Mechanical Properties 2016 PDF

‫ِبْس ِم اللِه‬ ‫ي‬ ‫ِح‬ ‫ال‬ ‫َّر ِن َّر ِم‬‫م‬ ‫ْح‬ ‫ال‬ MECHANICAL PROPERTIES  Mechanical Properties  These are properties that are related to force Application.  Force  It is the external action which produces , tends to produce or change the motion of a body....

‫ِبْس ِم اللِه‬ ‫ي‬ ‫ِح‬ ‫ال‬ ‫َّر ِن َّر ِم‬‫م‬ ‫ْح‬ ‫ال‬ MECHANICAL PROPERTIES  Mechanical Properties  These are properties that are related to force Application.  Force  It is the external action which produces , tends to produce or change the motion of a body. Units: kg, lb, MN.  Stress  It is the internal reaction due to an external applied force.  It is equal in magnitude and opposite in direction to the external force. Force Stress = Area Units : Kg/cm2 or lb/in2 or MN/m2 (MPa) A B  Types of stress 1. Tensile stress. F1 F2 2. Compressive stress. F1 F2 3. Shear stress. F1 F2 4- Complex stresses. F F F  Strain  It is the change in length per unit length when stress is applied. ε= (Deformation) L f –Lo Lo 5 mm 7 mm 7- 5 2 Strain = = = 0.4 5 5 Units: Unitless or Cm/ Cm or m/m or in/in  Types of strain  Elastic (temporary) strain.. Plastic (permanent) strain.   Relation between stress & strain. 1- Proportional limit. σ (Sigma) P.L. 310 MPa 300 MPa Is the maximum stress a material can withstand without deviation from the law of proportionality between stress & strain. ε (Epsilon) 0.0 0.01 0.02 0.03 0.04 0.05 0.06 0.07  Relation between stress & strain. 2- Elastic limit. σ E.L. Is the maximum stress a material can withstand without permanent deformation. ε What is the difference between P.L and E.L ? σ P.L. = E.L. 310 MPa 300 MPa ε Proportional limit describes the Relation between σ and ε while elastic limit describes the Behavior of the material σ stress ε strain Elastic strain Plastic strain 3- Yield stress. σ Y.S 305 MPa 300 MPa It is the stress at which the material begins to function in a plastic manner. ε 0.2 % offset 4- Ultimate strength. σ.U.S The maximum stress that a material can withstand. ε Clinically : yield strength is more important than u.s.? 5- Fracture strength. σ F.S Fracture is complete at necking area The stress at which a material fails. ε 6. Modulus of Elasticity (E). It is the constant of proportionality between stress & strain. σ Stress E = Strain (Within the elastic slope) ε E: measures rigidity or stiffness of the material. Units: MN/m2 (MPa) or lb/in2 or Kg/cm2 Stiff vs. flexible materials stress A B strain Elastic modulus  Slope  -: Denture bases should be rigid in order to.distribute masticatory forces -1.be used in thin section -2 7- Ductility & Malleability. 7- Ductility & Malleability. Plastic deformation A σ B It is the property of materials to withstand plastic deformation under tensile load without fracture ε Ductility is measured by ……….? 8. Elongation %. E % = Lf – Lo X 100 Lo.Elongation % = (E%).8 Material A 10 mm 15 mm 15 – 10 E% = X 100 50% = 10 Material B 10 mm 11 mm 11 - 10 E% = X 100 % 10 = 10 9- Brittleness. It is the property σ of materials that show no or very little permanent deformation on application of load. ε  Ductile material Brittle material Withstand elastic &plastic deformation Withstand elastic deformation only i.e. Fracture occurs far away from the i.e. fracture occurs at or near P.L P.L Necking takes place before fracture No necking but crack propagation takes place till fracture Examples are gold alloys & stainless Examples are amalgams & porcelains steel.  10- Resilience. σ 300 MPa It is the amount of energy absorbed by a material when it’s stressed to its P. L 50 ε Resilience = ½ Base X Height = ½ Strain X Stress Unites = m/m x MN/m2 = mMN/m3 Clinical importance:- Porcelain & Orthodont acrylic ic Wires denture Teeth  11- Toughness. σ ε It is the amount of energy absorbed by a material to the point of fracture. Stress Stress (2) (1) Strain Strain Strong-Weak Rigid-Flexible Ductile-Brittle ? More Resilient MECHANICAL TESTING Brittle materials as amalgam and ceramics are not Necessarily Week. They are Week in tension & Strong in compression. 1- Diametral compression test ( Indirect Tensile Test , Brazilian test ) P= 2P Tensile stress = ПDT Tensile stress T D A test used to measure the …?… for …?… materials. 2- Transverse strength test ( 3 - point loading = flexural strength test)  2- Transverse strength test 3 X load X length Trans. Stress = ----------------------------------- 2 X width X Thickness2 3 XF X L T. S = ----------------------- 2 X W X Th2 load X length3 Deformation = ------------------------------------- 3 4 X E X Width X Thickness F X L 3 Deformation = ----------------------- 4 X E X W X Th3 -:Fatigue strength.3 Stress It is the fracture of a material when MPa subjected to repeated (cyclic) 5000 small (below the P.L.) stresses. 4000 2000 1000 Fatigue limit. 500 200 300 500 1000 5000 Number of cycles Clinical significance Clasps of partial dentures Impact strength.4 4.Impact Strength Is the amount of energy required to fracture the material when subjected to sudden force.  4.Impact Strength Is measured by either Charby or Izod tests.  5. 1- It is theHardness resistance of the material to scratching , indentation or penetration. 2- It is a surface property not related to any other mechanical property…e.g. strong or stiff materials are not necessarily hard. 3- It cannot be seen or calculated from stress-strain curve but only by using one of the following tests:  Brinell hardness test.  Rockwell hardness test. B  Vickers hardness test.  Knoop hardness test.  Shore A hardness tests.  a. Brinnel hardness test : Steel ball Load BHN = -------------------------- Area of indentation Smaller indents Larger indents mean higher mean less hardness hardness D D D a. Brinnel hardness test :- Disadvantages: 1- It is difficult to measure the indentation area. 2- Not suitable for measuring hardness of brittle materials……….. (Fracture). 3- Not suitable for measuring hardness of elastic materials ……….(Recovery). It is used only for metals and alloys. b. Rockwell hardness test :- Advantages: It is directly measured using a dial gauge. Disadvantages: As for the Brinnel test, Dial gauge Rockwell test is not suitable for brittle and elastic materials. c. Vicker hardness test (Square base 1:1 )  Easy test ( sharp indents)  Suitable for brittle materials but not for elastic materials (large recovery). d. Knoop hardness test: Cone with a base = 7:1 Advantages: 1- Easy measuring of indentation area. 2- Can test hardness of brittle materials without fracture. 3- Can test hardness of elastic materials with minimum recovery. e. Shore A hardness test:  The previous hardness tests cannot test hardness of rubbers.  An instrument called Shore A is used in the rubber industry to determine its hardness.  The indenter is attached to a scale that is graduated from 0 to 100. 6-Strain–Time Relations A. Elastic materials: a. When load is applied: i. Deformation (strain) occurs immediately. ii. No increase in strain with Strain time. b. When load is removed: Strain will disappear immediately and completely. Time To T1 B. Viscous materials: a. When load is applied, Strain Strain increases as the time of load application is increased (strain is time dependant). b. When load is removed, Time No strain recovery occur. To T1 C. Anelastic materials: a. When load is applied, Strain increases with time. Strain b. When load is removed, There is complete but delayed strain recovery. Time To T1 D. Viscoelastic materials: Materials such as amalgam and elastic impression materials, exhibit a combination of, elastic, anelastic and viscous behaviors. Strain a. When load is applied, Strain of the elastic part occurs immediately while strain of the viscous and anelastic parts are time dependant.  When load is removed The elastic strain is completely & immediately recovered , the anelastic strain is gradually recovered ,however the viscous Time strain is not recovered which T0 results in some permanent deformation.