Damage Detection Through Fretting Emission in Center-Hole CFRP Skin of Cantilever Wing Box Under Constant-Moment Fatigue Loading

摘要

This study addresses the effect of damage accumulated during fatigue loading of a carbon-fiber reinforced polymer (CFRP) panel on acoustic emission (AE) waveforms. A 610-mm by 1,067-mm, 18-ply, nearly quasi-isotropic CFRP panel containing a 79-mm diameter hole at its center was attached as a top-side component (e.g., skin) of a cantilevered wing box structure subjected to constant-moment fatigue loading. The fatigue loading consisted of three 55,000-cycle test segments, sequentially increasing the maximum load, at a constant R-ratio of 0.1 and at a load frequency of 0.375 Hz. The fatigue test was interrupted periodically to conduct quasi-static loading/unloading to assess strain redistribution via digital image correlation (DIC). AE was monitored throughout the test sequence, using six R151 sensors, installed concentric to the central hole at a 406 mm diameter, equally spaced. Flash thermography and pulse echo and phased array ultrasonic inspections were used to intermittently assess the state-of-damage in the vicinity of the hole. Special attention was given to identify the emission generated by fretting among the newly formed delaminated surfaces. It is shown that fretting emission is dominant during cyclic loading of large composite panels and it can be used to pinpoint existing damage.