Fundamentals - Preventive Biomechanics - page 28

Fig. 3.6 Qualitative stress-
strain responses of human
soft tissue and soft polymer
foam
behaviours to a greater or lesser extent, depending on the loading situation. One or
the other characteristic may be neglected, depending on measurement accuracy.
Behaviour of Human Soft Tissue and Polymer Soft Foam. The mechanical
behaviour of human soft tissue and of soft polymeric foam (rubberlike) material
exhibit non-linear material behaviour. Biological soft tissue material displays a
J-shaped curve in the stress-strain diagram, whereby at increasing stress the
corresponding strain change decreases, cf. Fig. 3.6 . This stress-strain character-
istic applies for soft tissues such as fat or (transversally loaded passive) muscle
tissue (e.g. in the buttock and upper leg region) as well as for arteries. In addition,
the stress-strain behaviour of pathological arterial tissue, such as that encountered
in aneurysms, is characterized by a distinct moderate initial stress increase (almost
horizontal tangent) in the origin. Stress-strain characteristics of soft polymeric
foams and rubberlike materials, in contrast, exhibit S-shaped curves with an
inflection point and plateau where relatively small changes in load result in large
changes in strain., cf. Fig. 3.6 . Both materials are discussed in more detail in
Chaps. 4 and 5 .
Both stress-strain characteristics in Fig. 3.6 represent the state of equilibrium
elasticity (after relaxation) and can thus generally be classified as non-linear
elastic. The actual material behaviour of such materials however, generally is
viscoelastic and is characterized by hysteresis. The hysteresis slope is formed by
relaxation termination points which mark the corridor in which the equilibrium
material response is located, cf. Chaps. 4 and 5 .
3.2.2.2 Time-Dependent Behaviour
To describe the mechanical behavior of many materials, stress-strain character-
istics are not sufficient and further constitutional diagrams must be considered. The
mechanical behavior of polymers at room temperature is strongly time-dependent