Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials

• Mechanical properties describe a material's ability to resist forces and their effects on the material.
• Stress is the force per unit area induced in a body due to an externally applied force.
• Strain is the change in length or deformation per unit length caused by an applied force.
• Tensile stress results from forces pulling away from each other in the same line.
  • Examples: enamel (10 Mpa), dentin (106 Mpa), amalgam (32 Mpa)
• Compressive stress results from forces pushing toward each other in the same line.
  • Examples: enamel (384 Mpa), dentin (297 Mpa), amalgam (388 Mpa)
• Shear stress results from forces acting parallel to each other but not in the same line, causing molecules to slide over each other.
  • Examples: enamel (90 Mpa), dentin (138 Mpa), amalgam (188 Mpa)
• Flexural stress (bending stress) results from a bending moment, causing tensile, compressive, and shear stress at the same time.
• Torsion stress results from twisting a body, causing a deformation.
• Elastic strain is temporary and recoverable, meaning the material returns to its original length after the force is removed.
• Plastic strain is permanent and unrecoverable; the material does not return to its original length after the force is removed.

Stress-Strain Curve

• A stress-strain curve is a graphical representation of the relationship between stress and strain in a material.
• It helps compare the mechanical properties of materials by analyzing the applied force and resulting stress and strain.
• Proportional limit is the greatest stress a material can withstand without deviating from proportionality between stress and strain.
• Elastic limit is the maximum stress a material can withstand without permanent deformation.
  • The area before the proportional limit is the "elastic region."
  • The area after the proportional limit is the "plastic region."
• Ultimate strength is the maximum stress a material can withstand before failure.
  • Examples: acrylic (8000 PSI), Co/Cr (100000 PSI), stainless steel (15000 PSI)
• Fracture strength is the stress at which a material fractures.

Additional Mechanical Properties

• Modulus of elasticity (Elastic Modulus) represents the stiffness or rigidity of a material within the elastic range.
  • It is the ratio of stress to strain in the linear region of the stress-strain curve.
  • Examples: enamel (84 Gpa), dentin (17 Gpa)
• Flexibility is the amount of elastic strain a material can withstand before reaching its proportional limit.
• Ductility is the ability of a material to undergo permanent deformation under tensile stress without fracture.
  • Gold is highly ductile.
• Malleability is the ability of a material to undergo permanent deformation under compressive stress without fracture.
  • Gold is highly malleable.
• Brittleness is the opposite of ductility, characterized by a lack of plasticity.
  • Examples: porcelain, acrylic, cement, gypsum products
• Resilience is the amount of energy absorbed by a structure when stressed within its proportional limit.
• Toughness is the energy required to fracture a material.
  • It is represented by the total area under the stress-strain curve.
• Fatigue strength is the ability of a material to withstand repeated (cyclic) small stresses below the proportional limit before failure.
  • This is related to the material's resistance to change in shape due to frequent force application.