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Fatigue Fracture Mechanics
 

Fatigue failures, both for high and low cycle, all follow the same basic four step process. To begin cracks must nucleate within a material- either at stress risers in metallic samples or due to void coalescence in polymer samples. These cracks propagate slowly at first during stage I crack growth along crystallographic planes with high shear stresses until they reach a critical size, then quickly during stage II crack growth in a direction perpendicular to the applied force. These cracks can eventually lead to the ultimate failure of the material, often in a brittle catastrophic fashion.

==== Crack Initiation ==== The formation of initial cracks precluding fatigue failure is a process in and of itself generally consisting of four discreet steps in metallic samples. The material will develop cell structures and harden in response to the applied load. This causes the amplitude of the applied stress to increase given the new restraints on strain. These newly formed cell structures will eventually break down with the formation of Persistent Slip Bands (PSBs). Slip in the material is localized at these PSBs, and the exaggerated slip can now serve as a stress concentrator for a crack to form. Nucleation of a crack to a detectable size accounts for most of the cracking process. It is for this reason that cyclic fatigue failures seem to occur so suddenly- the bulk of the changes in the material are not visible without destructive testing. Even in normally ductile materials fatigue failures will resemble sudden brittle failures.

PSB induced slip planes result in intrusions and extrusions along the surface of a material, often occurring in pairs. Slip induced intrusions and extrusions create extremely fine surface structures on the material. With surface structure size inversely related to stress concentration factors, PSB induced surface slip quickly becomes a perfect place for fractures to intiate.

It should be noted that these steps can be bypassed entirely if the cracks form at a pre existing stress concentrator either from an inclusion in the material or from a geometric stress concentrator such as a sharp corner or small radius. Crack initiation at a pre existing stress concentrator takes less energy than forming a Persistent Slip Band (PSB) and using it to form a crack. It is for that reason that part design and material quality must be scrutinized when producing parts that will be subjected to high cycle loading.

Stage I Crack Growth
Stage I crack growth refers to the phase where the radius of the crack tip is still below the critical size for rapid propagation. In cyclic loading this phase occurs in stages with the load cycles and leaves visible striations or ‘beach marks’ on the fracture surface. These concentric striations originating from the point of origin are a primary indicator of a fatigue failure. Crack growth in this stage is heavily affected by material properties, mainly slip characteristics and microstructure. The initial crack tends to leave a relatively smooth and faceted failure surface.

Stage II Crack Growth
The transition to stage II crack growth occurs when the crack tip radius exceeds the critical size. Once this threshold is reached crack growth is energetically favorable, and progresses to failure of the part in a very quick and normally catastrophic fashion. Since this crack progresses through a path of least resistance throughout the material the failure surface left behind is noticeably rougher.

Ultimate Failure
Ultimate failure occurs when cracks extend through the entire cross section of the part in question, normally resulting in full separation of material, or at the least rendering the work piece unfit for use.