becasue it was massively easier to size and fit the pipe than using a hand grinder to notch the joints. Still came out about a 1000 time stronger than needed. I don't think I will ever own a car with a cage, but if I did I would probably take the time to notch all the joints if for no other reason then to impress the type of people who are obsessed with pretty, rather than effective, welds.

[Guest gtmattz]

sorry that my contribution to this post is a bit lacking, but I just have to say...

 

Pop, I like your style ;D