Biomedical Engineering Reference
In-Depth Information
Van der Waals forces (gravity attraction) or hydrogen bridging (between proton
donor and a proton acceptor in polymers for example). The values of E-moduli
may be as low as 0.1 GPa for low-density polyethylene or as high as 1,000 GPa for
diamond, which has the highest modulus of any material.
Fatigue strength
:
f
,[ML
1
T
2
], Pa, kPa, MPa. Materials may fail at stresses
below the UTS or YS (see below) by exposure to repeated stress cycles. ASTM
defines fatigue strength as the limiting value of stress at which failure occurs as N
f
,
the number of stress cycles, becomes very large. It is the stress level for steel and
the like below which fatigue failure never occurs (<endurance limit). More ductile
materials such as aluminum do not have such a distinct limit and these are tested
on fatigue by subjecting to given stress amplitudes and up to a fixed number of
cycles, usually N
10
7
. Fatigue strength is an extremely important engineering
D
and design property.
Fracture toughness
: K
Ic
,[ML
3=2
T
2
], N m
3=2
or k- or MPa m
1=2
. Fracture
toughness is a property which describes the ability of a material containing a crack
to resist fracture. It is an important property for all design applications. The subscript
“Ic” denotes
mode I
crack opening under a normal tensile stress perpendicular to
the crack. Numerically it is always smaller than the yield strength. For a condensed
treatment of this property, we may refer for further reading to Ashby and Jones [
36
,
pp. 131-139].
Friction coefficient
: , [-]. The force that will just cause two materials to slide
over each other, is called the static force F
s
, and is proportional to the force acting
normal to the contact surface; once the sliding started, the frictional force decreases
slightly but remains proportional to the normal force acting on the sliding surfaces:
F
s
D
s
P
and
F
k
D
k
P;
(1.4)
where is the proportionality or friction coefficient, respectively, for the static and
the kinetic case. Numerical values are difficult to give because they depend on the
material, surface finishing and lubrication.
Rigidity modulus
: see Shear Modulus.
Shear modulus
: G,[ML
1
T
2
], MPa, GPa. While Young's modulus describes
the response of a material to linear strain, the response to shear strain is described
by the shear modulus.
Strain
: for nominal strain or for shear strain, [-], fraction or %. Materials
respond to stress by straining. The degree of strain depends on the elasticity modu-
lus: a stiff material strains less than a compliant material under a given stress. The
change of length l and radius r of a cylinder of a material is measured as function
of the applied stress. Strain is calculated by taking the ratio of change in length to
original length l
0
and of change in radius to original radius r
0
, called, respectively,
tensile strain or lateral strain:
l
l
0
r
r
0
tensile
D
and
lateral
D
:
(1.5)
l
0
r
0
