Experimental Investigation of Self-Loosening Behavior of Bolt Joints with Superelastic Shape-Memory Alloy by Macroscopic-Mechanical Response and Microscopic Evolution

The self-loosening process of a bolted joint due to ratcheting consists of two distinct external loading stages. The first stage of self-loosening, with relatively small loading magnitude, is due to the ratcheting deformation of the materials at the thread of bolt. The second stage of self-loosening, with relatively large loading magnitude, is characterized by the ratcheting deformation of the materials at the bolt bar. The present work concentrated on an experimental investigation of these two stages for self-loosening of a bolt joint with superelastic shape-memory alloy (SMA). The experiments consisted of two members joined by a SMA bolt and two nuts which were subjected to cyclic alternating tension loading. The stress-strain relationship curves of the stud were tested by strain gauges, one attached to the bar of the stud, and another attached to an aluminum washer, which served as a force-test apparatus. The larger preload force and magnitude of alternating loading produced a larger reduction of clamping force of the bolt. The martensite residual strain accumulation under an increasing number of loading cycles is suggested to be responsible for the reduction of the clamping force. A series of microscopic evolution observations for the change law of the residual martensite morphology, the dislocation density, and the phase transformation temperatures under different numbers of loading cycles and different locations of the bolt for the identical loading case were carried out to support this viewpoint and to further clarify the mechanism of the self-loosening behavior of bolt joints with superelastic SMA.