Supports - Preventive Biomechanics - page 158

Table 4.1 Specification of
the investigated foam
materials (CLD: compression
load deflection)

HR-Foam VE-Foam

Brand name RL 35034 apricot,

Richlux-comfort

Visco aerial,
aerial VE

Manufacturer Carpenter Dunlopillo

CLD-value 3.4 kPa (23C) 1.6 kPa (23C)

32 kg/m 3 56 kg/m 3 Density

Retardation time 0.007 s 2.4 s

in general exhibit viscoelastic behaviour, and that the term VE refers to the brand
name of the distributing company). Both foam types belong to the polyurethane
soft foam materials and are generally composed of polyetherpolyols and isocya-
nates. The mechanical properties of these foam materials are used, amongst others,
in Chap. 6 to simulate body-support interaction. Basic technical specifications of
both foam materials are provided in Table 4.1 .

Cyclic Preloading Process: Prior to the actual loading procedure the speci-
mens were preconditioned via the three deformation modes (1), (2) and (3) to
eliminate stress softening, a phenomenon referred to as the M ULLINS effect (Mullins
1969). Cyclic preloading was performed by compressing (in the case of (1) and
(2)) the foam specimen up to 64% to decrease the force response of the material at
the initial virgin state during the first load cycles at constant displacement
amplitude, to reach a stabilized response. The M ULLINS effect reflects material
damage incurred during previous loading, as depicted in Fig. 4.8 . After the
pre-loading phase and a following rest phase of 16 h, the actual loading process
with holding times begins.
In addition to the M ULLINS effect, from the force-displacement slope charac-
teristics in Fig. 4.8 b it can be observed that the particular foam material exhibits
time-dependent material properties.

Experiments with Holding Times: To determine the long-term time-inde-
pendent material response, stepwise loading and unloading cycles with holding
times were performed. This allowed relaxation of the non-equilibrium stress,
separating the elastic and inelastic material properties. (James and Green 1975)
and (Van Den Bogert and De Borst 1994) proposed this cyclic procedure to
characterize filled vulcanisates. It has also been applied by (Lion 1996) and
(Hartmann et al. 2003) for rubber-like materials and by (Ehlers 2003), and
(Schrodt et al. 2005), for cellular polymers.
The increments in loading and unloading were set to steps of 2 mm for 8% of
compression during the plate experiment and subsequently two steps of 4 mm until
64% compression, corresponding to a total indenter plate displacement of 32 mm
at an initial specimen height of 50 mm. The maximum extent of compression of
the test specimen was determined by the extent to which the foam was compressed
during physical buttock-support interaction. MR-imaging of buttock-support
interaction, when in the supine position, showed that maximum compressive
(nominal) strain did not exceed 60% at the gluteal region with a total body mass of