Determining the Microstrain of Biological Composites by X-Ray Diffraction

1. Determining the Microstrain of Biological Composites by X-Ray Diffraction Alyssa Roessler Stanford Synchrotron Radiation Laboratory August 13, 2008

2. Introduction • Mother of pearl has a unique microstructure and exhibits surprisingly high strength and toughness Biological composites often exhibit desirable properties, including emergent properties that are present in the composite but not in the constituent materials individually In order to replicate or improve upon these biological designs, understanding the function of the various components of the material is necessary

3. Nacre • The aragonite (>95% by weight) is in the form of hexagonal platelets about 10 μm wide and .5 μm thick [1] • The polymer acts as an adhesive between the aragonite platelets • Because nacre is layered, its properties are anisotropic Mother of pearl, otherwise known as nacre, is found in many mollusk shells and is also the composition of pearls Nacre is composed of two materials: the ceramic material aragonite (CaCO3) and a biological polymer.

4. Nacre Deformation • We wanted to determine the strain induced in nacre more precisely than could be achieved by conventional tests • Full 3D strain tensor • How much strain each component was absorbing • Stiffness of the material in the different crystallographic directions • At SSRL, we can use x-ray diffraction to directly observe the microscopic stain present in the aragonite portion of nacre • When thecrystal lattice is strained, thediffraction rings become distorted and the eccentricity can be measured • This eccentricity is a measure of the material strain

5. Sample Preparation Sections of Red Abalone shell and White MOP were ordered from Australia The samples were brought to the facilities at UC Berkeley and cut to the desired length (~7 mm) with a low-speed diamond saw.

6. Sample Preparation, cont. • The nacre plates were then cut into slices 480 microns wide, cleaned with acetone and distilled water, and stored for later use. The samples were applied to a grinding disk and sanded to the desired thickness, about 300 microns.

7. Bracket Design • A part was needed to attach the specimens to the load cell • Design parameters: 10 lbf load cell capacity, needed a 300 MPa stress, needed to avoid interference with x-ray beam, size constraints • An example design consideration: buckling. We need the sample to be thin for diffraction, small for the stress considerations, but as stable as possible in terms of buckling • The notch was designed to give the specimens maximum support

8. Experimental Setup

9. Testing Methodology • Pictures and aragonite diffraction patterns were collected periodically as strain was applied, typically every 3 N of applied force • Data was taken in both tension and compression, and all tests were done with the load applied in-plane with the aragonite platelets • All in all three samples each of white mother of pearl and red abalone were tested

10. Results • In tension, strain was hard to control due to the rubbery nature of the epoxy • The sample brackets held the specimens in place much better under compression, and significant strain was seen in the aragonite Discontinuous Rings show texture White Mother of Pearl Red Abalone Aragonite Diffraction Rings

11. Data Analysis The more strain present in the aragonite, the more distorted the diffraction rings become. The diffraction rings can be visually “unwrapped” to observe the eccentricity of the ring more clearly. The diffraction peaks were found and the full three-dimensional strain tensor was calculated Unstrained In Compression

12. Stress vs. Strain in Aragonite Each diffraction ring represents a different crystallographic direction Slope = E, Young’s Modulus

13. Material Stiffness • Since our experimentally determined Young’s modulus in each crystal direction was higher than the theoretical value, we can conclude that the aragonite portion of the material is in fact taking less than its weight percent in load • Literature cites a 92 GPa bulk Young’s modulus for aragonite [2] and a 70 GPa Young’s modulus for nacre as a whole [3]

14. Conclusion X-Ray diffraction allowed us to observe more detailed strain data than conventional methods, and the full strain tensor was computed for a variety of strain levels The percent of strain on each component of the material was found. While the polymer portion of nacre constitutes less than 5% of the material by weight, it carries between 6.5 and 16.5 percent of the load. This could explain the high toughness exhibited by nacre

15. Still to Come The relative loads carried by the aragonite and polymer sections of nacre in tension has yet to be explored Testing should also be done perpendicular to the platelets as well as in the in-plane direction Diffraction data should be collected from polymer portion of nacre to confirm the results suggested by the aragonite diffraction There are several species of nacre, so testing should be done on the other varieties for comparison Better methods of determining macroscopic strain on the material should be used. Digital Image Correlation (DIC) methods work well for this.

16. Acknowledgments • Thanks to my mentor Apurva Mehta, Matt Bibee, David Bronfenbrenner, and all the scientists and machinists at SSRL that provided help with this project. • Also thanks to the DOE Office of Science for funding this exciting research experience and to the SULI program directors for making sure this experience was so rewarding. References: [1] Chen, P.Y., A.Y. M. Lin, A.G. Stokes, Y. Seki, S.G. Bodde, J. McKittrick, and M.A. Meyers. "Structural Biological Materials: Overview of Current Research." JOM os June (2008): 23-32. [2] Bruet, B. J. F., Panas, R., Tai, K., Frick, L., Ortiz, C., Qi, H. J., and Boyce, M.C. “Nanoscale morphology and indentation of individual nacre tablets from the gastropod mollusk trochus niloticus.” Journal of Materials Research (2005). [3] Evans, A.G., Z. Suo, R.Z. Wang, I.A. Aksay, M.Y. He, and J.W. Hutchinson. "Model for the Robust Behavior of Nacre." Journal of Materials Research 16 (2001): 2475-2484.