Many elements of fracture have been used to describe and categorize the types of fractures encountered in the laboratory and in service. These elements include loading conditions, rate of crack growth, and macroscopic and microscopic appearance of fracture surfaces.
Failure analysis often find itself useful to classify fractures on a macroscopic scale as ductile fractures, brittle fractures, fatigue fractures and fractures resulting from the combined effects of stress and environment.
Many elements of fracture have been used to describe and categorize the types of
fractures encountered in the laboratory and in service. These elements include loading
conditions, rate of crack growth, and macroscopic and microscopic appearance of fracture
Failure analysis often find itself useful to classify fractures on a macroscopic scale
as ductile fractures, brittle fractures, fatigue fractures and fractures resulting
from the combined effects of stress and environment. The last group includes
stress-corrosion cracking and liquid-metal embrittlement, interstitial embrittlement,
corrosion fatigue and stress rupture.
Ductile fractures are characterized by tearing of metal accompanied by appreciable
gross plastic deformation and expenditure of considerable energy. Ductile tensile
fractures in most materials have a gray, fibrous appearance and are classified on
a macroscopic scale as either flat-face (square) or shear-face (slant) fractures.
Brittle fractures are characterized by rapid crack propagation with less expenditure
of energy than with ductile fractures and without appreciable gross plastic deformation.
Brittle tensile fractures have a bright granular appearance, are of the flat-face type,
and are produced under plain-strain conditions with little or no necking.
Fatigue fractures result from cyclic loading, and appear brittle on a macroscopic
scale. They are characterized by incremental propagation of cracks until the cross
section has been reduced to where it can no longer support the maximum applied load
and fast fracture ensues. In the fatigue fracture the fracture surface consists of
three distinct zones: a fairly smooth, multiple origin fatigue zone containing
"ratchet marks", a low cycle, rougher fatigue zone and a single cycle
final fracture zone.
When designing modern equipment to operate in severe environments, a designer is
confronted with many complex problems in selecting and evaluating materials, processing,
expected loadings and design stresses. Components in turbines, reactors, missiles,
submarines and cryogenic equipment may be subjected to such conditions as extremely
high or low temperature, corrosive liquids, high vacuum, progressive deterioration
due to radiation damage and surface wear. Materials selection must often be confined
to a small group of metals for outstanding resistance in one characteristic, such as
inertness to the environment in chemical processing equipment. However, many other
factors must be considered such as strength, toughness, fabricability and wear
resistance, before selection and design can be finalized.
Detailed analysis of failures encountered in developing a prototype (or in a service
component) is vital before appropriate changes can be made to assure a reliable
In general, service failures may arise from many causes. For mechanical equipment,
these causes might be broken down roughly into three categories, as follows:
Design inadequacies. Sharp corners or abnormal stress-raisers,
inadequate fasteners, wrong material or heat treatment, unforeseen conditions of
service, and lack of accurate stress analysis are included.
Processing and fabrication. About half of these may be due to
metallurgical factors such as quench cracks, improper heat treatment, forging or casting
defects, nonmetallic inclusions; the other half are due to misalignments, weld flaws,
improper machining or assembly, grinding cracks, cold straightening, and the like.
Environmental and service deterioration. These include overloads,
chemical attack, wear, corrosion, diffusion, and improper maintenance. A "failure"
usually occurs as:
- excessive deformation
The failure mechanism is usually a material failure that is controlled by the entire
environment and history.
Failures of Category I (design considerations) result from mistakes or incompetence of
the designer. Regarding failures due to flaws developed by processing or fabrication
(Category II), few, if any, standard tests cover all of the possible inherent defects
that may be induced by such operations as casting, forging, welding, machining,
grinding, heat treating, plating, chemical diffusion, or careless assembly operations.
Category III failures, caused by deterioration, can not be predicted by standard
tests that evaluate materials. In some instances unforeseen vibrations or overload
conditions may develop to cause failure. In others, service induced damage may develop
fatigue failure. Many service conditions involve extremely rapid rates of heating, or
include radiation damage, ablation, corrosion or various types of wear. Deterioration
during service in an aggressive environment needs to be given special consideration.
There are many types of surface disintegration, chemical activity or metal transfer
that affect stability of the component. These are influenced by the time, temperature
and dosage of the critical factors in the environment.