Comparative Study On Electromagnetic Ultrasonic Probes For In Service High Strength Bolt Inspection

Bolt connection is widely used in wind turbine,large bridge,airport waiting hall and other buildings of large steel structure.With the development of China,the quantity of skyscrapers,sea-crossing bridges and other large-scale & long-span projects are gradually increasing.In these buildings,the overload and years of service beyond the fixed number of years on high-strength bolt is more and more frequent,bringing many safety risks.From the present point of view,although there has been in-depth research on the detection of dismantled bolt in China and foreign countries,there is no effective and mature detection method for the detection of in-service high-strength bolt.Due to its unique energy exchange mechanism,EMAT(electromagnetic acoustic transducer)can excite ultrasonic wave on bolts without contacting the bolt surface.Therefore,there is no need to treat the bolt surface,and the detection process is simple and efficient.So it is very suitable for the crack detection of in-service high-strength bolts.In the process of EMAT testing,the transmitting and receiving of ultrasonic signals are realized by the probe,so a well-structured probe is very important for EMAT testing.There are several difficulties in the detection of in-service high-strength bolt cracks with EMAT probe: firstly,the available position of the in-service bolt for the EMAT probe is limited,and the size of probe is limited.At the same time,the acoustic energy of EMAT cannot be as concentrated as piezoelectric flaw detection,so the signal strength is weaker.How to excite a stronger amplitude with the limited volume of probe becomes a problem;Secondly,there are many kinds of EMAT probes,so it is difficult to select the probes with potential for in-service bolt crack detection;Thirdly,the installation position and size of probes are limited in in-service bolt cracks detection,so how to design the probe structure in limited space becomes a problem.In this paper,the bias magnetic field and the eddy current field are modeled and simulated.Firstly,the principle of crack detection with electromagnetic ultrasonic probe is expounded,and the performance of probes with different structures is analyzed qualitatively;Secondly,the simulation quality is controlled from the rationality of model simplification,the relationship between the bias magnetic field of bolt surface and the size of permanent magnet is obtained.By comparing the bias magnetic field produced by different permanent magnets,it is concluded that the cylindrical permanent magnet is more suitable for in-service high-strength bolt detection;Thirdly,the simulation quality is controlled from the mesh size suitability.The relationship between eddy current field and coil size is obtained,and the coil size suitable for in-service highstrength bolt inspection is determined.By comparing the eddy current field produced by different coils,it is concluded that the spiral coil is more suitable for in-service highstrength bolt inspection.This paper has guiding significance to engineering crack detection of in-service high strength bolts.