Dr. Omar S.M.J.Ali PhD Orthodontic Dental Material Dr. Omar S.M.J.Ali PhD Orthodontic

Proportional limit (P) The stress strain curve is a straight line up to point P after which it curves. The point P is the proportional limit, i.e. up to point P the stress is proportional to strain. Beyond P the strain is no longer elastic and so stress is no longer proportional to strain. Thus proportional stress can be defined as the greatest stress that may be produced in a material such that the stress is directly proportional to strain. The proportional limit deals with proportionality of strain to stress in the structure.

Elastic Limit (E) Below the proportional limit (point P) the material is elastic in nature, that is, if the load is removed the material will return to its original shape. Thus Elastic limit : define as the maximum stress that a material will withstand without permanent deformation. The elastic limit describes the elastic behavior of the material.

Yield strength The stress at which a material begins to deform plastically whereas yield point is the point where nonlinear (elastic + plastic) deformation begins. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible.

Ultimate tensile strength: It is the maximum stress that a material can withstand before failure in tension.   Ultimate compressive strength: It is the maximum stress that a material can withstand before failure in compression.

Permanent deformation (Plastic Deformation ) Once the elastic limit of a material is crossed by a specific amount of stress, the further increase in strain is called permanent deformation, i.e. the resulting change in dimension is permanent. If the material is deformed by a stress at a point above the proportional limit before fracture, the removal of the applied force will reduce the stress to zero, but the strain does not decrease to zero because plastic deformation has occurred. Thus, the object does not return to its original dimension when the force is removed. It remains bent, stretched, compressed, or otherwise plastically deformed.

Clinical Application An elastic impression material deforms as it is removed from the mouth. However, due to its elastic nature it recovers its shape and little permanent deformation occurs.

Resilience It is the amount of energy absorbed by a structure when it is stressed not to exceed its proportional limit. Resilience can be measured by calculating the area under the elastic portion (straight line portion)

Toughness It is the energy required to fracture a material. It is also measured as the total area under the stress strain curve (elastic and plastic portions of stress strain curve). Toughness is not as easy to calculate as resilience.

Brittleness It is the relative inability of a material to sustain plastic deformation before fracture of a material occurs.  Brittleness is generally considered as the opposite of toughness, glass is brittle at room temperature. It will not bend appreciably without breaking. It should not be wrongly understood that a brittle material is lacking in strength, from the above example of glass we see that its shear strength is low, but its tensile strength is very high, if glass is drawn into a fiber, its tensile strength may be as high as 2800 MPa.

Ductility It is the ability of a material to withstand a permanent deformation under a tensile load without rupture. A metal that can be drawn readily into a wire is said to be ductile. It is dependent on tensile strength. Ductility decrease as the temperature increased

Hardness The hardness is the resistance to permanent surface indentation or penetration. The value of hardness, often referred to as the hardness number, depends on the method used for its evaluation. Generally, low values of hardness number indicate a soft material and vice versa

Brinell test: A steel ball is used, and the diameter of the indentation is measured after removal of the indenter. Rockwell test: A conical indenter is impressed into the surface. Under a minor load (dashed line) anti a major load (solid line), and M is the difference between the two penetration depths. Vickers test: A pyramidal point is used, and the diagonal length of the indentation is measured.

Relaxation After a substance has been permanently deformed (plastic deformation), there are trapped internal stresses; such situations are unstable. The displaced atoms are not in equilibrium positions through a solid-state diffusion process driven by thermal energy, the atoms can move back slowly to their equilibrium positions, the result is a change in the shape or contour of the solid as the atoms or molecules change positions. The material warps or distorts.

Clinical Application This stress relaxation leads to distortion of elastomeric impressions. Waxes and other thermoplastic materials like compound undergo relaxation after they are manipulated.

Friction It is the resistance to motion of one material body over another. If an attempt is made to move one body over the surface of another, a restraining force to resist motion is produced; this restraining force is the (static) frictional force and results from the molecules of the two objects bonding where their surfaces are in close contact.

Wear It is a loss of material resulting from removal and relocation of materials through the contact of two or more materials

Adhesion and Cohesion Adhesion is the force which causes two different substances to attach when they are brought in contact with one another. When the molecules of the same substance hold together; the forces are said to be cohesion

Viscosity It is the resistance offered by a liquid when placed in motion, e.g. honey has greater viscosity than water.

Creep It is the increase in strain in a material under constant stress. It is time dependent plastic deformation or change of shape that occurs when a metal is subjected to a constant load near its melting point.