CompTest Residual Curing Stresses in Thin [0/90] Unsymmetric Composite Plates Marco Gigliotti°, Michael R. Wisnom, Kevin Potter Department of Aerospace Engineering, University of Bristol, UK °current address: Département MEM, Ecole des Mines de Saint-Etienne, France contact:

CompTest COMPAVS Research Program Uni of Bristol, Airbus UK, QuinetiQ, AugustaWestland, Bombardier Shorts Aim : Understanding, predicting and controlling residual stresses, distortions and variability coming from the cure of high temperature composite parts Understanding of basic phenomena is needed

CompTest Introduction 1. Introduction Experimental activity Simulations Conclusions

CompTest Main sources of residual curing stresses are: - Cooling (T cure -T room ) - Resin Chemical Shrinkage - Tool interaction - Thermal, degree of cure and V f gradients - …. AUTOCLAVE Tool Laminate Curing cycle t T P Generalities on Residual Curing Stresses AUTOCLAVE MOULDING TECHNIQUE Introduction (1/2) Tg

CompTest Introduction (2/2) Use of flat thin [0/90] unsymmetric samples Aim : Elimination or minimisation of many parameters, such as: - thermal, degree of cure and V f gradients through the thickness - cure shrinkage - tool interaction Investigation on the residual deformation of partially or totally cured samples Method :

CompTest Introduction 2.Experimental activity 2. Experimental activity Simulations Conclusions

CompTest Experimental activity (1/13) Physical principles : What we measure : - the curvature k after partial or total cure - the stress free temperature T sf, at which samples are flat - intermediate curvatures between T room and T sf 0°0°/90° TT 90°

CompTest Experimental activity (2/13) Physical principles : T sf  T g T Strain/Curvature T initial > T g Cooldown + Reheating T initial tT Tg

CompTest Experimental activity (3/13) Physical principles : A: only thermoelastic strains are in the structure B: non-thermoelastic strains are in the structure T initial T T initial  T g Strain/Curvature A B Cooldown tT Tg

CompTest Experimental activity (4/13) Physical principles : A: only thermoelastic strains are in the structure B: non-thermoelastic strains are in the structure T initial  T sf T initial  T g T Strain/Curvature T sf > T initial A B Reheating tT Tg

CompTest Experimental activity (5/13) Measurement apparatus : oven CCD video-camera PC L’ h t then k 

CompTest Experimental activity (6/13) Interrupted Cure Cycles (ICC) : material: AS4/8552, oven curingsamples: 300mm x 30mm x 1mm

CompTest Experimental activity (7/13) Results : Residual curvatures

CompTest Experimental activity (8/13) Results : Stress free temperatures The reaction rate slows down at the vitrification point T sf > T cure for samples cured beyond vitrification

CompTest Experimental activity (9/13) Results : Reheating sample B T sf The increase of T sf indicates post-cure

CompTest Experimental activity (10/13) Results : Post curing effects

CompTest Experimental activity (11/13) Results : Reheating curves Linear behaviour, curves have the same slope

CompTest Experimental activity (12/13) Results : Tool effect No significant differences (level of confidence 5%) Autoclave cured samples

CompTest Experimental activity (13/13) Results : Tool effect No significant differences (level of confidence 5%) Oven cured samples

CompTest Introduction Experimental activity 3.Simulations 3. Simulations Conclusions

CompTest Simulations (1/6) Model generalities A FE Abaqus code is used for modelling the thermoelastic behaviour of 0/90 thin plates during the cooldown from the stress free temperature - shell 4 node elements with reduced integration (S4R) - temperature differentials applied uniformly in one static step - option nlgeom (small strain, moderate rotations) Material properties AS4/8552

CompTest Simulations (2/6) Remarks on the thermoelastic behaviour of 0/90 thin plates: - according to the Classical Lamination Theory (small strain, small displacement) deformed shapes are saddles - due to large displacements, plates in some range of in-plane dimensions (or thickness) exhibit cylindrical deformed shapes and/or strong non-linear behaviour with temperature

CompTest Simulations (3/6) Remarks on the thermoelastic behaviour of 0/90 thin plates: For narrow plates (AR>10) the deformed shape is almost a saddle with curvatures which vary almost linearly with temperature. AR>10 principal curvature lateral bow For such samples, predictions from CLT are good

CompTest Simulations (4/6) Results :

CompTest Simulations (5/6) Results :  3* /°C The offset indicates the non-thermoelastic portion of residual curvature (< 5%)

CompTest Simulations (6/6) Results : HTA/913C Composite system (cured at 120°C)

CompTest Introduction Experimental activity Simulations 4.Conclusions 4. Conclusions

CompTest Conclusions (1/1) residual curvature and stress free temperature monitoring gives exhaustive information about the cure process of composites the stress free temperature of AS4/8552 samples cured beyond the vitrification point is found to be higher than T cure the stress free temperature of HTA/913C samples is found to be equal to T cure simulations allow us to find values of  T (below T g ) and to estimate non-thermoelastic sources of residual stress for the AS4/8552 non-thermoelastic sources of stress may be ascribed to resin chemical shrinkage