Mechanical Properties of Dental Materials

Mechanical Properties

• Mechanical properties are a group of properties dealing with force and its effect on dental materials.
• They are important because restorative materials need to withstand forces during placement and function (e.g., chewing).

Force

• Force is an external action that tends to produce motion in a body.
• Force is characterized by:
  • Magnitude
  • Direction
  • Point of application
• Force can be static or dynamic.
• Units: kg, pounds (Ib), Newtons (N).

Stress

• Stress is the internal reaction to an external force.
• It's equal in intensity but opposite in direction to the external force.
• Stress = Force/Area

Types of Stress

• Compressive stress: Two forces acting towards each other in a straight line. Tend to shorten the object.
• Tensile stress: Two forces acting away from each other in a straight line. Tend to stretch or elongate the object.
• Shear stress: Two forces directed parallel to each other, but not in the same line. Causes atoms to slide past each other, potentially leading to tearing.
• Complex stress: The result of multiple types of stresses acting on an object.

Deformation and Strain

• Strain is the change in length per unit length of an object when a stress is applied.
• Strain = (Final Length - Original Length)/Original Length
• Strain has no units.
• Strain is often reported as a percentage.
• Types of strain:
  • Elastic strain (temporary): Returns to original shape when stress is removed.
  • Plastic strain (permanent): Doesn't return to original shape when stress is removed.

Stress-Strain Curve

• Stress-Strain curves show the relationship between stress and strain for materials.
• Stress is plotted on the vertical axis; strain is plotted on the horizontal axis.
• Two types of curves:
  • False (engineering) curve: cross-sectional area is constant.
  • True curve: cross-sectional area changes during testing.

Stress Terms

• Proportional limit (P.L.): Maximum stress without deviating from the stress-strain proportionality.
• Elastic limit (E.L.): Maximum stress a material can withstand without permanent deformation.
• Elastic region: Region between 0 and the elastic limit.
• Plastic region: Region beyond the elastic limit.

Material Properties

• Yield Strength (YS): Stress at which a material begins to deform plastically. Important for dental materials to withstand stress without permanent deformation.
• Ultimate Strength: Maximum stress a material can withstand before fracturing.
• Fracture Strength: Stress at which a material fractures.
• Elastic Modulus (Young's Modulus): Measures stiffness or rigidity; the ratio of stress to strain.

Importance of Mechanical Properties

• Rigidity of major connectors in dentures affects stability.
• More rigid alloys allow for thinner sections, reducing risk of bending.
• Rigid cement bases increase the fracture resistance of fillings.

This quiz focuses on the mechanical properties relevant to dental materials, including definitions of force, stress, and the various types of stress that affect these materials. Understanding these concepts is critical for ensuring that restorative materials endure the forces they encounter during use. Test your knowledge of these foundational principles in dentistry!