Biomedical Engineering Reference
In-Depth Information
Tabl e 2. 2
Range of
E-moduli per bond type
[36, p. 60]
Bond type
Example
E(GPa)
Covalent
-C-C-
200-1,000
Metallic
Fe-Fe
60-300
Ionic
Li-Cl
32-96
Hydrogen bond
H
2
O-H-OH
8-12
Van der Waals
Between polymer chains
2-4
as: point, line (1D) or surface (2D) defects.
6
They have fundamental implica-
tions for the mechanical behavior of a metal. Some properties of single crystals are
highly anisotropic but a polycrystalline sample, consisting of many randomly orga-
nized grains, will exhibit isotropic properties. When, however, those microcrystals
exhibit some preferred orientation, the properties will become anisotropic and such
a microstructure is called
texture
. Texture can be the result of production processes
as solidification, annealing by thermal treatment or deformation by, for example,
drawing. It is a kind of structure or ordering at a higher length scale than the atomic
length scale, which dominates crystal structure.
For a theoretical treatment of structure the topic by Allen and Thomas can be
recommended [38], for thermomechanical treatment a new comprehensive topic on
the subject by Verlinden [53].
Both defects in crystals and texture are responsible for the particular properties
of metal alloys and for all other materials as well. Let us return to the stress-strain
curve shown in Chap. 1. In its
engineering form
, it is slightly different and mostly
the stress passes through a maximum before fracture. While in Fig. 1.6, the applied
force was divided at each point by the effective section of the specimen, i.e., the
true
stress
, the applied force is in the engineering convention only divided by the initial
section of the specimen, the so-called
nominal stress
,
n
F
A
0
D
plotted against
the nominal strain
n
l. No permanent strain remains after applying stresses
within the elastic region: that means that the material follows simply Hooke's law
for the ideal spring:
D
E
n
. The upper boundary of the elastic region is called
the
elastic limit
. The 0.2% proof stress is commonly used as yield strength (YS)
for materials yielding gradually (without distinct yield point). Any stress exceed-
ing this value induces permanent strain. Deformation beyond the maximum of the
(engineering) curve leads to fracture at the ultimate tensile strength (
UTS
). The
strain after fracture
f
is calculated by the sum of the lengths of the broken pieces
minus l
0
divided by l
0
. These parameters are characterizing strength, are extensively
tabulated and are experimentally measured by tensile/compression testing machines,
standard equipment in all engineering laboratories.
D
called perfect, although without any geometrically definable structure. When glasses are discussed
in later chapters, we will come back on this issue; perfection in the geometric sense is not meeting
real-life requirements. This statement can be quoted as one of the
benefits of chaos
.
6
Defect
has a pejorative meaning and therefore substituted by some authors by imperfection; see
former footnote.
