Biomedical Engineering Reference
In-Depth Information
Fig. 7.5
Graph of tensile stress versus strain. Tension is applied to a human humerus. The elastic
limit and the tensile strength are indicated
The forces that act on the bones are classified as longitudinal that include
compression and tension and transversal that are the shear forces. Tensile forces are
applied, for example, when traction is exerted on a leg in an equipment of physio-
therapy and compressive forces, when a person suffers a fall from a certain height or
when he or she jumps with a parachute, for example, and reaches the ground.
In accidents, involving a collision between two cars, the body of the driver goes
forward instantaneously with a small inclination and strikes the steering wheel. In
this movement, some vertebrae are compressed anteriorly and tensioned posteriorly
which causes horizontal fracture of vertebra, according to information given by the
orthopedist, Dr. Alexandre Sadao Iutaka,
2
a specialist in the spinal column.
Figure
7.5
shows the curve of the tensile stress applied to the human humerus
against strain. Similar curves are obtained for the radius and the ulna, with the main
difference in the value of the tensile strength.
Young's modulus of the human femur under traction is 1.7 times larger than that
under compression. This means that for the same value of stress, if the applied force
is of compression, the deformation produced is 1.7 times larger than that caused by
tension. From the tables, we verify that the compression strength of the human
femur is 1.38 times larger than the tensile strength, or in other words, the tibia or the
femur is more susceptible to fracture under tension than under compression. Note
that the compressive strength of the human femur (16.7
10
7
Pa) is about 1/3 of
2
We acknowledge the information given by Dr. Iutaka, orthopedist of Hospital das Cl´nicas of
Faculdade de Medicina of S. Paulo University.
