From eboltz@jhunix.hcf.jhu.edu Tue Jul 25 17:09:23 EST 1995
Article: 6378 of rec.climbing
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From: eboltz@jhunix.hcf.jhu.edu (Eric Scott Boltz)
Newsgroups: rec.climbing
Subject: Re: Rope Lifetime?
Date: 21 Jul 1995 09:55:31 -0400
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>In article <0040E5CC.fc@nile.com> Clyde_Soles@nile.com (Clyde Soles) writes:
>
>>Wrong on all counts. Read the rope review in the current issue of R&I.

Let's explore the background and data for your evaluation of ropes
Clyde. I'm a materials scientist who specializes in materials charaterization
and evaluation using non-invasive and non-destructive techniques which
are later validated using "conventional" destruction tests.

The first bit of data for our evaluation should be the number of
modern nylon ropes which have failed, in the field, due to tensional
loading (i.e. not cut). We will exclude any cases where said ropes were
compromised by such factors as long exposure to outdoor elements or
worn sheath material. In my ongoing observation of reported accidents I
have *never* heard of such a thing happening...have you? Certainly, 
ropes break (last year someone rapped off a year-old fixed line on
Everest and died), but I don't think any of these cases constitutes
a "normal use" situation.

Certainly manufacturers are inclined to recommend rope retirement after
minimal use and falls for reasons of both liability and profits, but
do they have externally (unbiased) verified data to support their specific
recommendations?

Consider also the nature of rope design and behavior under moderate
impulse loading. The braided internal structure of nylon ropes is 
quite elegant. Without any plastic deformation of the actual nylon
fibers this structure can stretch, thus absorbing tremendous energy. 

But how do we *know* that there isn't plastic deformation. Plastic 
deformation is the single most important parameter for determining
whether or not a structure has been comprimised. Well, the easiest, 
ball-park estimate is whether or not the rope returns to its original
length and diameter. Granted, the fracture of a few fibers is not
going to be measureable without some sophisticated techniques, but
a few fibers are not going to compromise the actual failure strength
of the overall structure considerably or else QC of rope manufacturing
would be a big problem.

So do we measure the length of our ropes after a fall? Well, it's hard
to measure 50+ meters accurately. What's easier is to feel the rope density
(hardness) in the area which took the brunt of the the stress while
the rope is under load. If there is noticeable flattening of the rope, 
this is bad :). Another qualitative measure of the damage done during
a fall is how jarring the fall feels. Dynamic ropes are called dynamic
because of their ability to stretch elastically. Once they reach the
end of they're elatic region they "stretch" plastically. At this point
the rate of absorption is much lower and much more jarring. Hard falls
will definitely dammage a rope. Keep in mind that even a moderate fall,
if followed shortly thereafter by another moderate fall, can do dammage.
This is why some sport climbers switch ends after a leader fall.

I'm quite certain there is a ton of research out there on rope failure
mechanisms and materials evaluation of ropes (I remember the DoD funding
a big project for this back in the eighties), so once I finish the two
papers I'm writing (that are due in a week - YIKES!) and return from the
conference, I'll start a literature search and come back with some
quantitative information based on research rather than bucks.

Till then perhaps Clyde would like to fill in some of us non-R&I supporting
folks on his own (or those given to him by rope manufaturers)
conclusions...but I doubt it.

Back to the coal mine,

-E

(Sorry if I sound pissy Clyde, but your post - like many - was not
exactly what I term "helpful" and seemed to constitute little more than
a plug for your mag.)