Acoustic emission applications to composite sandwich structures at room and low temperatures

This paper briefly states the results of experimental and numerical studies on the application of acoustic emission NDE to the fatigue behavior of composite sandwich (CS) beams at room temperature (RT) and low temperatures (down to -60 degree) with particular reference to the hull materials of ships which ply the Polar Regions. Four-point static and load-controlled fatigue bending tests were performed on PMI foam cored composite sandwich beams of the same dimensions but with two different skin materials (unidirectional carbon/epoxy and woven glass/epoxy). Effects of low temperatures on the flexural static behavior of the CS beam were observed from the increase in stiffness, strength, and elastic limit, and the decrease in the displacement to with decrease in temperature. Core shear was found to be the main failure mode under fatigue loading conditions at -60 degree, as early found by literature-reported room temperature studies. Brittle type core shear failure with notably increased useful 91 fatigue life increase manifested at lower temperatures for both types of CS beams. In this work, AE data was synchronously acquired as the mechanical test data, and AE parametric analysis, general joint time-frequency and especially wavelet transform techniques were applied to the data obtained from wide-band AE sensors. For both static and fatigue testing, all physically observed effects were portrayed very clearly by the AE parameters, which really gave insight into the characteristics of damage progression and the phenomena responsible for it. FEA models and analysis for the two types of CS beams revealed the effects of skin stiffness on the crack initiation location under fatigue loading conditions.