Materials can fail when the forces acting upon this material are greater than its strength. The fractures that result can occur in various shapes and sizes.
A crucial distinction in fracture behavior
is found between ductile and brittle fractures: the main difference is presence/absence
of plastic deformation: when a fracture is accompanied by significant
plastic deformation, it is considered a ductile overload. Without plastic
deformation, it is a brittle fracture.
Brittle Fractures
Brittle fractures are often (though not always)
the result of abrupt changes in force. Possible examples include sudden
pressure fluctuations in a system, a collision, or impact loading. Such
fractures are difficult to predict.
Ductile Fractures
Ductile fractures occur more slowly as
the material will first deform before breaking. Such shape changes are
therefore a warning from the material that it is being overloaded.
Other Factors Influencing
Failure
Failures can also occur when the forces
remain much smaller than the tensile strength of a material. A typical
example of this is fatigue. This is a damage mechanism that occurs under
the influence of alternating forces. Some examples are the forces acting on a
rotating shaft, or vibrations caused by a pump, but also stresses due to
temperature variations.
Analysis of Fractures
A lot of information about the cause of a
fracture can be gathered from the shape of a fracture. A fracture
surface inclined at 45° to the axis direction, for example, indicates a torsional
load. Other forms of force application cause a flat and/or symmetrical
fracture surface, which also provides information about the loading conditions.
In addition to the shape of the fracture
surface, there are other characteristics from which the type of fracture can be
deducted. Such a fracture surface examination, combined with a metallographic
examination, allows the cause of the fracture to be determined. This is
important to know what measures can be taken to avoid future failures: a
brittle impact fracture or a fatigue failure may seem very similar at first
glance but obviously require different preventive measures. It can often also
be determined whether the fracture was caused by a local material defect or by
incorrect material use.
Corrosion can also influence the fracture behavior of
materials. However, exploring this aspect fully merits its own dedicated
discussion. Stay tuned for a future article where we'll dive deeper into this
fascinating topic.
Want to learn more about failure analysis? Here we take a look at one of the most important tools in our failure lab: the SEM-EDX.
