Biomedical Engineering Reference
In-Depth Information
Fig. 1.7
Induction of shear
strain by shear stress
θ
The negative ratio of lateral strain to tensile strain at a given stress is called the
Poisson ratio
. Its value is
0.3 for most metals (steel, Ti, Cu, ...) in the elastic
range, for glass
0.5.
Shear strain
: When the sample is not subjected to tensile or compressive stresses
but to shear stress, the deformation is defined as shear strain. If a given shape, say,
a cube shears sideways by an amount
w
asshowninFig.
1.7
, the shear strain is:
0.2 and for rubber, an upper extreme,
w
l
D
D
'
tan
(1.6)
If the deformation is small, tan
.
Stress
: ,[ML
1
T
2
], Pa, kPa, MPa. When a force (F ) is applied to a cylinder of
material in tension or compression, it is said to be in a state of stress. It is calculated
by the force applied to the cylinder divided by the area (A) of a section perpendicular
to the direction of the force at any moment of the deformation. Why is this said?
Where the stress-strain curve starts leveling off (ductility zone), one is seeing a
reduction of section somewhere halfway between both ends, the so-called
necking
as shown in Fig.
1.6
. If the actual section at any moment of the curve is not taken
into account, the stress is called
engineering stress
. In that case the curve will go
through a maximum before fracture.
Thermal expansion coefficient
: ˛,[T
1
],
ı
C
1
(usually
'
10
6
). The linear expan-
sion coefficient is the ratio of the change of length per degree C to the length. It is
only approximately linear in function of temperature range. The volume expansion
coefficient is roughly three times the linear coefficient.
Yield strength
:YS,[ML
1
T
2
], Pa, kPa, MPa, Nmm
2
. Tensile yield is a given
value of stress where the stress-strain curve departs from linearity. From this value
on a permanent set or deformation remains after removal of stress. For practical
reasons, the stress inducing a plastic deformation of 0.2% is generally taken as the
yield strength.
Ultimate tensile strength
:UTS,[ML
1
T
2
], Pa, kPa, MPa, Nmm
2
.Itisthe
highest endurable stress at which the test specimen begins to neck in tensile or to
fracture (mainly for brittle materials) in compression.
Viscosity (dynamic)
: or ,[ML
1
T
1
], Pa s or kg/s m. It is a measure of the
resistance of a fluid which is being deformed by either shear stress or extensional
stress (see Sect.
2.3.1
).
