Fatigue Failure
Griffith criteria
Fracture toughness
How plane crash "COMET" influenced
the propagation of cracks in plastic materials, as a combination of the release of elastic deformation energy and the increase of surface energy as a result of the appearance of new surfaces on the faces of the crack, from a critical tension.
two criteria
A) the propagation of cracks in plastic materials, as a combination of the release of elastic deformation energy and the increase of surface energy as a result of the appearance of new surfaces on the faces of the crack, from a critical tension.
A relationship is obtained between the fracture tension and the crack size that has been repeatedly tested in fragile materials.
Due to a material fatigue problem, it was revealed that there were imperfections in the airplane structures and everything happened since the fuselage of the plane was improperly manufactured, these imperfections were concentrators of efforts that at the time of being twelve thousand meters The pressure increased in height, causing the fuselage of the plane to explode.
Fatigue failure typically occur in three stages:
Identification of failure due to fatigue
Measures the ability of a material containing a flaw to withstand an applied load. Note that this does not require a high strain rate (impact). A typical fracture toughness test may be performed by applying a tensile stress to a specimen prepared with a flow of known size and geometry. Fracture toughness depends on the thickness of the sample: As thickness increases, fracture toughness (Kc) decreases to a constant value.
Kc = K required for a crack to propagate
A tiny crack initiates or nucleates (at or near the surface) often at a time well after loading begins.
The crack gradually propagates as the load continues to cycle.
A sudden fracture of the material occurs when the remaining cross section of the material is too small to support the applied load.
Fatigue failures are often easily to identify. The fracture surface is typically smooth. The surface becomes rougher as the original crack increases in size and may be fibrous during final crack propagation. Microscopic and macroscopic examination (cross section) reveals a fracture surface including a beach mark pattern and striations.
Microscopic
Macroscopic
(a) Beach marks – Are normally formed when the load is changed during service or when the loading is intermittent.
(b) Striations – Show the position of the crack tip after each cycle.
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How to prevent fatigue fracture
• Alternating or cyclic voltage is another element to consider.
• Surface hardening through carburation and nitriding processes
• Improving the surface finish by polishing, to avoid small scratches or grooves that appear on the surface of the piece by cutting action.
• Consider The design also has a large influence on mechanical fatigue breakage.
• Consider the type of geometry of the piece, it influences the speed of propagation of the cracks
• Consider the variation of sections of a mechanical element where a stress concentration will be generated.
fissure growth factor
There are several factors involved in a fatigue breakage process apart from the stresses applied:
Se = Ka x Kb x Kc x Kd x Ke x S´e
Se: Limit of fatigue resistance at the critical location of a machine part in the geometry and condition of use
S´e: Fatigue resistance limit on rotating beam
Ke: Various effects modification factor
Kd: Temperature modification factor
Kc: Load modification factor
Kb: Size modification factor
Ka: surface condition modification factor