Characteristics of the ductile failure

Ductile fractures have characteristics that are different from those of brittle fractures. However, it must be recognized that many fractures contain some of the characteristics of both types. Ductile fractures have the following characteristics:
  1. 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.
  2. 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.
  3. 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.

The classic example of a ductile fracture is a tensile specimen that has "necked down," or deformed to form a "wasp waist" prior to fracture. A typical fracture of this type is the socalled cup-and-cone fracture characteristic of ductile metals pulled in tension. It is instructive to study this type of fracture in some detail:
  1. The narrowing, or "necking," indicates that there has been extensive stretching, or elongation, of the grains of metal in the reduced area, particularly near the fracture itself.
  2. As pointed out earlier, shear stress dominates deformation and ductile fracture. In most cases, the 45° plane of maximum shear stress components is not obvious or readily observed.
  3. A tensile cup-and-cone fracture originates with many tiny internal fractures called "microvoids" near the center of the reduced area. These voids occur after the tensile strength has been attained and as the stress (or load on the test machine) is dropping toward the fracture stress.
  4. A ductile fracture starts near the center of the reduced section in tensile loading and then spreads outward toward the surface of the necked-down area. Before the fracture reaches the surface, however, it suddenly changes direction from generally transverse to about a 45° angle. It is this slant fracture - frequently called a "shear lip" - that forms the cup-and-cone shape characteristic of many tensile fractures of ductile metal. This slant fracture is useful for study of many fractures, for it represents the end of the fracture process at that location.Tensile fracture of a relatively thin section of a ductile metal may be entirely slant fracture. As the thickness increases, however, the percentage of slant fracture around the central origin area will decrease, sometimes resembling a "picture frame," on a relatively thick rectangular section.
NOTE: Shear is defined as "that type of force that causes or tends to cause two contiguous parts of the same body to slide relative to each other in a direction parallel to their plane of contact. "
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