Striation (fatigue)
Striations are marks produced on the fracture surface that show the incremental growth of a fatigue crack.
The width of a striation is indicative of the overall crack growth rate but can be locally faster or slower on the fracture surface.
While striations are fairly straight, they tend to curve at the ends allowing the direction of crack growth to be determined from an image.
Striations generally form at different levels in metals and are separated by a tear band between them.
The source of the river pattern converges to a single point that is typically the origin of the fatigue failure.
There is some dispute as to whether striations produced on both sides of the fracture surface match peak-to-peak or peak-to-valley.
The unloading part of the cycle causing plastic deformation on the surface of the striation.
Variable amplitude loading causes cracks to change the plane of growth and this effect can be used to create marker bands on the fracture surface.
When a number of constant amplitude cycles are applied they may produce a plateau of growth on the fracture surface.
Fast fracture occurs through a process of microvoid coalescence where failures initiate around inter-metallic particles.
The F111 aircraft was subjected to periodic proof testing to ensure any cracks present were smaller than a certain critical size.
These loads left marks on the fracture surface that could be identified, allowing the rate of intermediate growth occurring in service to be measured.
[10] Marker bands may be used to measure the instantaneous rate of growth of the applied loading cycles.
They are produced instead of regular striations when there is insufficient atmospheric moisture to form hydrogen on the surface of the crack tip in aluminium alloys, thereby preventing the slip planes activation.
[14] When water vapour deposits onto the freshly exposed aluminium fracture surface, it dissociates into hydroxides and atomic hydrogen.
The growth rate increases typically by an order of magnitude, with the presence of water vapour.
