Substrate stiffness determination in curved layered composites using bending methods

Bending tests are conveniently used to determine mechanically the elastic modulus of materials. However, in the case of asymmetrical, coated specimens that exhibit warping due to mismatch in thermal or isothermal expansion of the individual layers, non-linear deformation behaviour is present and the stiffness evaluation becomes difficult with this method. A bending test procedure is outlined which uses a flat composite specimen obtained by joining two spherically curved layered specimen along their coating surfaces. The elastic behaviour of the joint specimen essentially reflects the stiffness of the substrate since the thin coatings and the thin adhesive layer are located close to the neutral specimen axis of the bending test. The method is illustrated using warped anode-substrate/electrolyte sheets of solid oxide fuel cells. The results of ring-on-ring tests with anode substrate or electrolyte under tension and with the joint specimen are presented and discussed.