An Experimental Study of the Initiation and Progression of Damage in Compressively Loaded Composite Laminates in the Presence of a Circular Cutout

A series of experiments have been performed to determine the mechanisms of failure in compressively loaded laminated plates in the presence of stress gradients generated by a circular cutout. Real time holographic interferometry and in-situ photomicrography of the hole surface, along with strain gage measurements are used t o study the progression of failure. The test specimens are multi-layered composite flat plates which are loaded in compression. The laminates are made up of two material systems, T300/BP907 and Ih17/8551-7. Two different lay-ups of T300/BP907 and two different lay-ups of IM7/8551-7 are investigated. The experimental results reveal that the failure is initiated as a localized instability in the 0' plies at the hole surface approximately at right angles t o the loading direction. This instability emanating a t the hole edge and propagating into the interior of the specimen within the 0' plies is found to be fiber microbuckling. Subsequent to the 0' ply failure, extensive delamination cracking is observed with increasing load. The delaminted portions spread to the undamaged areas of the laminate by a combination of delamination buckling and growth, the buckling further enhancing the growth. When the delaminated area reaches a critical size, about 75-100% of the hole radius in extent, an accelerated growth rate *Formerly, Graduate Research Assistant, GALCIT. Member AIAA tDeceased. Formerly Professor of Aeronautics and Applied Mechanics. $Professor of Aeronautics and Applied Mechanics. of the delaminated portions is observed. The culrnination of this last event is the complete loss of flexural stiffness of each of the delmainted portions leading to catastrophic failure of the plate.