ELASTO-MAGNETIC SENSOR UTILIZATION ON STEEL CABLE STRESS MEASUREMENT

Maintenance of the huge stock of infrastructures is one of the major concerns in the developed countries in this century. Structure Health Monitoring System development, such as, evaluation of the field condition of existing structures and to monitor the important engineering properties of new structures, can be considered as the first step in resolving of the above problem [1]. New innovative evaluation methods need to be devised to assess the deterioration of infrastructures such as steel tendons, cables in cable stayed bridge and strands embedded in preor post-tensioned concrete beams. However, no accurate and simple method is available for directly measuring the stresses in steel cable in cable-stayed bridges and suspension bridges [2]. The measurement of the stresses is important for monitoring excessive wind or traffic loading to gage the redistribution forces present after seismic events, and for detecting corrosion via loss of cross section of steel. These cables are often very large, containing several hundred wires of 7mm diameter. The cables are sheathed in a plastic protective cover filled with cement grout, and the wires may be coated with lubricating compounds. For these reasons, invasive methods such as strain gages that have been used for much smaller prestressing cables are impossible to use with cable stays. Vibration frequency measured at the middle region of a cable is often used to determine the tension force. The uncertainties of the values of parameters such as mass, length, and cross section area can introduce a significant error to the actual force. Moreover, the stress near the anchor point is of real concern. The Elasto-Magnetic technology is a novel new approach to monitor cable forces in pre-stressed structures and bridge cables [3,4]. This technology overcomes the above mentioned disadvantages related to vibrating frequency or strain gauge methods while still inhabiting the advantages of normal NDT methods [5]. The Elasto-Magnetic Phenomenon is a simple nondestructive evaluation technique for monitoring stress in steel cables [6]. An experiment to verify the utilization of Elasto-Magnetic phenomenon on stress monitoring of steel cables was constructed. The magnetization phenomenon is performed by two solenoids, i.e., a primary coil and a secondary coil. Basic material parameters such as magnetic permeability, intensity of magnetization and temperature were obtained from calibration tests on 7mm wire. Then, these basic material parameters were used to measure the real stress in a 37x7mm diameter cable. Comparison of measurement results with load cells yielded satisfactory agreements and thus it can be stated that utilization of Elasto-Magnetic Phenomenon is suitable for structural health monitoring.