Development of the Two-Step Delamination Identification Method by Resonant and Anti-Resonant Frequency Changes

Identification of the location and size of delamination is important for structural health monitoring of composite laminated structures, since delamination degrades compression stiffness of the structures. In the present study, a two-step delamination identification method using resonant and anti-resonant frequency changes is proposed. It is well known that resonant frequencies are changed depending on the location and size of delamination. As for anti-resonant frequencies, they also change mainly depending on the location of delamination. Since the resonant and anti-resonant frequencies are specified easily in the frequency response function, the present method is low cost and applicable to any structures. The present method is applied to delamination identification of carbon/epoxy cantilever beams, and effectiveness of the present method is discussed. Introduction Recently, composite laminated structures have been applied to many structures of vehicles because of its high specific stiffness and strength. Since interlaminar strength of composite laminated structure is relatively low, internal damage, such as delamination, can be easily induced in service. Since delamination degrades compression strength of the structure, it is necessary to identify the size and location of delamination nondestructively for the assessment of structural integrity. However, conventional nondestructive inspection methods, such as X-ray or ultrasonic inspection, are high cost, and application to large structures is difficult owing to size restriction of the apparatuses. The present study proposes a two-step delamination identification method using resonant and anti-resonant frequency changes. In order to examine effectiveness of the present method, delamination identification of a carbon/epoxy cantilever beam is conducted. As a preparation for delamination identification, the beam is divided into two domains based on the mode shapes. The delaminated domain in which delamination is supposed to exist is first identified from the anti-resonant frequency changes, and the delamination location and size are identified from the resonant frequency changes to the next. In order to analyze frequency response changes caused by delamination, finite element method was applied. Effectiveness of the present identification method was investigated both in analysis and experiment. Symmetry of Resonant Frequency Changes Fig.1 shows schematic of a carbon/epoxy cantilever beam with delamination. The beam is 1.45mm thick, 19mm wide and 200 mm long. The laminate configuration of the beam is [02/902]S, and a through width delamination is existed at one 0/90 interlayer. In order to examine effect of delamination location and size on the resonant frequencies of the beam, modal analyses of the beam were conducted by using ANSYS, a commercially available general-purpose finite element code. Table 1 shows material properties of uni-directional carbon/epoxy lamina used for calculation, where Key Engineering Materials Vols. 270-273 (2004) pp. 1852-1858 online at http://www.scientific. net © 2004 Trans Tech Publications, Switzerland Title of Publication (to be inserted by the publisher) subscript 1 means the direction parallel to the fiber and subscripts 2 and 3 means the direction perpendicular to the fiber. In the present paper, normalized values l/L and a/L are employed to express the delamination location and size respectively, where L is beam length. Fig.2 shows resonant frequency changes of the beam as a function of delamination location obtained by FEM analysis, where delamination size is constant value 0.1. In fig.2, abscissa means delamination location and ordinate means frequency value normalized by the intact value of each mode. In general, reduction of resonant frequency is caused by stiffness reduction of structures. For a composite laminated beam with delamination, reduction of the bending stiffness is caused when shear force acts on the delamination, and shear force distribution of each mode depend on each modal shape. For this reason, the resonant frequency changes of the cantilever beam shows quasi-symmetry in the region defined by 0.3≤l/L≤0.7 except for the first mode, though the cantilever beam is not a symmetric structure. This causes failure in identification of delamination location based on resonant frequency changes [1]. In order to make it possible to identify delamination location in the cantilever beam, another parameters that have information about delamination location and no symmetry. In the present study, anti-resonant frequency is adopted. (L=200mm, b=19mm, h=1.45mm) L h Delamination