Cantilever Beam Bending f Sft Tissue Intrductin Cantilever beam bending is a methd used t determine the mechanical prperties f a material. It is used because it is relatively inexpensive t perfrm and gives gd results quickly. It is beneficial t knw the mechanical prperties f a material because it allws us t predict hw they will react t an applied frce. Fr example, the material prperties f steel have been widely studied and this allws engineers t plan structures such as buildings and bridges such that sufficient supprt is built-in t accunt fr the lading. Hw d they use this infrmatin specifically? Think f a small steel pin. There is sme frce that yu culd apply t break the pin, but if the thickness f the pin were increased t the size f a blt, the same frce wuld have little impact n it. The amunt f frce required t damage any thickness f a material is described by its material prperties, such as Yung s Mdulus and Fracture Strength. Althugh standard materials like steel and aluminum have been tested s much that their prperties are cnsidered t be knwn facts, much less is knwn abut cmpsite materials such as mdern fiber-reinfrced plastics and living tissue. As Bimedical Engineers, we seek t describe the prperties f the different tissues f the bdy s that medical devices interacting with them d nt cause damage. Fr example, hw much frce is acceptable fr a knee jint replacement t place upn the femur? This study wuld require knwledge f the mechanical prperties f bth the replacement device and f the femur itself. Similarly, this prcess can be used t determine which material is mst suitable as an implant. Fr example, culd silk r rubber be used as a substitute fr a damaged tendn? Again, this wuld require knwledge f the prperties f the candidate replacement materials and the native tissue. One f the best ways t determine the prperties f the native tissue is using a cantilever beam test. The basic frmat f a cantilever beam test is that the specimen is fixed n ne end and free n the ther. Varius lads are then applied t the free end and the beam defrmatin is recrded. Special mdificatins are required fr testing n tissue, but the cncept is generally the same. The main cncern fr tissue testing is that dried ut tissue has different prperties than living tissue. Als, tissue is very weak and will ften fall dwn during a cantilever beam test under n lad at all. We get arund bth f these prblems by perfrming the experiment inside a water-filled tank. Because the tissue is relatively very weak, the lads that we add are very small.
Experiment Materials: 1. One strip f prcine (pig) arta 2. A ruler 3. A water-filled tank 4. A digital camera 5. A stage 6. A weight fr fixatin 7. A pencil r marker fr tissue marking Prcedure: 1. ****Always wear glves when handling animal tissue**** 2. Place the tissue specimen upn the stand such that ¾ r mre f the tissue hangs ff the side 3. Place the fixatin weight n tp f the tissue 4. Mark the line where the tissue meets the stage and weight 5. Place the stage n the bttm f the tank pressed against the wall that it mst flat and easiest t see thugh (this will aid in image acquisitin and prcessing later) 6. Place the tissue and weight n the stage, being careful t nce again align the line with the edge 7. Place the ruler in the water squarely n the bttm f the tank and with the edge aligned with the edge f the tissue (see belw) Tank Ruler Fixatin Weight Tissue Stage 8. Take the zer-lad picture 9. Remve the tissue frm the water and apply the first weight
The weight shuld be a small suture needle Sticking the needle halfway thrugh the tissue is enugh t secure it Try t stick it as clse t the free edge as pssible 10. Replace the stage, tissue, weight, and ruler back in the tank Be sure t align the previusly marked line with the stage edge again 11. Take the weight #1 image 12. Repeat steps 9 & 10 fr the secnd weight Try t use the same hle as in step 9 fr munting the secnd needle 13. Remve the tissue and lay it flat n a paper twel next t the ruler 14. Take a picture f the tissue, ensuring that bth the (entire) tissue and part f the ruler are in the image 15. Weigh the tw weights used in the experiment in Kg. Recrd these values. 16. Calculate the frce fr each weight, where Frce = weight * 9.81 17. Measure the thickness f the tissue in several places, recrd the average value (yu can use Micrsft Excel fr this averaging functin if yu d like) Image Analysis 1. Imprt the images frm the digital camera int the cmputer 2. Open Natinal Instrument s Visin Assistant prgram 3. Open the first image (unladed) by clicking 4. Scale the image by clicking 5. Nte the height f the tip f the free end 6. Repeat steps 3-5 fr the first and secnd weight-laded images a. Enter the heights and displacements in the table belw b. Displacement (Δ) = (riginal height) (new height) 7. Nw pen the image f just the tissue and the ruler 8. Calibrate the image using the ruler as in step 4 9. Measure and recrd the tissue effective length (frm the line t the free edge) and base width Length Thickness Base width
Table 1 Measured Values Measurement: Tissue Base Width (B): Tissue Length (L): Tissue Thickness (t): Weight #1: Frce #1: Weight #2: Frce #2: Undefrmed height: Defrmed height frm weight #1: Δ 1 = (undefrmed height) (defrmed height #1): Defrmed height frm weight #2: Δ 2 = (undefrmed height) (defrmed height #2): Recrded Value: (Kg) (N) (Kg) (N) Calculatins Calculate the gemetry cnstant (G) fr this material: GG = 4 LL3 BB tt 3 (1) L is the length yu measured in the image B is the base width that yu measured in the image t is the tissue thickness that yu measured using the calipers Yur value f G is: Calculate the Yung s Mdulus (E) fr weight #1 (it is expected that the number will be very large): EE 1 = (FFFFFFFFFF #1) (ΔΔ #1) GG (2) Repeat this calculatin fr weight #2: EE 2 = (FFFFFFFFFF #2) (ΔΔ #2) GG (3) Finally, determine an average Yung s Mdulus fr prcine arta by taking (E1 + E2)/2 E1 E2 Average E
Simulatin Nw that yu have experimentally determined the Yung s Mdulus f the tissue, yu can use this t simulate hw the material will perfrm under different lading. The first step is usually t simulate the experiment yu perfrmed t ensure that the results match. Fllw these steps t set up the simulatin: 1. Open ABAQUS/CAE frm Windws Start menu. 2. Select Create Mdel Database. 3. Frm the menu Part Create. In the Create Part windw shwn belw, change the Name t Tissue-beam and Apprximate size t 100.
4. Chse Cntinue 5. Frm the left tl bar click Create lines: rectangle. 6. T create the crss sectin f the tissue beam, yu need t input the x and y crdinates f the tp left crner and bttm right crner. Fr example, yu want t create a rectangle with height f 3 mm and width f 12 mm, at first yu need t input the tp left crner (0, 3) and Enter, and input the bttm right crner (12, 0) and Enter. 7. Click this