Rheological model incorporating gap element

Abstract

Viscoelastic materials undergo creep due to elasticity and viscosity, two intrinsic material characteristics, and a representative rheological model can be constructed for creep behavior analysis. Based on the viscoelasticity theory, this model simulates creep by formalizing elastic and viscous deformations in spring and dash-pot elements, respectively. However, most materials have internal gaps caused by the generation process, changes in the internal or external stress, etc. In this paper, a rheological model is proposed in which a gap is introduced; hence, the phenomenological behaviors are formalized using spring, dash-pot, and gap elements. The gap element has strain, but no material property. The opening or closing of the gap according to time history influences the structural changes in the model. Owing to the gap element, the energy due to external forces is dissipated before the gap is closed. After the gap is closed, it is converted and stored as internal strain energy in the spring element, which causes creep recovery. The proposed rheological model has two types, which depend on the element combination. Further, a generalized model is obtained by constructing n models on the basis of the element material properties. To validate the proposed rheological model for various stress conditions, the results predicted by three creep models (the proposed model, Burgers model, and the step-by-step method) are compared with the previously experimental results of concrete specimens because there is a large difference between the evaluations of internally stored strain energy by the creep prediction models. Analysis of a stepwise loading case and an unloading case reveal the following characteristics and mechanism of the model. When additional stress is applied, the principle of superposition is not applied to the proposed model. Instead, the model predicts creep by considering the linearity and structural change of each element. In addition, the proposed model simulates creep recovery and permanent strain more precisely compared to other creep models, which occur due to stress removal, by considering the structural change caused by the gap.