Mechanical Properties Part 1 KSIU Lecture Notes (PDF)

Faculty of Dentistry Dentistry Program Lecture : MECHANICAL PROPERTIES (part 1) Dr : Reem Ashraf Date : 3/11/2024  Objectives 1. Differentiate between the force, stress, and strain 2. Know the clinical significance of different mechanical properties 3....

Faculty of Dentistry Dentistry Program Lecture : MECHANICAL PROPERTIES (part 1) Dr : Reem Ashraf Date : 3/11/2024  Objectives 1. Differentiate between the force, stress, and strain 2. Know the clinical significance of different mechanical properties 3. Know how to extract material’s properties from its stress-strain curve 4. Practice to draw stress strain curve for a given properties  Mechanical Properties ▪ These are properties of materials which are related to force They describe the response of restorative materials to force application in service. Important in understanding and predicting behavior of restorative materials under loads if the maximum service is to be obtained Why Study Mechanical properties of Dental Materials…???  Dental materials are subjected to forces during Fabrication (Adjustment) Function (Mastication)  Average Biting force:  Molars: ~665 N  Premolars: ~450 N  Incisors: ~220 N  Varies between males, females & children  Force It is the external action which produces , tends to produce or change the motion of a body. Units: kg, lb.(pound), MN(newton).  If the body remains at rest the forces will cause its deformation. A force is defined by the following characteristics; speed, magnitude, point of application, and direction  Biting Force Biting Force = 77 Kg in the posterior part of the mouth. i.e. 1925 Kg/Cm2 on a single cusp of a molar tooth. Women < Man. Child < Adult. Removable restorations < Natural dentition.  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)  Stress = Force / Area ↑ Stress ↓ Stress A B  MN/m2 Kg/Cm2 lb/in2 (MPa)  Types of stress 1. Tensile stress. Tension results in a body when it is subjected to two sets of F1 forces directed away from each other in the same straight line. Tensile Stress →Elongation F2  2. Compressive stress. F1 Compression results when the body is subjected to two sets of forces directed towards each other on the same straight line. Compressive stress →Shortening F2  3. Shear stress. Shear is the result of two sets of F1 forces directed towards each other but not in the same straight line. Shear stress → Tearing Or Sliding F2  4- Complex stresses. F The forces applied to a dental restoration are resolved as a combination of compressive, tensile, and shear stresses (complex stresses) rather than F F pure single stress.  Strain ▪ It is the change in length per unit length when stresses applied. ε= (Deformation) L f –Lo Lo 5 mm 7 mm 7- 5 2 Strain = = = 0.4 5 5 Units: Unit less or Cm/ Cm or m/m or in/in  Types of strain ❖ Elastic (temporary) strain. ❖ Plastic (permanent) strain. Types of strain: Elastic strain: It disappears on removal of the external force. The material will return to its original shape. Plastic strain: It will not disappear on removal of the external force.  Hook’s Law  It states that strain is directly proportional to the stress until a stress value known as proportional limit  Stress- strain curve 1. Straight portion or linear relation Stress increased ; the strain increased Stress doubled, the strain will be doubled Hooks law ( strain is directly proportional to stress) 2 2. Curved portion 1 3. End point of the curve (fracture point) Failure or fracture of the material 1.Stress related properties  I. Properties obtained from stress axis 1. Proportional limit Stress f/A Mpa 2. Elastic limit plastic 3. Yield strength permenant Elastic temporary 4. Ultimate strength Strain ΔL/L 5. Fracture strength m/m Describes the relation between stress and strain. σ (Sigma) P.L. 310 MPa 300 MPa Is the maximum stress a material can withstand without deviation from the law of proportionality(hooks law) between stress & strain. ε (Epsilon) 0.0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Describes the elastic behavior of the material. σ E.L. Is the maximum stress a material can withstand without permanent deformation. ε Q. 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 Stress at which the material begins to function in a plastic manner. A limited permanent strain usually 0.1% or 0.2% of the total permanent strain occurs in the material σ Y.S 305 MPa 300 MPa It is the stress at which the material begins to function in a plastic manner. 0.2 % offset ε Offset yield strength:- stress at which material shows specific amount of permanent deformation (0.1%, 0.2%, 0.5%) Importance: 1-It is important for the dental material to withstand high stresses while in function without permanent deformation which is considered functional failure even without fracture. 2- during adjustments of restorations the stress induced must be greater than their yield strength to produce permanent deformation e.g. burnishing. 3- Burnishing of crown margins:  σ U.S. The maximum stress that a material can withstand. Clinically : yield strength is more important than u.s.? ε Because yield stress represents the clinical failure (functional failure). stress at which the material will fracture σ F.S Fracture is complete at necking area The stress at which a material fails. ε 2. Strain related properties  II. Properties obtained from strain axis 1.Stiffness and Flexibility 2.Ductility and Malleability 3.Brittleness It is the constant of proportionality between stress & strain. It represents the rigidity/stiffness of the material i.e. the resistance for elastic deformation. σ Stress E = (Within the elastic slope) Strain ε Units: MN/m2 (MPa) or lb/in2 or Kg/cm2 E: measures rigidity or stiffness of the material.  Stiff vs. flexible materials Materials with higher Young's modulus values are said to be more rigid (stiffer) than those of low Young's modulus values because they require much more stresses to produce the same amount of strain. stress A B A= stiff B= flexible ☺ strain Elastic modulus → Slope A stronger inter atomic bonding, a more rigid material  This property is independent of any heat treatment or mechanical treatment, but it is quite dependent on the composition of the material. The modulus of elasticity does not change either tested under tension or compression. E.g.: Rubbers have low modulus of elasticity (flexible) while metals have high modulus(stiff). Dental importance: Materials with high modulus of elasticity resist elastic deformation and shows even stress distribution over the area to which the load is applied 1. Bridges particularly long span bridges. 2. Denture base materials: Denture materials with high modulus of elasticity can be used in thinner section without fear of uneven stress distribution e.g. cobalt chromium denture base material compared to gold alloys. 3) Base under Amalgam should be rigid to increase fracture resistance of the filling. Denture bases should be rigid to distribute masticatory forces & used in thin section. High modulus of elasticity is required to allow proper stress distribution in case of long span bridges.  Maximum Flexibility: 1)Impression materials, must have the ability to spring back without suffering any permanent change in shape during removal. 2) Clasps are flexed during mastication Maxi. flexibility: it is the amount of elastic strain till the elastic limit. Ductility the ability to be plastically deformed under tension (drawn into wire) Malleability the ability to be plastically deformed under compression (hammered into thin sheets). They indicate the workability of metals and alloys.  Plastic deformation A σ B ε Ductility is measured by ……….?  It is the measure of ductility Importance 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  Elongation %. E % = Lf – Lo X 100 Lo 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 σ It Is the property of materials that show no or very little permanent deformation on application of load. ε  ductile and brittle fracture.png Ductile material Brittle material Withstand elastic and plastic deformation Withstand elastic deformation only i.e. Fracture occurs far away from the P.L i.e. fracture occurs at or near P.L Necking takes place before fracture No necking but crack propagation takes place till fracture Examples are gold alloys & stainless steel. Examples are glass ,amalgams , porcelains and composites. Energy related properties 1. Resilience 2. Toughness  The amount of energy required to deform the material to its proportional limit. It represents the resistance of the material to permanent deformation. σ 300 MPa Units are energy per unit volume (J/mᶾ) Amount of energy absorbed by a material 𝑃𝐿2 when it’s stressed to its P. L 𝑅= 2𝐸 R: resilience modulus ε PL: Proportional limit E: modulus of elasticity 50 Resilience = ½ Base X Height= ½ Strain X Stress Unites = m/m x MN/m2 = mMN/m3  Clinical Importance: 1.Orthodontic wires: stored energy can be released over time and move the teeth 2. Resilient denture base materials 3. Acrylic denture teeth are more resilient than porcelain teeth, so it absorbs most of masticatory forces and transmits less force to underlying bone, so preserves it Orthodontic Porcelain Wires Denture Teeth  Energy required to stress the material to point of fracture. represented by the area σ under the whole curve (elastic and plastic area) Units of toughness: mMN/mᶾ Tough material:has high proportional limit and good ductility ε It is the amount of energy absorbed by a material to the point of fracture. Fracture Toughness: It is the ability of the material to resist crack propagation materials fracture by Brittle materials fracture by crack propagation[Sudden catastrophic fracture] Fracture toughness gives a relative value of a brittle material's ability to resist crack propagation Ductile materials fracture by necking -Fracture toughness is of no value because the material has the ability of plastic deformation and stress redistribution  Importance of Fracture toughness Modification of the brittle materials composition to raise its fracture toughness by addition of some materials as * Filler in composite resin *Zirconia in ceramics Let's practice Stress Stress (2) (1) Strain Strain Strong-Weak Rigid-Flexible Ductile-Brittle ? More Resilient Stress Stress A B Strain Strain Strong = P.L. Weak = P.L. Stress Stress A B Strain Strain Rigid or stiff = E Flexible = E  Stress Stress B A Strain Strain Ductile = Plastic deformation. Brittle = little or no plastic deformation. Stress Stress A B Strain Strain Resilient = area of the Less Resilient = area of triangle below elastic the triangle below elastic slope slope Stress Stress A B Strain Strain Tough = area under the Less tough = area under the elastic and plastic (curve) elastic and plastic (curve) Stress Stress (2) (1) Strain Strain Strong-Weak Rigid-Flexible Ductile-Brittle ? More Resilient  stress flexible flexible flexible ductile ductile brittle weak strong strong resilient resilient strain stress flexible stiff stiff brittle ductile brittle weak strong strong tough strain stress stiff stiff ductile brittle weak weak strain