The effects of Mandrel material and tow tension on defects and compressive strength of hoop-wound, on-line consolidated, composite rings

Abstract

Even carefully controlled, on-line processing, which is superior to standard autoclave processing in a number of ways, is not immune to the development of process-induced defects. These include waviness, residual stresses and voids caused by poor process parameter selections. Several designed experiments and multivariate regression analyses were performed to formally examine the effects of mandrel material, tow tension, cooling rate and ring thickness (as well as their interactions) on the development of these defects and their effects on the compressive strength of thin and thick, hoop-wound rings. Cylinders were produced from T300/polysulfone powder-impregnated towpreg using on-line consolidation; they were wound using different mandrels (aluminum, steel), tow tensions (13.3 N, 44.5 N), cooling rates (natural and natural convection) and radius-to-thickness ratios (11, 5.5). Three or four rings per cylinder were failed using a new, compliant-ring-based, radial compression test method; the compressive hoop and radial stresses at failure were measured and compared based on either uniaxial longitudinal compressive failure (thin rings) or biaxial maximum work (or Tsai-Hill) theory (thick rings). In thin rings, independent changes in mandrel material and tow tension significantly affect the hoop and radial stresses at failure: decreasing with increasing mandrel coefficient of thermal expansion and decreasing tow tension. Void contents higher than certain threshold values (2–3% in this study) substantially degrade the rings’ compressive strengths by eroding its inter- and intralaminar shear strength. Changes in part thickness had a significant effect on the radial (but not hoop) stresses at failure. Waviness, which was isolated to the innermost six plies of any cylinder, was principally affected by mandrel material, with tow tension and thickness played lesser, moderating roles. Cooling rate had no discernable effect on waviness. Residual stresses were principally affected by tow tension and thickness, with the mandrel material's role difficult to de-convolve from its effect on waviness. Modest cooling rates did not appear to affect the residual stresses. Void contents decreased with increasing cooling rates and tow tensions in thin rings and with increasing radius-to-thickness ratios in thick rings. With the appropriate selection of process parameters, on-line processing can produce relatively ‘defect-free’ hoop-wound cylinders, which can match the compressive strengths of autoclave-consolidated, unidirectional plate laminates and be used to measure the laminates intrinsic compressive strength.