1. Biaxial bulge testing of thin films and foils Miroslav Cieslar Faculty of Mathematics and Physics Charles University, Prague Czech Republic J.L. Martin, A. Karimi: EPFL Lausanne, Switzerland C. Fressengeas: LPMM, Université de Metz, France

2. Schedule Introduction to small structure testing Bulge test Applications –Recrystallization of thin foils –Plastic instabilities in Al foils –Plastic deformation of thin metallic films

3. Most common experimental methods Films adhered to substrate –Nanoindentation (hardness, modulus) –Microbeam bending (fatigue, bending) –Wafer curvature (biaxial strain, thermal fatigue) Free standing films –Tensile test (difficult sample preparation) –Microbeam bending –Biaxial bulge test

4. Industrial requirements for reliable biaxial tests Biaxial bulge test Testing of membranes in micro- and nano-devices Finstocks for heat exchangers

5. Spherical cap model Biaxial bulge test

8. Biaxial test with a constant fluid flow-rate Time [s] Stress [MPa]

9. Biaxial test under constant stress-rate Time [s] Stress-rate [MPa/s]

10. Examples Thin Al-Fe-Si foils (thickness 8.5  m) ElementFeSiCuMnMgZnTiAl wt. %0.510.610.0070.0200.00660.0220.024bal.

11. Initial microstructure after homogenization 590 °C/30 min

12. Stress – strain curves obtained from bulge tests during prestraining and after annealing at indicated temperatures. Recrystallization of thin foils

13. Yield stress variation of predeformed aluminium foils with annealing temperature

14. Microstructure after predeformation and annealing 200 °Cinitial 380 °C 590 °C

15. Plastic instabilities in Al–Fe-Si foils

16. Instabilities after strain rate jump

17. Instabilities after an instant increase of stress by 3 MPa

18. Portevin – Le Chatelier effect?

19. Stress or strain oscillations?

20. Stability analysis Constitutive equation Evolution of perturbations Homogeneous solution

21. Stability analysis Instability grow The rate of perturbations growth  >0

22. Stability analysis Hill’s criterion For negative SRS For positive SRS  e ~  e n

23. Simulations Ring-shaped zone of localized intense strain rate

24. Simulations

25. Thin film plastic deformation Biaxial plastic deformation of Al thin films

26. Influence of temperature

27. Biaxial plastic deformation of Al thin films Creep-fatigue tests Variation of maximum strain with the number of cycles Deformation loops received during cycling

28. Deformation processes in Al-Zn-Mg- Cu thin films 4  m thin films from AA 7075 alloy