2 Composites forming technologies Table 7.7 Deformation mechanisms for continuous,aligned fibre based materials during forming Mechanism Schematic Characteristics Intra-ply shear Rotation of between parallel tows and at tow crossovers,followed by inter- tow compaction Rate and temperature dependent for prepreg Key deformation mode (along with bending)for biaxial reinforcements to form 3D shapes Intra-ply tensile Extension parallel to tow direction(s) loading Sunysijqnd For woven materials initial stiffness low until tows straighten;biaxial response governed by level of crimp and tow 2 compressibility 86-0 Accounts for relatively small strains but represents primary source for energy dissipation during forming wo'ssaudmaur'peaypoowy/:dy Ply/tool or Relative movement between individual ply/ply shear layers and tools Not generally possible to define single friction coefficient;behaviour is pressure and(for prepreg)rate and temperature dependent 名 Ply bending Bending of individual layers Stiffness significantly lower than in- plane stiffness as fibres within tows can slide relative to each other;rate and temperature dependent for prepreg Only mode required for forming of single curvature and critical requirement for double curvature Compaction/ Thickness reduction resulting in consolidation increase in fibre volume fraction and (for prepreg)void reduction For prepreg behaviour is rate and temperature dependent.Table 1.1 Deformation mechanisms for continuous, aligned fibre based materials during forming Mechanism Schematic Characteristics Intra-ply shear · Rotation of between parallel tows and at tow crossovers, followed by inter￾tow compaction · Rate and temperature dependent for prepreg · Key deformation mode (along with bending) for biaxial reinforcements to form 3D shapes Intra-ply tensile loading · Extension parallel to tow direction(s) · For woven materials initial stiffness low until tows straighten; biaxial response governed by level of crimp and tow compressibility · Accounts for relatively small strains but represents primary source for energy dissipation during forming Ply/tool or ply/ply shear · Relative movement between individual layers and tools · Not generally possible to define single friction coefficient; behaviour is pressure and (for prepreg) rate and temperature dependent Ply bending · Bending of individual layers · Stiffness significantly lower than in￾plane stiffness as fibres within tows can slide relative to each other; rate and temperature dependent for prepreg · Only mode required for forming of single curvature and critical requirement for double curvature Compaction/ consolidation · Thickness reduction resulting in increase in fibre volume fraction and (for prepreg) void reduction · For prepreg behaviour is rate and temperature dependent. 2 Composites forming technologies Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 6:57:44 PM IP Address: 158.132.122.4