Damage in laminated composite is not readily detectable, since the flaw is often interlaminar. Such a defect is commonly referred to barely visible impact damage (BVID) and, if not promptly identified, it may cause strength and stiffness reductions driving the structure to collapse. In the last few decades, a number of acoustic/ultrasonic-based Non-destructive (NDT) Evaluation techniques and Structural Health Monitoring (SHM) systems were developed to provide an early detection and warning of critical defects .
These techniques are based on the nonlinear material response of damaged composite structures under periodic excitation.
Indeed, damage induces a distortion in the measured signal, typical of systems with hysteresis, which can be observed in the frequency response as odd harmonics (for monotone loading) with respect to a fundamental frequency.
We developed an in-house explicit finite element code able to predict the nonlinear interaction of acoustic/ultrasonic waves with damage precursors and micro-cracks. The hysteresis is captured through a constitutive model (with memory) given by the application of a Preisach-Mayergoyz mesoscale model at the element level.
We have then validated experimentally on a composite plate undergone to a low velocity impact loading.