Subsurface Damage Detection in Sandwich Composites Using Three-Dimensional Laser Vibrometry Measurements with Nonlinear Vibration Response Characteristics

S. Saxton Underwood, Purdue University

Abstract

Composite materials are being used more frequently in commercial and military aircraft structures. However, a drawback to using sandwich composite materials for aircraft applications is that damage in composite materials often occurs beneath the surface, making it difficult to inspect these aircraft using visual or line-of-sight techniques. Other nondestructive techniques have been developed to inspect composite materials for damage, but these techniques are time consuming, expensive, and typically require reference standards in order for subsurface damage to be detected. This research aims at developing a reference-free method for detecting subsurface damage in sandwich composite materials by using a three-dimensional scanning laser vibrometer and using the nonlinear behavior indicative of subsurface damage in a composite as a means of detecting damage. Three different sandwich composite materials are obtained, including a fiberglass and two carbon fiber sandwich panels. The panels are damaged by placing cuts in the honeycomb core material to simulate disbond and core cracking damage mechanisms or through impact damage at controlled amplitude levels. A three-dimensional scanning laser vibrometer is used to measure frequency response behavior on the panels as they are excited at multiple amplitudes of excitation. The measured multi-amplitude frequency response data is analyzed to determine if subsurface damage in the panels displays localized nonlinear behavior and if that behavior can be identified and used to detect damage in the panels. Subsurface damage in each of the composite panels investigated is observed to display nonlinear response behavior due to a change in excitation amplitude. A detailed analysis of core cracking, disbond, and core crushing damage mechanisms showed that localized motion in the vicinity of damage is the mechanism that allows for nonlinear behavior at the damage location to be identified. A comparison of the frequency response functions measured on the composite panels showed that the nonlinear behavior can be attributed to a bilinear stiffness nonlinearity. This nonlinearity was simulated in an analytical model to show that using multi-amplitude frequency response functions to detect localized nonlinear behavior is a feasible approach. With this approach, subsurface damage was successfully detected in each of the sandwich composite structures investigated. This research concludes that localized nonlinear behavior in damaged composite specimens is able to be detected through a comparison of multi-amplitude frequency response functions and that identification of this behavior leads to successful detection of subsurface damage. This approach to detecting subsurface damage addresses many of the limitations with existing inspection techniques for composite materials, including the elimination of the need for reference signatures and a reduced impact of environmental factors on the accuracy of the damage detection result.

Degree

Ph.D.

Advisors

Adams, Purdue University.

Subject Area

Mechanical engineering|Engineering

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