Propagation of hole machining defects in pin-loaded composite laminates

The propagation of hole machining defects in pin-loaded carbon/epoxy laminates subjected to uniaxial tension fatigue loading was investigated using two nondestructive examination (NDE) techniques. The KTH-method, a new method which gives defect-free(1) holes, was used to machine holes in specimens. For comparison, holes were also machined using two traditional methods (DIXI and Dagger drills) causing varying extent of damage. X-ray radiography and computerised tomography were used to give two- and three-dimensional images, respectively, of defect propagation after 50,000-250,000-504000-cycles, and after failure. Permanent hole elongation was monitored during cyclic loading. The results showed that the propagation of defects occurred in a controlled and stable phase until a sharp fatigue limit was reached (about 400,000 cycles for the DW specimen and about 500,000 cycles for both the KTH and Dagger specimens). During the stable phase, the global structural response of the specimens was affected very little by the damage propagation. At the fatigue limit, extensive damage development and defect propagation occurred, resulting in similar large permanent hole elongations in all specimens.