Table 1.

List of definitions.

Elastic modulus	The stiffness or resistance to deformation of a material, which is calculated as the ratio of elastic stress to elastic strain [18].
Flexibility	The reciprocal property to stiffness of the material, which is the deformation obtained per unit of applied stress [19]. Flexibility may also be referred to as springiness.
Elastic limit	The stress at which some permanent deformation occurs (deformation is not recovered on unloading) [19].
Plastic deformation	Occurs when a load applied to a material takes it past the elastic limit, and the material does not return to zero strain on removing the load. This phenomenon occurs when some atoms or molecules cannot return to their original position on removing the load because they have gone past an energy maximum, and they continue to remain in their new position to become stable at that position [19].
Brittleness	The relative inability of a material to deform plastically before it fractures [18].
Ductility	The amount of plastic strain produced in the specimen at fracture due to tensile stress is called the ductility of the material [19].It is reported as percentage elongation.
Yield strength and ultimate strength	Yield strength is the maximum stress that a structure can withstand without sustaining a specific amount of plastic strain, and ultimate strength is the stress at the point of fracture [18].
Resilience	The ability of a metal to absorb energy when elastically deformed and then return it when it is unloaded. It is measured by the modulus of resilience, which is the strain energy per unit of volume required to stress the material from zero to yield stress [20].
Toughness	The ability of a material to absorb energy in the plastic range, measured as the total area under the stress–strain curve; hence, toughness is a function of both ductility and strength [20].
Fracture toughness	The ability of a material to absorb and/or dissipate energy, due to the applied stress, by elastic and plastic deformation before fracturing. This is measured by introducing a crack of known size and shape, and then measuring the stress required for this crack to grow [19].
Shear	Shear stress tends to resist the sliding or twisting of one portion of a body over another, where the layers of atoms or molecules of the material are envisaged as sliding over one another [19]. Shear stress can also be produced by a twisting or torsional action on a material [18].
Torsion	Rotational motion about the longitudinal axis of one end of the member relative to the other end. In torsion, each element of the material deforms in pure shear. The shear strain is directly related to the radius of the file to which it is clamped and the angle from the shank to the point at which it is measured, and inversely related to length of the file from the shank to the point of clamping [20].
Fatigue	The process of progressive, localized, and permanent structural change occurring in a material subjected to conditions that produce fluctuating stresses and strains at some point (or points), and that may culminate in cracks or complete fracture after a sufficient number of fluctuations [20].
Fatigue limit	Most materials show a continuously declining stress for failure as the number of cycles increases, but some materials show a stress below which no amount of load cycling produces a failure; this is the fatigue limit, or endurance limit [19].