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Axial Strain Measurement in Bolts

In mechanical structures such as bridges, expressways, cars, and airplanes, bolts are essentially used for connecting the structural members. They are designed to handle relatively heavy loads, while continuously bearing stress concentration, crack, corrosion and looseness. These external loads tend to affect the intensity of the axial force applied to the bolt. The inadequacy of this force can cause damages to the structure due to looseness. In a similar way, the excessive axial force could also impose stresses on the bolt, which in turn lowers its efficiency. Therefore, axial force monitoring is a significant factor in the detection and investigation of the looseness of bolted joints. 

A strain gauge is either embedded or bonded to a bolt to measure the axial force. These installing methods are chosen according to the conditions of use. TML offers BTM/BTMC series embedment type strain gauges that are inserted into the hole drilled in the center of the bolt and embedded with an adhesive. Using this method, the strain gauge can be prevented from being damaged by a washer, while fastening the bolt. This method is preferred when ordinary strain gauges can not be mounted on the bolt surface. 

The strain gauges from the F, QF, ZF, CF series from TML are bonded to the bolt for measuring axial force. Two strain gauges are bonded on both sides of the bolt shaft in axially symmetric positions to cancel the influence of bending. Also, for getting accurate measurements of the axial strain of a bolt, the bolt is calibrated with the specified load.

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