Biaxial bulge testing of thin films and foils

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

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

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

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

Biaxial bulge test Spherical cap model

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

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

Examples • Thin Al-Fe-Si foils (thickness 8. 5 mm) Element Fe Si Cu Mn Mg Zn Ti Al wt. % 0. 51 0. 61 0. 007 0. 020 0. 0066 0. 022 0. 024 bal.

Initial microstructure after homogenization 590 °C/30 min

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

Yield stress variation of predeformed aluminium foils with annealing temperature

Microstructure after predeformation and annealing initial 200 °C 380 °C 590 °C

Plastic instabilities in Al–Fe-Si foils

Instabilities after strain rate jump

Instabilities after an instant increase of stress by 3 MPa

Portevin – Le Chatelier effect?

Stress or strain oscillations?

Stability analysis Constitutive equation Homogeneous solution Evolution of perturbations

Stability analysis The rate of perturbations growth Instability grow w>0

Stability analysis For positive SRS se ~ є e n Hill’s criterion For negative SRS

Simulations Ring-shaped zone of localized intense strain rate

Simulations

Thin film plastic deformation Biaxial plastic deformation of Al thin films

Biaxial plastic deformation of Al thin films Influence of temperature

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

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