A viscoelastic model for time-dependent behavior of pultruded GFRP

Several works in the literature have investigated creep of pultruded glass-fiber reinforced polymer (pGFRP), but mainly to characterize the material in longitudinal direction. Moreover, the empirical Findley Power Law has been widely used to describe the creep response, but this approach is particular for each case and may result in little predictive capacity for different loading conditions, resin types and fiber architecture. In this work, a linear viscoelastic model combining elastic behavior of fiber and viscoelastic nature of matrix is proposed to represent the long-term behavior of pGFRP material. To validate the model, flexural creep and recovery experimental tests were carried out for vinyl ester (VE) and orthopolyester (OPE)-based composites for specimens oriented parallel and perpendicular to pultrusion direction. To reduce uncertainties with respect to the degree of curing of resin during fabrication, tests were conducted for two different conditions: as supplied and post-cured. Dynamic mechanical analyses (DMA) were performed to evaluate qualitatively the degree of cross-linking and the fiber architecture was studied by stereomicroscopy. Tests revealed greater compliances for OPE resulting from lower viscoelastic properties of this resin, as well as by lower fiber content and less effective fiber architecture. For transverse direction, matrix dominates the behavior and compliances were more pronounced. The thermal treatment adopted for post-curing resulted in damage formation or propagation, increasing the compliances. Satisfactory agreement was achieved between proposed model and experiments. (C) 2019 Elsevier Ltd. All rights reserved.