Twice-buckled cermet composite laminates under equibiaxial compression

Experimental data on the buckling and fracture of thin, layered composite plates under biaxial compression is presented and compared to results from both non-linear analytical and finite element models. The square plates considered are comprised of ceramic and ceramic-metallic (cermet) layers in a symmetric composite layup, and are fully clamped (in-and out-of-plane restraints) on all four edges. Under a state of equibiaxial compression, such plates are known to bifurcate at least twice: the first bifurcation is the classic out-of-plane mode with in-plane symmetry, while the second bifurcation involves out-of-plane deformations having an asymmetric in-plane spiral pattern. The composite laminates are formed using microfabrication methods, with the plate geometry being realized by anisotropic etching of the single-crystal silicon substrate. The equibiaxial stress is introduced via residual stresses from fabrication, as well as thermal cycling. Modeshapes, center deflections, and other critical points from both analyses compare well to the experimental data for a wide range of specimens (plate thickness and width are varied). Advantages and disadvantages of buckling experiments using microfabrication and thin-film processing are discussed, one advantage being that microfabrication allows many (100's) of buckling tests to occur on a single wafer providing good statistical significance. The plates considered have application as a portable fuel cell device, which must be designed to withstand residual stresses from fabrication as well as extrinsic stresses during temperature excursions of ∼.600°C during operation. While the composite plates are buckled (or even twice buckled), they are not failed and have operated as fuel cells. Future work includes design and verification of large-area ultra-thin plate structures in the post-buckling regime for various device applications.