Keywords: composite pipe, adhesive joint, structural damage evolution, life prediction, numerical analysis, finite element method.

Composite materials have been extensively used in piping systems for many years as the alternative to carbon and stainless steels in corrosive fluid transport for the petrochemical and pulp industries applications. Nowadays, composite pipe takes over its importance in the offshore oil and gas industries, too, due to its light weight and corrosion resistance. The continued integrity and long term durability of new composite pipelines depend mainly on the integrity of the adhesive bonds joining the particular pipe sections. The composite pipelines used in marine and oil industries applications, which are exposed to dynamic excitation and great displacements, exhibit the joints as the weakest link in a composite piping system, what emphasizes the importance of composite pipe joints reliability.

The main goal of this work is to present computational analysis of damage evolution in the adhesive joint connecting two unidirectional composite pipes subjected to tension with the constant amplitude. This approach needs the average shear stress criterion for a defect propagation in the adhesive layer and applies the continuum damage mechanics-like concept for a continuum crack-like damage representation in terms of the finite element stiffness as the damage metric [1]. The numerical studies have been performed by the commercial Finite Element Method (FEM) displacement-based program ANSYS [2] and its special purpose finite element birth and death option. Four node finite elements PLANE42 of three translational degrees of freedom at each node and the linear shape functions, have been used in the axisymmetric FEM analysis. Average values of the shear stress component have been computed by ANSYS in finite elements of the model and then have been compared to the static shear strength ( ) of the adhesive layer. Finite element stiffness has been multiplied by the reduction factor equal to after exceeding of this value in the finite element and element has been deactivated.

The tendency of the longitudinal crack-like damage propagation has been obtained from the computer analysis as the difference between crack-like damage tip at th and th cycle. The crack-like damage tip position has been chosen in the finite element centroid with reduced stiffness. Since the crack-like damage growth occurred from two opposite sides of the joint, thus two extreme longitudinal positions of the crack-like damage tips have been considered and summed up to give a single crack-like damage value. The prediction of composite pipe joint life has been described in terms of defect growth rate as a function of load cycles number. Finally, a relation between the mean applied stress and the crack-like defect growth rate has been obtained as a result of the computer simulation.

Computational approach proposed in this work made it possible to estimate numerically the fatigue crack-like damage evolution rate within composite pipe joint subjected to varying tensile load. This computational approach may especially be convenient to predict life of structures with the high stress concentration regions, where the internal stresses even under applied fatigue loading may be high enough to overcome material static strength in tension.

Qualitative numerical comparison of the fatigue crack-like damage evolution rate can be elaborated in the FEM displacement-based analysis using cohesive zone- based fracture mechanics tools [3]. In this case, the damage of adhesive layer can be represented by the single crack model and crack evolution can be numerically modelled e.g. through the common spring finite elements, interface finite elements or solid finite elements with embedded discontinuity based on the critical energy release rate growth condition.

A possible extension of the present work may account for damage evolution modelling through coupled damage mechanics approaches, in which damage variable is embedded within a constitutive relation. For that purpose and in order to avoid spatial discretisation dependence of results, the use of non-local and gradient- enhanced damage mechanics formulations is currently under study. Further research will also aim at understanding and possibly providing correlation between damage mechanisms at different structural materials scales, e.g. micro-macro. For these purposes, the investigation will switch to the micro-level and consider the material with certain micro-structural arrangements using the micro-mechanical cell models [4]. Next, the problem shall be extended to numerical analysis on fatigue damage evolution under random material parameters, and implemented and solved using the probabilistic finite element method based on the second order perturbation second- moment analysis or the Monte-Carlo simulation [5].

1

L.M. Kachanov, "Introduction to damage mechanics", Kluwer Academic Publishers, 1986.

2

ANSYS User's Manual, v.5.5, 1999.

3

T.L. Anderson, "Fracture Mechanics: Fundamentals and Applications", CRC Press, 1994.

4

T. Mura, "Micromechanics of defects in solids", Martinus Nijhoff, 1987.

5

M. Kaminski, "On probabilistic fatigue models for composite materials", International Journal of Fatigue, 24, 477-495, 2002. doi:10.1016/S0142-1123(01)00103-7

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