Mechanical Properties of Materials

Mechanical Properties of Materials

• Mechanical properties are crucial for understanding and predicting how a material behaves under stress.
• Key characteristics like force, stress, strain, strength, toughness, hardness, friction, and wear are vital to understanding material properties.
• These characteristics allow for the identification of materials such as polymers, ceramics, and metals.
• Understanding reasons for material failure and designing dental restorations and appliances are important applications of these properties.
• Standardization of laboratory tests is essential to control quality and allow consistent comparisons across different researchers.

Force

• Force is generated when one body interacts with another.
• Forces can be applied directly through contact or remotely, such as gravity.
• The SI unit of force is the Newton (N).

Stress

• Stress is an internal resistance to external forces applied to a body that is tending to deform it.
• It is denoted by σ (sigma).
• Stress = Force/Area.
• Pascals (Pa) is the unit for stress; 1 Pa = 1 N/m².
• Stress is commonly reported in MegaPascals (MPa); 1 MPa = 10⁶ Pa.

Types of Stress

• Axial Stress:
  • Compressive Stress: Occurs when forces push towards each other along the same line.
  • Tensile Stress: Occurs when forces pull away from each other along the same line.
• Non-Axial Stress:
  • Shear Stress: Occurs when forces act parallel to a surface but not along the same line.
  • Torsion: Occurs due to twisting of a body.
  • Bending: Occurs due to bending movement.

Strain

• Strain is the change in length per unit length of an object under stress.
• It's denoted by ε (epsilon).
• Strain = (Change in length) / (Original length).
• Strain is a unitless measure.
• Strain can be elastic, plastic, or both.

Stress-Strain Relationship

• Testing materials under tension, compression, or shear loads using universal testing machines measures stress and strain.
• A graph plotting stress against strain (stress-strain curve) is produced.

Stress-Strain Curve

• Proportional Limit (A): Highest stress point where stress is linearly proportional to strain.
• Elastic Limit (B): Maximum stress a material can withstand without permanent deformation.
• Yield Stress/Proof Stress (C): Stress where permanent deformation begins.
• Ultimate Strength (D): Maximum stress the material can withstand before failure.
• Fracture Strength (F): Stress at which the material fractures.

Stress-Strain Behavior: Types of Strain

• Elastic Deformation: Reversible; the material returns to its original shape when the stress is removed.
• Plastic Deformation: Irreversible; the material does not return to its original shape when the stress is removed.

Stress Terms

• Proportional Limit: The maximum stress below which stress is directly proportional to strain.
• The significance of the proportional limit is that any stresses beyond this cause permanent deformation in the structure. Materials that have high proportional limits can handle greater stress without permanent deformation.
• Elastic Limit: The maximum stress a material can withstand without permanently deforming.

Significance (Dental Application)

• Dental restorations are made with high proportional limits to prevent permanent deformation damage during mastication.
• Excessive stress beyond the elastic limit can lead to restoration failure and discomfort.
• For example, a fixed partial denture, if deformed by excessive occlusal force, exhibits altered occlusal contacts.
• Dental materials need high yield strength to prevent plastic deformation from occlusal forces.

Strain Terms

• Flexibility: Maximum strain a material can withstand before its elastic limit. A material with high flexibility is important in some dental restorations.
• Ductility: Amount of plastic strain a specimen experiences before fracture. It is the ability to be drawn into a wire.
• Brittleness: Shows minimal plastic deformation before fracture. A brittle material fractures near the proportional limit.

Elastic Modulus (E)

• Constant of proportionality between stress and strain.
• Represents the slope of the elastic portion of the stress-strain curve.
• Measures rigidity or stiffness; higher Young's modulus means a stiffer material.

Energy Terms

• Resilience: Represents the amount of energy required to deform a material to its proportional limit.
• This is measured by the area under the elastic part of a stress-strain curve.
  • Material resilience is important for denture linings, and tissue conditioners.
• Toughness: Resistance of a material to fracture, represented by the area under the elastic and plastic portions of a stress-strain curve.
  • Fracture toughness refers to the ability of a material to withstand crack propagation.

Analysis of Stress-Strain Curve

• A material with a long longitudinal part of a stress-strain curve is stiff and has high strength.
• A material with a short longitudinal part is weak.
• Graph shape helps determine how tough, brittle, or flexible a material is.

Dental Applications Summary

• High proportional limit is important for dental restorations to withstand chewing forces.
• High resilience is important in resilient-denture linings, maxillofacial pads, and tissue conditioners.
• High toughness is necessary for dental materials to resist fracture during use.
• Stiff materials are often used where large forces are needed, while flexible materials are used for smaller forces or for situations where slight deformation is acceptable.