Biomedical Engineering Reference
In-Depth Information
Compression
Tension
Bending
Ends
free
Shear
Torsion
Ends
constrained
FIGUre 2.17
fracture initiation patterns.
In torsion, the failure is usually a shear failure that follows a spiral
path. Such a spiral pattern is absolutely diagnostic of torsional failure,
even in combined loading situations. However, there may be an addi-
tional small vertical tensile failure component.
These are ideal patterns. The actual details of crack initiation and
propagation are complex, depending on the addition of stresses, on the
relationship between ultimate tensile and shear strengths for the mate-
rial, and on the presence of internal structural features and defects.
(These issues will be dealt with at greater length during the discussion of
fracture of bone [Chapter 5].)
Practical aspects of material deformation
Ideality
The discussion of deformation so far has dealt in ideal terms. In the real
world, a number of complicating factors arise that significantly affect
the results obtained, even in apparently simple circumstances. There are
three primary sources of these complications.
1. The materials discussed have been assumed to be uniform and
homogeneous internally. In fact, real materials contain many
structural and compositional inhomogeneities.
2. It was also assumed that the surfaces of materials have the same
properties as their interiors. This is also not so, leading to some
interesting difficulties.
3. Finally, it was assumed that forces applied to an object lead to uni-
form internal stress distributions, in the presence of homogeneous
material. This is not the case, even for the apparently simple cases
of compressive and tensile deformation.
These departures from ideality produce many practical problems in
the examination and analysis of the mechanical properties of material.
However, one of these problems plagues us in day-to-day life: the con-
centration of stress by inhomogeneities, leading to premature and unex-
pected failure.
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