Hi John,
I haven't had any lanyards fail yet but that really doesn't mean much. My experience with the high strength fibers has been in tthe marine industry; primarily sailing. Kevlar was the first of the new strong fibers and it had serious problems and failures. Due to it's extremly low stretch and elasticity, there were stories of hearing a kevlar rope breaking when bent on a small sheave, one strand at a time! Ping Ping Ping......
Technora has better properties and then there's vectran and some other new fibers out there now. Spectra has had wide spread success and its two weakness are creep under load as well as lower melting point (compared to aramids)
Kevlar is hydrophillic and that may be why you'fe heard that the vests don't do as well when wet. I don't know what is going on there.
For high temperature/ high strength fibers there aren't many choices. If high temperature isn't an issue for your boot laces, I think spectra would hold up better in the long run. I really don't think you will be breaking any of them. The kevlar is a nice flat tubular web, similar to the shoe strings. The 7/64" single braid spectra that I have used is a tighter round cord.
If anything, a caution should be considered in using these materials in so far that they are very strong and the "weak" link in a tether or lanyard system will likely be elsewhere and perhaps not weak enough.
I'm not sure if you want me to address boot laces or lanyards here but in either case, I suspect that the loads on the fibers will typically be well below the rated strengths of the materials and other considerations such as abrasion may have more bearing on the ultimate failure.
Rambling mode - off
Don
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