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The Res-Q-Jack engineering department recently completed a series of testing on straps, using a 15’ hydraulic press capable of applying 50,000 lbs. of tension and compression (shown below). Engineers at Res-Q-Jack use this machine to test struts and accessories in any position, to simulate any situation it may encounter in the field. Below, they report on their data.
 We tested the straps in a variety of ways, in order to better understand their properties and strength. One of the properties that we found to be interesting was the elastic nature of a strap. Straps act much like a spring when stretched and can be described in terms of F=K*x, the basic spring formula, where K is a constant and x is the change in length of the spring.When using a strap to secure a strut or prevent movement of a vehicle, it is put into tension and it stretches. For the sake of knowledge, we wanted to characterize this and apply it in some of our calculations for understanding the forces and stresses our struts and accessories experience while being used.
Unlike a simple spring, a strap can be used at many different lengths, which changes how much force it will take to stretch a given amount. This concept is analogous to having a bunch of small springs in a series. For example, a spring takes 10 lbs to stretch 1 inch. If you have 10 of these same springs connected in a row, this new long spring will stretch 10 inches when you put 10 lbs of load on it. So we must modify the simple F=K*x equation to account for the different lengths in which a strap can be used.
We can describe the change in length as a percent of the overall length. This value becomes X (uppercase “X” for our new equation) and has units of in/in. This is a common engineering method. To be able to use the equation we must also then change the units of the spring constant K. In the original form, K has units of lbs/in. Now our constant must have units of lbs*in/in and must be found experimentally.
To experimentally determine the value of k (lowercase “k” for our new equation) for our 2 inch straps, we stretched various lengths of webbing to create a diverse data set. By applying loads in intervals we were able to record the change in length and corresponding load. Plotting this data on a graph allowed us to create a trend line that described the stretch of a strap in the F=k*X form. We found in our first tests
that a strap, which had been frequently used during our extrication events, had an elastic constant (k) of approximately 78052 lbs*in/in. For comparison, we look at a suspension spring for a car, which has a spring rate of around 300 lbs/in. That suspension spring is about 24” in length and will stretch 1” under 300 lbs of load. A 24” sample of webbing will take 3,200 lb to stretch 1”, which signifies that it’s about 10x less “stretchy” than the suspension spring.  Since the strap we tested first was well used, we were wondering if there would be a difference when compared to a fresh, out-of-the-box strap. The new strap had a much different feel then the older strap, clearly showing that the straps do undergo some type of “breaking-in,” which might change the elastic properties of the webbing.
We repeated the same test as we did with the older strap and found that the new strap had a constant of 38444lbs*in/in. This measurement indicates that the new webbing takes about half the amount of force as the old webbing, to stretch the same amount.
To better illustrate this, the chart below shows the applied force verse the percent elongation of the strap.  The graph shows that a new strap is about twice as stretchable as an older strap. The most likely hypothesis? As the strap is used, the woven fibers adjust and move into a tighter fit. In other words, when you pull on the strap, the fibers get tighter, but in the strap there is nothing to force the fibers to go back to their original location. This causes the strap to become broken in, and, therefore, have less stretch the next time it is loaded.
However, we found that this process does not happen instantly. The new strap we tested did not have much variance even after four test runs loading the strap to 2000 lbs. We also observed a very interesting phenomenon in the new strap, which was that after loading the strap, if the load was removed quickly, the strap did not instantly spring back to its original length. It would first sag as though it was permanently deformed, and then slowly return to its original length in an unloaded position.
While performing this test we developed an interest in what else may affect the properties of a strap. Because we constantly use straps over and around things, causing concentration points for stress, there are many possible effects and possible concerns. We continue to test different variables in strap wear-and-tear, to ensure that we never use straps when they are wornout, over-worked, or exposed to the wrong environment.
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