Development of Evaluation Methods for Microscopic Damage Initiation and Growth in Composite Material Systems for Simultaneous Material/Structure Optimization

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Microscopic damage initiation and growth procedure is the most important information for optimum design of damage-tolerant composite material systems as well as composite srucural members. In the present study, in-situ microscopic evaluation methods were developed in order to experimentally characterize the microscopic damege evolution in carbon fiber reinforced high-temperature polymer matrix composite laminates under loading. Three types of in-situ measurement techniques were used : in-situ optical microscope (OM), in-situ scanning electron microscoped (SEM) and in-situ scanning acoustic microscopes (SAM). Cross-ply laminates were selected for quantitative measurements of (1) initiation of transverse cracks in 90-degree laminas, (2) multiple transverse crack evolution, and (3) delamination from the tips of transverse cracks. Based upon the above observation, several theoretical models were proposed to explain the microscopic damage evolution in CFRP laminates.[1] High-temperature epoxy (TGDDM/DDS), toughened or modified TGDDM/DDS,and interleaved prepregs were used for experiments to study the effects of toughened interleaves on delamination suppression.[2] In-situ OM,SEM,and SAM were sucessfully used for damage evolution measurements in CFRP laminates. Each method provided complementary information.[3] Micro-line or micro-grid methods were sucessfully developed for in-situ SEM observation of interlaminar deformation and damage evolution.[4] Damage evolution under fatigue loading was quantified and compared with that under monotonic loading. Theoretical models were established based upon the qualitative imcroscopic measurements.