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OCR for page 60
A-6 Once the stress is found, the modulus is determined by Test to Determine Slow-Crack-Growth Resistance of HDPE dividing by the strain (0.02). Resins or HDPE Corrugated Pipe. The NCTL test is ASTM D5397, Standard Test Method for Evaluation of Stress Crack Resistance of Polyolefin Geomembranes Using Notched Constant A.2.9 Oxidative Induction Temperature Tensile Load Test. The appendix of D5397 describes a single (OITemp) (ASTM D3350) load test that is very similar to F2136. The procedure for this test is found in ASTM D3350, Spec- The main difference between the two test methods is the ification for Polyethylene Plastics Pipe and Fittings Materials. applied load. For D5397, the applied loads are based on percent- An unweighed chip of material was placed in an open DSC ages of the measured yield stress of the material to be tested. sample pan and heated at 20C/min in air until an exother- For F2136, the applied load is a constant 600 psi (4.14 MPa). mic oxidation occurred. The onset temperature of the oxida- This is the applied load that would be used in D5397 if the tion peak is reported. measured yield stress was 4,000 psi (27.59 MPa) and the load was 15% of the yield stress. The constant applied stress of 600 psi has been accepted for pipe resins in the density range A.2.10 Oxidative Induction Time (OIT) of 0.9480.955 g/cm3. (ASTM D3895) For most of this study, the NCTL test was performed at an This test was done in accordance to ASTM D3895, Test applied stress equal to 15% of the materials yield stress. This was Method for Oxidative-Induction Time of Polyolefins by Differ- done for all materials submitted for characterization. The rea- ential Scanning Calorimetry. A sample weighing from 510 mg soning was that samples of recycled resins were obtained with was placed in a DSC under nitrogen and heated to 200.00C. yield stresses from 3,300 to 4,500 psi (22.7631.03 MPa). Apply- Once the temperature was steady, the atmosphere was changed ing a constant stress of 600 psi would load these materials from from nitrogen to oxygen. The test continued until an exo- 13 to 18% of their yield stress. Since stress crack times are highly thermic peak was obtained. The time from the introduction dependent on applied stress, it was believed that the NCTL test of oxygen to the onset of the exothermic peak is the oxidative at an applied stress of 15% of yield would produce more accu- induction time (OIT). rate results. Eventually, however, resin blends for considera- tion for AASHTO will require NCLS test results. Besides how the load is determined, the tests are run basi- A.2.11 Pipe Deflection Tests (ASTM D2412) cally the same way. Small dumbbell shaped specimens are cut The stiffness and flattening characteristics were determined and a face notch equal to about 20% of the specimens thick- by ASTM D2412 as modified by AASHTO M294. The testing ness is cut in a controlled manner with a razor blade and a machine used was a Lo-Tes Model LT-10, manufactured by notching device. Five specimens are then hung on a lever-load Plowman Brothers (now Varicore). Duplicate pipe samples, frame and placed in the 10% Igepal CO-630 surfactant solu- about 12-in. long were deflected at a rate of 0.5 in./min, with a tion. The loads are applied and the timers reset to zero hours. 5 lb preload and the load/deflection curve obtained. One pipe The variables that need to be controlled in this test include specimen was tested along the mold lines and the other was the plaque preparation, the notches, the loads and the expo- tested 90 offset from the mold lines. The test was continued sure environment. Of these, the environment seems to be the up to a 40% linear deflection of the inside diameter occurred. most dynamic. It was determined that for these laboratories, The pipe stiffness was calculated by: the Igepal baths required changing about every 45 weeks. PS ( psi ) = Load ( lb ) Deflection ( in.) Length ( in.) A.3.2 BAM Stress-Crack Test (TRI Method) The peak load and peak deflection were also recorded, which This test was introduced by researchers in Germany at the can be considered the buckling point of the pipe. Bundesanstalt fr Materialforschung und-prfung (BAM). It was developed for evaluating textured coatings for Geo- membranes (landfill liners). It was used successfully by the A.3 Stress-Crack Tests researcher on both textured Geomembranes and heat-bonded seams (1, 2). A.3.1 Notched Stress-Crack Tests (NCLS and The test involves the use of the same load frame as the NCTL)(ASTM F2136, D5397) NCTL or NCLS tests, except that special clamps are needed to There are two notched stress-crack tests that have been test the 15.24-cm 1.27-cm (6-in. 0.5-in.) test specimens. used during this project. The NCLS test is ASTM F2136, Stan- The test is normally run under a constant load of 580 psi dard test Method for Notched, Constant Ligament-Stress (NCLS) (4MPa), at 80C in 5% Igepal CA-720. However, different

OCR for page 60
A-7 temperatures were explored to determine appropriate condi- 1 /16-in. all thread. The back plate of the bottom grip was 1.5- tions for evaluating recycled materials. A different surfactant in. (3.81-cm) long and had a 3/16-in. (4.76-mm) hole at the is necessary because the cloud point of CO-630 is 55C, so bottom so it could be mounted to the frame. The grips are it is immisible at 80C. The cloud point of CA-720 is 82C. shown in Figure A-6. The ones pictured show serrations on Because there is no notch in the test specimens, cracks will the outside. The insides of the grips are smooth. only grow if there are critical defects in the specimen. This The test requires more weight than can be added to a makes the test useful to evaluate the effects of contamination, typical NCTL weight tube. The tubes used had 1/8-in. all- especially for recycled plastic. thread running through the length of the tube. The tube Test results on geomembrane seams showed clearly that was reduced in length by 4 in., and in its place were added the test was sensitive to both the inherent stress-crack resis- two or three, 1-in. 1-in. 2-in. brass blocks. A combina- tance of the sheet and the defects where cracks could initiate tion of brass blocks and zinc coated steel shot was used to (2). The results also showed that very good stress-crack resis- obtain the required loads. This is shown in Figure A-7. tance resins were much more forgiving, in terms of critical Exposure bath: A Blue M Model 1140A was used to per- defect size. Therefore, by choosing a better resin, one can lessen form the tests. A solution of 5% Igepal CA-720 in de- the effects of defects that are present. ionized (D.I.) Water was the test solution. The solution was found to have a limited life of around 300 h, so it was made fresh at the beginning of each test. The bath had an BAM Test Procedure external stirrer along with a magnetic "flapper" to help 1. Specimen preparation: control bath temperature. Five samples, 0.6 in. 6 in. (1.52 cm 15.24 cm) were 3. Procedure: The test specimens with the shaved edges were cut from the plaque with a die. The edges of these samples carefully mounted in the grips. The pair of screws closest were shaved with a sledge microtome to remove any defor- to the ends of the specimens were tightened first, followed mations caused by the cutting die and to reduce the sam- by the pair of screws closest to the center. The amount of ple width to about 0.5 in. (1.27 cm). The shaving operation pressure on the screws is critical. If the grips are too loose, is shown in Figure A-5. The result is five test specimens, the specimen slips out. If the grip is too tight, a stress crack 0.5-in. wide by 6-in. long with perfectly smooth edges. is initiated at the edge of the grip. 2. Test Apparatus: Once the specimens are mounted, the five station load The test was run with the use of a BT Technology stress frame is placed in the bath, the pre-weighed weight tubes crack fixture that had been modified with special grips and hung immediately, and the timers reset to zero. This way, as weights. The top grips were simply 1-in.2 (2.54-cm2) smooth the test specimens heated up, the load was applied gradually. faced, stainless steel grips with a screw in each corner for After the specimens failed or 300 h was reached, the test tightening. These can be easily made from 1-in. bar stock, was terminated. The failed specimens were examined to 1 /4-in. thick. The top of the back plate was tapped to accept be sure they did not break at a grip, and the crack faces Figure A-5. Edge shaving of BAM test specimens.