API CEAC-TR-96-0102-1996 LEAKAGE FAILURE OF THREADED FIBER-COMPOSITE JOINTS UNDER COMBINED INTERNAL PRESSURE@ AXIAL AND MAKEUP LOADING: EXPERIMENTS AND ANALYSES
INTRODUCTION The demand of alternative and/or better materials to meet stringent performance and cost-effectiveness requirements in petroleum exploration and production operations has led to consideration of light-weight@ fiber composites for a variety of aggressive@ load-bearing applications. Joining of composite tubing@ especially the threaded composite joints@ has been a major concern because of the complexities involved. It is recognized that design and analysis methodologies for monolithic metallic joints have been practiced for a long time and well established. However@ the mechanics and failure behavior of threaded composite tubular joints for load-bearing applications has not been well understood. Rigorous analytical and experimental methods to study the mechanical response of a threaded composite joint simply do not exist@ mainly because the following difficulties and complexities: 1) Complexities in fiber composite joint materials. Composite materials have inherently inhomogeneous microstructures and anisotropic properties. An accurate leakage failure study of composite joints requires knowledge of nonlinear anisotropic material properties@ complicated failure criteria@ and nonlinear thread contact mechanics. 2) Lack of understanding of threaded joint contact mechanics; Complexities of the local joint geometry and lack understanding of the thread contact pose a fundamental problem for evaluation and prediction of joint failure. The connection threads are recognized to be the weakest link in a composite joint system. A detailed study of a threaded composite joint made of different kinds of composite materials is generally tedious@ difficult and very complex. 3) Complexities in leakage failure modes: The three-dimensional states of stress and deformation in a threaded region of a fiber composite joint are known to be very complex. Various leakage failure modes may be developed under a combined internal pressure@ axial loading and make-up interference. Very little is understood about the failure modes and the effect of the combined loading modes on the joint failure. 4) Unknown long-term deformation and leakage failure resistance; Compared to monolithic metallic joints@ the threaded composite joints exhibit more complicated stress states and failure modes@ in addition to the issue of material degradation. Long-term deformation and failure behavior of threaded composite joints are not fully known at present. 5) Lack of accurate methods for analysis and design: The complexities of the joint materials@ geometry@ and loading and difficulties in handling the contact mechanics and failure modes in a threaded composite joint have laid serious barriers in the development of sound analysis and design methodologies. The analysis and design methodologies for traditional threaded metallic joints are not able to nor suitable for the threaded fiber composite joint problem. In-depth knowledge of leakage-failure mechanics of a threaded composite joint is needed to develop proper design and analysis methodologies.