Human Body Models: BOSS-Models - Preventive Biomechanics - page 204

Fig. 5.30 In vivo gluteal tissue compound force-relaxation curves a force-time, b force-
displacement obtained at a constant ramp speed of 0.68 mms -1 and various ramp displacements,
and c normalized (by the time at peak force and the peak force) filtered experimental force-time
responses from experimental data provided in Fig. 5.30 a
5.2.4.2 Relaxation Experiments
The employed test devices, protocols including indentation locations and MR-
settings of relaxation testing are similar to the protocols applied for long-term
material parameter evaluation as described in Sect. 5.2.2 .
Stepwise Indentation Procedure: In the first step, long-term elastic tissue
material parameters were established analogue to the procedure described in Sects.
5.2.2 and 5.2.3 . Stepwise and cyclic tissue indentation with holding periods was
performed at a constant loading and unloading speed to separate the elastic from
the inelastic tissue material properties.
Relaxation Experiments: After long-term material parameter evaluation,
displacement-controlled ramp-and-hold indentation testing at the buttocks was
performed. Using the MR-compatible loading device (cf. Fig. 5.13 a), separate
tissue indentation with a maximum ramp displacement of 10-, 20-, 30-, 40 mm at a
constant ramp speed of 0 : 68 mm s 1 (equivalent to an (engineering) strain rate of
0 : 75 % s 1 along the indenter axis) followed by a hold phase of 180 s was per-
formed. Corresponding tissue deformation after each ramp phase was MR-scanned,
Fig. 5.29 . Indenter force and displacement were recorded with a time resolution of
0.05 s. The recorded force-time relation at 40 mm ramp displacement together
with the MR-scanned individually deformed tissue geometries after the ramps
served as constraints in the parameter optimization process.
Force-Time-Data: Between the indentation tests, a 20 min pause was introduced
to allow for tissue regeneration (shown to be sufficient since the single ramp phases of
all tests coincided, cf. Fig. 5.30 a). Resulting tissue force-time and force-displacement
responses are depicted in Fig. 5.30 a and b. Significant force-relaxation occurred
during ramp displacement and at constant indenter displacement after the ramp,