Abstarct: Monitoring the health of mechanical structures is a vital issue during their operation and due to the sensitivity of structures and facilities, they should be constantly reviewed by users, engineers and their quality supervisors. There are several methods for assessing the health of structures, each of which has limitations on opening structures from the installation site, transporting them to a laboratory environment, radiation imaging, or not exposing them accurately during the operation of the structure. The structural health monitoring methods introduced in this research provide the identification of structural health disorders in a non-destructive manner and in a very short time at the place of application of the structure. In this research, the fault detection in the structure has been done by using the property of ultrasonic waves in a sheet and also by the method of nonlinear characteristic detection as sideband frequency components in the vicinity of the detection signal, using frequency analysis of return signals from the structure. This system is portable and installed on structures and is able to quickly detect the health of structures and the occurrence of disturbances in them during their operation and continuously. By implementing this plan, the health of sensitive mechanical systems such as air and space systems, high pressure tanks, high pressure transmission pipelines, weld fractures and other systems under their operating conditions will be continuously evaluated. In this research, by analyzing the return signal from the sheet, its healthy and defective state has been investigated. Also, using frequency analysis of the return signal from a mechanical structure, by examining the nonlinear components, its healthy and defective states have been investigated.