Why Does The Connection Between The Coupler And The Self-Drilling Hollow Bar Fail?

The self-drilling anchor bolt is assembled and used in conjunction with a hollow bar with external threads on the surface and couplers, drill bits, centers, nuts, and other accessories. As an important part of the self-drilling anchoring system, the coupler has an important influence on the strength of the entire anchoring system. For self-drilling bolts, it is required that the coupler and the hollow bar meet the requirements of equal strength. The strength of the thread connection between the coupler and the hollow bar directly affects the overall strength of the self-drilling bolt, and is also an important basis for determining whether the coupler and the hollow bar are equally strong.


Failure Analysis of The Coupler Connecting


Deformation Under Different Tensile Loads

Under the action of axial tensile load, the coupler and the hollow bar are connected by threads, and corresponding deformation will inevitably occur when bearing the axial tensile load. Under the action of axial tensile load, the diameter of the hollow bar tends to shrink inward, and this tendency becomes larger as the load increases. Correspondingly, the coupler has a tendency to expand in the radial direction, and the radial deformation of the orifice is relatively large, forming a bell shape, and the radial deformation will continue to increase as the load increases. Under the combined action of these two deformations, the phenomenon of internal and external thread tripping will eventually occur.


plastic deformation

As the axial tensile load increases, when the stress reaches the yield limit of the material, plastic deformation will occur locally on the hollow bar and the coupler. Plastic deformation first occurs at the position of the hollow bar and the entrance of the coupler. When the load continues to increase, the plastic deformation gradually spreads to the end.



When the self-drilling anchor bolt bears the axial tensile load, there is a stress concentration area in the threaded connection between the coupler and the hollow bolt body. The stress at the end of the coupler and the upper internal thread is relatively large, and the stress at the outlet of the coupler on the hollow anchor body is the largest. Large stresses lead to large radial deformation and plastic deformation of both the hollow anchor body and the coupler, which promote the failure of the connection between the coupler and the hollow anchor body.

When the tensile load is large enough, the plastic deformation of the thread and the deformation of the rod body and the coupler will cause the bite strength of the internal and external threads to become smaller, and it is easy to pull off. In addition, when the load is excessive, the rod itself will shrink due to stretching, and in severe cases, the will cause the hollow bar breakage.

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