Material Properties of the Post-Mortem Small Intestine in High-Rate Equibiaxial Elongation

Motor vehicle collisions (MVCs) are the most common cause of small intestine injuries and the accompanying mesenteric injuries that occur due to blunt trauma. Crash-induced injuries of the intestine include lacerations, serosal tears, and perforations. To characterize the biomechanical response of the small intestine associated with these injury modes, the multidirectional failure properties of cruciate tissue samples were investigated with high-rate equibiaxial stretch. Tissue testing was conducted on intestine samples harvested from four post-mortem human surrogates. Cruciate samples were stamped from the tissue with the material and stretching axes aligned with the visible longitudinal fiber direction or offset by 22 degrees. Sample arms were gripped in four low-mass tissue clamps and simultaneous motion of four carriages applied equibiaxial stretch in orthogonal directions. Tests were conducted at a target strain rate of 100s to investigate tissue failure properties at rates expected to be experienced in MVCs. Load and acceleration were measured at each carriage. Laser displacement sensors were used to obtain changing sample thickness. All data were captured using 20ksps. True stress and 2 Piola Kirchhoff stress were computed and transformed to align with the material axes. Overhead highspeed video captured at 2500fps provided optical marker displacement data. Marker positions were tracked using motion analysis software. Displacement data were input into LS-DYNA, and average Green-Lagrange strain was calculated at 0.05-ms intervals. All data were truncated at tear initiation determined from high-speed video analysis. Results from 28 small intestine tests indicated an average maximum principal strain rate of 86.7±25.9s and an average maximum principal failure strain of 0.270±0.083. Average Green-Lagrange failure strain was 0.169±0.060 in the circumferential direction and 0.169±0.068 in the longitudinal direction. Average peak 2 Piola Kirchhoff stress ranged from 3 to 5MPa with a trend toward a stiffer response in the longitudinal direction. Tests conducted in this study identify failure thresholds for the small intestine loaded in equibiaxial stretch at rates expected to be experienced in MVCs.