Ask an Engineer: Are Differences Among Webbing Types Important?

[freemexy]

The following guest article was inspired by an enlightening conversation between SRN editor Denise Donaldson and Dave Sander, CPST-I and engineer (formerly with Evenflo, but employed elsewhere at the time this article was written).Custom made Webbing

Have you ever given close attention to the webbing used for car seat harnesses, LATCH straps, or vehicle seat belts? If so, you may have noticed that some are wider or feel thicker, smoother, or rougher than others. You may have also noticed that some have stripes (actually called panels), and that those panels vary in appearance and number.

If you have noted these things, I congratulate you on your keen sense of observation! These differences are not random or decorative; each detail in webbing has been intentionally designed to affect how it will perform, especially in a crash.

FMVSS 213 stipulates certain webbing characteristics of CRs. It defines the minimum width of the webbing used in harnesses, tethers, and LA straps. It also says that new webbing must meet a minimum strength requirement of 11,000 Newtons for harness webbing and 15,000 Newtons for LA and tether webbing. To get an idea of how strong that is, you could basically pick up a Honda Accord with a strap made out of LA or tether webbing!

CR manufacturers purchase this strong webbing, and most also do their own internal testing to doubly ensure compliance with the standard. FMVSS 213 specifies that this be done using what’s called a quasi-static test. This is simply a test in which the webbing must not break, at the specified load, when a device attached to the ends pulls it apart at a slow and steady rate.

The quasi-static test is beneficial as a consistent benchmark for measuring performance criteria among all the different webbings that a company might use. However, CR manufacturers also assess webbing during dynamic testing of car seats during sled tests run at a very high rate of speed. Quasi-static results typically do not match these high-speed results, in that the amount of elongation (or stretch) seen during the quasi-static test is likely to differ from the amount during a sled test—it could be more or less. Since the amount of stretch is a key characteristic with respect to how webbing manages crash forces, it is helpful to know the results of both types of testing.