Early damage accumulation caused by cyclic loading of composite plate with hole

Novel experimental technique, which provides quantitative description of damage indicators evolution related to cyclic loading of composite specimens with stress concentrators, is developed and implemented. Involved indicators represent by itself deformation response to narrow notch inserting. This notch is emanated form the edge of through circular hole, located in the centre of plane rectangular coupon, under constant external load. The first indicator is the notch mouth opening displacement. In-plane displacement component directed along the notch line as measured in the notch top serves as the second damage indicator. Both parameters follow from direct physical measurements by counting interference fringes, which are visualized on the base of electronic speckle-pattern interferometry. Damage indicator values are obtained for array of specimens with different damage levels. Dependencies of both parameters from loading cycle number are constructed. Theoretical background essential for quantitative analysis of damage accumulation process proceeding from experimental data, which represent evolution of damage indicators caused by loading cycle number increase, is presented. The approach developed is implemented to a quantitative description of damage accumulation process related to cyclic loading of thin rectangular plates with central through hole. Each plate is made from orthotropic composite material. Specimens under study, which are manufactured from layers with three different angular orientations, are tested under fatigue loading with stress range 262.5 MPa and stress ratio –6.0. The point corresponding to the specimen with maximum cycle number, for which damage indicator values are obtained, is decided as conventional limiting case responsible for explicit form of damage accumulation function. Chosen value is equal to 150,000 cycles that comprises 41% of average lifetime of tested specimens. There are no evidences of any delamination inherent in all investigated specimens. Damage accumulation function is constructed in an explicit form for prescribed cycle range. It is established that both indicators provide the same results. Data obtained show that damage accumulation rate is constant for investigated loading cycle range. Accordingly to currently adopted opinion, revealed character of damage accumulation is related to the first stage of the process considered. It is shown that a decrease in the value of deformation response to artificial notch inserting, which occurs after only 1000 cycles applying, is produced by relaxation of residual stress component directed along external force acting line.