Dental Materials and Fracture Types

Study Notes

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

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.

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

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: 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.

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: 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.

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.