Dental Materials and Fracture Types

Failure of Dental Materials

• Dental materials can fail in various ways, including tarnish, corrosion, wear, fracture, and crazing.

Types of Fracture

• Fracture is the process of creating new surfaces within a material.
  • Tensile fracture mode: Occurs during direct loading, like tensile testing.
  • Controlled crack propagation: Occurs when a predetermined crack is introduced, as in impact resistance tests.
  • Fatigue fracture: Results from cyclic loading.
  • Creep fracture: Happens after prolonged static loading, leading to plastic flow and eventual fracture.
  • Stress corrosion cracking: Occurs when a material is in a hostile environment under stress, causing degradation and weakening.

Appearance of Fatigue Fracture

• Fatigue fracture patterns show initiation, crack propagation, and catastrophic rupture stages. Images of these stages are presented.

Ductile Fracture

• Ductile Fracture: Materials fracture after plastic deformation, showing slow crack propagation. The broken surface looks dull or fibrous.
  • Occurs when the material is in a plastic condition.
  • Characterized by cup-and-cone formation due to high stress.
  • Cross-sectional area decreases. Examples include most metals.

Brittle Fracture

• Brittle Fracture: Materials fracture with little or no plastic deformation. The surface is shining and crystalline.
  • Occurs when the material is in an elastic condition.
  • Characterized by clean separation under tensile stress.
  • Cross-sectional area remains unchanged. Examples include ceramics.

Stages of Ductile Fracture

• The stages of ductile fracture, from initial necking to crack propagation are illustrated.
• Necking is the narrowing of a specimen.
• Cavities form at points of stress.

Crazing

• Crazing: Fine cracks appearing on a denture surface due to tensile stresses, causing polymer chain separation. This substantially weakens the denture.
  • Commonly found in thin layers of denture base resin around artificial teeth.

Causes of Crazing

• Repeated drying and wetting: Creates alternating stresses that cause contraction and expansion in the denture.
• Thermal expansion differences: Differences in the coefficient of thermal expansion between denture base acrylic and porcelain teeth.
• Solvent action: Solvents like alcohol, acetone, or monomer (especially during repairs) can weaken the resin, leading to crazing.
• Cross-linked dentures are less prone to crazing.

Explore the various failures of dental materials, focusing on different types of fractures including tensile, fatigue, and creep fractures. This quiz covers the mechanisms behind each fracture type and their appearances, along with practical implications in dental practice.