Module 5: Experimental Modal Analysis for SHM
  Lecture 35: Experimental modal analysis for damage detection in composite plates using laser                    doppler vibrometer
 

Review of literature related to experimental modal analysis ( EMA )

Damages in a structure may alter its modal parameters (i.e. modal frequencies, mode shapes and modal damping) and structural parameters (i.e. mass and stiffness). These dynamic characteristics are used to study the condition of the structure as they reflect the state of the whole structure.

Some of the researches related to damage detection using dynamics based approach are summarized as follows:

Presence of localized damage reduces stiffness and increases damping at that region.

  • Cawley and Adams [1979] have noted that ratio of frequency changes in two modes is a function of damage location. Location of damage zone is identified where the theoretically determined ratios of frequencies and experimental values are equal.
  • Vandiver and Mitome [1979] have used the same principle of changes in natural frequencies to detect damage in offshore platforms.
  • Swamidas et al [2003] have explained experimental procedure for crack detection in beams using changes in frequencies and corresponding amplitudes in frequency response functions.
  • Pandey et al [1991] have proposed the use of curvature mode shapes for damage detection in concrete beam sections. Curvature mode shapes are related to the flexural stiffness of the beam cross sections. Curvature of the mode shape increases due to reduction of stiffness at the damage region. Absolute change in the curvature for damaged and undamaged beam can be used for measuring the degree of damage. Curvature changes increase with the increase in the damage size.
  • Valdes and Scoutis [1999] have applied resonant ultrasound spectroscopy for obtaining modal frequencies of delaminated specimen. Resonant ultrasound spectroscopy is based on the study of spectra obtained by forced mechanical resonance of test objects using swept sine excitation. It is used to determine the elastic constants of a test specimen from its resonance spectrum. In this method, the test object is placed on actuator-sensor couple. The actuator is excited with a sine wave producing mechanical vibrations within the specimen at the same frequency as the actuator and the sensor detects the amplitude of the induced vibration. The complete response spectrum of the object is obtained by this method. It has been reported that the effect of delamination is pronounced in the high frequency region. For low frequency region, the positioning of actuating and sensing devices is critical for excitation of specific modes.
  • According to Tracy and Pardoen [1989], delamination has no more than twenty percent effect on the first four natural frequencies of the delaminated beams compared to the undamaged composite beams.
  • Hanagud and Luo [1993] have proposed a method of delamination coefficients to study the existence of delamination in composite plates without visualizing the mode shapes. Higher values of these coefficients are used as quantitative measure of delamination in the composite plates.
  • Yam et al [2004] have proposed a mode dependent energy dissipation method for locating the delamination in cantilever composite plates. For a given mode shape, there is an associated strain field related to the displacements at different sections of the object. The modal strain energy stored in different sections gives unique information regarding energy distribution for a given mode shape. Presence of a defect induces more damping leading to more energy dissipation in that region. Intact and delaminated composite plate specimens are analyzed using modal stain energy distribution for locating the delamination.
  • Kessler et al [2002] in their pioneer work have used Laser Doppler Vibrometer for observing changes in natural frequencies and mode shapes for various types of damages such as impact, cut-out and delamination in graphite-epoxy composite beams. Frequency response based methods are found to be more reliable than mode dependent damage detection methods as coalescence of higher frequency modes makes it difficult to analyze the true nature of the damages.
  • In-depth information regarding mechanics of delamination and its effects and vibration based detection methods are found in the reviews of Bolotin [1996] and Zou et al [2000].