Mineral grit affects webbing strength

It is hard to find a standard way to dirty cordage.

For this test, I sewed full-strength loops on the ends of six 5/8" pieces of climbspec tubular webbing. I kept 3 as clean controls, and very slightly dirtied the other 3, by swirling them for 5 minutes in a slurry that consisted of two liters of water, fine backyard sand, plus 30 g of Black Diamond climbing chalk (Magnesium Carbonate, supposedly 10-25 micron average). After the dirtied samples were dried, I brushed off all loose particles; they really didn't look dirty except for a gray tinge.

I used my car as the force, and Amsteel soft shackles (1/4" cord, sheathed in 1" nylon climbspec webbing) were used to connect the test pieces to a thick semi-dynamic rope on the back of my car, and to a linescale 3 (attached to a solid anchor)

The plot below tells the story. This simple dirtying, barely observable, decreased the webbing strength  by roughly 20%. It isn't clear if the mineral particles decreased strength by cutting the nylon filaments, or by increasing the friction among filaments, so that shear forces were more easily transmitted.


(click above to see full-sized plot)