Residual Curing Stresses in Thin [0/90] Unsymmetric Composite Plates

1. 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: gigliotti@emse.fr CompTest 2003

2. 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 2003

3. 1.Introduction 2. Experimental activity 3. Simulations 4. Conclusions CompTest 2003

4. Curing cycle T Main sources of residual curing stresses are: Tg - Cooling (Tcure-Troom) P - Resin Chemical Shrinkage t - Tool interaction Laminate - Thermal, degree of cure and Vf gradients Tool AUTOCLAVE Introduction (1/2) Generalities on Residual Curing Stresses AUTOCLAVE MOULDING TECHNIQUE - …. CompTest 2003

5. 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 Vf gradients through the thickness - cure shrinkage - tool interaction Method : Investigation on the residual deformation of partially or totally cured samples CompTest 2003

6. 1.Introduction 2.Experimental activity 3. Simulations 4. Conclusions CompTest 2003

7. 0° 0°/90° 90° Experimental activity (1/13) Physical principles : DT What we measure : - the curvature k after partial or total cure - the stress free temperature Tsf, at which samples are flat - intermediate curvatures between Troom and Tsf CompTest 2003

8. T Tg t Experimental activity (2/13) Physical principles : Strain/Curvature Tinitial > Tg Cooldown + Reheating T Tsf  Tg Tinitial CompTest 2003

9. T Tg Strain/Curvature t Cooldown B Tinitial Tg A Tinitial T Experimental activity (3/13) Physical principles : A: only thermoelastic strains are in the structure B: non-thermoelastic strains are in the structure CompTest 2003

10. T Tg Strain/Curvature t Reheating B Tinitial Tg A T Tinitial  Tsf Tsf > Tinitial Experimental activity (4/13) Physical principles : A: only thermoelastic strains are in the structure B: non-thermoelastic strains are in the structure CompTest 2003

11. oven CCD video-camera PC L’ then k  h t Experimental activity (5/13) Measurement apparatus : CompTest 2003

12. Experimental activity (6/13) Interrupted Cure Cycles (ICC) : material: AS4/8552, oven curing samples: 300mm x 30mm x 1mm CompTest 2003

13. Experimental activity (7/13) Results : Residual curvatures CompTest 2003

14. Experimental activity (8/13) Results : Stress free temperatures The reaction rate slows down at the vitrification point Tsf > Tcure for samples cured beyond vitrification CompTest 2003

15. Tsf Experimental activity (9/13) Results : Reheating sample B The increase of Tsf indicates post-cure CompTest 2003

16. Experimental activity (10/13) Results : Post curing effects CompTest 2003

17. Experimental activity (11/13) Results : Reheating curves Linear behaviour, curves have the same slope CompTest 2003

18. Experimental activity (12/13) Results : Tool effect Autoclave cured samples No significant differences (level of confidence 5%) CompTest 2003

19. Experimental activity (13/13) Results : Tool effect Oven cured samples No significant differences (level of confidence 5%) CompTest 2003

20. 1.Introduction 2. Experimental activity 3.Simulations 4. Conclusions CompTest 2003

21. Simulations (1/6) A FE Abaqus code is used for modelling the thermoelastic behaviour of 0/90 thin plates during the cooldown from the stress free temperature Model generalities - 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 2003

22. 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 2003

23. Simulations (3/6) Remarks on the thermoelastic behaviour of 0/90 thin plates: AR>10 lateral bow principal curvature For narrow plates (AR>10) the deformed shape is almost a saddle with curvatures which vary almost linearly with temperature. For such samples, predictions from CLT are good CompTest 2003

24. Simulations (4/6) Results : CompTest 2003

25. a 3*10-5 1/°C The offset indicates the non-thermoelastic portion of residual curvature (< 5%) Simulations (5/6) Results : CompTest 2003

26. Simulations (6/6) Results : HTA/913C Composite system (cured at 120°C) CompTest 2003

27. 1.Introduction 2. Experimental activity 3.Simulations 4.Conclusions CompTest 2003

28. 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 Tcure the stress free temperature of HTA/913C samples is found to be equal to Tcure simulations allow us to find values of aT (below Tg) 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 CompTest 2003