What does a fatigue fracture look like?

What does a fatigue fracture look like?

A quick analysis of the fracture surface of a fatigue failure will often show features casually referred to as “beach marks”. These indicate the propagation of the failure from the initial cracks. Once the crack size has reached a critical level, it will propagate very rapidly until the fracture is complete.

How can you tell fatigue cracks?

Fatigue cracks are sometimes easy to recognize and sometimes much more challenging. Beach marks, ratchet marks and smooth surfaces are all keys that may point to the crack having propagated over time in fatigue due to cyclic stresses rather than due to a single, large load cycle.

What is distinctive about the surface appearance of a fatigue fracture?

The surface is characterized by the presence of multiple and superficial nucleation sites, radial ratchet marks in the crack propagation region, and a rough feature in the final fracture zone due to overloading.

What are fatigue cracks?

Fatigue cracking is one of the primary damage mechanisms of structural components. Fatigue cracking results from cyclic stresses that are below the ultimate tensile stress, or even the yield stress of the material. This leaves tiny steps in the surface that serve as stress risers where tiny cracks can initiate.

What are the characteristics of fatigue failure?

Microscopic characteristics of fatigue failure

• On very hard or very soft metals.
• Artifacts caused by rubbing or other postfracture damage may produce parallel ridges that resemble striations. Certain lamellar microstructures in metals, resemble fatigue striations.

What is the nature of fatigue fracture?

Fatigue fracture is the result of repetitive cyclic short-time stress or tensile stress, or deformation well below the tensile or flexural strength of the material (78). Fatigue is thus an important and complex phenomenon as it accounts for 80% of all structural failures (79).

Where does fatigue crack start?

Fatigue cracks in typical aircraft alloys usually start from statistically distributed small material discontinuities or surface defects. Over a low stress concentration region, the largest crack that dictates fatigue life is often the fastest growing crack that starts from one of the worst discontinuities.

Where do fatigue cracks occur?

Fatigue cracks of metals usually initiate from the surface of a component, where fatigue damage initiates as shear cracks on crystallographic slip planes. The surface shows the slip planes as intrusions and extrusions.

What is surface fatigue wear?

Surface fatigue, or fatigue wear, is the cracking and subsequent pitting of surfaces subjected to alternating stresses during rolling contact or the stresses from combined rolling and sliding. Fatigue wear is produced when the wear particles are detached by cyclic crack growth of microcracks on the surface.

What are the properties of fatigue?

In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.

What is a ratchet mark?

Lines on a fatigue fracture surface that result from the intersection and connection of fatigue fractures propagating from multiple origins. Ratchet marks are parallel to the overall direction of crack propagation and are visible to the unaided eye or at low magnification.

What features distinguish fatigue fracture surfaces from monotonic fracture surfaces?

12.3. 2 Fatigue/cyclic loading. Fatigue failure differs from monotonic fracture mainly in the early crack propagation phase and is more likely to exhibit multiple crack nucleation sites. Crazing, as for the monotonic case, is the dominant mechanism of crack propagation.