Biomedical Engineering Reference
In-Depth Information
s
for second; the first figure refers to the SI unit); definition.
The dimensions are use-
ful to test the consistency of the units in a result of calculation through an equation
or to define the units after deduction of an equation. Test specimens for the determi-
nation of mechanical properties are normally machined to standardized shapes and
by standardized procedures. They can be found in national and international stan-
dards for testing materials listed, e.g., by Helsen and Breme [
35
, pp. 67-71]. The
main units used and conversion factors in the text are summarized in Tables A3 and
A4 (Appendix
A
).
Creep
: , [-], %. Creep is a slow continuous deformation or strain under constant
stress. It is function of stress, temperature and time. It only represents a problem
for metals and ceramics near their melting or softening temperature. For polymers,
however, is it sizable at low temperature (<200
ı
C)andassuchamajordesign
parameter. Glass transition temperature is a criterion for creep resistance. A spe-
cial kind of creep to mention is viscous flow and, it is, like diffusion, an exponential
function of temperature and a linear function of stress:
d
dt
D
Ce
RT
(1.3)
with t time, C a constant for a given polymer, Q the activation energy for vis-
cous flow, R the universal gas constant and T the temperature in
ı
K. C and Q are
supplied by producers.
Density
: %,[ML
3
], kg m
3
or with the same numerical value g dm
3
;kgdm
3
,
gcm
3
, g mL. It is a property that depends on the way atoms or molecules are
spatially organized. It is measured as mass per unit volume; sometimes expressed
as multiples of the density of a standard material (usually water at 4
ı
C).
Ductility
: A, [-], %. Ductility or elongation at fracture means strain at fracture,
i.e., the strain at the high end of the stress-strain curve in Fig.
1.6
. The reduction in
area of the specimen at that point is a measure of ductility. It is the ability of a mate-
rial to undergo large plastic deformation before it is breaking. In some instances, in
addition to tensile tests, the reduction in area in the necking zone is given.
Elasticity modulus (Young's modulus)
: E,[ML
1
T
2
], Pa, kPa (kilo-), MPa
(Mega-), GPa (Giga-). The linear part of the curve below the elastic yield point
is described by the famous law of the English seventeenth century scientist Robert
Hooke (1635-1703):
ut tensio, sic vis
('as the extension, so the force', published
in 1676 in anagram form
ceiiinossttuuv
). The modulus of elasticity is determined
by measuring the slope of the stress-strain curve, where strain is effectively pro-
portional to stress. Measurement of sound velocity or vibration frequency are other
methods to determine E.
Young's modulus is the macroscopic translation of the magnitude of interatomic
forces and the packing of atoms (number of atoms per unit volume) and as such,
it represents a fundamental physical property of matter. Atoms are held together
either by strong primary bonds, ionic (Na
C
Cl
), covalent (as in diamond or in
chains of carbon in polymers: -C-C-) or metallic (Fe-Fe), or by secondary bonds,
