ABSTRACT
Material nonlinearity occurs when properties of the material change significantly under load. For example, in stress analysis, continued loading of a ductile material beyond the elastic limit results in a plastic contribution to the total strain, which then depends on the current state of stress. In seepage analysis, hydraulic conductivity may depend on fluid pressure. Desaturation occurs with the result that a much higher pressure gradient is required to move water through the film adjacent to the solid skeleton of the partially saturated rock mass. Resistance to fluid flow is greatly increased, while hydraulic conductivity is greatly reduced under these conditions. In both cases, the onset of material nonlinearity is marked by a limiting value of one of the unknowns. There are two common approaches to material nonlinearity in finite element analysis; these are incremental and iterative approaches. Both require reassembly of the master stiffness matrix, although a modified Newton– Raphson approach avoids reassembly when the material nonlinearity is not widespread.
