This study focuses on dynamic stability of sandwich beams with orthotropic faces and electro-rheological (ER) core subjected to a harmonic axial load. ER layer is adhered to the orthotropic beam to control vibration and consequently improve dynamic stability of the beam. In this study behavior of ER core is modeled based on Bingham equation. To obtain FEM formulation a sandwich beam element is driven. Dynamic stability investigation has been done based on transient response of the beam. Dynamic critical status of the beam happens when vibrations amplitude becomes large. By obtaining dynamic critical loads for different load frequencies, boundary of stability region is achieved. Obtained results show, ER layer mostly affects damping properties of the system such that by increasing thickness of ER layer and applied electric field, dynamic stability of the beam increases. By increasing the angle of layers, stiffness of the beam in axial direction decreases and as a result natural frequencies and dynamic stability of the beam decrease.

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