Mechanical Properties OF Dental Materials PDF

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MECHANICAL PROPERTIES OF DENTAL MATERIALS What does the average biting force depend on?    

Location in the dentition Types of food Condition of the dentition Sex age race

When is deformation elastic? When the deformed material after the removing load restores to its original form and size When is deformation plastic/residual? When the stress is enough to deform the metal permanently When does the destruction come as a result of? Breaking between the atomic and molecular links in the material What do mechanical properties indicate? The degree of load which generates deformations and destructions of the material What is strength? Ability of materials to resist against any deformation and failure load How is strength tested static load; dynamic load and fatigue strength What is tensile strength ability to withstand pulling forces Equation for tensile strength ultimate tensile strength (N)/cross section (mm^2) Which materials can be tested by tensile strength metal alloys and resins Tests for tensile strength diametrical tensile strength test or diametrical compressive test Does dentin or enamel have a higher tensile strength Dentin What is compressive strength ability to withstand pressing forces Test for compressive strength increasing driving force causes the material to deform and crack and destroy Equation for compressive strength downward force/cross section

Which materials can be tested by compressive strength restorative, refractory investment, gypsum materials and resin Does dentin or enamel have a higher compressive strength dentin What is flexural strength ability to withstand a perpendicular force to a longitudinal axis Equation for flexural strength 1.5x(max load of fracture x distance between supports)/ cross section of bar What are the stresses on the upper and lower surface called upper-> compressive lower-> tensile What materials are tested for flexural strength porcelain materials Does dental enamel or dentine have a higher flexural strength enamel What is shear strength ability to withstand opposing shear forces What does shear strength investigate forces of adhesion among materials Materials with highest shear strength alloy for metal ceramic and feldspar porcelain What does stress-strain diagram represent and axis deformation changes y axis: stress applied (MPa) x axis: deformation What is proportional limit greatest amount of stress where the amount of stress is proportional to strain What is elastic limit maximal stress to which the material is elastic, stress is in elastic region What is yield strength maximum stress that causes 0.1-0.2% plastic deformation What is maximum strength maximum strength which the material can withstand without being destroyed What does elasticity and rigidity indicate degree of plastic deformation under some load in the region of proportionality Relationship between elastcitity and rigidity increase in rigidity-> decrease elasticity

What is elasticity represented by angle of straight line in region of proportionality Which is more elastic dentin or enamel dentin (smaller angle) What happens when E (young modulus) increases elasticity decreases for thinner prosthetics Advantages of high modulus of elasticity thinner prosthetics, no danger of elastic deformations during chewing What happens with greater elasticity deforms more so ruptures adhesive bonds and collusive forces What is brittleness material destroyed without deformation Brittle materials dental enamel, dental ceramics, zinc phosphate cement, acrylic resin and some composite materials What is toughness ability to absorb energy under some stress Why do tough materials need energy for destruction they have great elastic and plastic deformation How is toughness determined the index of fracture toughness and area under stress strain graph What is fracture toughness and what does it take into account resistance of material to destruction in presence of cracks, takes into account effects of fracture Which is tougher dentin or enamel Dentin- because it is more brittle What is ductility degree of plastic deformation under stress over proportional limit How is ductility determined in tensile strength test plastic deformation causes elongation of test body. Ductility is proportional to relative elongation Equation for ductility length of body after/ length of body before x 100 Why are the values for relative elongation useful help find materials for cold work e.g. cutting drilling, bending, hamering

Relationship between ductility and elastic and plastic deformation decrease ductility= increase elastic deform+ decrease plastic deform What is impact strength tests for stress required to destroy material using sudden max impact Test for impact strength sharpy-type impact test: pendulum attached and material is fixed on both ends in the tester. Pendulum released where potential energy converted to KE. Destroys test body. Less the deviation of the pendulum the stronger the material Equation for impact strength weight of pendulum x (height of base line- height after rupture)/ cross section of body Difference between Izod method and sharpy test body fixed unilaterally in Izod What is fatigue strength resistance to cyclic loads where it doesn’t destroy the material What is the fatigue of materials due to the reduction of the forces of adhesion among its building elements Scheme of fatigue strength microscopic cracks appear, deepen and widen until material breaks under minor influence What materials are tested for fatigue strength ceramic, plastic and metal dental materials What is hardness resistance to penetration of a harder body Scratching method for hardness -> has diamond pyramid called microcharacter: bierbaum microcharacter test Difference between bier baums test and turners b. baums measures depth of furrow and t. turners’ measures width of furrow Indentation method for hardness pressing body of specific shape using specific load into test body causing plastic deformation How is hardness measured using size of deformation and applied force Describe Brinell’s test steel ball, tests metal alloys, BHN determined after measuring diameter of indentation Describe Vickers test square based pyramid (136 angle), tests dental tissue and material. VHN is determined after measuring length of the diagonals of indentation. Small loads=9.8N max, investigated using a microhardness tester

Knoop’s Test diamond tipped pyramid; impression is rhombic, longest diagonal of indentation is used. KHN tests brittle materials Rockwell’s test diamond cone/ steel ball. RHN is depth of indentation Why is hardness of materials important gives estimate to materials strength properties and wear resistance Formula of VHN 1.05 x BHN Formula of BHN VHN/1.05 What is A-Shore used for determining hardness of elastic materials where the penetrating body leaves no indentation Describe A-Shore cylindrical penetrating body, connected to graduated scale 0 to 100. Penetrating body is pressed into material scale is read. Soft= increase penetration and low values What is wear resistance ability to resist abrasion. Wear occurs during mastication between antagonistic teeth What is the ability to cause wear called abrasiveness Different factors affecting wearing corrosion, fatigue of material, increased load, pressure of lubricating fluid, bad habits What is physiological abrasion 15-30 um/year 2 bodied abrasion particles from harder materials erode softer materials, abrasive tools based of this 3 bodied abrasion abrasive environment between 2 surfaces, brush and polishing paste Adhesion abrasion cold welding friction between 2 surfaces in certain conditions and forces. Substances from 1 to other or removed Fatigue of material strikes with a force of less than max strength, causes microcracks deepening with fraction-> fragmentation Tribochemical chemical degradation, exposes new surfaces to degrade and abrade

Ways wear resistance can be tested: Clinical observations most accurate, in vivo investigations, takes several years, very expensive and slow Lab apparatus rotating cylinder covered in abrasive material, moved along test body, coolant flushed, heat increases rate of wear How is wear resistance measured loss of height/weight; pressure of compression; time of abrasion; abrasion material used; rotation speed; coolant used Dental simulators reproduce mastication mechanically. Quick easy, cheap. In vitro investigation What should an ideal restorative material possess a wear resistance similar to that of enamel What can the inadequate wear resistance of dent materials lead to reduced durability, exchanging of shape during mastication, TMJ disorder, endodentric complications What does the optimal treatment with a max durability require right choice of materials according to wear resistance and abrasiveness, functional distribution of masticatory forces and polishing of restorations...