Mechanical Properties & Viscoelasticity PDF

Introduction Mechanical properties are a group of physical properties that describe the behavior of materials under force or load.  Force Definition: 1. Speed: static or dynamic force. 2. Magnitude 3. Point of application. Normal or Tangential 4. Direction ...

Introduction Mechanical properties are a group of physical properties that describe the behavior of materials under force or load.  Force Definition: 1. Speed: static or dynamic force. 2. Magnitude 3. Point of application. Normal or Tangential 4. Direction  Force On applying force on a body, it may cause: 1. Displacement.  Force On applying force on a body, it may cause: 1. 2. Acceleration.  Force On applying force on a body, it may cause: 1. 2. 3. Deformation  Stress Definition: It is an internal reaction to the external applied force. It is equal in intensity and opposite in direction to external force. Stress = Force/Area σ = F/A Unit: Pa = N/m2 MPa = MN/m2  Stress Factors affecting stress: 1. Force varies directly. 2. Area varies inversely.  Stress Types of stress: 1. Tensile stress. 2. Compressive stress. 3. Shear stress. 4. Complex stresses.  Stress 1.Tensile stress: 2 sets of forces. On the same line. Away from each others. Cause......... Elongation.  Stress 1.Compressive stress: 2 sets of forces. On the same line. Toward from each others. Cause.......shortening.  Stress 1.Shear stress: 2 sets of forces. Not on the same line (parallel). Toward from each others. Cause.......sliding or tearing.  Stress 1.Complex stress: Combination between 2 or more types of stresses. The stresses in oral cavity are complex.  Strain The stress causes distortion or deformation. Definition: It is the change in length per unit length. Strain = Deformation Original length ε= l final – l original l original  Strain Types of strain: a) Each type of stress can produce corresponding strain i.e: tensile strain, compressive strain ….. b) Each type may be: i. Elastic Strain (Temporary Strain): recovered after load removal. ii. Plastic Strain (Permanent Strain): not recovered after load removal.  Strain Types of strain:  Strain Poisson’s ratio (μ): During axial loading (tension or compression), there is a simultaneous axial and lateral strain. Within elastic range: μ = lateral strain / axial strain. For most of dental materials μ = 0.3  Stress-Strain curve: In order to study the properties of the material of dental interest, stress is applied over the material and the value of strain is recorded. Stress is plotted in the vertical axis. Strain is plotted in the horizontal axis.  Properties gained from Stress-Strain curve: 1. Proportional limit: It the maximum stress the material can withstand without deviation from Hook’s law or the law of proportionality between stress and strain. If stress is doubled.............. the strain is doubled.  Properties gained from Stress-Strain curve: 2. Elastic limit: It is the maximum stress the material can withstand without permanent deformation. The proportional limit and the elastic limit represent the same value. They differ in the fundamental concept.  Properties gained from Stress-Strain curve: 3. Yield strength: The stress at which the material begins to function in a plastic manner (permanent deformation). The amount of permanent deformation is arbitrarily selected (may be 0.1%, 0.2% or 0.5%) (Called percent offset).  Properties gained from Stress-Strain curve: 4. Ultimate strength: The maximum stress the material can withstand before fracture. Yield strength is more important than ultimate strength. Because It represents a functional failure (clinical failure) of restoration (The restoration can’t be used in patient mouth)  Properties gained from Stress-Strain curve: 6. Modulus of elasticity (Young’s Modulus) “E”: It is the constant of proportionality between stress and strain. It represents the stiffness of the material within the elastic range. It represents the slop of the elastic portion of the stress-strain curve. It is not change either tested under tensile of compressive test.  Properties gained from Stress-Strain curve: 6. Modulus of elasticity (Young’s Modulus) “E”: It depends on: 1. Inter atomic or intermolecular forces of the material. 2. Composition of the material. 3. Heat treatment. 4. Mechanical treatment.  Properties gained from Stress-Strain curve: 6. Modulus of elasticity (Young’s Modulus) “E”: Clinical Importance: a) Denture base should be constructed of a rigid material to be used in thinner sections without the risk of bending. b) Long span bridges are constructed of a rigid material to allow proper stress distribution. c) Rigid base should be used under restorative filling material to increase the fracture resistance of the filling.  Properties gained from Stress-Strain curve: 7. Flexibility The maximum strain that occurs when the material is stressed to its proportional limit. Clinical Importance: a. Flexibility of Elastic impression materials indicates easily removal from the mouth. b. Clasps of partial denture should be flexible to be easily removed from the tooth undercuts. c. Flexibility of endodontic files provides easily preparation of curved root canals.  Properties gained from Stress-Strain curve: 8. Brittleness: The brittle material is the material that show no or very little plastic deformation. Brittle materials are weak in tension but strong in compression. Brittle materials fracture by crack and crack propagation. Dental amalgam compressive strength is 6 times higher than tension strength.  Properties gained from Stress-Strain curve: 9. Malleability and ductility: Malleability: The ability of metal or alloy to be hammered into thin sheets without fracture (withstand compressive stresses). Ductility: The ability of metal or alloy to withstand tensile stresses (drawn into wire) (withstand tensile stresses) These are properties of metals and alloys They indicate the workability e.g.: The burnishability of the alloys  Properties gained from Stress-Strain curve: 10. % Elongation: It is the deformation resulted from application of tensile forces. It is an indication of the workability of alloys. % elongation = (increase in length)/(original length) x 10 Dental importance: a. Dental gold alloys % elongation = 20 %......ductile b. Nickel-chromium alloys % elongation = 1 %......brittle.  Properties gained from Stress-Strain curve: Brittle fracture Ductile fracture Occurs in brittle Occurs in materials as composite ductile materials as resin and metals ceramics The material shows no The material or little plastic shows great plastic deformation deformation Characterized by crack Characterized and crack propagation by necking Properties gained from Stress-Strain curve:  Properties gained from Stress-Strain curve: 10. Resilience It is the amount of energy needed to deform the material to its proportional limit. This energy is stored energy because when the load is removed the energy is released causing complete recovery of the deformed material. It is represented by area under the straight portion of the stress-strain curve  Dental importance: a. It is important in orthodontic wires as they store energy and release it over a required time to move teeth. b. In removable dentures, acrylic teeth are more resilient than porcelain teeth. Therefore, they absorb masticatory forces and transmit less force over the residual ridges. c. Resilience of mouth guard allows absorption of undue energy and protection of teeth from damage.  Properties gained from Stress-Strain curve: 11. Toughness: It is the amount of energy required to stress the material to fracture. It is represented by the area under the elastic and plastic portions of stress-strain curve. It is calculated graphically by calculating the number of squares X area of each square.  Properties gained from Stress-Strain curve: 11. Fracture Toughness: It is the amount of energy required to stress the material to fracture. It is the amount of energy required to fracture the material in the presence of cracks or flaws. Crack act as a stress concentration factor…..less forces are needed to fracture the material. It is more obvious in the brittle materials as the ductile material can be plastically deform and redistribute the stresses.  Properties gained from Stress-Strain curve: 11. Fracture Toughness: Dental importance: 1. Presence of fillers in resin composites deflect cracks. 2. Presence of crystalline phases of ceramics deflect cracks 3. Presence of zirconia particles heal cracks.  Other Mechanical Properties and Tests: Diametral Compression Test (Brazilian Test) (Indirect Tensile Test): It is used to determine the tensile strength of brittle materials in an indirect way. A compression load is applied on a cylindrical specimen, so tensile stresses are introduced in a perpendicular plane on the applied load  Other Mechanical Properties and Tests: Transverse strength (Modulus of rupture) (Flexure strength) (Three-point loading) Simple Beam is supported at both ends, and subjected to static load in the middle. The test gives the flexural strength and the accompanied deformation. The deformation is more important as it represent the functional failure of the restoration  Other Mechanical Properties and Tests: Transverse strength (Modulus of rupture) (Flexure strength) (Three-point loading) Dental importance: a. It is important in denture base and long span bridges design as the deformation is varies with cube of length and thickness of the beam. b. The operator should choose a material with high modulus of elasticity to avoid its deformation during service.  Other Mechanical Properties and Tests: Transverse strength (Modulus of rupture) (Flexure strength) (Three-point loading)  Other Mechanical Properties and Tests: 3. Cantilever Bending: a. The sample is clamped at one end and load is applied on the other free end. As the force increase, the angle of bending increased. Curve is drawn with bending moment versus angle of bending Dental importance: The bending properties is very important in endodontic files  Other Mechanical Properties and Tests: 4. Fatigue strength: Definition: It is the stress at which the material fracture progressively under repeated cycling loading. Fatigue strength test is performed by subjecting the specimen to repeated cyclic stresses under the proportional limit until fracture.  Other Mechanical Properties and Tests: 4. Fatigue strength: If the applied stress is high......the material needs less cycles of load. if the applied stress is lowered...... the materials needs more cycles of load to fracture. Until a stress the material will not fracture with infinite numbers of cycles and called endurance limit or fatigue limit  Other Mechanical Properties and Tests: 4. Fatigue strength::  Other Mechanical Properties and Tests: 4. Fatigue strength: Dental importance: The dental materials should have fatigue limit above the masticatory forces to withstand unlimited numbers of cyclic loading.  Other Mechanical Properties and Tests: 5. Impact strength: Test : 1) Charpy test: The specimen is supported at both ends and struck at the middle. 2) Izod Test: The specimen is supported at one end and struck at the other end  Surface Mechanical Properties and Tests: 1. Hardness: Definition: It is the resistance to permanent indentation, penetration or scratching. Test principles: The hard materials are difficult to be penetrated, so they resist the action of the indenter (produce little indentation). The hardness tests differ in the indenter shape and material  Surface Mechanical Properties and Tests: 1. Hardness Tests: 1. Brinell 2. Rockwell 3. Shore A 4. Vickers 5. Knoop  Surface Mechanical Properties and Tests: 1. Hardness Dental Importance: 1. Natural teeth should not be opposed by harder material as porcelain. 2. Avoid scratch of soft materials as model and die. 3. Restorative hard materials are difficult to polish, but they preserve their polished surface.  Surface Mechanical Properties and Tests: 2. Wear Definition: It is loss of material resulting from mechanical action. Types: a. Physiologic: normal mastication leads to wear of the teeth. b. Mechanical: improper use of toothbrush. c. Pathologic: bruxism. Dental importance: It is undesirable process except in polishing.  Viscoelasticity: The viscoelastic materials: Slow rate of loading........... behave in a ductile manner. Rapid rate of loading...............behave in a brittle manner.  Viscoelasticity The viscoelastic materials exhibit combination of three behaviors: a) Ideal elastic b) Anelastic c) Ideal viscous Viscoelasticity  Viscoelasticity Viscoelastic behavior: 1. When stress is applied: Immediate strain (elastic part). Gradual increase in strain (anelastic and viscous parts).  Viscoelasticity Viscoelastic behavior: 2. When stress is removed: Immediate recovery (elastic part). Gradual recovery (anelastic part). Permanent deformation (viscous part).  Viscoelasticity Dental application: 1. Elastic impression materials..... Should be removed rapidly from patient mouth (sharp snap removal). a) To decrease the deformation produced by viscous part. b) To increase the tear strength of the impression material.  Viscoelasticity Dental application: 2. Elastic impression materials: Should have time before pouring with gypsum To give time for recovery of the anelastic part.  Viscoelasticity Dental application: 3. Creep Definition: It is time dependent permanent deformation as a result of long stresses below yield strength.  Viscoelasticity Dental application: 3. Creep Conditions of creep: 1. Viscoelastic material. 2. Stresses below P.L. 3. Temperature near softening temperature.  Viscoelasticity Dental application: 3. Creep Polymers and dental amalgam are viscoelastic materials, have softening temperature near the mouth temperature, so they can creep if stresses are applied over them. Thank you