Abstract
Nonlinear dynamic response represents the most important studies for structures subjected to a dynamic mo-tion so that it provides the researcher by an excellent information especially at critical design levels. The un-predictable nonlinearity in the structure appears when damage is inherited. Most times, the failure of the structure is related to the dynamic nonlinearity. With regard to the breathing phenomena for nonlinear struc-tural systems, very little is known about how the nonlinearities influence the response and the dynamic char-acteristics of cracked structures. In this research, dynamic nonlinearity is presented in damaged structure due to presence of a crack. The crack is assumed to be open and close simultaneously and then breathing. Effect of breathing phenomenon was studied deeply. Crack breathing is simulated at the crack surfaces using con-tact elements. The contact, geometrical, penalty, and spin stiffnesses are taken in consideration. In addition, effect of several important parameters such as rotor angular velocity and crack ratio are studied. The study showed that the breathing natural frequency of any structure is ranged between opened (no contact) and closed crack natural frequencies. The larger crack length, the more nonlinear disturbance in the dynamic re-sponse behavior. Also, at a critical crack length, some mode shapes tend to exchange and pass over with other modes. The presence of the mode interchanging and mode crossover was a guide on the nonlinear re-sponse for the cracked structure. The numerical modeling is achieved using ANSYS finite element program. Experimental data are used for validating the accurate use of contact elements in ANSYS environment.
Highlights
A crack, one of the most common defects in a structure, may result in a dangerous effect on the dynamic behavior of structures
The increasing of crack ratio from 0 to 0.2 exhibits no effect on the mode shape but an increasing in the amplitude of the deformation will occur
At a crack ratio of 0.4, an exchange in the mode shapes occurs, so that the 5th mode will veering to the 4th mode and transform from sin bending mode to in plane one
Summary
A crack, one of the most common defects in a structure, may result in a dangerous effect on the dynamic behavior of structures. They incorporated the breathing crack model into a single-degree-of-freedom system as a harmonic change in the beam stiffness Their analysis showed that the natural frequency reduction for a fatigue crack (breathing crack) was much smaller than that for an open crack. Reference [12] simulated a breathing crack state in a single degree of freedom of T-beam using piece wise linear time record based on bilinear stiffness model The eigen-frequency changes due to a breathing edge-crack are shown to depend on the bi-linear character of the system They showed that using an open-crack model assumption to interpret vibration measurements for a fatigue-breathing crack will lead to the incorrect conclusion that the crack severity is smaller than what it really is. The breathing effect of the crack, the effects of rotation, and crack ratio are considered in this study
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