Fluid-ingression phenomena in composite sandwich structures is a concern for components employed in the aerospace industry. Poor design/material selections, impact damages, thermo-mechanical loads occurring during ground-air-ground (GAG) cycling and changes to internal pressure within the sandwich structure all lead to localized transverse cracks, delaminations, facesheet/core disbonds and core damages which create more passageways for fluid migration. In some cases, damage growth due to latter two cases is stable and occurs over a period of flights, but may not be readily detected on the ground when the thermo-mechanical and internal pressure loads are released. Hence, the structure will continue to sustain load-carrying capabilities until the damage size reaches a critical damage threshold (CDT) which leads to catastrophic failure. Unless such damage is detected and repaired prior to reaching CDT, GAG effects will further increase the damage size and threaten the structural integrity and safety of the aircraft. Current research provides an analytical method to predict facesheet debond based on the Winkler foundation approach that agrees well with both numerical and experimental results and evaluates the capabilities of cohesive zone modeling to predict onset of damage growth. Also, a standardized procedure and test apparatus for GAG testing to simulate damage under a mixed-mode stress state caused by the pressure differential and in-plane mechanical loads is developed. Current phase of the research focusses on investigating the effects of GAG cycling on sandwich structures by subjecting them to combined pressure and in-plane mechanical loads and investigate the conditions leading to the onset of damage, and to develop a method to conduct GAG simulation based on SCB model calibration. The information gathered through this research will be instrumental in developing analytical methods and validating finite element analysis procedures to further investigate the damage growth mechanics of sandwich composite structures. © 2019 by DEStech Publications, Inc. and American Society for Composites.