International Journal of Fracture 108: 1–23, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. Delamination growth in long composite tubes under external pressure HAYDER A. RASHEED 1 and JOHN L. TASSOULAS 2 1 Assistant Professor of Civil Engineering, Kansas State University, Manhattan, Kansas 66506, USA 2 Professor of Civil Engineering, The University of Texas at Austin, Austin, Texas 78712, USA Received 7 July 1998; accepted in revised form 2 April 1999 Abstract. Delamination growth is a phenomenon known to reduce the integrity of laminated composite structural elements and may lead to premature failures. In the present study, state of the art procedures of delamination growth analysis are overviewed. The energy release rate calculation is formulated for composite delaminated tubular cross sections and specialized to a finite element model for delamination buckling and growth analysis of long laminated composite tubes taking into account initial geometric imperfections, large deformations, contact between delamination faces and material degradation. It is, then, used to study the potential of delamination growth in a hybrid composite tube. Parametric studies are conducted to assess the effects of delamination length, location and geometric imperfection on growth. Key words: Composite cylinders, contact, delamination buckling, delamination growth, external pressure, initial imperfection. 1. Introduction The use of composites in constructing laminated tubes is increasing and expanding to a wide range of engineering applications. This includes aerospace, automotive, marine and civil engineering industries. In addition, the use of such structural components has been potentially considered for deep-water offshore drilling (Sparks, 1986). Despite its advantages, composites suffer, like other laminated materials, from layer separation or delamination which leads to loss of integrity and, perhaps, premature failure. Such delaminations may pre-exist in composite tubes due to manufacturing defects (poor interply bonding) (Tarnopolskiy, 1978), manufacturing consequences (residual radial stresses) (Kachanov, 1974), transportation im- pacts (Chai and Babcock, 1985) and environmental effects during the service life of the structural component (Sloan and Seymour, 1990). It has been observed that thin delaminated layers tend to locally buckle inwards, in rings and tubes under external pressure (Kachanov, 1975), see Figure 1. On the other hand, thick delaminated layers may have no tendency to buckle, yet, they may still reduce the capacity of such tubes, due to the considerable reduction in the flexural stiffness of the delamination region. The importance of delamination buckling of thin layers, in rings and cylinders under ex- ternal pressure, has been recognized in the last two decades (Kachanov, 1975). However, only few studies attempted to estimate the pressure at the onset of delamination buckling. Simpli- fied analytical expressions were suggested with many restricting assumptions and conditions, e.g. small tube deformations and homogeneous isotropic or orthotropic material properties (Kachanov, 1988; Chen and Simitses, 1988; Marshall and Moshaiov, 1990; Kardomateas and Chung, 1992). Kachanov used a strain energy criterion and the elastica solution, to estimate the