Question

11. What would the following features on the fractured surface face of a broken shaft tell you about the mode of failure and the levels of stress that that shaft experienced? (a) ratchet marks (b) beach marks (c) size of final fracture area (7 marks)

Answer 1

a) When examining fatigue failures, marks are commonly found on the outer perimeter of the fracture surface of the failed material; these marks can often look like small steps on the periphery of the failure surface and usually vanish moving towards the interior. These are called Ratchet Marks. Usually these are observed on a microscopic level but sometimes they can be seen with the naked eye depending on the material and size of the failure fracture face. Ratchet marks originate when multiple cracks, commenced at different points, join together, creating this common step like appearance on the fracture surface. The number of ratchet marks found can be closely related to the number of crack origins and can often play a key role in helping identify the failure mode, type, origin location and root cause.

Fatigue is a very common failure mode for metal. Although there are plenty of other modes for metal failure, fatigue can be commonly found in items that are used on a regular basis. For example, springs, gear shifters, door handles, light poles, airplane wings, locomotive parts, wires and cables, hinges, anything that is subject to repeatedly applied (cyclical or alternating) loads over time are perfect vessels for fatigue. The fatigue failure usually occurs in the area that the material is the weakest, and/or the applied stress is the highest or most frequent. The applied stresses are cyclic in nature.

b) Beach marks are macroscopic progression marks on a fatigue fracture or stress-corrosion cracking (SCC) surface that indicate successive positions of the advancing crack front. They take the form of crescent-shaped macroscopic marks on fatigue fractures representing positions of the crack propagation, radiating outward from one or more origins.

Beach marks are concentric rings in a fatigue region which resemble tide marks on a beach. Crack advancements are caused by changes in environment and load.

Whenever there is an interruption in the propagation of a fatigue fracture, a unique feature of macroscopically visible marks or ridges may be found. Beach marks must not be confused with striations, although they frequently are present on the same fracture surface; there may be many thousands of microscopic striations between each pair of macroscopic beach marks. Beach marks do not occur if the part has been operated continuously, or with only brief interruptions in service.

Beach marks arise from two unrelated sources:

Selective oxidation at the tip of the crack when the component is in an unloaded state
Rapid changes in applied-stress intensity

Beach marks from oxidation are created only when the loading is intermittent. There is no opportunity for beach mark formation at a constant load in a dry environment.

It is a macroscopic fatigue defect and caused due to continuous cyclic loads over a period of time.

C) The fracture area varies with different types of loading and stresses. The fracture area reduces significantly when a tensile load is applied. The mode of failure is due to shear stresses induced within the material. The special shape formed during fracture is called cup and cone shape.When is the fracture area is negligible then the failure mode is due to normal stresses caused due to torsion.