Stress, Strain and Creep Analysis of the Layered Composite Structures under Sustained Loading

The paper proposes numerical and analytical models of the time-dependent analysis of a multilayered composite. These models enable us to predict the time-dependent behaviour of composite structures made of materials having age-variable properties. The proposed numerical model deals with a numerical solution of linear Volterra integral equation implying the validity of the principle of superposition for the layers. Meanwhile, the proposed analytical approach is based on the application of the age-adjusted effective modulus method for the composite. The validity of these models has been proven by a direct calculation of the time-dependent stresses and strains for the three-layered composite wall structure comparing the obtained theoretical values with those determined numerically. In particular, it was demonstrated that the analytical approach gave the identical values of the time-dependent stress strain state parameters. Relying on the results obtained, the concluding remarks have been summarized as follows: a) Despite the fact of sustained load, acting on the composite made of layers with different time-dependent deformational properties does not always hold time-invariant layer stress. In particular, the layers possessing a higher strain tend to produce relaxation while the layers with a lower strain are undergone the stress increase to maintain the equilibrium between internal and external forces (Fig. 4, b, e). Therefore, the failure should primarily be originated from the layer with a lower strain; b) Time variation in the elastic modulii of the layers is responsible for the increased stresses at time t in a higher strain layer due to a partial recovery of instantaneous strains. This effect is more progressive at a young age of the composite loaded (cf. Fig. 4, b and e); c) The ageing coefficient between the layers is different and is mainly subjected by the history of the ageing process of the layers. However, at the age of loading exceeding 28 days, the difference in the ageing coefficient of the layers is insignificant (Fig. 4, a). Moreover, it should be emphasized that the ageing coefficients of the layers are coincident with each other for the case of the time invariant elastic properties of the layers (Fig. 4, d). d) For the most practical purposes requiring long creep periods (exceeding 360 days), an approximate value of the ageing coefficient of the layers equal to 0.8 should be recommended predicting the stress-strain state of the layered composite using the proposed analytical method. More sophisticated functions for the ageing coefficients proposed by Lacidogna and Tarantino (1996), Ghali et al. (2002), ACI 209R-92 may be implemented for the sake of analysis accuracy. e) The values of the creep coefficient for the whole composite are mainly within the values of the creep coefficients of the layers.