Evaluation of durability and strength of stitched foam-cored sandwich structures

The static and fatigue characteristics of polyurethane foam-cored sandwich structures are investigated. Three types of specimens with glass fabric faces and polyurethane foam core are used; non-stitched, stitched and stiffened sandwich specimens. The bending strength of the stitched specimen is improved by 50% compared with the non-stitched specimen, and the stiffened specimen is over 10 times stronger than the non-stitched specimen. After fatigue loading of 10(6) cycles, the static bending strengths of all specimens decrease compared with those of the static test. To verify the aging effect of polyurethane foam, ultrasonic C-scanning equipment is used to detect damage of the skin laminate alone after the fatigue test for non-stitched specimens. From the results of UT C-scanning images, no damage is found to have occurred during the fatigue test. Results indicate that the decrease in bending strength of foam-cored sandwich structures is caused by the degradation of stiffness due to the aging of the polyurethane foam core during fatigue cycles. To investigate the effect of distance and diameter of stitching thread, four types of stitched specimens are used. The strength of stitched specimen is improved by increasing the stitching thread diameter and decreasing the stitching thread distance. But fatigue characteristics are not predominantly affected by the variation of stitching thread diameter and distance. (C) 2000 Published by Elsevier Science Ltd.