Truss Cracks

[LavaASU]

So, I've heard multiple times about the possibilities of "internal fractures" and "stress fractures" on aluminum truss. I've heard both that they can develop from an impact/shock load/overloading and that they just naturally develop through use and weaken the truss over time. I'm not talking about cracked welds (or other issues) that can be spotted on visual inspection.

What is the real deal on this? Is there truth to this (and what parts)?

 

[MNicolai]

I'm not qualified to answer your exact question, but there is a point I'd like to make based on how you prefaced your question...

Degradation due to impacts, shock loads, or overloads should not be considered a "natural development." Events such as those could be described many alternative ways, but "natural" is not one of them.

That's not to say you will never see degradation to trusses/materials that have, to the best of your knowledge, always been used within their ratings, but in an ideal world, you shouldn't. This, however, is not an ideal world, which is why routine inspections by a qualified, competent party should be made to all rigging mechanisms and structures.

 

[LavaASU]

I'm not qualified to answer your exact question, but there is a point I'd like to make based on how you prefaced your question...

Degradation due to impacts, shock loads, or overloads should not be considered a "natural development." Events such as those could be described many alternative ways, but "natural" is not one of them.

That's not to say you will never see degradation to trusses/materials that have, to the best of your knowledge, always been used within their ratings, but in an ideal world, you shouldn't. This, however, is not an ideal world, which is why routine inspections by a qualified, competent party should be made to all rigging mechanisms and structures.

I was not saying impacts, shock loads and overloads are natural developments, rather that I've heard two versions of this. One being that those events cause the damage and the other that it occurs over time through normal use.

Obviously all truss needs to be inspected. I am curious specifically about damage that would not be detected in routine visual inspection.

 

[Flyboy]

I will begin by saying that, although I have been rigging for many years, I am not a certified rigger or rigging inspector, and that all rigging inspections should be carried out by a Qualified Person. But this is how it was explained to me:

I don't know about "stress fractures", but one thing to be concerned with about all metals is "strain hardening." All materials have a degree of "elasticity," which is the measure of how far a material can be strained beyond it's original shape while still being able to naturally return to its original shape when the strain is removed. Think about a rubber band: it has high elasticity, which means that it can be strained quite a bit past its original structure, but it still returns to its original size and shaped when the strain is removed.

When materials are strained beyond their elastic limit (as in when a truss is overloaded), their shape is permanently deformed (think about the rubber band that has been "stretched out"). In metals particularly (and possibly in other materials, I'm not sure), this molecular deformity causes a permanent decrease in elasticity (hardening), which results in the material being more brittle. This hardening happens a little at a time, based on how much something is overloaded, how often, and what direction the strain is in.

As an example, think about a paper clip. Paper clips are very flexible, but they have a low elasticity: they do not return to their original shape after they've been bent. If you take a straight section of a paper clip and bend it in half, it doesn't break, but it is permanently deformed. If you straighten it back out, you see that it's shape has been changed...it can't be fully straightened again. Now, if you keep bending and straightening the paper clip in the same spot, it eventually breaks. This is the result of brittleness caused by strain hardening.

So to answer your question: yes, there is truth to this. Even though truss is less flexible than a paper clip, it still has a degree of flexibility designed into it. When strained beyond its elastic limits, or in other words, when overloaded, the truss undergoes a molecular change which makes it less flexible and more brittle. Flexible objects bend when strained, brittle objects break.

Shock loading can cause the same damaging issues, but to a much more serious degree which is exceptionally difficult (is it even possible?) to measure. Because of this, I have always been taught to immediately retire any equipment that has been subject to a shock load.

 

[Focus]

Im not sure how much credence there is to damage that is caused by normal use over time. Certainly some items will suffer from this, such as slings and other soft rigging, that to degrade do to light abrasion, and UV damage. Theoretically, Metals utilized within their elastic range should not be damaged with normal use, but research and development on rigging is ongoing, so things are subject to change. Fairly recently, Extreme Structures re-engineered their 12" box truss, It now has many fewer ladder bars on the top and bottom, and no vertical members on the sides, compared to the older design, however they discovered that reducing the weight and the number of welds yielded a stronger truss.

All of this said, Rigging should be regularly inspected per the manufactures guidelines, and also physically inspected by the end user with each use. If I see something that is unsafe, like a spanset with the care exposed, regardless of ownership, I have no problem taking it and cutting it into several pieces, and returning it to its owner. There is no reason to take any risks in rigging.

If you want a definite answer I would contact the truss manufacture and ask.