Click on pictures below for larger images.

This aluminium alloy mountain bike seat post failed at the point where it entered the steel frame. It had been machined to size and the rider had marked the preferred height setting with PVC tape as shown.

The combination of the damp conditions under the tape, the machining marks and the galvanic cell produced from the aluminium seat post and steel frame caused reverse-bend corrosion fatigue to occur that enabled a crack to nucleate and grow so that the remaining ligament could no longer support the applied load.

This photograph shows the machine marks on the outer surface and to the rear of the photo the ductile failure as the last material fractured.

The plan view of the fracture surface shows the half moon corrosion fatigue crack at 9 o'clock and the fast fracture surface it triggered.