LABORATORY MODULE UNCONFINED COMPRESSION OF HYDROGEL DISKS

Transcription

1 LABORATORY MODULE UNCONFNED The unconfined compression test is by far the most popular mechanical testing configuration to measure the mechanical properties of cylindrical samples. amples with non-planar geometry can also be tested using this configuration, but data analysis requires more complex theoretical modelling to extract the mechanical parameters. A disk is compressed between two flat platens and is free to expand in the radial direction (slipping boundary conditions). This test configuration is normally performed under displacement control, as in this laboratory module. The sample's stiffness is determined by the slope of the load vs displacement curve. By considering the geometry of the disk sample (radius and thickness), the Young's modulus can be calculated. For purely elastic materials, the Young's modulus is independent of the strain rate in compression, while for non-purely elastic materials (viscoelastic and poroelastic), the Young's modulus can vary with the strain rate. This laboratory module consists of assessing mechanical properties of both rubber and hydrogel samples. Throughout this laboratory, each team will: Prepare the rubber and hydrogel samples; Rubber and hydrogel disk-shaped samples 1

2 LABORATORY MODULE UNCONFNED Test each sample in unconfined compression with various stain rates; train Rate Compression Variation 0.1 % of (thickness) per second 5 % of (thickness) per second 20 % of (thickness) per second Extract slopes from the load-displacement curves using Mach-1 Analysis software to obtain the rubber and hydrogel samples' stiffness for various strain rates; Calculate the corresponding Young's modulus considering the sample geometry; Discuss the effect of stain rate on the Young's modulus for each material; Conclude with the effect of strain rate on Young's modulus for materials with elastic vs. non-elastic behavior. Learning Goals 1. Become familiar with the safe operation of a mechanical tester during an unconfined compression test. 2. Learn how to prepare disk-shaped samples for an unconfined compression test. 3. Differentiate between stiffness and Young's modulus. 4. Measure the stiffness and Young's modulus of disk-shaped samples. 5. Understand the effect of stain rate on the measured Young's modulus for materials used in biomedical research. 6. Understand how the structure and composition of materials determine their behavior during compression testing. 2

3 TABLE OF CONTENT LEGEND... 3 KEY CONCEPT... 4 MATERAL... 5 DEFNTON... 5 AMPLE PREPARATON... 6 RUBBER... 8 TETNG PROCEDURE... 8 ANALY PROCEDURE AMPLE DATA HYDROGEL TETNG PROCEDURE ANALY PROCEDURE AMPLE DATA AMPLE DATA QUETON QUETON ADDTONAL REOURCE LEGEND For instructors For students This laboratory module can be used as a whole by the instructor. The student version of the laboratory module should only include pages 1-11, 13-16, and 24. Page 12, 17 and should NOT be included since they contain solutions. 3

4 KEY CONCEPT Rubber: A material composed of polymer chains. Rubber is an elastomer; therefore, it possesses predominant elastic properties caused by the stretching of polymer chains under deformation or loading. Hydrogel: A network of hydrophilic polymeric chains that can associate with a large quantity of water without dissolving. The polymeric chains of the gel can be engineered to give a wide spectrum of properties to the material; mechanical and chemical, including biocompatibility. Hydrogels make very interesting platforms for the development of various bioengineering applications: 3D scaffolds for cell cultures, cell encapsulation, nanoparticles, etc. Hydrogels generally express a non-purely elastic behavior due to the viscoelasticity of the polymeric chains and the poroelasticity brought on by the presence of fluid. tiffness : A structural property of a sample. By plotting force vs. displacement during an unconfined compression and extracting the slope, we obtain the stiffness of the sample. n Mach-1 Analysis, the slope is in gram-force per mm (gf/mm). The gram-force is 1 gram multiplied by the acceleration due to gravity on Earth, exactly m/s². Young's modulus : ntrinsic mechanical property of a material independent of its geometry. From the slope of the stress vs. strain graph during an unconfined compression and considering the geometry of the sample, we can calculate the Young's modulus. Elasticity: Property of a material to deform under load or strain and to return to its original shape when the stress is released. Viscoelasticity: Property of a material to exhibit both viscous and elastic characteristics when undergoing deformation. uch material will show time-dependent mechanical behavior. Poroelasticity: Property of an elastic or viscoelastic material with an incompressible free fluid phase. When such a material is put under stress, fluid is released from the solid matrix. The frictional drag generated from the movement of the fluid through the matrix affects the mechanical behavior of such material. These materials are normally tested in a bath of fluid. train rate: Change in strain or deformation of a material with respect to time. 4

5 MATERAL High-Temp. ilicone Gasket Material, 6 x 6", 1/16" thickness, McMaster-Carr: 8525T42 Miltex Dermal Biopsy Punch 8 mm, REF Cutting board 37.5 x 27 cm AMPLE PREPARATON tainless teel Rod for Extraction of ample, D = 1/16", L = 6", McMaster-Carr: 88915K11 D Water Agar powder, any brand (e.g. Telephone Brand Agar-Agar Powder) cale, 0.1 g minimum accuracy 200 ml beaker 100 ml graduated cylinder poon or equivalent for agitating the solution Plastic protector for sample extraction (e.g. packaging of the Biopsy Punch) Tweezers Petri dish, 10 cm diameter quirt Bottle filled with D water MECHANCAL TETNG AND ANALY Mechanical tester model Mach-1 V500c (MA001) or higher models Mach-1 Motion software (W326) Mach-1 Analysis software (W186) ingle-axis load cell 250 N (MA297) or equivalent with at least 100N in compression o Note: All single-axis load cells use units of "gf" instead of "N". n Mach-1 Motion, ensure to use the proper units to avoid any damage to the load cell. Calibration holder and weight 500 g (MA327 or MA337) Testing chamber, D = 98 mm (MA626) ample holder and chamber, D = 37 mm (MA740) Flat indenter, D = 12.5 or mm (MA262 or MA263) DEFNTON D RT gf Deionized Room Temperature Gram-force: 1 gf = N 5

6 RUBBER AMPLE AMPLE PREPARATON 1. Place the rubber sheet on a cutting board. 2. Position the 8-mm biopsy punch perpendicular to the surface of the material. 3. Push through the surface using a rotating motion to perforate the gasket material. 4. nsert the metal rod through the top end of the biopsy punch to extract the sample. 5. Repeat steps 2 to 4 twice to obtain three disk-shaped rubber samples. Extraction of the rubber sample HYDROGEL AMPLE AMPLE PREPARATON 1. Weigh 6 grams of agar and measure 100 ml of D water. 2. Combine the D water and agar in a 200 ml beaker and stir the solution. 3. Heat the solution in a microwave for 30 seconds and stir. 4. Heat the solution in a microwave for 10-second intervals, approximately five times, stirring in between until complete melting of the agar and thickening of the solution (the solution should be close to boiling but do not let it boil because air bubbles will form). 5. tir the mixture for 1 minute. 6. Pour into a petri dish to obtain an ideal 3 mm thickness. 7. Let cool for 15 minutes at RT. 8. Let rest at RT for 12h covered with a plastic film. 3-mm thick hydrogel (5% agar) in petri dish 6

7 AMPLE PREPARATON HYDROGEL AMPLE 9. Using tweezers, unmold the hydrogel and place it on a cutting board. 10. Using an 8-mm biopsy punch, first punch the plastic to create a disk that will protect the hydrogel during sample extraction when using the metal rod. 11. Ensure that the plastic disk is inside the biopsy punch and then punch the hydrogel. 12. nsert the metal rod through the top end of the biopsy punch to extract the hydrogel disk. 13. Repeat steps 11 and 12 twice to obtain three hydrogel disk-shaped sample. 14. tore the hydrogel sample in D water. Extraction of the hydrogel sample 7

8 RUBBER TETNG PROCEDURE RUBBER AMPLE Test setup for unconfined compression of disk-shaped rubber samples 1. Turn ON the Mach-1 controller and wait until initialized. 2. Open Mach-1 Motion software. 3. Using manual controls in the software, raise the vertical stage to its maximum height using "medium" speed. 4. Verify the load cell calibration as per Mach-1 user manual. trongly recommended before each test session. 5. ecure the testing chamber onto the base of the Mach-1 using four screws. 6. crew the sample holder onto the testing chamber. 7. Gently screw the flat indenter into the thread of the load cell and lightly fingertighten to secure it. 8. Using manual controls, lower the stage at "medium" speed to approximately 20 mm above the sample holder. Lower the stage at "low" speed to approximately 2 mm above the sample holder. 8

9 TETNG PROCEDURE RUBBER AMPLE 9. Find the vertical position of the bottom platen and set it as the reference by performing the following test sequence. Note: ince the upper flat platen indenter and the bottom of the sample holder need to be in contact (metal-on-metal), this step should be executed very carefully to minimize the risk of damaging the load cell or the accessories. We suggest that this step be executed or supervised by a qualified laboratory technician. Functions Parameters Zero Load No parameter Find Contact tage Axis: Position (z) Load Cell Axis: Fz Direction: Positive Velocity: 0.01 mm/s Contact Criteria: 3.75 gf tage Limit: 3 mm tage Repositioning: 2X Load Resolution Zero Position tage Axis: Position (z) 10. Using manual controls, raise the stage at least 50 mm above the sample holder. 11. Place the first 8-mm disk-shaped rubber sample in the center of the sample holder using tweezers. 12. Using manual controls, lower the stage at "medium" speed to approximately 20 mm above the sample. Lower the stage at "low" speed to approximately 2 mm above the sample. 13. Perform the following test sequence to measure the disk thickness L 0 : Functions Parameters Zero Load No parameter Find Contact tage Axis: Position (z) Load Cell Axis: Fz Direction: Positive Velocity: 0.1 mm/s Contact Criteria: 3.75 gf tage Limit: 3 mm tage Repositioning: 2X Load Resolution 14. Report the current position of the stage axis (z) as the thickness of the sample in Table 1 near the end of this document. 9

10 TETNG PROCEDURE RUBBER AMPLE 15. Proceed with the first unconfined compression test using the following sequence. Functions Parameters Zero Load No parameter tress Relaxation tage Axis: Position (z) (Pre-Compression) Load Cell Axis: Fz Ramp Amplitude: 3% of Ramp Velocity: 0.4% of per second Number of Ramps: 1 top Based On: Fixed Relaxation Time Fixed Relaxation Time: 60 s ave results as: 1 st Test: "GrXX_TeamXX_Rubber_01.txt" 2 nd Test: "GrXX_TeamXX_Rubber_5.txt" 3 rd Test: "GrXX_TeamXX_Rubber_20.txt" tress Relaxation tage Axis: Position (z) (Compression) Load Cell Axis: Fz Ramp Amplitude: 10% of Ramp Velocity: 1 st Test: 0.1% of per second 2 nd Test: 5% of per second 3 rd Test: 20% of per second Number of Ramps: 1 top Based On: Fixed Relaxation Time Fixed Relaxation Time: 60 s ave results as: 1 st Test: "GrXX_TeamXX_Rubber_01.txt" 2 nd Test: "GrXX_TeamXX_Rubber_5.txt" 3 rd Test: "GrXX_TeamXX_Rubber_20.txt" 16. Using the manual controls, raise the stage at least 50 mm above the sample holder. 17. Repeat steps 11 to 16 with the two remaining rubber disk samples using the parameters for the second and third unconfined compression tests. 10

11 ANALY PROCEDURE tructural properties will be extracted from the test result files generated during the unconfined compression of the disk-shaped rubber samples for each strain rate. Each sample's shape and dimensions will later be considered to obtain the mechanical properties in terms of strain rates. RUBBER AMPLE 1. Open Mach-1 Analysis software and open the Folder Path containing the Mach-1 results files. 2. From the File List, select the results file "GrXX_TeamXX_Rubber_01.txt". 3. elect the second "tress Relaxation" function. The first "tress Relaxation" being the results from the pre-compression, it will not be analyzed. 4. Choose "Fz, N" for the Y-Axis and "Position (z), mm" for the X-Axis. 5. From the "Analysis" dropdown menu, select lope. Use the cursors to choose the appropriate part of the curve for analysis. Recommended range for the extraction of the Young's modulus is the last 50% of the slope. A blue line corresponding to the slope in the chosen range will be superimposed on the experimental curve and the results will be computed. 6. Report the slope as the stiffness (gf/mm) in Table 1 near the end of this document. 7. Repeat steps 2 to 6, choosing the results files "GrXX_TeamXX_Rubber_5.txt" and "GrXX_TeamXX_Rubber_20.txt" from the "File List". Load vs. position graph for slope analysis in Mach-1 Analysis software 11

12 HYDROGEL TETNG PROCEDURE HYDROGEL AMPLE Test setup for unconfined compression of disk-shaped hydrogel samples 1. Open Mach-1 Motion software. 2. Using the manual controls in the software, raise the vertical stage to its maximum height using "medium" speed. 3. ecure the transparent wall onto the sample holder (being careful not to touch the load cell or indenter). 4. Place the first 8-mm disk-shaped hydrogel sample in the center of the sample holder using tweezers. 5. Using manual controls, lower the stage at "medium" speed to approximately 20 mm above the sample. Lower the stage at "low" speed to approximately 2 mm above the sample. 13

13 TETNG PROCEDURE HYDROGEL AMPLE 6. Perform the following test sequence to measure the disk thickness L 0 : Functions Parameters Zero Load No parameter Find Contact tage Axis: Position (z) Load Cell Axis: Fz Direction: Positive Velocity: 0.1 mm/s Contact Criteria: 3.75 gf tage Limit: 3 mm tage Repositioning: 2X Load Resolution 7. Report the current position of the stage axis (z) as the thickness L 0 of the sample in Table 1 near the end of this document. 8. Fill the sample holder chamber with D water. Pour the solution slowly to avoid sample movement. Make sure no air bubbles get trapped under the platen and that the solution level is at least 5 mm above the cylindrical plate portion of the flat indenter to minimize volumetric artifacts in the load. Disk-shaped sample between the upper flat indenter and the bottom of the sample holder filled with D water 14

14 TETNG PROCEDURE 9. Proceed with the unconfined compression test using the following sequence. HYDROGEL AMPLE Functions Zero Load tress Relaxation (Pre-Compression) tress Relaxation (Compression) Parameters No parameter tage Axis: Position (z) Load Cell Axis: Fz Ramp Amplitude: 10% of Ramp Velocity: 0.4% of per second Number of Ramps: 1 top Based On: Fixed Relaxation Time Fixed Relaxation Time: 60 s ave results as: 1 st Test: "GrXX_TeamXX_Hydrogel_01.txt" 2 nd Test: "GrXX_TeamXX_Hydrogel_5.txt" 3 rd Test: "GrXX_TeamXX_Hydrogel_20.txt" tage Axis: Position (z) Load Cell Axis: Fz Ramp Amplitude: 10% of Ramp Velocity: 1 st Test: 0.1% of per second 2 nd Test: 5% of per second 3 rd Test: 20% of per second Number of Ramps: 1 top Based On: Fixed Relaxation Time Fixed Relaxation Time: 60 s ave results as: 1 st Test: "GrXX_TeamXX_Hydrogel_01.txt" 2 nd Test: "GrXX_TeamXX_Hydrogel_5.txt" 3 rd Test: "GrXX_TeamXX_Hydrogel_5.txt" 10. Using the manual controls, raise the stage above the sample holder. 11. Unscrew the sample holder from the testing chamber and dispose of its contents. 12. Repeat steps 4 to 11 with the two remaining hydrogel disk samples using the parameters for the second and third unconfined compression tests. 15

15 ANALY PROCEDURE tructural properties will be extracted from Mach-1 result files generated during the unconfined compression of the disk-shaped hydrogel samples for each strain rate. Each sample's shape and dimensions will later be considered to obtain the mechanical properties in terms of strain rates. 1. Open Mach-1 Analysis software and open the "Folder Path" containing the Mach-1 results files. 2. From the "File List", select the result file "GrXX_TeamXX_Hydrogel_01.txt". 3. elect the second "tress Relaxation" function. The first "tress Relaxation" being the results from the pre-compression, it will not be analyzed. 4. Choose "Fz, N" for the Y-Axis and "Position (z), mm" for the X-Axis. 5. From the "Analysis" dropdown menu, select lope. Use the cursors to choose HYDROGEL AMPLE the appropriate part of the curve for analysis. Recommended range for the extraction of the Young's modulus is the last 50% of the slope. A blue line corresponding to the slope in the chosen range will be superimposed on the experimental curve and the results will be computed. 6. Report the slope as the stiffness (gf/mm) in Table 1 near the end of this document. 7. Repeat steps 2 to 6, choosing the results files "GrXX_TeamXX_Hydrogel_5.txt" and "GrXX_TeamXX_Hydrogel_20.txt" from the "File List. Load vs. position graph for slope analysis in Mach-1 Analysis software 16

16 AMPLE DATA Table 1: Data from the rubber and hydrogel samples train rate (% of per second) Parameter Values for rubber samples Values for hydrogel samples ample thickness (mm) 0.1 lope (gf/mm) ample thickness (mm) 5 lope (gf/mm) ample thickness (mm) 20 lope (gf/mm) 18

17 QUETON 1. Complete the following table by extracting the Young's modulus from the stiffness of each sample for various strain rates. Use the values in Table 1 and the space provided below to demonstrate your calculations. Table 2: Young's modulus of rubber and hydrogel train rate Young's modulus (% of per second) Rubber Hydrogel

18 QUETON 2. Compare the Young's modulus obtained for both materials in terms of strain rates. Comment on the materials' behaviors. 3. Explain how the structure and composition of poroelastic materials (like hydrogels) affect their mechanical properties under varying stain rates. 20

19 QUETON 4. Discuss what could be expected if the hydrogel had been tested in air instead of water. 5. Determine two error sources that you think could have been significant during experimental manipulations and data analysis. 21

20 ADDTONAL REOURCE Biomomentum's website ( contains more information on unconfined compression test and the Mach-1 mechanical tester. There, you can find case studies as well as literature related to this laboratory module. 24