Issue 45

C. Huang et alii, Frattura ed Integrità Strutturale, 45 (2018) 108-120; DOI: 10.3221/IGF-ESIS.45.09 110 Figure 1 : Typical creep curve. Figure 2 : Burgers model. The creep acceleration is: 0 2 2 2 ( ) 0 t e t      − −  =  (3) where  0 is the loading stress; t is the loading time; E 1 and E 2 are the elastic modulus of elements 1 and 2, respectively;   and   are the viscosities of elements 3 and 4, respectively; and  =E 2 /  2 . In Eqn. (1), the permanent deformation of FRAC is a linear function of time t and the reciprocal of viscosity 1/  1 . With time elapsing, the viscosity increases while the permanent stress increment decreases. Both the stages of the stable creep and accelerated creep do not exist during the creep process. There is no inflection point in the creep deformation-time curve noticing from Eqn. (2) and (3). Therefore, the model does not present the rheological time characteristics of viscoelastic material and does not match the creep deformation characteristics of FRAC. Based on Burgers model, a modified model with four-unit and five-parameter was proposed through a nonlinear modification on the viscosity of the external damper with the viscous flow deformation characteristics of material [2], as shown in Fig. 3. The viscosity of the external damper in this model is: