Although bolted joints represent one of the most common types of connections used in mechanical engineering, there is scanty experimental evidence of their actual working conditions due to the fact that the contact interface between the two members is closed to the outside world, thus making its direct observation impossible. Nevertheless, it is well known that the strength of the joint and its heat transfer and electrical conductivity characteristics are basically entrusted to a small area located around the bolt where contact pressure is likely to reach very high values. Therefore, it is essential to have available experimental information on phenomena originated by the contact. Engineering practice has developed a number of effective techniques to assess the reliability of the assembly through estimation of bolt tension by measuring either the applied torque (e.g., by means of a torque wrench) or the elongation of the bolt (using strain gages, acoustoelastic techniques, etc.), but it would be quite useful to have a tool capable of visualizing the contact area (and possibly giving a rough idea of contact pressure distribution) while the joint is working. The study described herein aims at visualizing the contact area of closed steel bolted joints by means of a high-frequency ultrasonic scan. The method, which exploits the property that ultrasounds are reflected differently by a stressed interface depending on the local contact pressure value, has been applied to commercial four-hole steel flanges. A number of tests have been carried out under different values of applied torque to obtain graphic reproductions of the contact area, which are then compared with maps acquired with a commercial pressure-sensitive film. Results show that the ultrasonic method may represent a useful tool to supply information about the contact status of bolted joints and, primarily, be very effective in detecting irregular and unexpected contact conditions, which have the potential to compromise the reliability of the connection.
Visualization of contact areas in bolted joints using ultrasonic waves
PAU, MASSIMILIANO;BALDI, ANTONIO;LEBAN, BRUNO
2008-01-01
Abstract
Although bolted joints represent one of the most common types of connections used in mechanical engineering, there is scanty experimental evidence of their actual working conditions due to the fact that the contact interface between the two members is closed to the outside world, thus making its direct observation impossible. Nevertheless, it is well known that the strength of the joint and its heat transfer and electrical conductivity characteristics are basically entrusted to a small area located around the bolt where contact pressure is likely to reach very high values. Therefore, it is essential to have available experimental information on phenomena originated by the contact. Engineering practice has developed a number of effective techniques to assess the reliability of the assembly through estimation of bolt tension by measuring either the applied torque (e.g., by means of a torque wrench) or the elongation of the bolt (using strain gages, acoustoelastic techniques, etc.), but it would be quite useful to have a tool capable of visualizing the contact area (and possibly giving a rough idea of contact pressure distribution) while the joint is working. The study described herein aims at visualizing the contact area of closed steel bolted joints by means of a high-frequency ultrasonic scan. The method, which exploits the property that ultrasounds are reflected differently by a stressed interface depending on the local contact pressure value, has been applied to commercial four-hole steel flanges. A number of tests have been carried out under different values of applied torque to obtain graphic reproductions of the contact area, which are then compared with maps acquired with a commercial pressure-sensitive film. Results show that the ultrasonic method may represent a useful tool to supply information about the contact status of bolted joints and, primarily, be very effective in detecting irregular and unexpected contact conditions, which have the potential to compromise the reliability of the connection.
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