A study on change in modal parameters with damage in composite specimens

This paper deals with studies of changes in modal parameters due to damage in specimens made of fibre-reinforced components. The vibration response of glass reinforced polyester laminates kt the form of cantilever beams is investigated Mechanical as well as modal tests were carried out on the beams in the undamaged state as well as in two or three additional states of progressive damage produced by loading the specimens to approximately 60, 70 and 80% of the ultimate strength. At each stage of damage, dye penetrant tests were carried out to determine the crack density along the length of the beam. The natural frequencies, damping ratios and mode shapes were noted as the damage accumulated The tests were conducted on three types of glass reinforced plastic specimens namely laminates with random mat, woven roving mat and unidirectional mar. In all the above types of specimens, if is found that the natural frequencies decrease by about 9% and damping ratios increase by about 98% for the state of 80% damage. Changes in mode shapes are compared first by noting nodal points and then by assessing Modal Assurance Criterion (MAC).