OBSERVATIONS OF MICROSCOPIC DAMAGE EVOLUTION IN OFF-AXIS PLIES UNDER FATIGUE LOADING

In the present work properly designed glass/epoxy specimens with lay-up [45(2)/-45(2)/0(2)](s) were tested under uniaxial fatigue loading with the aim to observe damage evolution at the microscopic scale in the 45 degrees ply. Microscope observations revealed that the first damage event was the initiation of microcracks in the matrix between the fibres. Macro-cracks then formed by coalescence of these microcracks and propagated along the fibres direction with the same mechanism. These experimental evidences confirm some assumptions at the basis of a recently developed criterion (Carraro and Quaresimin, 2014), which was indeed based on the idea that a macro-crack initiation under fatigue loading is driven by damage accumulation at the microscopic scale in the form of micro-cracks normal to the local maximum principal stress. The same specimens were tested in the presence of voids, revealing that the micro-scale damage mechanisms do not change in this condition.