Mechanical Properties II PDF

MECHANIAL PROPERTIES II by Dr/Reem Gamal Hassan  ENERGY RELATED PROPERTIES I-Resilience The amount of energy absorbed by structure when it is stressed not to exceed P.L. or Resistance of the material to permanent deform...

MECHANIAL PROPERTIES II by Dr/Reem Gamal Hassan  ENERGY RELATED PROPERTIES I-Resilience The amount of energy absorbed by structure when it is stressed not to exceed P.L. or Resistance of the material to permanent deformation under sudden impact. Called stored energy because when the load is removed its released causing complete recovery of deformation it may described as spring back potential It depend on P.L and E Dr. Reem Gamal  Dental application Dr. Reem Gamal  II-Toughness It's the amount of energy absorbed by the material when its stressed to point just before fracture point It depend on Strength and ductility or malleability of the material. It is measured by the total area under there stress strain curve(elastic and plastic) Unite is mMN/m³=energy per unit volume Dental importance:- mica crystal in ceramic Dr. Reem Gamal  III-Fracture toughness the amount of energy required to fracture sample with crack Or It is the ability of the material to resist fracture through its resistance to crack propagation. its proportional to the energy consumed in plastic deformation so brittle material has low fracture toughness and ductile material has high one due to cracks has no effect on ductile material because they able to plastic deform Dr. Reem Gamal  Clinical Significance Design basis against this type of failure can be made to increase the lifetime of the material In composites The presence of glass particles increase the fracture toughness because glass will stop crack propagation. In porcelain The addition of zirconia’s particles increase the fracture toughness because zirconium particles absorb the energy needed for crack propagation In amalgam low copper amalgam has greater fracture toughness than high copper Dr. Reem Gamal  IV-Impact strength Is the amount of energy absorbed to fracture material when subjected to sudden Force. Fracture happened due to crack propagation by the energy of the sudden impact(impact strength can't initiate cracks) The units are joules (J=Nm) Dental application :- -It predict the behavior of the material under such condition, -Fracture of complete denture when dropped on a floor corrected by using rubber modified acrylic resin denture base. Dr. Reem Gamal Impact test A Charpy test Izod test Dr. Reem Gamal  Cantilever bending A beam of a material is fixed at one end and subjected to load at the other end. F bending moment=force x distance, A plot of bending momentum versus angle can obtained and have the similar appearance of stress strain curve an instrument permanently bent if the bending angle exceed the value at the end of linear proportion of the curve Dental consideration: fixed free bridge endodontic file and reamers Dr. Reem Gamal  Transverse strength test, three point bending(3PB), Flexural strength, modulus of rupture(MOR) This test is sensitive to surface condition specially on tensile side Dr. Reem Gamal From this test Stress= 3x load x length 2x width x thickness² Deformation= load x length³ 4x E x width x thickness³ Dental application:- Dr. Reem Gamal  Tear strength The resistance to tearing It depend on the rate of loading (strain rate dependent ) Important in impression materials especially in interproximal Tear strength increase by removing the impression with sharp snap parallel to the long axis of the teeth Dr. Reem Gamal  The diametral compression test (indirect tensile test , Brazillian test ) Tensile stress σ = 2P πxDxT D=Dr.diameter Reem Gamal T= thickness  Fatigue Fatigue Is the fracture of a material when subjected to repeated cyclic small stresses (dynamic fatigue) or constant load (static fatigue) below the proportional limit.(called catastrophic fracture) depend on:- the magnitude of the load the number of loading cycle repetitions. The mechanism:- repeated application of small stress to an object causes tiny microcracks which grows and coalesce to form macrocracks until the material breaks. (surface condition as roughness and sharp angles promote fracture) Dental application Dr. Reem Gamal  fatigue test Fatigue tests performed by subjecting a specimen to alternating cyclic stress application below the yield stress until fracture occurs The curve show that when stress is high the material fail at low no. of cycle endurance limit, Fatigue limit is the stress can be applied an infinite no of times without causing material failure Endurance limit of restoration should be higher than masticatory force Dr. Reem Gamal  Surface mechanical properties I-Hardness resistance of the material to scratching, permanent indentation or penetration Its cant be determined from stress strain curve important to dentistry. 1-cutting, finishing, and polishing, e.g cobalt chromium its 2-natural teeth should not be opposed by porcelain, 3-model and die materials Dr. Reem Gamal  Shore A hardness test Dr. Reem Gamal  Shore hardness test The instrument consists of a blunt – point indenter 0.8mm diameter, that taper to a cylinder 1.6mm. Indenter is attached by a lever to a scale that is graduated from 0- to 100 units to masseur the depth of penetration. The usual method is press down firmly and quickly on the indenter and record the maximum reading. If indenter completely penetrates the sample reading of 0 is obtained if no penetration occurs Reading of 100 units results. Is used in the rubber industry to determine the relative hardness of Elastomers and rubber viscoelastic material. Evaluate soft denture liners , mouth protectors and maxillo- facial Elastomers. Dr. Reem Gamal  Brinell Vickers Knoop Rockwell Shore A durometer Shape of the 1.6mm ball 136º pyramid Pyramid with a Cone Blunt pointed. indenter: with a square rhomboidal Or base. base. Ball Material of the Steel Diamond Diamond Diamond Steel indenter: Measurement and Load/ area of Load/ area of Load/ area of Depth of Depth of units: indentation indentation indentation penetration penetration (kg/m2) (kg/m2) (kg/m2) (0-100 unite) Expressed as: BHN VHN KHN Letter for load 0_ very soft Number for 100_ very hard hardness Uses: - Ductile materials - Ductile - Ductile - Elastic materials. Viscoelastic materials. materials. - Plastic materials. - Brittle materials. - Brittle materials. materials. - Elastic materials. - Enamel. - Dentin. Dr. Reem Gamal  Wear Wear is a surface loss of material resulting from mechanical action Wear is usually undesirable due to produce shape changes in the object that can affect toothbrush and wear can produce particles that can elicit an inflammatory response Under controlled conditions during finishing and polishing, wear can be very useful. Wear dentally may result from mechanical abrasion (improper use of toothbrush), physiological attrition (mastication), or pathological conditions (bruxism) Dr. Reem Gamal  Rheology, Creep and Flow: Rheology: the study of the flow of the material Viscosity: the resistance to flow of the material. Caused by internal frictional forces in the liquid. Units: Centipoise(CP) which= Mpa.Sec or MN sec/m Shear stress/ shear strain rate diagram( the slope of the line)  Viscoelasticity. describes materials that exhibit characteristics of both elastic solid & viscous fluids. Viscoelastic materials are strain-rate dependent. Dr. Reem Gamal  Mechanical models of viscoelasticity. 1-Ideal elastic (spring): Strain is independent of the rate of loading or the length of time in which the load was applied strain time t0 t1 Dr. Reem Gamal  2-Ideal viscous(dashpot): Total strain directly proportional to the total time of loading strain time t0 t1 Dr. Reem Gamal  3-Ideal anelastic(spring-dashpot parallel) non linear increase of strain on a given stress with time non linear but complete gradual recovery after removal of stress if there is enough time (time dependent) strain time t0 t1 Dr. Reem Gamal  4-Ideal in series (spring dashpot in series) strain t0 t1 time Dr. Reem Gamal  5- Viscoelastic On loading instantaneous strain then nonlinear strain and on removal of stress; instantaneous elastic recovery then anelastic recovery after that permanent strain representing the viscous part Dr. Reem Gamal  removing the elastic impression should be with sharp snap parallel to the long axis of the teeth due to more permanent strain occur in these impression with longer time applied during removal of them from patient mouth (load application) Also they should be given a time to recover before die can be poured Dr. Reem Gamal  Creep Its time dependent plastic deformation that occurs in a viscoelastic material subjected to a small load Static or dynamic below its E.L.(P.L.) near its melting T Importance: Most metallic and ceramic restorations do not creep in the oral environment. 1-Dental amalgam creep WHY ? it contain component with melting T slightly above mouth T in class II lead to lose of its marginal adaptation (marginal overhang).loss of contact , loss anatomy, gingival inflammation 2-Sagging in ceramometalic restoration Dr. Reem Gamal  Flow Term flow use rather than creep in time dependent plastic deformation for amorphous material as polymer (wax, rubber & plastics ) Dr. Reem Gamal  Important notes Failure of the material below P.L may be due to Fracture as in, -Fatigue -Impact strength or Deformation as -Creep -wear Dr. Reem Gamal  Energy related properties Resilience, Toughness, Impact strength Dynamic related properties Fatigue, Creep, Impact strength , Wear Dr. Reem Gamal  In summary Three interrelated factors should be considered in selection of dental materials; 1-Type of the material(material choice), 2-Geometry of restoration and underlying structure, 3-Component design. Dr. Reem Gamal © M.Lenhard Dr. Reem Gamal Heidelberg , Germany