Nondestructive evaluation of damage in composite structures using modal parameters

The problem of using measured modal parameters to detect and locate damage in structures made of fiberreinforced composites is investigated. Recent work in this area using modal sensitivity equations is used in conjunction with internal-state variable constitutive theory to derive a set of damage-detection equations which are used to predict, from changes in measured modal parameters, the current value of the internal-state variables in each finite element. The value of the internal-state variable determines the extent of damage at a given location. Numerical examples involving damaged composite beams are used to demonstrate the capability of the theory to predict the exact location and the severity of damage. To provide experimental evidence to support the theory, mechanical and modal tests are performed on a [0,90₃] _s laminated composite beam in the undamaged state and in three additional states of progressive damage. At each stage of damage, edge replications are taken to determine the crack density along the length of the beam. The predicted values of the internal-state variables, obtained from the modalsensitivity equations using measured modal information, are compared with the values of the internal-state variables obtained from crack-density measurements along the length of the beam. Good agreement between the predicted and the measured values is found.