The pipe walls begin to lose their shape, and the outer border seems to "fall" inside the pipe, and it tends to take the form of an oval in the cross-section. The forces acting on the pipe walls on the inside of the bend compress the metal, and since it cannot be compressed indefinitely, at some point, it starts to get into an "accordion." In the case of
lakeside-hire.co.uk, the situation is exacerbated by the fact that the main force when bending is applied to the tube in one very narrow area - the center of the bend. This is where most of the defects occur. And if the pipe breaks, the fracture also appears in this area.