PSI - Issue 24

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

605

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time-temperature superposition principle (TTSP).

Table 4. Boundary conditions of creep tests. Specimen Temperature [°C]

Load [N]

PC1 PC2 PC3 PC4 PC5 PC6 PC7 PC8

40 40 35 35 30 30 25 25

460 460 460 460 460 460 460 460

Displacement and strain measurements are acquired during the tests using both 2D digital image correlation (DIC) technique and strain gauge data acquisition system.

1 2 3

L 0 /3

L 0

4

Fig. 3. Scheme of ROIs’ location for displacement evaluation.

The Ncorr algorithm (Babler et al. (2015)), modified via an in house made Matlab routine, was used for DIC analysis. The resulting displacement vector for each ROI was found by evaluating the mean value of the computed data in the neighborhood of ROI center. Four small (3 mm diameter) circular regions of interest (ROI) as in Fig. 3 were considered for displacement measurements. The distance L 0 between the extreme ROIs was equal to 40 mm. 2.2. Time-temperature superposition principle Polymers’ viscoelastic properties are strongly dependent on temperature. The time dependent strain response under constant stress (creep) or the time dependent stress response under constant strain (relaxation) are very useful data for a proper composite structure design. Hence, prediction of long-term behavior of these structure plays a crucial task in structural strength reduction evaluation. Unfortunately, outcomes from conventional creep or relaxation experiments do not supply an accurate prediction for the composite structures lifetimes of 20 to 50 years. The time-temperature superposition principle (TTSP) provides a method for predicting the long-term behavior from short-term tests, i.e. accelerated characterization, of the so called “thermo-rheologically simple” materials. At first, this technique was applied to the creep compliance and the relaxation modulus. Afterward, it was adopted for other properties such as failure. The TTSP relates the influence that temperature and time (or its inverse, i.e. frequency) have on the rheological properties of polymeric materials, considering that the variation of a physical quantity, such as creep compliance,