| Description |
TThe mechanical effects of varying stitch angles in stitched sandwich structures were investigated. A repeatable fabrication method was developed for stitching Kevlar yarn through sandwich panels of polyurethane foam and dry woven carbon fiber facesheets. Unstitched, 30°, 45°, 60°, and 90° stitch orientations, measured from the horizontal, were used throughout the study. Mechanical tests of flatwise tension, flatwise compression, and core shear were performed to observe the stiffness and strength effects of the different stitch configurations. With only a measured areal density increase of 2-7%, stitches oriented at 90° were found to nearly double out-of-plane tensile stiffness and strength while compressive stiffness and strength increased only modestly. Stitches oriented at 30° and 45° were found to increase shear stiffness and strength by as much as 70% and 100%, respectively. Analytical models were developed to predict out-of-plane tensile modulus and shear modulus using a rule-of-mixtures approach to account for the separate constituent properties. As expected, the model predicts 90° as the optimal stitch orientation for flatwise tension stiffness; however, 35.2° was found to be optimal for shear stiffness. Agreement between model predictions and experimental measurements were within 15% for the core shear model and within 4% for the flatwise tension model when compared to the average. Creep behavior of the varying stitch angle reinforced sandwich structures was also investigated. Portable spring loaded creep test fixtures were specially developed to test reinforced polymer core sandwich panel configurations under flatwise compression and core shear. Traditional creep test methods of using dead weight were not practical for testing multiple specimens simultaneously at ‘high' loads (above 3.5 kN in flatwise compression and 5.3 kN for core shear). The required 60 tests at 150 hours made use of servo-hydraulic and electromechanical machines for creep testing not feasible. Low cost creep test fixtures were developed to accurately apply and sustain load, allow for creep measurement, and be compact enough to fit six test fixtures simultaneously in a large convection oven approximately. Use of the developed creep test fixtures resulted in typical creep curves for polymers loaded under compression and shear for the collected creep data with only a 3.8% and 1.2% load loss for flatwise compression creep and core shear creep testing, respectively. Measurements of creep under constant flatwise compressive stress indicate stitch angles of 90° decreased creep by approximately 75% when compared with unstitched sandwich specimens. Measurements of creep under core shear loading indicate stitch angles 30°, 45°, and 60° decreased creep by approximately 70% when compared to unstitched sandwich specimens. |
