International journal of COMADEM Image reconstruction of subsurface damage in unidirectional fiber composite laminate by dynamic shear strain analysis of lamb wave- a numerical approach M. Sanaul Rabbi a *, K. Teramoto a , and Md. T. I. Khan a a Department of Advanced Technology Fusion, Graduate School of Science and Engineering, Saga University, 1 Honjo-machi, Saga 840-8502, Japan * Corresponding author. e-mail: pavel_rabbi@yahoo.com 1. Introduction Nowadays, CL materials have been a growing popularity in a wide spectrum of the industries. They have been extensively used in engineering process and become an important material in aircrafts, power plants, constructional structures, ships, cars, rail vehicles, robots, prosthetic devices, sports equipment, etc. CL based products offer a better strength-to-weight and stiffness-to- weight ratios due to reduced weight of the structure. The major advantage of such kind of material is the ability of controllability fiber alignment. Other advantageous properties include good acoustic and thermal insulation, low fatigue and corrosion. CL can be damaged during manufacturing as well as in service by many ways and these damages are most likely depend on the working environment experienced and the sensitivity of the particular materials used. The mechanism of damage includes static overload, impact, fatigue, hygrothermal effects, overheating, lighting strike, creep, and etc. It can be either local or covering a wide area. Damage constitutes a severe discontinuity because they do not transfer interlaminar shear stresses and, under compressive loads, they can cause rapid and catastrophic buckling failure [1]. The integrity of such structures need to be determined NDT to assure the performance in service. NDT methods are developed to detect and characterize flaws and also to determine the material properties of test specimen. The development of computational tools, along with a more widespread understanding of the nature of lamb waves made it possible to devise techniques for NDT. The use of lamb waves is potentially an effective solution to detect the damage in CL with traction free boundaries [2]. In this type of elastic wave, particle motion lies in the plane that contains the direction of wave propagation and the plate normal (the direction perpendicular to the plate) [3]. Typical dispersion curve of Lamb wave always exhibits two modes of propagation, i.e. Symmetric (S) and Anti- symmetric (A) [4], whereas each mode contains several orders depending on the propagating frequency. Modes of certain frequencies can therefore be selected according to their sensitivity to the defects of interest. Lamb wave based NDT technique always deals with shorter wavelength than the dimension of the part being inspected. The advantages of lamb waves over bulk waves for damage detection are variable mode selection by using various combinations of different striking incident angles and frequencies. This type of wave can travel long distance with little attenuation and they have been studies intensively by many researchers worldwide for Structural Health Monitoring (SHM). Lamb waves produce stresses throughout the plate thickness; therefore surface as well as internal defects can be detected. The S 0 mode was used for damage detection in aircraft structures [5] and, the longitudinal modes were employed for plate inspection [6]. This type of waves allows for the inspection of structures over reasonably long distances, and can be used even if local access to that inspected part is not possible [7]. A 0 and S 0 modes were used in a synthetic aperture array system based on full matrix capture methods for detection of corrosion defects [8]. Kundu et. al. [9] has identified the most efficient lamb wave modes for detecting defects. Rose [10], described that, the particle motion of wave distribution through the thickness of the plate can be varied to COMADEM International Journal of Condition Monitoring and Diagnostic Engineering Management ABSTRACT Composite laminates (CL) widely used in various engineering applications due to their high strength-to-weight ratios. Defects could inadvertently be produced in those, either during the manufacturing process or in the course of normal service lifetime of the component. Damage detection drawn much more attention during the design, operation, maintenance and repair of equipment involving CL. Moreover, subsurface damage could be endangering for the structural integrity. The dynamic shear strain analysis is a damage identification technique dealing with the overlapping of incident wave with the scattered wave from the finite defect in a plate. This investigation presents an application of such technique to detect the subsurface defect in horizontally transverse isotropic (HTI) media focusing on unidirectional fiber CL. Several numerical experiments carried out considering the fiber direction of the plate. A0 mode of Lamb wave chosen over S0 mode due to its shorter wavelength and high sensitivity to identify subsurface damage. The analytical data of out- of-surface displacement obtained by using the multiphysics simulation software package LS-DYNA. The defect detection algorithm is based on the recorded time series signals of the orthogonal pair of out-of-surface shear strains. The numerical simulated result of the algorithm is presented in the form of image of the shape of the defect. Keywords: Composite laminate, Subsurface damage, Lamb wave, Finite element analysis. 3