Tension Tests of UHMW PE

`Tension Tests of UHMW PE July 2009 Rebecca Bemrose-Fetter 1. Purpose: The QUIET experiment uses a window composed primarily of Ultra-High Molecular Weight Polyethylene (UHMW PE). The window is subject to a vacuum pressure. We designed a series of tension tests to analyze the strength of the UHMW PE window, and how it evolves over time.

2. Stress and Yield Strength:

The QUIET experiment’s window is primarily UHMW PE with Teflon bonded to each side by Low Density Polyethylene (LDPE). The circular window is bolted to the spool, protecting the black body inside. The spool will be placed under a vacuum pressure that will result in a tensile force of approximately 2900psi on the UHMWPE window.

According to MatWeb, a website devoted to the mechanical properties of materials, the tensile yield strength of UHMWPE is 2030-5800psi, with an average of 2960psi (see Appendix). A plastics manufacturer in San Diego claims natural UHMWPE has a tensile yield strength of 3100psi (see Appendix). The manufacturer of the UHMWPE that we purchased lists a tensile yield strength of 2800 psi. As a result of this range of values, we will be conducting our own tests.

We will do tension tests on 25 samples of the material that have been under a load equal to 2900psi for varying amounts of time. Specifically, we will test after 1, 2, 4 and 8 days under load, as well as an initial sample loaded to the yield point without prior subjection to the pre- load. We wish to examine the behavior of the material under load over time, and the change in the yield point that occurs.

3. Setup

We have followed ASTM D638-08 in our tension testing setup. We will be using Type I dog bones 6.5” long x 0.75 with 0.5” wide x 2.25” reduced section. We have designed these in strings of five with intermittent sections 2” wide between each dog bone. The string will be clamped at the top and hung from the crane in Cleanroom A. A clamp at the bottom of the string of samples provides a place to hang the load from.

We will do a gage measurement of the length of the 2.25” reduced section before and after applying the pre-load. The load will be applied for 0, 1, 2, 4 or 8 days. Then the string of samples will be removed from the crane and the samples will be cut apart and labeled with position in the string and time under the pre-load. We will test one string of five samples for each time length.

All samples will be stored in the humidity- and temperature-controlled cleanroom before, during and after the pre-load is applied. When all samples are ready, they will be taken to Technical Division, acclimatized to their testing environment, and then pulled to the yield point. Observations/comments on pre-load initial test: The crane uses a cable which has twist, giving the length of dog bones a tendency to twist while the load is applied. Clearly we need to put some thought into preventing twisting of the samples while under load.

The material is very ductile. We struggled to keep the load off the ground, as the dog bones would simply stretch to the point that the load was again resting on the ground. Interestingly, the elongation was not the same from one sample to the next. This kind of ductility is consistent with the MatWeb specifications. They list an elongation at break of 50-800% and a elongation at yield of 14-22%. Aug 11 2009 David Butler

Hung another single dog bone. Put reduced load on it-359lbs=2872psi. Dog bone instantly stretched. Stopped. Put 253lbs=2024psi. Dog bone still stretching too quickly. Aug 13 2009 Hung another new dog bone and put on it 147lbs=1176psi. 2 29/32” initial lgth 3 1/32” immediately after picking up load 9:00am 3 5/32” 11:00 am 3 5/32” 15:00 3 11/32” 8/14/09 15:00 3 11/32” 8/17/09 0700 (measurement doesn’t look right-probably error in reading) 3 14/32” 8/18/09 11:00 3 16/32” 8/21/09 09:00 (~120% original lgth) 3 16/32” 8/24/09 15:00 3 16/32” 8/25/09 07:25 Add one 53lb block 07:27 Add one 53lb block 07:28 253lbs total. This increased tension to 2024psi 3 26/32” 07:38 3 30/32” 07:48 4 18/32” 09:05 5 23/32” 11:48 7 22/32” 14:35 Raised load to 5” above floor for overnight. 9 29/32” 8/26/09 08:00 9 31/32” 10:40 10 2/32” 15:00 Raised load to 6” above floor (SKIPPED ONE DAY) 10 13/32” 8/28/09 06:30 10 13/32” 09:50 Add two blks increasing load to 2872psi 10 20/32” 09:58 10 28/32” 10:08 11 1/32” 10:18 11 5/32” 10:28 11 8/32” 10:38 11 12/32” 10:54 11 13/32” 10:58 12:20 Discovered dog bone broken. Aug 28 2009 Installed single new dog bone and hung 2024psi load Preload measurement: 3.0” Pickup load 13:18 13:20 3 7/32” 13:25 3 9/32” 13:30 3 11/32” 13:45 3 17/32” 14:15 3 28/32” 14:30 4 4/32” 8/31/09 day3 07:30 11 11/32” Load was touching floor 07:35 lifted off floor 08:15 11 14/32” Removed to set up for next test. I think that if this load was lifted higher to begin with, it would have broke over the past weekend.

8/31/09 Setup new string of five dog bones. Put measuring stops on each one, set to 3.0” Also, took overall measurements with no load. Load=1824psi Position No load W/load 5 - 1 41 11/32” 42 28/32” 5 – 2 31 9/32 32 30/32” 5 – 3 18 10/32” can not reach 5 – 4 8 22/32” can not reach 10:00 #1 3 28/32” #2 3 25/32” #3 3 25/32” #4 3 30/32” #5 4 2/32”

11:45 #1 4 8/32” #2 4 11/32” #3 4 13/32” #4 4 12/32” #5 4 13/32” Next day Sept 1 Load was touching floor. 07:30 #1 6 27/32” #2 9 23/32” #3 10 12/32” #4 8 6/32” #5 7 21/32” Remove load. Remove #1 dog bone. Reconnect and apply load. 07:50 07:55 #2 9 12/32” #3 10 1/32” #4 7 22/32” #5 7 10/32” Next day Sept 2 07:45 #2 10 4/32” #3 10 24/32” #4 8 29/32” #5 8 18/32” Remove load. Remove #2 dog bone. Reconnect and apply load.

Next day Sept 3 07:15 #3 10 28/32” #4 9 2/32” #5 8 22/32” Remove load. Remove #3 dog bone. Reconnect and apply load. Next day Sept 4 07:00 #4 9 6/32” #5 8 25/32” Remove load. Remove #4 dog bone. Reconnect and apply load.

Sept 8 07:30 #5 8 29/32” Sept 9 07:20 #5 8 31/32” Sept 10 07:30 #5 9” Sept 14 07:00 #5 9 1/32” Sept 14 End test. Alum measuring stops fell off. 9/3/09

Another Test: Hang single dog bone in the Hi Bay area. 1800psi load. 9/3/09 14:30 Next morning, 9/4/09 Found load touching floor. Dog bone stretched from 90mm initial length to 290mm. Lifted of floor. (Change measurement convention to metric.) After three day weelkend,9/8/09, found dog bone broke.

Hung new single dog bone and lifted load higher(do not want it to touch floor before max amount of stretch) Initial length:187mm.(Measurements taken between clamps rather than taping on aluminum pieces) Next morning,9/9/09: 345mm 9/10/09 362mm 9/21/09 390mm 9/11/09 368mm 9/28/09 394mm 9/14/09 386mm 10/12/09 397mm 9/15/09 387mm 10/16/09 398mm 9/16/09 388mm 10/22/09 398mm 9/17/09 389mm 11/17/09 400mm 9/18/09 389mm 12/21/09 401mm

9/16/09

Start new long term tension test Sept 16 Hang 200lbs=1600psi Established measurements marks at 153.5mm spacing. Pick load at 11:30 #1 159mm #2 159mm #3 157.5mm #4 158mm #5 159mm

Sept 16 Day1 11:30 #1 187.5mm #2 188.5mm #3 191.5mm #4 189.5mm #5 185.5mm Remove dog bone #1 and reapply load.

Sept 20 Day4 11:15 #2 198mm #3 230mm #4 208mm #5 193mm Remove dog bone #2 and reapply load.

Oct 2 Day 16 #3 326mm #4 308mm #5 201mm Remove dog bone #3 and reapply load

Oct 16 (Day 30) #4 318m #5 300mm?(bad number)

Nov 3 Day 48 #4 318mm #5 209mm Remove dog bone #4 and reapply load

Nov 17 #5 212mm Nov 25 #5 214mm Dec 21 #5 227mm Remove dog bone #5 This phase of the test is completed Started new test. Apply 12.86 lbs to poly strip 1”wide, suspended 12’’ with fixed ends.

Dim from table top To underside of strip:

Sept 28 11:00 125mm no load 11:05 103mm load applied. Sept 29 10:30 98mm Oct 5 Discovered weight has fallen off. The Poly strip twisted. 10/8/09 Made new fixture setup to test two ploy strips at the same time. The weights are hanging. Ends are securely attached. Weight: 13lbs=1872psi

Strip A Strip B 592mm Before Load 591mm 582mm 13lb load 580mm 575mm 10/12/09 575mm 575mm 10/16/09 575mm 574mm 10/22/09 574mm 574mm 11/3/09 575mm 574mm 11/17/09 574mm 573mm 11/25/09 573mm Remove strip B Install strip C Strip A Strip C 573mm 11/25/09 579mm 573mm 12/11/09 573mm Remove strip C Install strip D 12/11/09 Strip D 578mm Remove strip D 12/15/09 Install strip E 1/11/10 Strip E 580mm Remove Strip E 1/12/10 575mm Remove strip A 1/12/10 Final measurement 570mm Final crosshead displacement at breaking: DB-1 org 7.342 in DB-1 Te 7.284 in DB-2 Te Unknown DB-3 Te 4.525 in. DB-4 Te 4.434 in. DB-5 Te 6.783 in. DB-ABe 3.359 in. DB-BBe 3.000 in. DB-CBe 8.050 in. DB-DBe 6.100 in. DB-EBe 7.962 in.