Biomedical Engineering Reference
In-Depth Information
Tensile
Shear
Fracture
stress
σ
τ
E high
G high
E low
G low
ε
γ
Fig. 3.1.2-6 Stress versus strain for elastic solids.
average, these aggregates exhibit isotropic behavior at the
macroscopic level, and values of E and G are highly re-
producible for all specimens of a givenmetal, alloy, or ceramic.
On the other hand, many polymeric materials and most
tissue samples are anisotropic (not the same in all di-
rections) even at the macroscopic level. Bone, ligament,
and sutures are all stronger and stiffer in the fiber (longi-
tudinal) direction than they are in the transverse direction.
For such materials, more than two elastic constants are
required to relate stress and strain properties.
than the specimen to be tested ( Fig. 3.1.2-7 ). One cross-
bar or cross-head is moved up and down by a screw or
a hydraulic piston. Jaws that provide attachment to the
specimen are connected to the frame and to the movable
cross-head. In addition, a load cell to monitor the force
being applied is placed in series with the specimen. The
load cell functions somewhat like a very stiff spring scale
to measure the applied loads.
Tensile specimens usually have a reduced gage section
over which strains are measured. For a valid determination
of fracture properties, failure must also occur in this
reduced section and not in the grips. For compression
testing, the direction of cross-head movement is reversed
and cylindrical or prismatic specimens are simply squeezed
between flat anvils. Standardized specimens and pro-
cedures should be used for all mechanical testing to ensure
Mechanical testing
To conduct controlled load-deflection (stress-strain)
tests, a load frame is used that is much stiffer and stronger
Table 3.1.2-2 Mechanical properties of some implant materials and tissues
Elastic modulus
(GPa)
Yield strength
(MPa)
Tensile strength
(MPa)
Elongation
to failure (%)
Al 2 O 3
350
-
1000 to 10,000
0
CoCr Alloy a
225
525
735
10
316 S.S. b
240 (800) c
600 (1000) c
55 (20) c
210
Ti-6Al-4V
120
830
900
18
Bone (cortical)
15 to 30
30 to 70
70 to 150
0-8
PMMA
3.0
-
35 to 50
0.5
Polyethylene d
0.6-1.8
-
23 to 40
200-400
e
Cartilage
-
7 to 15
20
a 28% Cr, 2% Ni, 7% Mo, 0.3% C (max), Co balance.
b
Stainless steel, 18% Cr, 14% Ni, 2 to 4% Mo, 0.03 C (max), Fe balance.
c
Values in parentheses are for the cold-worked state.
d
High density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (UHMWPE).
e
Strongly viscoelastic.
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