Metal fatigue is the process by which a material is slowly and progressively damaged by stresses and strains not strong enough to cause immediate breakage.
In HDD drilling where the drill pipe rotates during drilling, this rotation continues through the bend as it passes through the curved part of the drill path. Pipe “in the bend” compresses on the inside of the curve and stretches on the outside. During each rotation, the pipe compresses and stretches. This stresses the metal and leads to fatigue.