Effects of Manufacturing Defects and Scaling on Glass Fiber Composite Sandwich Panels in General Aviation Aircraft Structures

摘要

This report documents the collaborative effort between the United States and Canada to investigate the impact of process-induced defects on strength and stiffness under static and fatigue loading and for low-cost composite aircraft structures, in particular, general aviation aircraft. The emphasis was on safety, reliability, and certification. The research involved manufacturing sandwich panels with process-induced defects (porosity) and used destructive and nondestructive analyses to characterize the defects. The sandwich panels were also tested under compressive loading to measure the severity of the defect for strength and stiffness assessment. The test results on monolithic and sandwich beams indicated that the failure mode under compression load of the sandwich, in most realistic cases, will be dictated by the longitudinal stiffness of the sandwich facesheets, the shear modulus of the core, and the geometry of the sandwich. Although the porosity acts to reduce the static strength of the facesheets (20% at 4% porosity), it had no significant effect on the factors that affect the static strength of the sandwich, such as longitudinal stiffness. For the scaling/size effect study, sandwich specimens of various sizes (geometrically scaled in the ratio 1:2:4:8) were subjected to eccentric axial compression and tested to failure. The sandwich core consisted of a closed-cell polyvinyl chloride foam, and the facesheets were woven glass/epoxy laminates, scaled by increasing the number of plies. The test results revealed a size effect on the mean nominal strength, which was strong enough to require consideration in design. The observed size effect was fitted with the size effect law of the energetic (deterministic) size effect theory. However, because of inevitable scatter and limited testing range, the precise form of the energetic size effect law to describe the test results was not unambiguous. The Weibull-type statistical size effect on the mean strength was ruled out because the specimens had small notches that caused the failure to occur in only one place in the specimen. The observed failure mode was shear band propagation, previously shown to cause energetic size effect.