A Coupled Experiment-finite Element Modeling Methodology for Assessing High Strain Rate Mechanical Response of Soft Biomaterials

The overall goal of this procedure is to obtain the uni axial high strain rate, true stress strain response of the soft biomaterials being tested. This is accomplished by first testing a brain sample in a split hopkinson pressure bar or SHPB apparatus, and then calibrating the one dimensional version of the material model with experimental true stress strain response. The finite element or FE simulation of the SHPB setup is then performed to evaluate the sample's average loading direction, stress, and center line stress.

The final step is to iteratively compare the one dimensional material model fit and FE simulation center line stress, and the three dimensional FE loading direction, stress and experimental true stress strain response until good agreement between the aforementioned results are obtained. Ultimately, the coupled experimental finite element methodology is used to show that the uniaxial true stress strain response for soft biomaterials can be obtained using an SHPB. The main advantage of this technique over existing methods like shock tube tests is that this technique renders the high strain rate uni axial mechanical response of a soft biomaterial.

The implications of this technique extend towards the diagnosis of traumatic brain injury through virtual human body modeling. Because the data obtained from the current method can accurately capture local injuries and damage arising from trauma to the human head. We first had the idea for this method when working on correlating the brain high strain rate experiment data with the results from thyroid and modeling, Demonstrating this procedure will be veterinarian Dr.Jim Cooley, graduate student, Mr.Soro Patnaik and research associate Mr.Wilburn Wittington Under the supervision of a veterinarian, surgically extract porcine organs such as brain, liver, muscle, fat, or tendon.

Then place them in containers filled with phosphate buffered saline or PBS for temporary storage. Store the PBS containers in an iced cooler and immediately transport them to the testing facility for sample preparation and split Hopkins in pressure bar or SHPB testing. Once in the testing facility, remove the porcine organ from the PBS container and place it on a sterile surface.

Identify the primary fiber orientation and locations for each test sample. Then use a cylindrical dye with a 30 millimeter inner diameter to dissect the test sample from the porcine organ. Next, use a scaffold to trim the sample to the prescribed thickness and aspect ratio for SHPB testing of porcine samples.

The thickness is 10 to 15 millimeters while the aspect ratio is point 33 to 0.5. Use calipers to measure the thickness and diameter at three different locations. Place a striker bar, incident bar and transmitted bar in the metal extensions for SHPV testing.

Ensure that bars are free moving to the touch and that their interfaces are aligned with each other. Provide a stopper for the transmitted bar for safety. Connect the strained gauges, adhere to the incident and transmitted bars to the signal amplifier.

Turn on the signal conditioning amplifiers and the data acquisition module computer. Then initialize the high speed data capturing software. Verify the live capturing of the signals to see if they lie within the normal range and nullify the noise signals by clicking the zero icon.

Load the striker bar adjacent to the pressure chamber. Build the pressure chamber to a desired pressure by opening the nitrogen tank's nozzle and then the pressure chambers inlet valve zero out the laser speed meter by pressing the zero button and set it to read the striker bar velocity by setting the reflector strip on the striker bar. Behind the laser sensors.

Place the sample confinement chambers such that it does not hinder the move movement of incident and reflected bar bar. Then place the incident bar in contact with the transmitted bar for calibration purposes. Run a test without a sample by turning on the trigger switch for the pressure chamber on the Stryker bar.

Following calibration, place a cylindrical sample between the incident and transmitted bar and then close the sample confinement chamber. Ensure that no preconditioning is performed on the sample by avoiding loading and unloading of the sample multiple times and perform the test after testing is complete. Use disposable sanitary wipes to remove sample debris from the incident bar, transmitted bar and sample confinement chamber.

Dispose of all debris and wipes in biohazard safety bags. Sanitize the bars and sample confinement chamber. Using a 70%ethanol cleaning solution and sanitary wipes.

Open the MSU high rate software for analysis of Hopkins in bar waves. Begin the software by examining the settings window and choosing the tension compression option in the mode tab for uni axial testing. Also, select two gauges in the gauges tab and click continue In the main window, select the open file one tab and navigate to the incident wave data from the strain gauge record on the incident bar, select the open file two tab for importing the transmitted bar strain gauge record.

Next, select the parameters tab in the main window and input the physical parameters of the test setup, including bar dimensions, voltage to strain factors, strain gauge positions, and viscoelastic dispersion constants. Click continue. Then select the select data tab in the main window and use the cursor bars to reduce the dataset to only the amount of data containing the incident reflected and transmitted waves.

Click continue. At this point, navigate to the select waves tab in the main window and use the cursor bars to confine the incident wave in the incident wave graph, the reflected wave and the reflected wave graph and the transmitted wave and the transmitted wave graph. Then click continue.

After that, select the correct tab in the main window to allow the software to correct for the viscoelastic dispersion. Now select the shift tab in the main window In the wave graph. Use the cursor to drag the incident reflected and transmitted waves to the same initial position in time.

By selecting each one individually. In the wave select tab, view all of the waves in the data graph. Once completed, click continue in the results file.

Save the load placement position and velocity profiles by clicking save as using commercial finite element or FE software. Create an FE model of the SHPB setup. Then run an FE simulation.

Verify the strain gauge measurements in the experiment and FE simulation are in good agreement. Incorporate the biomaterial sample into the FE model of the SHPB setup. Assign a three dimensional implementation of the internal state variable material model to the biomaterial sample.

Calibrate the true stress strain curve of the experiment with the model's true stress strain curve by adjusting the internal state variable material models parameters. Then assign the internal state variable material constants to the biomaterial sample in the FE model for the SHPB setup. Next, run the FE simulation with the Stryker bar velocity and sample deformation strain rate corresponding to the SHPB tests at the same strain rate.

In the second step of the FE model calibration, run the FE simulation strain gauge data. SHPB experiment post-processing software MSU high rate software. Finally perform a volume average of the loading direction stress along the center line elements of the FE model sample.

The effectiveness of the coupled methodology is exemplified here. The SHPV experimental stress strain response for the brain is at a lower stress state in comparison to the stress state of the one dimensional material point simulator, which is akin to the FE sample center line stress state average. This is due to the multiaxial nature of deformation.

That soft BioMAT exhibit shown here are representative material constants for the brain obtained through the coupled SHPB experiment F FE simulation methodology. The virtual finite element analysis strain gauges are compared to the experimental strain gauges and then simultaneously the material point simulator and the FE center line average are compared to achieve good agreement. Here the material constants are varied until more than two are in good agreement.

This figure shows that the SHPB experimental true stress strain curve actually measures the first invariant of stress rather than the uniaxial loading direction stress strain behavior. Hence use of the SHPB experimental results alone would be erroneous if it was not coupled with FE type modeling to assess the uni axial behavior. Once mastered, this technique can be done in 12 hours if it is performed properly.

While attempting this procedure, it's important to remember that experiments e multiaxial strate when testing soften wire materials.