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  1. Ductile fracture is the most common failure mode in aerospace metal alloys and polymers. A ductile crack in a metal usually starts at an existing flaw, such as a brittle inclusion within a grain, a precipitate at a grain boundary, or a void. The stress ahead of the crack is not distributed evenly. The stress in the region immediately ahead of the crack tip is much higher than the nominal stress that is applied.
  3. Ductile fractures have many characteristics that shows the differences between itself and other fractures. However, they do share some of the same characteristics as brittle fractures. Here are just some of the ductile fracture characteristics below;
  4. • There is considerable gross permanent or plastic deformation in the region of ductile fracture. In many cases, this may be present only in the final rupture region of a fracture that may have originated with a fatigue or brittle fracture.
  5. • The surface of a ductile fracture is not necessarily related to the direction of the principal tensile stress, as it is in a brittle fracture.
  6. • The characteristic appearance of the surface of a ductile fracture is dull and fibrous. This is caused by deformation on the fracture surface, which will be discussed in the section on the microstructural aspects of ductile fracture.
  8. Ductile failure is one of the most key concepts in material engineering. A ductile failure is clear as a body going through separation due to imposed stresses. All engineering materials undergo only two types of fracture modes, and they are ductile and brittle fracture.
  9. Ductile materials display massive amounts of plastic buckling or deformation in comparison to brittle materials. In ductile failure, the crack grows at a slow pace and is accompanied with a great deal of plastic deformation. In this case, the crack does not expand except when high levels of stress are present.
  15. Brittle Fracture
  16. A brittle fracture is the fracture of a metallic object or other material without significant plastic deformation. It is a break in a brittle piece of metal that failed as stress exceeded solidity. The brittle fracture of normally ductile steels occurs primarily in big structures such as:
  17. • Box beams
  18. • Bridges
  19. • Pressure vessels
  20. • Pipes
  21. • Ships
  22. • Tanks
  23. • Other restrained structure
  24. Brittle fractures that happen in service are invariably started by defects that are initially present in the manufactured product, or by defects that develop during service.
  26. In brittle materials, fractures can happen by cleavage as the result of a tensile stress acting normal to crystallographic planes with low bonding. In shapeless solids, the lack of a crystalline structure results in a conchoidal fracture. With results in cracks proceeding normal to the applied tension.
  28. Brittle fractures show either trans granular or intergranular fractures. This depends upon whether the grain limits are stronger or weaker than the grains:
  29. • Trans granular fracture - The fracture travels through the grain of the material. Cracks choose the path of least resistance.
  30. • Intergranular fracture - The crack travels along the grain boundaries, and not through the actual grains. This usually occurs when the phase in the grain boundary is weak and brittle.
  32. A brittle fracture is a breakage of a material into visible parts, from a clean break. It is characterized by quick crack propagation with low energy release and without major plastic deformation. The fracture may have a bright gritty look to it. The fractures are generally of the flat type and chevron patterns may be present.
  38. Fatigue
  39. Fatigue is the progressive structural damage that will happen when a material is subjected to cyclic loading. If the local stresses are high enough, this leads to the initiation of a crack, the growth of the crack and finally fracture. Fatigue can worsen structural damage in the presence of corrosive environments.
  41. Many dramatic and severe failures happen by fatigue, and it is essential that structural engineers include fatigue into designs. Some materials like steel and titanium alloys exhibit a theoretical fatigue limit below which continued loading does not lead to fatigue failure.
  43. Fatigue is the weakening of a material. It is an increasing effect causing a material to fail after repeated submissions of stress, none of which beats the ultimate tensile strength. The word fatigue is based on the idea that a material becomes worn out and fails at a stress level below the average strength of the material.
  45. Corrosion fatigue is the result of the joint action of an alternating or cycling stresses and a corrosive environment. The fatigue process is thought to cause separation of the protective passive film, upon which corrosion is faster. If the metal is at the same time exposed to a corrosive environment, the failure can take place at even lower loads and after a shorter time.
  47. Fatigue failure occurs in three stages:
  48. • Crack initiation
  49. • Slow, stable crack growth
  50. • Rapid fracture
  52. Most engineering let downs are triggered by fatigue. The type of fatigue of most concern in circuit boards, gasoline, diesel, gas turbine engines and many industrial submissions is thermal fatigue. Thermal fatigue can rise from thermal stresses produced by cyclic changes in temperature.
  57. Creep
  58. Creep refers to a material science idea that describes the likelihood of a material to deform under an applied force of mechanical stress. Creep may also be identified as material creep or cold flow.
  60. Creep happens due to prolonged exposure to applicable forces below the yield strength of the receiving metal. It can be observed to have greater impact when a metal is visible to increased levels of heat.
  62. Creep is a kind of deformation that is vital and experienced in a wide range of industries ranging from nuclear power plants, jet engines and even heat exchangers.
  63. There are three main stages of creep:
  64. • Primary creep - Starts at an increased rate and slows with time due to material hardening.
  65. • Secondary creep - Has a relatively steady rate.`
  66. • Tertiary creep - Has an accelerated rate and ends when the material breaks.
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