Stress state ice failure criteria with applications for plastic limit analysis of ice forces on offshore structures
Description
An inclusive review of the engineering methods for predicting ice forces on structures in ice infested waters is presented. Interaction forces, particularly in the horizontal direction, are described by initial impact and by pressure fields induced by the crushing, bending and fracture strength of the ice moving past the structure. Important parameters for the description of ice forces are identified and discussed. Methods of analysis and published experimental results are addressed with frequent references given to pertinent literature Because ice strength, modes of failure and deformation mechanics play important roles during ice-structure interaction, a general summary of the strength and properties of ice is presented. Of particular importance is the description of ice failure criteria for complex stress states. Two failure criteria, a modified Von Mises function and a modified Drucker-Prager function, are introduced as approximate methods of representing ice failure in ductile and brittle modes while retaining the simplicity of conventional criteria. Their suitability to ice mechanics is enhanced by their capability of being adapted to a wide range of tension-compression failure envelopes. Such criteria offer a mathematical representation of the strength of particular ice types under multiaxial loading conditions These failure criteria together with an appropriate approximation of the rheological behavior of ice allow for the use of plastic limit analysis methods to predict the collapse pressures required to indent an ice medium. For this purpose the mechanical behavior of ice is approximated as an elastic-perfectly plastic material. Such analytic studies are of interest as an idealization of the process of an ice sheet crushing against a vertical structure. Ice indentation forces can thus be estimated by establishing the penetration pressures of a flat indenter into an ice sheet A study is presented of the effects of ice strength parameters on the indentation pressure for ice sheets under plane stress and plane strain conditions. Results indicate the dependence of indentation pressures primarily upon confined and unconfined compression strength and less upon tensile strength. Lower bound solutions are obtained using adaptations of the modified Von Mises criterion and the modified Drucker-Prager criterion as well as criteria employed previously in ice mechanics