Damage Identification of Composite Laminate Impact Delaminations with Dual-Tone Steady-State Ultrasonic Excitation and Nonlinear Signature Analysis

Abstract

Composite layered materials offer many engineering advantages including high strength-to-weight ratio, tailorable material properties, and high fatigue resistance. Detection of small intra-layer delaminations that occur during manufacturing or while in the service environment are critical to ensure that the structure maintains its designed performance. Nondestructive evaluation (NDE) methods, such as ultrasonic testing, can identify hidden defects and damage, but detection can be challenging because the separated layers often remain in physical contact and prevent ultrasonic waves from scattering. Simultaneous multi-frequency excitation has the ability to incite contact acoustic nonlinearities and unique ultrasonic scattering patterns which can be observed within the ultrasonic signatures of the harmonics and mixing frequencies. Acoustic steady-state excitation spatial spectroscopy (ASSESS) is a full-field ultrasonic NDE inspection technique. ASSESS implements a steady-state ultrasonic excitation using a transducer and rapidly measures a structure’s surface velocity response using a laser Doppler vibrometer (LDV). The measurement product is a complex-valued wavefield response mapped to the surface geometry. This research investigates the presence of nonlinear signatures in a dual tone ASSESS measurement of a composite laminate panel with three impact delaminations. From a single measurement, complex velocity wavefield maps are calculated at the two excitation frequencies and additional harmonic and mixing frequencies. Contact acoustic nonlinearity signatures are identified as changes in response amplitude over certain frequencies. Spatial and temporal frequency and image processing methods are applied to individual wavefield images to identify damage locations.