PSI - Issue 2_B

T. Šarac et al. / Procedia Structural Integrity 2 (2016) 2405–2414

2409

Author name / Structural Integrity Procedia 00 (2016) 000–000

5

6

1400Gy/h, 70C ° , 160kGy

non-aged

4

450Gy/h, 40C ° , 388kGy

1400Gy/h, 40C ° , 600kGy

2

1400Gy/h, 70C ° , 600kGy

Stress (MPa)

0

0

200 400 600 800 1000 1200

Strain (%)

Fig. 2. The engineering stress versus engineering strain of neat EPDM polymer ( Nordel).

Fig. 3. Necking behaviour observed in neat polymer aged to low dose (white samples). The total recovery in industrial EPDM (black samples).

hibit macroscopic plastic deformation / necking. On the contrary, at high irradiation dose both NORDEL and industrial EPDM brake in elastic regaion of the stress strain curve. The e ff ects of dose rate and temperature are analyzed by observing the change of ultimate tensile versus total dose. In Figure 4, the ultimate tensile stress data for industrial EPDM are presented. By increasing the dose, the ultimate tensile stress first increases and then decreases, exhibiting a local maximum. According to the literature, this maximum corresponds to the transition (cross-over) between the regime where cross-linking dominates, and the regime where chain scission dominates the degradation mechanisms Rivaton et al. (2005); Gueguen et al. (1994.); Cellete aet al. (2004.); Seguchi et al. (1983.). The ultimate tensile stress of the industrial EPDM samples, aged at di ff erent temperatures at the medium dose rates is shown in Fig.4a). By increasing the ageing temperature the cross linking region is gradually suppressed, and the local maximum is shifted to a lower dose. Dose rate e ff ect, or the shift of the cross-over as a function of the dose is also analyzed at low Figure 4b) and high ageing temperatures Figure 4c). For the samples aged at low temperatures, e.g. in the range of 25-40 0 C , an increase of the dose rate leads to the slight gradual shift of the cross-over to higher dose. With respect of the samples aged at high temperatures, e.g. in the range of 75-85 0 C , the dose rate e ff ect is observed to be much stronger. The shift of cross-over is not more evidenced at low dose rate of about 106 kGy / h. Generally, one ca say that cross-linking region (increase of the ultimate stress) is promoted at high dose rates.