| Couple: | (english) A system of forces composed of two equal forces of opposite direction, offset by a distance. A couple is statically equivalent to a moment whose magnitude equals the magnitude of the force times the offset distance. |
| Deflection: | (english) This word usually carries the same meaning as displacement, although it is sometimes used in place of deformation. |
| Deformation: | (english) A change in the shape of an object or material. |
| Degree of Freedom: | (english) A displacement quantity which defines the shape and location of an object. In the two dimensional plane, a rigid object has three degrees of freedom: two translations and one rotation. In three dimensional space, a rigid object has six degrees of freedom (three translations and three rotations). |
| Displacement: | (english) A change in position. A displacement may be a translation a rotation or a combination of those. |
| Distributed load: | (english) An external force which acts over a region of length, surface, or area: essentially any external force which is not a concentrated force. |
| Ductility: | (english) Ductility generally refers to the amount of inelastic deformation which a material or structure experiences before complete failure. Quantitatively, ductility can be defined as the ratio of the total displacement or strain at failure, divided by the displacement or strain at the elastic limit. |
| Dynamic equilibrium: | (english) Equilibrium which includes inertial forces. |
| Elastic: | (english) A material or structure is said to behave elastically if it returns to its original geometry upon unloading. |
| Elastic energy: | (english) The energy stored in deformed elastic material (e.g., a watch spring). Elastic energy equals where k is the stiffness, and is the associated deflection. Elastic energy is sometimes called elastic potential energy because it can be recovered when the object returns to its original shape; see potential energy. |