Wavy-ply sandwich with crushable core:
CompTest 2015 Madrid, 10th April 2015 Wavy-ply sandwich with crushable core: design, simulation and testing Soraia Pimenta [email protected] Paul Robinson Full paper in: Pimenta S, Robinson P (2014), βWavy-ply sandwich with composite skins and crushable core for ductility and energy absorptionβ, Composite Structures, vol:116, pages:364-376. DOI: http://dx.doi.org/10.1016/j.compstruct.2014.05.020 Towards ductile compositesβ¦ Brittle conventional composites π (MPa) 1000 ο½ Damage / defect sensitive CFRP (AS4/M56) 750 ο½ Difficult to inspect Aluminium 2024-T3 500 ο½ Catastrophic failure 250 π (%) 0 0 5 10 15 20 Programme Grant The concept Initial configuration Wavy composite skins Foam core Response under loading ο Unfolding of skins ο Extra extension ο Crushing of core ο Extra energy dissipation πβ πβ Outline ο Design ο Simulation ο Testing Design definition Wavy skins Foam core ο ο Increase failure strain ο maximise excess length ο large waviness Preserve high strength ο minimise bending stresses ο small waviness Materials ο Skins M21/T800 UD π‘ p = 0.193 mm ο Wave geometry sinusoid 2π΄w Core Rohacell foam 25 5 Design of wave geometry Selected configuration 0 sinusoid unfolding 2π΄w 1 25 tension 2 πβ (MPa) Analytical modelling ο 5 Failure criterion: πunfolding + πβ = πskin ο Failure stress: πβ = πskin β πunfolding ο Failure strain: πβ πβ = πunfolding + πΈskin 2500 π΄w ο 2000 1500 π΄w = 5 mm 1000 500 πβ (%) 0 0 5 10 Design of bridging region Bridging region premature delamination crushing foam core opening stresses Avoid premature delamination ο Soft, crushable foam wavy skin foam core ο Epoxy fillet at bridging region epoxy fillet Design definition Wavy skins Foam core ο ο Increase failure strain ο maximise excess length ο large waviness Preserve high strength ο minimise bending stresses ο small waviness Materials ο Skins M21/T800 UD π‘ p = 0.193 mm ο Wave geometry ο Epoxy fillet sinusoid 10 Core Rohacell foam Density? Bridging region 25 5 π‘f Outline ο Design ο Simulation ο Testing FE model epoxy cohesive elements foam cohesive elements CFRP skin foam core epoxy fillet foam core Modelling the crushable foam Crushable foam with volumetric hardening von Mises stress uniaxial tension Densification πc (MPa) 6 pure shear 4 uniaxial compression hydrostatic tension hydrostatic compression hydrostatic pressure οΎ Uniaxial crushing yield stress datasheet οΎ Hydrostatic yield pressure π(crushing yield stress, shear strength, tensile strength) οΎ Hydrostatic tensile strength π(crushing yield stress, shear strength, tensile strength) πc = 0.075 g/cm3 2 πc = 0.052 g/cm3 πc = 0.032 g/cm3 0 0.0 ο 0.2 0.4 0.6 πc 0.8 Strain hardening: Constant hydrostatic tension Self-similar flow Simulated response of wavy sandwich πβ (MPa) 1500 1000 500 πβ (%) 0 0 2 4 6 8 plastic deformation in foam Simulated response of wavy sandwich πβ (MPa) 1500 1000 500 πβ (%) 0 0 2 4 6 8 degradation of skin-core interface foam crushing Simulated response of wavy sandwich πβ (MPa) 1500 1000 500 πβ (%) 0 0 2 4 6 8 foam densification Simulated response of wavy sandwich πβ (MPa) 1500 1000 500 πβ (%) 0 0 2 skin tensile failure 4 6 8 Effect of foam core πβ (MPa) πc = 0.052 g/cm3 πc (MPa) 1500 6 1000 4 500 2 πc = 0.075 g/cm3 πc = 0.032 g/cm3 0 0 2 4 οΎ Lighter foam: lower stresses 6 8 πβ (%) 0 πc 0.0 0.2 0.4 0.6 0.8 ο½ Denser foam: delamination Effect of epoxy fillet πβ (MPa) π‘ f = 0.5 mm 1500 π‘ f = 0.7 mm 1000 π‘f 500 πβ (%) π‘ f = 0.3 mm 0 0 2 4 6 οΎ Thicker fillet: stress concentrations 8 ο½ Thinner fillet: delamination Design definition Wavy skins Foam core ο ο Increase failure strain ο maximise excess length ο large waviness Preserve high strength ο minimise bending stresses ο small waviness Materials ο Skins M21/T800 UD π‘ p = 0.193 mm ο Wave geometry ο Epoxy fillet sinusoid 10 Core Rohacell foam ππ = 0.052 g/cm3 Bridging region 0.5 25 5 Outline ο Design ο Simulation ο Testing Manufacturing CNC-machined foam cells CNC-machined aluminium moulds Test specimens ο ο 240 mm Set A: as designed Set B: larger fillet Overcoming premature delamination πβ (MPa) 1500 1000 500 0 0 2 4 6 8 πβ (%) Test specimens ο ο 240 mm Set A: as designed Set B: larger fillet Experiments vs FE As-designed set (0.5 mm fillet) πβ (MPa) Modified set (larger fillet) πβ (MPa) Experimental 1500 1000 1000 500 500 0 2 4 6 π‘ f = 0.6 mm 1500 FE 0 π‘ f = 0.5 mm 8 πβ (%) 0 0 2 4 6 Large deformations and crushing of the core 8 πβ (%) Deformation fields Outline ο Design ο Simulation ο Testing Potential of wavy-ply sandwich Withstand large deformations Energy absorption by core πβ (MPa) other contributions π (kJ/kg) 10 1500 elastic energy 8 6 1000 4 500 2 0 0 2 4 6 8 πβ (%) plastic dissipation 0 0 2 4 6 8 πβ (%) Potential applications ο Blast protection ο Protective casing of pressure vessels Correlation between FE and experiments πβ (MPa) Experimental FE 1500 1000 500 0 0 2 4 6 8 πβ (%) Potential applications ο Blast protection ο Protective casing of pressure vessels Acknowledgments Evonik Industries AG ο Rohacell foam materials Soraia Pimentaβs Research Fellowship This work was supported by the EPSRC Programme Grant EP/I02946X/1 on High Performance Ductile Composite Technology in collaboration with the University of Bristol. Programme Grant
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