PSI - Issue 24

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

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obtained at a chosen frequency and varying the temperature, can be inferred from test at a reference temperature and variable frequency. Derived from the TTSP, short-term creep tests at various temperatures are sufficient to provide a master curve that defines the long-term deformation at imposed temperature. The construction of a master curve takes in shifting the timescale of the measured creep curves to belong the creep behavior at a reference temperature. The matching shift factor � is defined as: � ( � , � ) = ( , ) (1) where J r is the creep compliance at the reference temperature T r , t is the time and J is the compliance at the increased temperature T . More specifically the horizontal shift factor was formulated by Williams et al. (1980), for amorphous polymers above the glass transition temperature, by the following equation: log( � ) = ���∙(��� � ) ������ � (2) where C 1 and C 2 are material constants. Another common model for the shift factor evaluation is present in literature, the so called Arrhenius equation: log( � ) = � � � � � � − � � � � (3) where E f is the activation energy associated with the relaxation, R is the gas constant, T is the measurement temperature, T r is the reference temperature. The equation (3) is typically used for describing the material behavior outside the glass transition region. Thus, considering that the operating temperature of the tested resin in industrial and civil structures is below its glass transition temperature (50°C), in this work the horizontal shift factor was evaluated by means of the Arrhenius equation (3). 3. Results and discussion A dead-weight load corresponding to the 20% of short-term strength, as resulting from the tensile tests, was applied to the creep samples. A couple of specimens was tested for each temperature level, showing a good repeatability in terms of creep strains.

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Fig. 4. Total strain vs. time curves at various temperatures.