Tensile Testing BACKGROUND INFORMATION. Harper, A., and Nickels, K Queensland University of Technology.

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Tensile Testing TEACHER WORKSHEET 5-10 BACKGROUND INFORMATION Polymers, or macromolecules, are of great importance in the technical and scientific communities.

1 Tensile Testing TEACHER WORKSHEET 5-10 BACKGROUND INFORMATION Polymers, or macromolecules, are of great importance in the technical and scientific communities. There is an abundance of new and useful polymeric structures that generate corresponding new, interesting and potentially useful properties. The subcategories that will be emphasized in this experiment divide solid material polymers into: Fibres (low elongation, high stiffness and modulus) Plastics (intermediate elongation, generally intermediate stiffness) Elastomers or rubbers (high elongation, low modulus) Many demands are placed on each specific polymer, polymer blend, or polymeric composite. Essentially all materials, be they substitutes for brick, wood, glass, or other structurally demanding materials, must have and reasonably maintain specific mechanical properties. One way of measuring a range of some mechanical properties is to perform tensile and/or compressive testing. This experiment describes a simple laboratory version of a common sophisticated tensile machine, Instron Tensile Testers. In this experiment tensile properties are tested in a uniaxial (stretching) mode. As weight is added to the bucket, a specific load is applied, which upon using the known cross-sectional area of the sample is converted to stress (force/unit area). By measuring the initial length of the sample and its incremental increase with each new load, strain is also recorded. Upon completing the process of adding weights and measuring each new length, all necessary data for a standard stress/strain curve are generated. Some samples will break with the maximum weight applied; others may not break under these conditions. During the tensile testing basically three events occur that can lead to fracture or breakage. Disentanglement and stretching of the polymer: random coils stretch and align in somewhat parallel fashion, and crystalline polymers in a lamellar arrangement unalign and go towards a stretched-out somewhat linear configuration Breakage of the covalent bonds break under the applied stress Chain slippage (where secondary bonds eg. Dipole-dipole interactions, are broken)

2 Tensile Testing TEACHER WORKSHEET 5-10 WHAT YOU NEED Lab clamp 1x 3L Bucket with handle 1L Measuring jug 200mL measuring cylinder Paper clip Water Scissors Polymer samples: 1 elastic strap (about 21cm long) with metal hooks at ends (eg. Bike strap or bungy cord) Rubber band Regular photocopying or printer paper A3 Polyethylene film (0.8ml thickness eg. Gladwrap) Cellophane WHAT TO DO Part 1: Testing properties of different polymers 1. Prepare the paper, gladwrap and cellophane samples so they are uniform in dimensions. Cut each sample to be 2.5cm wide and 30cm long 2. Reinforce 10cm from the top and bottom of each sample with Scotch tape, leaving 10cm of the sample exposed. 3. Starting with the elastic strap. Attach one hook of the elastic strap to a lab clamp. 4. Adjust clamp location so that with strap extended and attached to the bucket with its handle, the bottom of the bucket is about 20cm from the bench top, alternatively face the lab clamp so that the bucket hangs over the edge of the bench. 5. Measure the length of the strap excluding the metal hooks. 6. Incrementally add specific loads of 150mL of water using the measuring cylinder. Have another student stand with hands under the bucket ready to catch the bucket of water if the material fails. 7. After each additional load measure the new length and record the total weight. 8. Continue steps 5-7 until the material breaks or the bucket is full with 3000mL of water. 9. Using the paper samples wrap the top taped portion of the paper around the lab clamp and secure it be reinforcing that portion with more tape, making a loop around the clamp. 10. Repeat step 9 for the bottom portion and slip this loop through the handle of the bucket. 11. Measure the length of the paper excluding the portions that have been reinformed by Scotch tape (should be ~10cm). 12. Repeat steps Using the polyethylene, repeat steps Then steps Using the cellophane, repeat steps Then steps Using the rubber band, repeat steps Then steps Record your findings in the observation sheet.

3 Tensile Testing TEACHER WORKSHEET 5-10 Part 2: Testing properties of different polymers under different conditions Use the following materials for this part of the experiment: Rubber band, Paper width, paper length, Polyethylene film, and Cellophane 17. Repeat as per Part 1 instructions. Prior to adding the initial load expose the material to a few droplets of water. You can do this by placing your fingers in the jug of water and then flicking the water onto the material from a distance of 25cm. 18. Record your findings in the observation sheet. 19. Repeat as per Part 1 instructions. Prior to adding the initial load make a small incision in each of the materials (Make sure the incision is the same length for each and is around 2 mm). 20. Record your findings on the observation sheet. QUESTIONS 1. Are the results for paper different if measured along the length or along the width of the sheet? Why would they be different? 2. Which of the materials is most likely to be sensitive to the relative humidity at the time of testing? 3. Which of the materials is least sensitive to the presence of an incision in the edge of the sample when tested? CURRICULUM Strand: Natural & Processed Materials Core learning outcomes: 3.1, 3.2, 4.1, 4.2, 4.3, 5.3 REFERENCES Carter Gilmer, T. & Williams, M. (1996). Polymer mechanical properties via a new laboratory tensile tester. Journal of Chemical Education. 73:

4 PART 1 OBSERVATIONS Incremental loads Initial Bungy Rubber paper paper Polyethylene film Cellophane band Length Weight Length Weight Length Weight Length Weight Length Weight Length Weight 150mL 300mL 450mL 600mL 750mL 900mL 1050mL 1200mL 1350mL 1500mL 1650mL 1800mL 1950mL 2100mL 2250mL 2400mL 2550mL 2700mL 2850mL 3000mL

5 PART 2 OBSERVATIONS (water droplets) Incremental loads Initial Rubber band Paper (width) Paper (length) Polyethylene film Cellophane Length Weight Length Weight Length Weight Length Weight Length Weight 150mL 300mL 450mL 600mL 750mL 900mL 1050mL 1200mL 1350mL 1500mL 1650mL 1800mL 1950mL 2100mL 2250mL 2400mL 2550mL 2700mL 2850mL 3000mL

6 PART 2 OBSERVATIONS (2mm incision) Incremental loads Initial Rubber band Paper Paper Polyethylene film Cellophane (width) (length) Length Weight Length Weight Length Weight Length Weight Length Weight 150mL 300mL 450mL 600mL 750mL 900mL 1050mL 1200mL 1350mL 1500mL 1650mL 1800mL 1950mL 2100mL 2250mL 2400mL 2550mL 2700mL 2850mL 3000mL

Chapter 15 Part 2. Mechanical Behavior of Polymers. Deformation Mechanisms. Mechanical Behavior of Thermoplastics. Properties of Polymers

Chapter 15 Part 2. Mechanical Behavior of Polymers. Deformation Mechanisms. Mechanical Behavior of Thermoplastics. Properties of Polymers Mechanical Behavior of Polymers Chapter 15 Part 2 Properties of Polymers Wide range of behaviors Elastic-Brittle (Curve A) Thermosets and thermoplastics Elastic-Plastic (Curve B) Thermoplastics Extended