Linear Abrasion Testing PVC/Polymer Blend Hoses

[mkematt96]

Working on a research project to develop a test standard for abrasion testing pressure washer hoses. Ideally this could be used on any sort of hose. We have a set procedure that has worked on a couple hoses that looks like this:

.Weight applied is constant
.Cycle speed is constant
.Constant abrasive type used
.Hose sample is weighed pre and post test to see weight lost per cycle
.Hose is filled with rod to simulate it being filled with a fluid
.A fixture that holds acid flux brushes automatically cleans the test surface as the abrasive passes.
.Run 500 cycles. Stop, clean abrasive media with a wire brush and wipe down sample
.Run another 500 cycles. Repeat previous step. Then resurface abrasive media with a tool provided by the machine manufacture.
.Repeat these two steps until failure occurs ( you can see through the jacket into the braided inner hose)

Here is what the machine looks like. This isn't our set up***


Here is the abrasive****
https://www.taberindustries.com/con... Abraser/01 Abradants/H-22 Taber Wearaser.JPG

This has worked great except on two hoses.

.Hose sample A melts onto and clogs the abrasive media after 10 cycles. So the remaining 490 cycles in the step don't really do anything. This occurs no matter what the speed is or the weight applied is. We also used a finer media provided by the manufacturer but it still occurred. The weird thing is we tested the same hose type under the same conditions but a different color (RED) and it tests no problem (fails within 2500 cycles). Could they be different materials even though the specifications on the drawing say they are the same? Could the coloring of the hose effect this?

.Hose sample B I cut up and tested immediately out of the box. The first two tests were successful (failed within 6500 cycles) I let the samples sit over the weekend and continued the test. When I went to continue the test with a fresh sample ( As in hasn't been tested yet ) that sat in the lab over the week it had the same results as hose sample A. The first 200 cycles would behave normally but then it would start melting to the abrasive. The samples that work and failed in a normal manner were off the assembly line but they were able to sit multiple days without any problem.What could cause this? Would the lab conditions cause this to occur?

 

[jrmichler]

One failure mode of thermoplastics is melting at greater than a certain PV (PV: pressure times velocity) value. Sliding friction causes heat. When the PV exceeds a certain value, the increased temperature from the friction heating causes the plastic to soften, the softening increases the friction, the increased friction increases the temperature even more, and you get thermal runaway. Because of thermal runaway, the wear life can be years at a certain PV, then drop to minutes with only a small increase in PV. The exact PV that triggers thermal runaway is a function of friction coefficient and ambient temperature.

I think that you are testing right at the critical PV value. Try dropping the speed or load in half, and see what happens. Keep in mind that your test is only valid if the failure modes are the same failure modes that are seen in the field.