21 st Annual International Conference on Mechanical Engineering-ISME2013 7-9 May, 2013, School of Mechanical Eng., K.N.Toosi University, Tehran, Iran 1 ISME2013-984 Guided Wave Propagation in Stepped Beams Mehdi Behzad 1 , Hamid Reza Mirdamadi 2 , Amin Ghadami 3 1 Professor of mechanical engineering, Sharif University of Technology, Tehran, Iran m_behzad@sharif.edu 2 Assistant Professor of mechanical engineering, Isfahan University of Technology, Isfahan, Iran hrmirdamadi@cc.iut.ac.ir 3 M.Sc. Student of mechanical engineering, Sharif University of Technology, Tehran, Iran ghadami_amin@mech.sharif.ir Abstract In this paper, Lamb wave scattering in beams having step discontinuity will be discussed. The wave propagation problem in several cases of symmetric and asymmetric stepped beam is studied. Furthermore, the lamb wave mode conversion phenomenon is investigated in the stepped beams. The obtained results show that lamb wave mode conversion occurs in beams with asymmetric geometric. The results are completely applicable in health monitoring of structures with thickness variation, such as structures having thickness reduction caused by corrosion, which is very common in industry. Keywords: Lamb wave, Stepped beam, Mode conversion Introduction Ultrasonic guided waves have become very common in industries for structural health monitoring purposes because of their numerous advantages. One of their advantages is their applicability that means they can be easily generated into the structure. Another important advantage of the ultrasonic guided waves is their ability of global inspection. We can inspect the health of a large area of structure using a limited number of sensors and actuators. Lamb waves are widely used in non-destructive evaluation (NDE) of engineering structures. They can be easily generated into the structure using small pieces of piezoelectric ceramics. However, there are some impediments that make problems in using lamb waves. The main restriction of lamb waves is that there are a large number of wave modes that could propagate in the waveguide while usually only one of them is suitable for damage detection purposes. Generation of ideal Lamb wave mode in structures having simple geometry is feasible. Beams and plates with constant thickness are examples of these simple structures. There are numerous researches that have been done on the Lamb wave propagation in beams and plates with constant thickness and damage detection in them [1-3]. In industry, many of important engineering structures have thickness variation. Therefore, it is vital to study lamb wave propagation in such structures. One of the main challenges in structures having thickness discontinuity (like step) is mode conversion phenomenon. Cho [4] studied high frequency wave propagation and mode conversion in a plate having thickness discontinuity using Boundary element method (BEM). Cho and Rose [5] studied the lamb wave mode conversion phenomenon on the edge reflection using BEM. Lee, etal. [6] studied the formulation of wave propagation in non-uniform one-dimensional waveguides. In this paper, lamb wave propagation and mode conversion in a beam with step discontinuity is studied. The beam is modeled in ABAQUS ® software. Lamb wave is generated using modeling the effect of piezoelectric actuators on the beam. Finally, the obtained signals are processed using MATLAB ® software and the effect of symmetric and asymmetric step discontinuity on the wave propagation is investigated. Theory Lamb wave particle displacement in structures is superposition of guided longitudinal and transverse shear waves. They can propagate in symmetric mode, anti-symmetric mode or a combination of symmetric and anti-symmetric modes. To find the propagation specification of symmetric and anti-symmetric modes, the Rayleigh–Lamb equation is used as follows [6]; , ) ( 4 tan tan 2 2 2 2         d d (1) , 4 ) ( tan tan 2 2 2 2         d d (2) Where d is the half-thickness of the plate,  is the wavenumber,  and  are given as follow; , 2 2 1 2 2      c (3)