Like many finite element applications, all nonlinear elements have associated with them a material. It is the material in conjunction with the element geometry that provides the force-displacement response of the element. In OpenSees materials are divided into three general types:
uniaxial materials which define a uniaxial (1 dimensional) stress-strain relationship.
nDimensional materials which define multi-dimensional (plane stress, plane strain, or 3d) stress-strain relationships.
sections which define coupled moment-curvature and axial-deformation relationships for beam column elements.
For simple investigation of material/section behavior, try the built-in material testing commands.