Thinking about this some more, I have convinced myself that for the same static (slow pulling) ultimate breaking strength, it makes sense that steel chain holds up better under impulses of over-loads than a nylon strap does.
This difference between static and dynamic strength is why a truck full of steel coils can flop over and the chains hold while the nylon straps break (assuming the peak impulse loads easily exceed the static ratings of both the chain and the strap).
Between the elastic point and the ultimate failure point, the steel chain will deform quite a bit - all those links being stretched into slightly longer, narrower shapes. It takes energy to deform the steel - a force applied over some distance. If the overload situation is limited in time duration or in the stretching distance, the chain will halt the movement of the thing pulling on it before it breaks, being permanently deformed in the process.
Meanwhile, with the same over-load the nylon strap has some elastic give, but there is a lot less margin betwen its elastic limit and ultimate failure. Both because of its material and its construction being straight strands vs. rounded chain links. Once the strands have stretched as far as they will go, the force curve quickly takes a dive as the material becomes plastic, flowing and deforming to the point of failure with little or no additional force. The strap either springs back or it fails - but never ends up permamently stretched very much. Once past its elastic limit, the nylon strap has very little ability to absorb even short impulses of additional energy.
- Rick
This difference between static and dynamic strength is why a truck full of steel coils can flop over and the chains hold while the nylon straps break (assuming the peak impulse loads easily exceed the static ratings of both the chain and the strap).
Between the elastic point and the ultimate failure point, the steel chain will deform quite a bit - all those links being stretched into slightly longer, narrower shapes. It takes energy to deform the steel - a force applied over some distance. If the overload situation is limited in time duration or in the stretching distance, the chain will halt the movement of the thing pulling on it before it breaks, being permanently deformed in the process.
Meanwhile, with the same over-load the nylon strap has some elastic give, but there is a lot less margin betwen its elastic limit and ultimate failure. Both because of its material and its construction being straight strands vs. rounded chain links. Once the strands have stretched as far as they will go, the force curve quickly takes a dive as the material becomes plastic, flowing and deforming to the point of failure with little or no additional force. The strap either springs back or it fails - but never ends up permamently stretched very much. Once past its elastic limit, the nylon strap has very little ability to absorb even short impulses of additional energy.
- Rick
