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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elements (Citarella and Cricrì (2014), Corato et al. (2015), Cricrì et al (2015), Cricrì and Perrella (2017)). Moreover, adhesive bonding is presently a very popular technique in rehabilitation and repair of existing civil structures, due to the development of advanced FRP materials (Ascione and Berardi (2011), Perrella et al. (2018), Berardi et al. (2019)). Many experimental data are required for an optimal composite structure design. Indeed, many mechanical properties of composite materials components, such as matrix resin, are affected by degradation by fatigue and creep effects, among others. The failure behavior of composite systems was extensively investigated by several researches, instead of the long term mechanical behavior of reinforcement materials (Sá, M.F. et al. (2016), Berardi et al (2017), Emara, M., et al. (2017), Fischer, J. et al. (2019)). Within this context, it is well-known the relevant role of materials viscosity in design/verification processes, and strengthened members analyzed by fewer authors with particular reference to the reliability and durability of these elements. In the case of composite structures used in the field of civil engineering, the different rheological properties of the structural components (e.g. composite tendons or laminates, reinforced polymer concretes) can lead to a stress migration from the reinforcement to the original structural element and an increase of the displacements, compromising the efficacy of the strengthening technique (Ascione et al. (2011), Berardi and Mancusi (2012), Berardi and Mancusi (2013), Mancusi et al. (2013), Silva et al. (2014), Lou et al. (2016), Wu at al. (2016), Berardi et al (2018)). Industrial, automotive, naval and aerospace structural applications also require long term verification, because the deferred strains of viscous materials could affect dimensional stability. A creep test program on commercial epoxy resin samples was carried out by the authors at the Design Machine Laboratory of University of Salerno under constant environmental conditions. An in house made test equipment was realized and the Digital Image Correlation (DIC) technique was adopted for evaluating short-time deformations. Afterwards, the long-term behavior of the tested resin was obtained by accelerating the creep rate by a thermally activated process, i.e. using the Time Temperature Superposition Principle (TTSP).The TTSP was firstly described by Leaderman (1944). Tobolsky and Andrews (1945) used this method to superpose the individual creep compliance curves into a single reference curve. According to Leaderman’s study the creep compliance vs. log (time) curves at different temperatures for materials with a viscoelastic behavior show the same shape, but the increasing of temperature permits a time scale contraction, which can be considered by introducing the shift factors. Many details on this method can be found in Markovitz (1975). The relationship between shift factors of all curves and temperatures can be described by some mathematical equations, i.e., the Williams-Landel-Ferry (WLF) model (Williams et al. (1955), Adams and Gibbs (1965)) or the Vogel-Fulcher-Tammann-Hesse (VFTH) formula (Vogel (1921), Fulcher (1925)) in glass-forming and viscous systems and the Arrhenius equation for secondary relaxations (Shaw and MacKnight (2005)) or terminal flow of polymers (Sperling (2005)). Many studies were presented on the TTSP methodology and the equations describing the relaxation properties of polymers (Kapnistos eta al. (1996), Kim et al. (1998), Zheng and al. (2001), Tajvidi et al. (2005), Yang, Z.; Han (2008), Hu (2009), Nakada (2019)). By using the WLF equation, Wise et al. (1997) calculated the diffusion rate constant during cure and obtained the chemical rate constant for the reaction by the Arrhenius equation in epoxy-amine resins. Shangguan et al. (2017) developed the WLF equation to describe the temperature dependence of relaxation property for the polymer ranging from secondary relaxation to terminal flow, and proposed its necessary criteria for its general application. The experimental data, reported in this paper, could be used for practical applications, as in predictive models, to be implemented into FEM codes (Calì et al. (2010)), for residual strength evaluation of bonded structures.

Nomenclature T temperature � t time ( ) creep compliance � shift factor

reference temperature