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Research Papers

Damage Localization in Composite Structures Using Nonlinear Vibration Response Properties

[+] Author and Article Information
Sara S. Underwood

Purdue Center for Systems Integrity,
Purdue University,
1500 Kepner Drive,
Lafayette, IN 47905
e-mail: sara.s.underwood@gmail.com

Janette J. Meyer

Laboratory for Systems Integrity and Reliability,
Vanderbilt University,
2301 Vanderbilt Place,
Nashville, TN 37235
e-mail: janette.j.meyer@vanderbilt.edu

Douglas E. Adams

Laboratory for Systems Integrity and Reliability,
Vanderbilt University,
2301 Vanderbilt Place,
Nashville, TN 37235

1Corresponding author.

Contributed by the Technical Committee on Vibration and Sound of ASME for publication in the JOURNAL OF VIBRATION AND ACOUSTICS. Manuscript received December 9, 2013; final manuscript received October 28, 2014; published online February 2, 2015. Assoc. Editor: Walter Lacarbonara.

J. Vib. Acoust 137(3), 031015 (Jun 01, 2015) (8 pages) Paper No: VIB-13-1421; doi: 10.1115/1.4029001 History: Received December 09, 2013; Revised October 28, 2014; Online February 02, 2015

Subsurface damage in composite materials is difficult to detect using visual techniques, and other current inspection methods lack the ability to perform quick, wide-area inspections without the need for reference signatures or baseline measurements. This paper presents a method for detecting and locating subsurface damage in composite materials without historical reference measurements by considering the nonlinear behavior of the material in the vicinity of damage. Nonlinear behavior is identified by comparing frequency response functions measured at different input amplitudes. It will be shown that the nonlinear behavior of the material is most evident in the areas nearest to the damage. The proposed inspection method is demonstrated both analytically and experimentally. First, a finite element model of a sandwich beam is developed using Bernoulli–Euler beam elements to represent each layer of the beam and springs to represent the interface between the layers. A bilinear stiffness nonlinearity is simulated to represent disbond damage between the top and core layers of the beam. The simulated disbond damage is localized by identifying degrees of freedom which indicate significant nonlinear response through a comparison of frequency response functions measured at various input amplitudes. Next, the method is demonstrated experimentally by identifying disbond damage in a fiberglass sandwich panel. A three-dimensional scanning laser vibrometer is used to measure the forced frequency response of the panel in its damaged state as it is excited at two or more amplitudes of excitation by a piezoelectric actuator. Comparisons of the frequency response functions measured at different input amplitudes show that the subsurface damage introduces nonlinear behavior which resembles a bilinear stiffness nonlinearity, and the differences in the frequency response functions are largest in the vicinity of the damage location. In addition, it was found that improved localization of the damage is achieved by investigating the response at higher frequencies. This work has application as a nondestructive method for detecting and locating subsurface damage in composite materials and, by using a laser vibrometer for noncontact measurement, allows for quick, wide-area inspection of composite materials without the need for reference signatures or baseline measurements.

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References

Figures

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Fig. 1

Schematic of a sandwich composite beam modeled with Bernoulli–Euler beam elements and translational and rotational springs

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Fig. 2

Frequency response functions for the three force levels (0.1 N: dark gray, 1 N: medium gray, and 10 N: light gray) for the nonlinear case with Δ = 1 × 10−9 m compared to the linear case (black) where (a) shows the frequency range of interest and (b) shows the bilinear stiffness behavior at a peak in the response

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Fig. 3

DI results (healthy face sheet element: dark gray, damaged element: light gray, damaged element (edge): medium gray, healthy core element: black) for simulation of the sandwich beam with simulated disbond damage

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Fig. 4

DI results (damaged element: light gray, healthy element: dark gray, difference: dotted line) of a degree of damage study for simulated disbond damage at an element

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Fig. 5

(a) Fiberglass sandwich panel dimensions and actuator location and (b) a disbond damage mechanism

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Fig. 6

(a) Measurement area on the fiberglass panel showing points (on top of damage: light gray, on the edge of damage: dark gray, away from damage: black) used in the frequency response comparison and (b) frequency response function comparison for three points on the fiberglass panel at the disbond damage location which shows the low (thin) and high (bold) amplitude frequency response functions and the absolute value of the difference between the frequency response functions (dashed) for each point considered

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Fig. 7

(a) Single degree of freedom model (healthy) and (b) the single degree of freedom model indicating an open gap

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Fig. 8

Frequency response function comparison (low amplitude (thin line), high amplitude (thick line), and difference (dashed line)) in the vicinity of a peak for (a) the disbond damage mechanism, where line colors correspond to the measurement points on top of the damage (light gray), on the edge of the damage (dark gray), and away from the damage (black) and (b) the single degree of freedom model with a simulated bilinear stiffness nonlinearity

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Fig. 9

DIs obtained for disbond damage in the fiberglass panel showing the effect of different analysis frequency ranges

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