2. • Fracture Mechanics is the field of mechanics concerned with the
study of the propagation of cracks in materials. It uses methods of
analytical solid mechanics to calculate the driving force on a crack and
those of experimental solid mechanics to characterize the material's
resistance to fracture.
4. • There are three ways of applying a force to enable a crack to
propagate:
• Mode I – Opening mode (a tensile stress normal to the plane of the
crack),
• Mode II – Sliding mode (a shear stress acting parallel to the plane of
the crack and perpendicular to the crack front), and
• Mode III – Tearing mode (a shear stress acting parallel to the plane of
the crack and parallel to the crack front).
6. 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: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.
• 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.
• 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.
7. • 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:
• 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.
• 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.
• 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.
• 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.
10. • Brittle Fracture is the sudden, very rapid cracking of equipment under
stress where the material exhibited little or no evidence of ductility or
plastic degradation before the fracture occurs. Unlike most other tensile
failures, where the material plastically strains under overload conditions
and becomes thinner until the point of rupture, when a piece of equipment
suffers a brittle fracture, there is no thinning or necking down. Rather, this
damage mechanism often causes cracking without warning, sometimes
fracturing equipment into many pieces.
• Brittle fracture is often caused by low temperatures. If the steel
temperature is at or below its brittle-to-ductile transition temperature,
then it will be susceptible to brittle fracture. Combine this with a critical
sized flaw and high stress on that flaw (either applied or residual), and then
you are likely to experience a brittle fracture.
