Mechanical Properties of Dental Materials Exam 2025

Force - Correct Ans-Generated by one body pushing or pulling on another Defined by: -point of application -magnitude -direction of application Average maximum sustainable biting force - Correct Ans-756 N (170 lbs) Stress - Correct Ans-Force/Area... (sigma) Units = N/m^2 = Pascal Types of stress - Correct Ans-Axial (tensile) - elongation Axial (compression) - shrinkage Shear - shear Twisting moment - torsion Bending moment - bending Strain - Correct Ans-A body undergoes deformation when a force is applied to it. Change in length per unit length of the body when it is subjected to a stress... Epsilon = deformation/original length = (L-L_o)/L_o Elastic limit - Correct Ans-The maximum stress that a material will withstand without permanent deformation Proportional Limit - Correct Ans-The greatest stress that a material will sustain without a deviation from the linear proportionality of stress to strain. (differs by definition from elastic limit) Yield strength - Correct Ans-The stress value at which a material exhibits a specified permanent strain *Usually 0.2% (percent offset) is used Ultimate tensile strength - Correct Ans-The maximum stress that a material can withstand before failure in tension Fracture strength - Correct Ans-The stress at which a material fractures *Not necessarily equal to ultimate strength Stiffness - Correct Ans-Elastic modulus - represents the stiffness within the elastic range Stress/strain = E... a greater E means a stiffer material A more flexible material will experience more strain with less stress placed on it. Brittle vs. Ductile fracture - Correct Ans-Brittle - breaks quickly? Ductile - able to deform before breaking... Brittle ex: amalgam, composites, cements, ceramics Ductility - Correct Ans-The ability of a material to be plastically deformed Burnishing - Correct Ans-Closing the metal margin of a metal crown... Recovery of elastic strain but not of plastic strain Resiliency - Correct Ans-The resistance of a material to permanent deformation important in orthodontic wires! May be thought of as energy absorbed per volume of material Toughness - Correct Ans-The resistance of a material to fracture Also thought of as energy absorbed per volume of material Strong vs. weak - Correct Ans-A strong material can withstand a higher amount of stress before experiencing permanent deformation. A strong material can withstand a higher amount of stress before entering the "Plastic" region. Ultimate compressive strength - Correct Ans-The maximum stress that a material can withstand before failure in compression. This is a factor in determining stress-bearing capability in a restorative material. Impact strength - Correct Ans-The energy required to fracture a material under an impact force Popular for comparison of denture base materials Diametral compression test - Correct Ans-(Indirect tensile test) Useful for testing brittle materials to obtain a tensile strength Not valid if the material deforms significantly before failure Apply load/compression support, which in turn applies tensile stress... see how much is needed to cause a tensile failure. Shear strength - Correct Ans-The maximum stress that a material can withstand before failure in shear mode Used for bond strength testing -Porcelain/metal interface -Interface between tooth and orthodontic brackets, composite resins, and adhesives Microtensile bond strength - Correct Ans-Alternative to shear bond testing Typically used for adhesives. Composite:Dentin... "pull" or "stretch" this bonding until failure. Bending - Correct Ans-Combination of compressive and tensile loads with a neutral axis separating the two. Bending moment = force*distance (b/n applied force and fulcrum) Cantilever bending tests - Correct Ans-Pure bending: Force stays perpendicular to object Transverse bending: Force stays vertical Angular deflection curves - Correct Ans-Bending moment vs. bending angle If bending angle exceeds the linear portion (elastic strain), it experiences permanent strain/bending **Larger wires = stiffer and enter into permanent strain at lower bending angles. 3 point bend test - Correct Ans-Also called transverse strength or flexural strength Used for denture based materials and long bridge spans **Higher compressive or tensile stress towards surface (further away from neutral line) Torsion - Correct Ans-Twisting Important in endodontic files and reamers rotated in the root canal Produces a shear stress, from zero at the center and increasing radially outward to a *maximum at the surface Torsional moment = shear force * distance b/n location of force and center (r or radius) = F * r Fatigue - Correct Ans-Progressive fracture under repeated loading Stresses frequently vary cyclically (mastication) Failure occurs below the single load strength (static), e.g. ultimate tensile strength Failure typically due to crack propagation Factors affecting fatigue - Correct Ans-Environmental agents: temperature, humidity, pH, aqueous media, biological substances Surface features: Smooth surfacerough surface Material history Fracture toughness (K_IC) - Correct Ans-A measure of a material's resistance to brittle fracture when a crack is present Useful for brittle materials because failure is frequently flaw dependent (fail due to crack propagation) Viscoelasticity - Correct Ans-Having appreciable and conjoint viscous and elastic properties Behavior is intermediate between elastic solids and viscous liquids Combination of elastic & plastic deformation and elastic recovery when stresses are eliminated. Creep - Correct Ans-The time-dependent plastic strain of a material under a static load or constant stress (even if below elastic limit) Occurs in materials at temperatures above ~ 1/2 melting temperature - problem seen in amalgams (Hg has a low melting point) although composition changes (increase in copper) have decreased the amount of creep Stress relaxation - Correct Ans-The reduction in stress in a material subjected to constant strain Useful in evaluating orthodontic modules or chains - some modules may sustain forces longer than other ***Ex: 2 elastic bands extended/stretched a fixed amount... the force needed to keep each extended decreases over time Hardness - Correct Ans-The resistance to permanent surface indentation Tests differ based upon indenter material, geometry and load. ** Smaller indentations -- higher hardness values = harder materials Dental importance of hardness - Correct Ans-Some (not direct) relation to wear Affects ability to finish or polish a material, which is desired esthetically Gives indication to resistance to in-service scratching -Scratches may decrease fatigue strength and lead to failure. Brinell hardness test - Correct Ans-Used for metals A hardened steel ball indenter is used and the DIAMETER of indentation is measured. Brinell hardness number (BHN or HB) is reported. Rockwell hardness test - Correct Ans-Used for metals & plsatics A ball or metal cone indenter is used and the DEPTH of indentation is measured Rockwell A, B, C... G, based upon indenter geometry and load. Rockwell hardness number (RHN or HR) is reported Vickers hardness test - Correct Ans-Micro indentation test Used for metals & plastic The indenter is a 136 deg pyramidal diamond that forms a square indent Vickers hardness number (VHN or HV) Knoop hardness test - Correct Ans-Micro indentation test Used for variety of materials The indenter used is a rhombic-based pyramidal diamon that produces an elongated diamond shaped indent Knoop hardness number (KHN or HK) Barcol & Shore - Correct Ans-Additional hardness tests Barcol - dental material applications include measuring resin composites Shore - used for rubber (denture liners and mouth protecters) Wear - Correct Ans-A loss of material resulting from removal and relocation of materials through the contact of two or more materials Bruxism - pathological form of wear Most common dental - abrasive wear Abrasive wear (two-body) - Correct Ans-Two surfaces rubbing together. The harder surface may indent or cut away the softer material from the other surface. Ex: tooth to tooth contact Abrasive wear (three-body) - Correct Ans-When objects (abrasive agents) are between the two surfaces (don't have to be in contact) Ex: Food during mastication, toothpaste between tooth & toothbrush.