PSI - Issue 12

Massimiliano Avalle et al. / Procedia Structural Integrity 12 (2018) 19–31 Massimiliano Avalle / Structural Integrity Procedia 00 (2018) 000 – 000

30 12

(a) Sample CR_EPS1_003

(b) Sample CR_XPS1_001

(c) Sample CR_PUR2_011

(d) Sample CR_EPP70_01

Fig. 7. Unloading curves with the four materials: (a) Expanded polystyrene (EPS); (b) Extruded polystyrene (XPS); (c) Expanded polyurethane (PUR); (d) Expanded polypropylene (EPP).

Table 4. Identified parameters of the unloading curves of Eqs. (7)-(8).

Property

Expanded polystyrene (EPS)

Extruded polystyrene (XPS)

Expanded polyurethane (PUR)

Expanded polypropylene (EPP)

A p,u (MPa) B p,u (MPa) A s,u (MPa) B s,u (MPa) A m,u (MPa) B m,u (MPa)

0.034 ± 0.008

0.11 ± 0.0454 0.23 ± 0.042 0.631 ± 0.3019 − 0.246 ± 0.2808 27.409 ± 4.6976 − 11.534 ± 4.3682

0.104 ± 0.0241 0.11 ± 0.024 0.062 ± 0.1008 0.007 ± 0.0528 21.204 ± 4.672 − 8.546 ± 4.1787

0.064 0.345 0.223

0.1 ± 0.006

0.222 ± 0.1149 − 0.06 ± 0.0947 26.842 ± 6.094 − 16.012 ± 5.0783

− 0.023 34.725 − 15.555

In practice, the trend is similar in almost all the four materials: the plateau stress in unloading clearly decreases with increasing initial strain: for the expanded polystyrene the increase is even more important than for the other materials. The slope instead is generally decreasing with increasing initial strain with the exception of the expanded polyurethane: it must be noted, however, that the slope is very small for this material, like in loading. The exponent