Damage Characterization and Classification for Composite Honeycomb Structures

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Advanced composite materials are increasingly being adopted for applications in airframe structures due to their high stiffness-to-weight and strength-to-weight ratios as compared to metallic materials. However, advanced composites are susceptible to barely visible impact damage, which could lead to a catastrophic failure if their initiation and growth are not monitored. Structural Health Monitoring (SHM) systems based on acousto-ultrasound methods have emerged as a promising new nondestructive inspection (NDI) technique for monitoring the onset and progress of structural damage. Lamb wave based baseline-subtraction techniques are widely used for distinguishing the damage from other structural features. However, these techniques are only suitable for monitoring impact damage on large composite structures but cannot characterize and classify debond, delamination and core crushing types of damages in honeycomb structures. This paper presents an innovative damage characterization and classification techniques that are capable of detecting, locating and characterizing different types of damage in carbon fiber honeycomb structural components of rotorcraft vehicles. Tests were conducted on the system for characterizing and classifying debond, delamination and core crushing type damage on carbon-fiber Nomex-honeycomb-core structures. Details of the technique along with the results will be presented and discussed in this paper. However, composite and honeycomb structures are susceptible to impact damage due to foreign object impact on the structures. Debond between dissimilar materials is one of the common problems along with honeycomb core crushing and delamination within the plies for composite based airframe structures. Impacts on structure can create delaminations between plies (affecting in-plane stiffness of the structure) and/or debonds between skin and stringers or skin and honeycomb core which can affect the bending stiffness of the structure. The consequences of these damages and the corrective maintenance procedures differ significantly. The reduction in residual strength of the structure due to these impact damages could greatly affect the remaining useful life of the structure. Therefore, accurate detection, localization, quantification and classification of these damage types are important and critical for selecting appropriate life cycle management measures and for Prognostics. Acousto-Ultrasound (AU) based SHM techniques [1-4] using built-in piezoelectric sensors and actuators can be used to monitor damage, reduce inspection time and structural downtime, and move towards predictive maintenance. However, further refinements to the technology are necessary in order to classify damage, especially delamination, debonding and core crushing, for practical aircraft applications. doi: 10.12783/SHM2015/366