Study on the bending behavior of reinforced thermoplastic pipes (RTPs): A theoretical continuum damage model and experimental tests

A novel theoretical model is proposed to study the continuum damage mechanical (CDM) behavior of reinforced thermoplastic pipes (RTPs) under bending moments, in which stress analysis of composites, failure evaluation and stiffness degradation are combined in loop calculation. Based on the existing homogenization assumption, the stress distribution of every ply could be calculated according to the equilibrium equations between the RTP and a hypothetical homogenous pipe. Once stresses of composite plies satisfy Hashin-Yeh failure criterion, dominant failure modes are determined by filtering failure coefficients. Subsequently, the stiffness degradation model would be performed, in which a sine weight function is employed to consider the damage distribution along the hoop direction. Meanwhile, the von Mises criterion and Ramberg-Osgood curve are used to simulate the material nonlinearity of liner and coating. Four-point bending tests and numerical simulations were conducted to verify the proposed theoretical model. A user-defined VUMAT subroutine was employed to simulate the progressive failure of 3D composites. Compared with experimental tests and numerical simulations, the proposed model could give accurate predictions on the linear and nonlinear responses, such as the bending stiffness, the stress field and the damage propagation. Furthermore, different methods for the four-point bending test were also compared and good correlation found.