This is not good, didnt expect this to break

VortecJeep

VortecJeep

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Aug 24, 2005
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Concord, NC
Since the Trailblazers ride last weekend, Lot of noise, vibration, and the gear shifter rises about 3 inches when I let the clutch out. Thought tranny mount was broken. Under it today to pull the rear driveshaft to replace u-joints, (will do tranny mount later), and found the crossmember for the lower rear control arms broken on both the passenger and driver sides!

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That's a lot of force for one tube, but definitely fixable.
 
That's actually impressive to me. Hats to the compentent welder. None of that failed. But curious if the upper inside corner was the starting place......and if so was it right at the end of a weld?

First glance appears to show no weld intersects the fractured area.
 
Kamel said:
For sheer curiosity; What tube? Guessing 1.75" x .120" wall??
Click to expand...

Geez I hope not. That would explain the failure if so, but to withstand the forces the suspension applies it needs to be .250 wall minimum.
 
Looking at a bigger screen and paying attention. Up in the corner of both have a built in stress riser. Notice the "high build" of the weld and very little wetting out of the toes.
A very restricted corner. Joint design or weld choice would have helped.
Coupled with probably a thinner tube choice.
Still the over all average was decent due to the failure point and shape.
@Mac5005
Your heat affected zone theory would make more sense in my mind if it ran more parallel to the weldment and slightly beyond the actual toes of the weld. Being short circuit Mig as it looks the heat soak was minimal. The amount of dilution and time on part to cause viable shift of a martinsetic material in the iron phase diagram required for ductility issues of that nature was probably not present here. A higher alloyed element would be much more sensitve in the time and temperature ranges for what I see. We did testing on specifically mild steel weldments using all processes to dispel a lot of "student" therory and excesses for coupon failure. It was great metalurgy content for classes.
With such a low carbon content "most" of the alloys chosen by normal builders is highly forgivable.
What is not still remains in all welds. Fusion and wetting of weldtoes to give the best mechanical properties.
 
That's exactly where it would be expected to break as a fatigue failure.

You've got a tube that's rigidly supported at the ends, but only supported at the ends (it appears). The tube is loaded in the middle, and it's loaded mostly in bending because it's only supported at the ends.
The bends in the tubing also add a torsional load component (like a crank) because the bending loads are now offset whatever distance from where the ends are supported.

It's going to fatigue at that transition from stiff weld to flexible tubing, because that's where the stress concentration is. Exactly where it starts and how it propagates depends on welding and metallurgy, but none of that changes the fact that there's a stress riser in that area because of how the tubing is loaded and supported.
Once one end starts to crack, the load on the other end starts to increase, and the same thing happens there.

That crossmember should have some extra support added (triangulate to where the lower arm mounts are?), which changes the loading at the ends more into tension/compression than bending.
Or, should be made out of something that's got more strength along the bending axis, like thicker/bigger tubing, or a stiffer/stronger tube profile, etc.

The tubing is obviously strong enough not to yield in bending or buckling, but it's shown that the fatigue loads are too high unless something is changed.
 
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Definitely a fatigue issue. The crack was propagating from the toe of the fillet weld. Probably best to cut out and drop in a different crossmember with mo beef :huggy:
 
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