Structural health monitoring for damage localization and imaging using a linear and nonlinear ultrasound subharmonic and modulation method

Structural health monitoring (SHM) of components is fast becoming a vital part of the in-service maintenance and evaluation of components and structures throughout various industries. This work presents a linear and nonlinear ultrasound subharmonic and modulation method for the localisation and imaging of barely visible impact damage in composite materials. Both linear and nonlinear techniques are used in order to combine the high sensitivity of nonlinear techniques with the accuracy of linear methods in damage cases where large impedance mismatches occur. A sparse array of ultrasound transducers and sensors was used to evaluate the linear and nonlinear subharmonic and modulated elastic responses of a complex composite stiffener panel using a nonlinear multi-path reciprocal (NEMR) methodology. The reciprocal method determines the location of damage between two paths, with higher nonlinear responses occurring at sensors close to the damage and vice versa for linear measurements. The reciprocal method is combined with a statistical method to group individual frequencies which relate to the damaged region and creates a point cloud which locates damage according to coordinate positions (x, y) and images damage according to a linear or nonlinear amplitude (z). Traditionally, SHM methods only provide damage localisation information, the proposed method not only provides imaging of damage region but does so without a priori knowledge of the wave velocity and without baseline measurements of undamaged samples.