Evaluation of the three-dimensional properties of Kevlar across length scales

Abstract

In this study, nanoindentation was utilized to measure the local, three-dimensional properties of Kevlar 49 and Kevlar KM2 on the length scales of the fiber microstructure. First, atomic force microscopy-based methods were used to explore the extent of property changes with respect to radial position in the fibers’ axial and hoop planes. From these measurements, no significant change in response was found for Kevlar 49 fibers, consistent with transverse isotropy. However, a reduced stiffness “shell” region (up to;300–350 nm thick) was observed for KM2 fibers. Instrumented indentation was then used to evaluate fiber response with respect to orientation and contact size and establish a critical contact size above which the response is independent of indenter size (i.e., “homogeneous” behavior). A previously proposed analytical method for indentation of a transversely isotropic material was used to estimate the local material properties of the Kevlar fibers from the measured homogeneous response.

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Acknowledgments

QM and JWG gratefully acknowledge sponsorship by the Army Research Laboratory under cooperative agreement W911NF-06-2-0011. The views and conclusions contained in this paper should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes not withstanding any copyright notation herein.

The authors wish to thank Dr. Gheorghe Stan for his efforts in obtaining the CR-AFM results. The authors would also like to thank Dr. Kenneth Strawhecker for useful discussions in data interpretation and Dr. Joseph Deitzel for help in formulating a proper mounting system.

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Correspondence to John W. Gillespie Jr.

Appendix Definition of Elastic Stiffness Constants, Cij

Appendix Definition of Elastic Stiffness Constants, Cij

Equations (A1)(A6) define the shorthand used for the elastic constants in Eqs. (4)(6):

$${C_{aa}} = {C_{aaaa}},$$
((A1))
$${C_{rr}} = {C_{rrrr}},$$
((A2))
$${C_{ra}} = {C_{rraa}} = {C_{aarr,}}$$
((A3))
$${C_{r{{\theta }}}} = {C_{rr{{\theta \theta }}}},$$
((A4))
$${C_{ar}} = {\left( {{C_{rr}}{C_{aa}}} \right)^{1/2}},$$
((A5))
$${C_{rara}},$$
((A6))

Equations (A7)(A12) define the elastic constants, Cij, in terms of the material properties:

$${C_{aa}} - \frac{{{E_{aa}}\left( {1 - v_{{\text{r}\theta }}^2} \right.}}{\Delta }$$
((A7))
$${C_{rr}} = \frac{{{E_{rr}}\left( {1 - \frac{{{E_{rr}}}}{{{E_{aa}}}}v_{ar}^2} \right)}}{\Delta },$$
((A8))
$${C_{ra}} = \frac{{\left( {{v_{ar}} + {v_{ar}}{v_{r{{\theta }}}}} \right){E_{rr}}}}{\Delta },$$
((A9))
$${C_{r{{\theta }}}} = \frac{{\left( {{v_{r{{\theta }}}} + \frac{{{E_{rr}}}}{{{E_{aa}}}}v_{ar}^2} \right){E_{rr}}}}{\Delta },$$
((A10))
$${C_{rara}} = {G_{ar}},$$
((A11))
$$\Delta = 1 - 2\frac{{{E_{rr}}}}{{{E_{rr}}}}v_{ar}^2 - v_{r{{\theta }}}^2 - 2\frac{{{E_{rr}}}}{{{E_{aa}}}}v_{ar}^2{v_{r{{\theta }}}},$$
((A12))

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McAllister, Q.P., Gillespie, J.W. & VanLandingham, M.R. Evaluation of the three-dimensional properties of Kevlar across length scales. Journal of Materials Research 27, 1824–1837 (2012). https://doi.org/10.1557/jmr.2012.80

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