Dynamics of Slender Structural Member having Cracks

Abstract

Structural discontinuity present in the structural member can potentially threaten the safe performance of the component and components may also fail during the service, which may be dangerous to life. It is desired to monitor the component and detect the damage or discontinuities at earliest possible stage. A wide group of Non-destructive testing (NDT) techniques used to detect these discontinuities by evaluating the properties of a material, component or system without causing damage. Common NDTs are time consuming, laborious and expensive for long components like pipes, rails and slender beams.

The main focus of the thesis is to develop the method to detect the location and the size of the crack in slender structural members using changes in natural frequency of transverse vibration. The cracks considered, are external crack which are normal to the axis and has straight front.

To model the crack present in the slender structural beam, rotational spring approach has been incorporated. Also, the effect of the crack location and the crack depth on the natural frequency is presented for various end conditions. The effectiveness of this method is investigated experimentally and through simulation. First three natural frequencies of the cracked beam have been obtained through experiments and by simulation and they are used for detection of crack location and size. Detected crack locations and size are compared with the actual results and found to be in good agreement with the actual condition. The same approach has been applied to detect the crack present in the straight pipe and results are found to be in good agreement with the actual crack parameters.

Approach for the detection of the multiple cracks has been analyzed and the results involving up to two cracks are presented to illustrate the effectiveness of the method.

Also, the approach for the detection of the crack location and the crack size for the multi-span beam has been developed. Effect of the crack location and the crack size on the natural frequency has been studied and presented for various end conditions for multi-span beam. The same approach is verified to demonstrate the effectiveness of the crack location.

The whole study indicates that the detection of crack condition i.e. crack location and size is possible based on changes in the natural frequency of the transverse vibration.