Nondestructive testing principle and technology of tension of anchor bolts

Based on its multi-contact surface characteristics, an indoor model is built to study the vibration characteristics of the anchorage system of an anchor bolt. By installing an acceleration sensor at the top of the unstressed section of the anchor bolt to test the time-history signals, the vibrating spectra at the top of the unstressed section of the anchor bolt are obtained through the fast Fourier transform. The dominant frequencis of the spectra have good identification, then the variation law of the dominant frequencies with the tension force and the unstressed length of the anchor bolt are obtained. But the dominant frequencies in the unstressed section are not the multi-order vibrating ones of the tension section of the anchor bolt. On this basis, the elastic vibration model for the anchor bolt is established by regarding the nut as an elastic foundation, and the rigid vibration model is established by assuming the nut and the anchor bolt to rotate relative to the contact surface between the nut and the spherical washer, then the frequency equations for the models are obtained, respectively. Based on the identified dominant frequencies, the stiffness parameters in the frequency equations can be solved. The indoor and field tests show that there is a good linear correlation and monotonic increasing relationship between the stiffness parameters and the tension. Further indoor model tests show that its relationship characteristic has no obvious correlation with different media in contact with the butterfly pallet and different lengths of tension section of the the anchor bolt. Therefore, the nondestructive testing principle, method and technical route for the tension of the anchor bolt are put forward. The field small-scale tests show that the proposed method is reliable.