Criteria for Yielding or Failure of Cellular Materials

Sandwich structures are used extensively in marine structures and other applications. Cellular materials such as foams, honeycomb, and balsa wood used as core materials in sandwich structures are subjected to multiaxial loadings. Concentrated loads, impacts, and explosions can result in large deformations and damage, and yield or failure criteria are needed to predict the end of the elastic phase. Yielding or failure of isotropic foams is generally dependent on the hydrostatic stress, a feature that these materials share with many polymers, concrete, and soils. Appropriate criteria which are different from those traditionally used for metals have been classified in four major categories. Similarities between seemingly different criteria and between foams and other materials are brought out. Foams can also be anisotropic as honeycomb and balsa wood, and the anisotropy has to be accounted for in addition to the differences in strength in tension and compression in various directions. Suitable anisotropic criteria are shown to be the extensions of criteria previously used for isotropic materials and methods to present these criteria in a unified manner or to systematically extend isotropic criteria are discussed. Finally the strain rate effects and size effects are discussed.