Saddle bolt failure

Click on pictures below for larger image.

The bolt securing this saddle to the seat post broke while the rider was cycling. The bolt may have been loose causing an increased bending load on the screw thread. This failure occured only three months after purchase.
The bolt's function is to fasten a clamp onto the saddle rails, this transmits the seat loads to the seat post (not represented on the picture).

Because the lower part of the clamping system is serrated, it is not free to move if the bolt is only slightly loose. A bending moment is applied to the screw if the cyclist transfers their weight forward or backwards in the saddle. The geometry of the component loading therefore suggests that the failure might be caused by bending fatigue.

These magnified views of the fracture surface show two zones that confirm the hypothesis of bending fatigue. The fatigued zone (left half of the image) seems to have several initiation points. However, by the time of failure they have all amalgamated into one linear crack front, the line of which is perpendicular to the applied bending stress.

 The fatigue strength of this bolt is severleycompromised by its threads. One possible improvement to its design would be to ensure that the par tof the bolt that was likely to be exposed to bending stresses was not threaded.