Pop Grooved rivet:
It is designed to set within a hole in fibrous materials such as wood. When the rivet sets, the body shortens and the fibres of the material are grasped between the closed grooves.
Note: that the rivet is set WITHIN a hole rather than passing right through the components and setting against the blind side. If installed THROUGH a hole, the grooves will not close, and therefore the rivet will not function correctly.
The rivet must be long enough to enter well into the hole, to give it enough space to expand and shorten without coming too close to the surface of the fibrous material.