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The Guardian Fall Team Blog

All of the products whose manufacture is governed by ANSI standards must go through rigorous compliance testing to ensure their integrity and performance. Although the specific test and performance parameters vary for each standard, tests can be generally divided into two types: static and dynamic. Why two types of tests? In a nutshell, because the tests tell us quite different things, for an explanation of how, read on.

Static Tensile Strength Test

A static tensile strength test is commonly used to determine two things: whether a material can withstand a specified load, and, what is the ultimate breaking (tensile) strength of a material. These are not necessarily the same thing. In the first test, a pre-determined static tensile load is indicated (e.g. 1,000 lbs.), which is then applied to the material (or product) for a specified period of time to ensure it can withstand that minimum load without breaking (or failing some other parameter). These tests are often used to determine the acceptable working loads of component materials, or in Guardian’s case, entire product assemblies.

On the other hand, an ultimate tensile strength test determines the absolute strength of a material. It defines the point at which a material will no longer elastically deform (return to original state), plastically deform (does not return to original state), and fail. Instead of having a pre-determined test load (like the 1,000 lbs. above), the material is loaded until it fails by breaking completely. The ultimate tensile strength of a material will usually far exceed that of the working load of a material. This difference helps to establish a safety margin to offset any variance in the material during manufacture or other factors including temperature and moisture, which may cause a material to perform slightly differently. As an example, Guardian web SRLs are statically tested to withstand 3,000 lbs. of tension force, however, the web lifeline component itself must have a minimum breaking strength of 4,500 lbs.

The video below demonstrates a static test of one of our popular CB anchors. Note that the CB anchor deflects (bends) slightly as the tension is increased, but returns to normal once tension is released. This is elastic deformation, and is perfectly normal. In fact, deformation is one of the methods an anchor absorbs the forces generated during a fall. Forces that elastically (or plastically) deform an anchor cannot travel to the worker. It is similar to a crumple zone on an automobile; as the car’s body deforms, it is using energy that otherwise would be transferred to the driver. F1 racing cars are famous for their ability to shed energy rather dramatically during high-speed crashes.

Incidentally, you might be wondering how a static (meaning no movement) test can induce tension, which requires the attachment points of the test equipment to pull apart from each other as you just witnessed in the video. How can static also be pulling? The answer is the rate of the pulling. ANSI specifies that, “The static tensile test equipment shall pull at a uniform rate of no greater than 2 inches (51mm) per minute….” By keeping the rate of pulling very low, we avoid changing from a static test to a dynamic test. Once that happens, all kinds of interesting physics starts happening.

Dynamic Strength Testing

Unlike a static tensile strength test, which applies tension at an even and slow rate, a dynamic test imparts the tension load rapidly. Although the specific setup changes depending on which type of equipment is being tested, the general method specified by ANSI is to attach one end of a connector to a test tower via a load-measuring cell, a 282 lb. weight to the other end, and drop it from a prescribed height.

By increasing the speed at which the tension is imparted, we are testing the ability of the material (or entire system) to respond to a sudden spike of stress, as is found during a fall. The faster a load is generated, the more that is demanded from the material (e.g. lifeline) or system to prevent failure. Even if the peak tension force in a dynamic load is identical to that of a static load, the fact that the load occurs over a much shorter time means that in effect, the load can be much higher. In other words, pulling a cable to a 1,000 lb. peak force over a minute is very different than the same load over a fraction of a second. We seem to know this intrinsically already. When given a string to break, if at first a slow pull does not break it, on our next attempt, we will try by some means to “snap” the string by quickly pulling our hands apart. A more fun example is Silly Putty – yes Silly Putty. Remember how if you slowly pulled the putty it stretched, but if you pulled very fast it would snap clean? That’s the concept we are trying to engage by performing a dynamic tension test. When the Silly Putty stretches (plastic deformation), the molecules are able to reform without breaking their bonds completely. When the putty finally separates after this stretching, this is called ductile failure. But when you pull the putty apart fast (say that three times fast!), you are left with a nice clean break line because you have exceeded the putty’s ability to plastically deform and a fracture occurs. The is called brittle failure. Both, of course, are to be avoided during testing of fall protection equipment.

More Than The Lifeline

Don’t get me wrong though, dynamic testing is not all about the lifeline in the case of an SRL, although that, of course, is a major focus. It is just as important that the snap hook, housing, and internal workings also withstand the test. In fact, in order for an SRL to successfully pass a dynamic drop test, the SRL must continue to function as normal, and in the cases of a leading edge SRL test, the lifeline must maintain a residual strength of 675 lbs.

Of the two types of tests, it’s clear that the dynamic tests are the more exciting to perform and to watch, sometimes with dramatic results. As a matter of fact, our test technicians often say they are “Headed down to the lab to watch some grass grow,” when preparing for a static strength test. But just because it might not be exciting doesn’t mean it’s not important; they know quite well that testing is the backbone of bringing a product to market and ensuring it performs as it should. I’m glad to know they take their work seriously, as should every worker at height. Now…if I can just get them to give me my Silly Putty back, all will be right with the world…