PSI - Issue 24

Michele Perrella et al. / Procedia Structural Integrity 24 (2019) 601–611 Author name / Structural Integrity Procedia 00 (2019) 000–000

607

7

The trends of total strains over time at the imposed temperature of 25°, 30°, 35° and 40°C are highlighted in Fig. 4. The plots of Fig. 4 show very high long-term strains at 40°C, reaching the value of about 14000 e after 5 days. The corresponding primary creep stage ends after about 1 day for the whole of tests. Lower strain levels are recorded at 20° and 25°C without a clear transition point between primary and secondary stages. Upper creep compliance limit values (end-life criterion) J i (for = 1, … ,3 ) were set in order to guarantee the proper resin performances and from the experimental J(t) curves the corresponding time values t i at the temperature T i were selected (Fig. 5). By means of the experimental points (ln(t i ), T i -1 ) a linear regression law was obtained, i.e. the Arrhenius curve (Fig. 6).

0.00E+00 2.00E-04 4.00E-04 6.00E-04 8.00E-04 1.00E-03 1.20E-03 1.40E-03 1.60E-03 1.80E-03 Creep compliance J [MPa -1 ] 25°C 30°C 35°C 40°C

1E+03

1E+04

1E+05

1E+06

log(time) [s]

Fig. 5. Creep compliance vs. log(time) curves at various temperatures.

Specifically, for the construction of the master curve at the reference temperature of 25° C, the experimental J(t) curves at the temperatures T 1 =40°C, T 2 =35°C and T 3 =30°C were considered. The Arrhenius graph is defined by an arithmetic scale of the inverse of temperature plotted on the abscissa axis and a logarithmic scale of time on the ordinate axis.

30

25

20

15

ln(t)

10

5

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

1/T [°C -1 ]

Fig. 6. Shift factor a T trend from Arrhenius curve.

By using the linear Arrhenius curve it is possible to extrapolate the value of the creep compliance or strain for any temperature T i . The shift factor, according to eq. (1) is: � = � � � (4)