Creep Evolution Analysis of Composite Cylinder Made of Polypropylene Reinforced by Functionally Graded MWCNTs

Polypropylene is one of the most common, fastest growing and versatile thermoplastics currently used to produce tanks and chemical piping systems. Even at room temperature creep is considerable for polypropylene products. The creep behavior of strains, stresses, and displacement rates is investigated in a thick-walled cylinder made of polypropylene reinforced by functionally graded (FG) multi-walled carbon nanotubes (MWCNTs) using Burgers viscoelastic creep model. The mechanical properties of the composite are obtained based on the volume content of the MWCNTs. Loading is composed of an internal pressure and a uniform temperature field. Using equations of equilibrium, stress-strain and strain-displacement, a constitutive differential equation containing total creep strains is obtained. Creep strain increments are accumulated incrementally during the life of the vessel. Creep strain increments are related to the current stresses and the material uniaxial Burgers creep model by the well-known Prandtl-Reuss relations. A semi-analytical solution using Prandtl-Reuss relation has been developed to determine history of stresses, strains and displacements. The results are plotted against dimensionless radius for different volume content of MWCNTs. It has been found that the creep radial and circumferential strains of the cylinder reduce with increasing content of carbon nanotubes. It has also been concluded that the uniform distribution of MWCNTs reinforcement does not considerably influence on stresses.