Structural parts made of composite materials are in general subjected to in-service cyclic loads which can bring the part to the degradation of its strength and stiffness and eventually failure. In fact it is widely documented in the literature that the fatigue life of composite multidirectional laminates is characterised by the initiation and propagation of multiple cracks in the off-axis plies. Their accumulation leads to the degradation of the laminate stiffness, much before the final failure. Mainly in a stiffness-based design against fatigue it is fundamental to predict damage initiation and accumulation in composite laminates under generic loading conditions.
With this aim, an extensive experimental investigation was carried out by the authors on composite materials in the form of tubular specimens under bi-axial loads and flat laminates under off-axis loading to characterise the fatigue crack initiation and propagation phenomena. On the basis of the damage mechanisms observed at the micro-scale a damage-based criterion was developed to predict the initiation of off-axis cracks under multiaxial fatigue loading.
Then an analytical model was developed to correlate the stiffness of a generic laminate to the density of off-axis cracks, also accounting for the interaction between cracks in different layers.
Eventually a procedure, based on the crack initiation criterion, on a Paris-like law for crack propagation and on the developed stiffness model was defined to predict the crack density evolution and consequent stiffness degradation in multidirectional laminates under fatigue loading.
Marino Quaresimin, PhD
Professore di Costruzione di Macchine
Università di Padova – Dipartimento di Tecnica e Gestione dei sistemi industriali
Stradella S. Nicola, 3 – 36100 VICENZA
TEL. +39 0444998723
FAX +39-0444-998888
