Failure of Dental Materials PDF

Failure of dental materials By Dr. Reham Mohammed Abdallah Associate professor of dental biomaterials  Items to be covered Tarnish and corrosion Wear Fracture Craze Fracture: Is a process of creating new surfaces within a body. Modes of fracture 1.Tensile fracture mo...

Failure of dental materials By Dr. Reham Mohammed Abdallah Associate professor of dental biomaterials  Items to be covered Tarnish and corrosion Wear Fracture Craze Fracture: Is a process of creating new surfaces within a body. Modes of fracture 1.Tensile fracture mode Occurs during direct loading, such as mechanical tensile testing. 2.Controlled crack propagation Occurs when a predetermined crack is made such as during impact resistance test. 3.Fatigue fracture Occurs when cyclic loading is applied. 4.Creep fracture Occurs after a long time of constant static loading leading to plastic flow of the material and eventually the material will fracture. 5.Stress corrosion cracking Occurs when the material is present in a hostile environment and under stresses leading to degradation and weakening of the material. 1. 3. 2.  Ductile fracture Brittle fracture  Material fractures after plastic  Material fractures with very little deformation and slow or no plastic deformation. propagation of crack  Surface obtained at the fracture  Surface obtained at the fracture is is dull or fibrous in appearance shining and crystalling appearance  It occurs when the material is  It occurs when the material is in in plastic condition. elastic condition.  It is characterized by the  It is characterized by clean formation of cup and cone due separation by normal applied to increased applied stresses tensile stress.  There is reduction in cross –  There is no change in the cross – sectional area of the specimen sectional area.  Ex. Most of metals  Ex. Ceramics  Stages of ductile fracture Cup and cone fracture (a) Necking, (b) Cavity Formation, (c) Cavities coalesce  form crack (d) Crack propagation, (e) Fracture Brittle fracture Crazing It is defined as fine cracks that may appear on the surface of the denture due to the tensile stresses that cause separation of the polymer chains. Crazing has a weakening effect on the denture. Crazing usually occurs in thin layer of a denture base resin surrounding the artificial teeth in a denture. Causes I- Continuous stresses due to repeated drying and wetting of the denture which cause alternate contraction and expansion. II- Difference in coefficient of thermal expansion between acrylic denture base and porcelain teeth. III- Action of solvents e.g. alcohol acetone, chloroform, or monomer e.g. when a denture is repaired, some monomer comes in contact with the resin and may cause crazing. N.B: Cross linked denture are less likely to craze.