M.G. Holmquist et al./ Composites: Part A 34(2003)163-170 the edge of the cloth, they would become major flaws controlling the onset of delamination The failure condition was determined using the tial stress of an internally pressurised thin-walled B strained tube, given by the equation where Pis the pressure difference across the tube wall, r, the radius of the tube wall and t its thickness. By substituting tube dimensions and failure pressures from Table I into Eq A (3), one find that the tubes failed at an average tangential stress of 47+7 MPa. This value is substantially lower than the expected tensile strength of flat panels (i.e. >200 MPa) 6, 20]. Even if the end constraints from the glued joints are taken into account, the calculated tangential stress will not increase more than -4% 31. It is hypothesised that the initial crack forms at the outer surface at the 'step' generated by the outer edge the spiral wrapped cloth. At this location there will be stress concentration due to the discontinuity. The propagation of the crack can be analysed using the concepts for fracture mechanics of layered materials developed by Hutchinson and Suo [34]. It is reasonable to approximate the tube wall with a straight section under tensile loading since there are no other stresses Fig. 6.(a)Cross section of tested tube ng delamination failure. The crack has progressed from A to A, starting from the wrap termination on the over the tube wall than circumferential in the case of outer surface. Another crack has started at the termination on the in internal pressure loading. One can derive an expressic surface, propagating from B to B.(b) Enlargement of the area around A for the steady-state energy release rate for the delamina the arrows indicate the propagation path of the delamination crack. tion crack as it is growing [34 the wound prepreg on the inside of the tube as shown in ig.6. However, it was not possible to assess at which stage with oe the tangential stress in the tube, h, the thickness of the test this crack was formed In an effort to block the of one ply and t, the wall thickness, i.e. two plies delamination failing mode, several strategies have been (0.49 mm), and E is the Youngs modulus in the employed with more or less success. When the tube edge tangential direction. Any bending moments arising from was glued with epoxy, the pressure reached 6 MPa without misalignment of loading and neutral axes are neglected Youngs modulus was experimentally measured to 100 GPa for a similar material [6]. Using this value, the actual dimensions of the tube and a tangential stress 4. Discussion and analysis of 47 MPa gives a steady-state fracture energy release rate of 1. 8 J/m". The analysis given by Hutchinson and Suo also indicates that the transient zone is small and The tubular structures made from wrapped prepregs have steady-state is reached when the delamination is about a microstructure comparable to flat panels made by the same three times longer than the thickness of a ply process[23]. The technique is comparable to that used with A value of.8 J/m- was determined by Radsick [31] polymer-based prepregs and the freezing step is a con for the critical strain energy release rate using his hoop practice to store those prepregs. The flexibili stresses measurements in nearly identical experiments and deformability of the prepregs during thawing is well Eq(4). One explanation for the smaller value is that the for making more complex tubular shapes impregnation with alumina precursor was carried out for The major defects in the microstructure are the large substantially shorter periods, which resulted as smaller ores formed during the wrapping process. These closed weight gains, and thus a weaker matrix pores are not believed to affect significantly the initial Fracture toughness measurements have recently been permeability of the composite as they do not create a done on matrix material having the same constituents as the through-thickness short-circuit. However, if located near matrix material in this study [40]. The fracture energy wasthe wound prepreg on the inside of the tube as shown in Fig. 6. However, it was not possible to assess at which stage of the test this crack was formed. In an effort to block the delamination failing mode, several strategies have been employed with more or less success. When the tube edge was glued with epoxy, the pressure reached 6 MPa without failure. 4. Discussion and analysis The tubular structures made from wrapped prepregs have a microstructure comparable to flat panels made by the same process [23]. The technique is comparable to that used with polymer-based prepregs and the freezing step is a common practice to store those prepregs. The flexibility and deformability of the prepregs during thawing is well suited for making more complex tubular shapes. The major defects in the microstructure are the large pores formed during the wrapping process. These closed pores are not believed to affect significantly the initial permeability of the composite as they do not create a through-thickness short-circuit. However, if located near the edge of the cloth, they would become major flaws controlling the onset of delamination. The failure condition was determined using the tangen￾tial stress of an internally pressurised thin-walled uncon￾strained tube, given by the equation su ¼ Pr t ð3Þ where P is the pressure difference across the tube wall, r, the radius of the tube wall and t its thickness. By substituting tube dimensions and failure pressures from Table 1 into Eq. (3), one find that the tubes failed at an average tangential stress of 47 ^ 7 MPa. This value is substantially lower than the expected tensile strength of flat panels (i.e. .200 MPa) [6,20]. Even if the end constraints from the glued joints are taken into account, the calculated tangential stress will not increase more than ,4% [31]. It is hypothesised that the initial crack forms at the outer surface at the ‘step’ generated by the outer edge of the spiral wrapped cloth. At this location there will be a stress concentration due to the discontinuity. The propagation of the crack can be analysed using the concepts for fracture mechanics of layered materials developed by Hutchinson and Suo [34]. It is reasonable to approximate the tube wall with a straight section under tensile loading since there are no other stresses over the tube wall than circumferential in the case of internal pressure loading. One can derive an expression for the steady-state energy release rate for the delamina￾tion crack as it is growing [34] Gss ¼ s2 ut 2E 1 1 þ ðh=tÞ  ð4Þ with su the tangential stress in the tube, h, the thickness of one ply and t, the wall thickness, i.e. two plies (0.49 mm), and E is the Young’s modulus in the tangential direction. Any bending moments arising from misalignment of loading and neutral axes are neglected. Young’s modulus was experimentally measured to 100 GPa for a similar material [6]. Using this value, the actual dimensions of the tube and a tangential stress of 47 MPa gives a steady-state fracture energy release rate of 1.8 J/m2 . The analysis given by Hutchinson and Suo also indicates that the transient zone is small and steady-state is reached when the delamination is about three times longer than the thickness of a ply. A value of ,0.8 J/m2 was determined by Radsick [31] for the critical strain energy release rate using his hoop stresses measurements in nearly identical experiments and Eq. (4). One explanation for the smaller value is that the impregnation with alumina precursor was carried out for substantially shorter periods, which resulted as smaller weight gains, and thus a weaker matrix. Fracture toughness measurements have recently been done on matrix material having the same constituents as the matrix material in this study [40]. The fracture energy was Fig. 6. (a) Cross section of tested tube showing delamination failure. The crack has progressed from A to A0 ; starting from the wrap termination on the outer surface. Another crack has started at the termination on the inner surface, propagating from B to B0 : (b) Enlargement of the area around A0 ; the arrows indicate the propagation path of the delamination crack. 168 M.G. Holmquist et al. / Composites: Part A 34 (2003) 163–170