Mechanical Properties PDF

MECHANIAL PROPERTIES By Dr.Reem Gamal Hassan MECHANIAL PROPERTIES They are properties that describe the behavior of material under a force or a load The are determined by 1-The nature and strength of inter-atomic binding forces 2-Structural arrangement of atoms....

MECHANIAL PROPERTIES By Dr.Reem Gamal Hassan MECHANIAL PROPERTIES They are properties that describe the behavior of material under a force or a load The are determined by 1-The nature and strength of inter-atomic binding forces 2-Structural arrangement of atoms. Dr. Reem Gamal  Force It is the external action that produces or tend to produces a change in the state of rest or motion of body Effect of the force Displacement Acceleration deformation The force is defined by four characters. Magnitude Point of application Direction of application. Speed Units of force: Pound(Ib), Newton(N) 1 Ib= 4.4 N Kg=1000g 1Kg=10 N Dr. Reem Gamal  Biting force Average biting forces of adult decrease from molars to anterior teeth eg;- molar area 665 N premolar area 450N anterior area 220 N Male 89N more than female and children less than the adult Patient with fixed bridge exert 40% of the force exerted by patient with natural teeth patient with removable denture exert 15%of the force exerted by patient with natural Dr. Reem Gamal  Pressure (P) it is the force per unit area. Dental significance: 1- Technique of condensation of filling materials as amalgam (beginning with smaller condensers then larger ones. 2-Maneuver of extraction of teeth. Dr. Reem Gamal  Stress δ It is the internal reaction of structure to the external applied force on this structure; which is equal in intensity and opposite in direction to the external applied force. σ = (force / area) = (F/A) Unite:- N/m2 ,Ib/in2 (psi=pound per square inch),Kg/Cm2, Pa MPa=MN/m2 = N/mm2 MPa= 10.1 Kg/cm2 (Mega = 106) It is obvious that the surface area in dentistry is small so it is more suitable to use the unit of MPa(N/mm2) Dr. Reem Gamal  stress PURE COMPLEX Dr. Reem Gamal  Types of stress I- pure stresses Dr. Reem Gamal  A-Normal or axial stress 1-Tensile stress δt Tension results in a body when it is subjected to two sets of forces - Directed away from each other - In the same straight line (same plan). -Tending to increase in the length of the body -The intermolecular bond try to resist pulling away Dr. Reem Gamal  2- Compressive stress δc When the body is subjected to two sets of forces -directed towards each other -in the same straight line. -tends to decrease the length of the body The intermolecular bond try to resist rupture. Dr. Reem Gamal  B-Tangential stress Shear stress When two sets of forces are -directed to or away from each other -not on the same straight line -a motion that would cause twisting or sliding of one portion of the body over The intermolecular bond try to resist sliding one another. Dr. Reem Gamal  II-Complex stresses Combination of more than one type of stresses It’s the most common type of stress Eg: 1- if we pull a wire The observed stress will be predominantly tensile but the cross section of the wire will decrease indicating the presence of compressive stresses. 2- in fixed fixed bridge 3- Restoration on occlusal surface Dr. Reem Gamal  Strain Ɛ It is change in length per unit length of body subjected to stress. Ɛ = deformation/ original length = ∆L/Lo it has no units it is dimensionless it can be expressed as percent Dr. Reem Gamal  Types of strain Elastic strain (temporary) Plastic strain (Permanent) Reversible Irreversible Totally disappears upon removal of Doesn’t disappear upon removal of external load. external load. The applied force is weaker than the The applied force is greater than the interatomic force, so atoms remain in interatomic force, so atoms are forced their equilibrium position. away from their equilibrium position. Upon removal of force, atoms return Upon removal of force, atoms do not to their original position. return to their original position. Dental considerations: Dental considerations: a. Clasp of partial denture. a. Shaping of an orthodontic wire. b. Removal of impression materials from undercut areas. Dr. Reem Gamal  Poisson’s Ratio μ It is the ratio of the lateral strain to the axial strain within the elastic range. μ=lateral strain Axial strain It is constant for each material. Most dental materials have approximately a ratio of 0.3 Dr. Reem Gamal  Stress –strain curve: It describes the relation between stress and strain of individual material stress is directly proportional to strain within certain limit Hook's Law Dr. Reem Gamal  POINTS ON STRESS STRAIN CURVE 1-Proportional limit (P.L) It’s the maximum stress that produced in a material without deviation from the proportionality of stress to strain. NB: Below PL, strain is elastic. Above PL, strain is both elastic and plastic. Dr. Reem Gamal  The elastic limit (E.L) The maximum stress that a material will withstand without permanent deformation resulting. It is the same point of P.L but different in the concept P.L describe the proportion of stress to strain Elastic limit describe the elastic behavior of the material Dr. Reem Gamal  Yield strength Yield strength is the stress at which the material begins to function in a plastic manner (exhibit limited deviation from P.L). It’s usually detected by predetermining amount of permanent strain, usually 0.1, 0.2 or 0.5%, which is called percent offset and then deducing the corresponding stress. It’s easier to be measured than PL and EL. Dr. Reem Gamal  Dental consideration of Y.S: 1-Permanent deformation of dental appliance under masticatory forces during function if it’s larger than Y.S of the material that the appliance formed from.(Destructive) 2- Shaping of orthodontic wires , During adjusting restoration adjustment of clasps of RPD. (Constructive) NB: Y.S is the most important point on the curve because beginning of undesired permanent deformation is considered the beginning of functional failure of the material,.i.e. it defines transition from elastic to plastic behavior. Dr. Reem Gamal  Ultimate Strength (U.S.) It is the maximum stress that a material can withstand before fracture or rupture it is determined by dividing the maximum load in tension or compression by the original cross sectional area of the test sample Dental application It’s important in determination of size or cross sectional area of restoration.. Dr. Reem Gamal  Fracture strength: the stress at which a material fracture or rupture Dr. Reem Gamal  Modulus of elasticity, young's modulus, elastic modulus It is the constant of proportionality between stress and strain with in the elastic limit or. It is the slope of the linear part in the stress strain curve It is constant for the material. E= δ /έ = MPa = MPa or N/mm2 dimensionless Its depending on chemical structure (bond type & strength ) (the stronger the basic attraction force the higher the value of modulus of elasticity ). independent on heat treatment(age hardening) and mechanical treatment (cold work) Its unrelated to P.L and strength stress type (tensile or compressive) It measure rigidity (stiffness ) of the material within the elastic range Dr. Reem Gamal  Material elastic modulus Cobalt-chromium 218.2 Gold Type IV 99.3 Field Spathic Porcelain 69.0 Amalgam 27.0 Composite Resin 16.6 Acrylic Denture 2.65 Silicone Rubber 0.002 Dr. Reem Gamal  PROPERTIES DERIVED FROM THE CURVE 1-Rigidity(stiffness): Rigid material is the material which need high stress to make small elastic strain It’s measured by modulus of elasticity. The steeper the of stress strain curve or the higher the E the stiffer the material It’s opposite to flexibility. Flexible material :-material demonstrate large elastic strain with slight stress Dr. Reem Gamal Dr. Reem Gamal  Dental important of rigidity Material with high elasticity modulus Effect an even stress distribution over the area which load applied this important in 1- long span bridge 2-Base under restoration should be rigid 3-Denture base material Cobalt-chromium 218.2 Gold Type IV 99.3 Cobalt-chromium can be used in half thickness( thinner section) of gold without fear of uneven stress distribution Dr. Reem Gamal Clinical importance of flexibility Dr. Reem Gamal  Strength A material is said to be strong when it has high value of Y.E Its opposite is weak. Dr. Reem Gamal  Modulus of shear (G): G= E 2(1+Poisson ratio) Because both modulus of elasticity and Possion's ratio are constant for each material the modulus of shear is constant. Its usually 40% of E N.B:- E for uniaxial stress ,G for shear stress Dr. Reem Gamal Material constant properties (fingerprints of the material) are: 1. Density, 2. Specific heat, 3. Poisson' ratio, 4. Modulus of elasticity, 5. Modulus of shear Dr. Reem Gamal  Ductility The ability of a material to withstand considerable permanent deformation (drawn into wires ) under tensile load without fracture Necking tacks place before fracture in ductile material ↑ Temperature ↓ductility It is important in determine the workability burnish ability of the material in the mouth Most ductile metals in dentistry are: Au > Ag > Pt Dr. Reem Gamal  Measurements of ductility: 1- Percent elongation after fracture Elongation:-the deformation in the material in response to tensile load. Percentage of elongation = Increased in length x 100 Original length 2- Reduction in the area of the fractured ends (necking) 3- Cold bend test(test sensitive to specimen diameter and ductility Dr. Reem Gamal  Importance of ductility in dentistry Clasps can be adjusted, orthodontics appliances can be prepared, crowns or inlays can be burnished if they are prepared from alloys of high values of percentage elongation. Dr. Reem Gamal Dr. Reem Gamal Brittleness o Material showed no or very little plastic deformation on application of load. o A brittle material fractures at or very near to its proportional limit. o Brittle materials not necessary lack of strength. o Brittle materials fractured by crack propagation tacks till fracture leave granular fracture surface o Brittle materials can withstand high compressive strength why? o Example: most dental materials are Dr. Reembrittle Gamal as Dental amalgam  Malleability The ability of a material to withstand permanent deformation (hammered into thin sheets) under compression load without fracture ↑ Temperature ↑ Malleability N.B:- Not all malleable material are ductile, but all ductile material are malleable Dr. Reem Gamal  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 It is measured graphically by the area under the elastic (linear) portion of the stress strain curve. Area of triangle= ½ base x height = (½ strain x stress ) at P.L= ½ m x MN =m MN/m³ m m² = energy per unit volume 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 Dr. Reem Gamal