Preliminary Study Of The Delamination Inspection Of Laminated Structures By Vibrothermography

In this paper, based on a great deal of experimental results abroad, the feasibility and fundamental principle of vibrothermography method for nondestructive testing of laminated structures with delamination are discussed in detail, and features of the vibrothermography method are preliminarily studied.Delamination is one of the main types of failure of laminated materials. The delamination would inevitably affect the mechanical behavior and dynamical properties of laminated structures to some extent. Thus investigating the influence of the delamination on natural frequencies of laminated structures will be useful to non-destructive testing method. In this paper, firstly the influence of the delamination upon the natural frequencies of a laminated beam is studied in detail. By partitioning the delaminated beam, its natural frequencies are obtained. Numerical results reveal that the delamination decreases the natural frequencies of the laminated beam, especially for the lower order natural frequencies; and short delamination has no appreciable influences on its natural frequencies. These conclusions will be valuable for the non-destructive testing techniques based on the vibrothermography method to some extent.Compared with other conventional detection method (such as ultrasonic scanning, X-ray radiography, and acoustic emission, etc.), the vibrothermography method presented in 1980s is rapid, safe, non-destructive and applicable to the structures' global detection. Its principle is: a laminated structure (beam or plate) with delamination consists of undelaminated region and delaminted region which can be divided to upper and lower parts. When a low-amplitude and high-frequency excitation has the frequency close to some part's natural frequency, this part will resonate and certain amount of mechanical energy will convert to heat so that the delaminated region's temperature exceeds greatly that of the undelaminated region. Therefore the delamination can be located by means of infrared thermograph. Based on the results of previous researchers, the following instigations have been carried out in this paper:(1) Employing a reasonable local resonance model which can accurately embody the heat-generation behavior due to resonance and vibration-heat mode in delaminated region, the dynamic response and temperature distribution in the small delamination region of a laminated beam are formulated. Influence of the excitation frequency on the temperature gradient formation is analyzed numerically, and several meaningful conclusions are drawn.(2) For laminated plates with delaminations, due to complexity of the analytical solution, the finite element method is employed to analyze their temperature distributions and satisfactory results are also obtained.