Copyright c  2007 Tech Science Press CMES, vol.22, no.1, pp.77-89, 2007 Modeling of Degraded Composite Beam Due to Moisture Absorption For Wave Based Detection. Shamsh Tabrez, Mira Mitra and S. Gopalakrishnan 1 Abstract: In this paper, wave propagation is studied in degraded composite beam due to mois- ture absorption. The obtained wave responses are then used for diagnosis of the degraded zone. Moisture absorption causes an irreversible hy- grothermal deterioration of the material. The change in temperature and moisture absorption changes the mechanical properties. Thus this affects the structure in dimensional stability as well as material degradation due to reduction in mechanical properties. Here, the compos- ite beam is modeled as Timoshenko beam us- ing wavelet based spectral finite element (WSFE) method. The WSFE technique is especially tai- lored for simulation of wave propagation. It in- volves Daubechies scaling function approxima- tion in time and spectral finite element approach. The simulated wave responses are then used as surrogate experimental results to predict degrada- tion using a measure called damage force indica- tor (DFI). Numerical experiments are presented for moisture absorbed composite beam due to modulated sinusoidal excitation. The responses are studied for different environmental conditions in term of relative humidity and at a temperature. Keyword: Composite; moisture absorption; material degradation; damage detection; wave propagation; spectral finite element 1 Introduction In the recent years there has been tremendous growth in composite technology as well as it’s uses due to it’s excellent strength to weight ra- tio, tailor-ability, thermal insulation to name few. 1 Department of Aerospace Engineering, Indian Institute of Science, Bangalore, India 560012 Corresponding author E-mail: krishnan@aero.iisc.ernet.in Fax:+91-80- 23600134 These days it is extensively used in automobile and aerospace industries. Besides the advantages mentioned it has some inherent disadvantages too. Here, we will discuss about the degradation in material property due to moisture absorption. When composite material is exposed to humid at- mosphere, many polymeric matrix composites ab- sorb moisture by instantaneous surface absorption followed by diffusion through the matrix. Mois- ture will enter the interface of fiber and matrix due to the capillary action [Jihua and Maosheng (2004)]. The polymer matrix and the interface between the matrix and fiber can be degraded by hydrolysis reaction of unsaturated groups within the resin [Kootsooks and Mouritz (2004); Vera and Vazquez (2004)]. Debonding may occur at fiber/matrix interface [Imielinska and Guillau- mat (2004)]. Composite material degradation occurs as cracks of the matrix material or/and fiber/matrix debonding, resulting from differen- tial swelling of fiber and matrix. The weaken- ing of bonding between fiber and matrix and soft- ening of matrix material are also the reasons be- hind decrease in composite strength. In order to utilize the full potential of composite materials for the structural applications, the moisture con- tent of composite material has to be well defined in advance. Few researchers have done experi- ments in standard laboratory conditions to estab- lish the effects of moisture concentration on the tension modulus of composite material [Tsai and Hahn (1980)]. The hygrothermal deformation of an unidirectional composite is much higher in the transverse direction than in the longitudinal direc- tion. Such difference in deformation in two direc- tions induces residual stresses in composite lam- inates. This is because the multi directionality of fiber orientation resists free deformations. Also, the change in temperature and moisture absorp-