Identification of the torsion coefficient of bolts in slip-critical connections

Abstract

Introduction. The article proposes a method for identifying the coefficient of torsion of a high-strength bolt in the laboratory environment. The coefficient is found by determining the relative deformation of a drawn bolt using load cells. This method requires specialized equipment and a test bench, and it can be used to study the behavior of bolts. The value of the torsion coefficient is necessary to ensure the appropriate bolt tension force, since according to current friction concepts of bolted connections involving high-strength bolts, the design behavior of a bolted connection is ensured by the appropriate bolt tension force, determined using the required torque. Otherwise, the actual behavior of a bolt differs from its design behavior, since only a pre-set tension force ensures the necessary bearing capacity of a bolted connection in the friction planes.
 
 Materials and methods. Specimens of bolts under study are made according to the German standard DIN931 with a strength of 10.9, and the size M14 × 220; their nuts comply with the German standard DIN934 with a strength of 10 and the size M14. Instron 1000HDX tension testing machine was used to make a diagram describing the performance of steel bolts. Nut tension was determined using NOGRAU NTW24-034R torque control key, and bolt elongation was controlled using FLA 5-11 strain gauges and the National Instruments data collection system.
 
 Results. A new method for monitoring the measurement of the torsion coefficient of high-strength bolts was tested and proposed in furtherance of the research conducted by the authors.
 
 Conclusions. As a result of testing, a change in the torsion coefficient can be evaluated depending on the torque value.