Dental Biomaterials I Lectures 3 & 4 PDF

12 October 2024 DR/ FAHIEM M. EL-SHAMY 1 Dental biomaterials I course # DEN-DBM201-1 Lectures 3 &4: MECHANICAL PROPERTIES OF DENTAL MATERIALS by DR/ Fahi...

12 October 2024 DR/ FAHIEM M. EL-SHAMY 1 Dental biomaterials I course # DEN-DBM201-1 Lectures 3 &4: MECHANICAL PROPERTIES OF DENTAL MATERIALS by DR/ Fahiem M. El-Shamy Lecturer in Dental Biomaterials Faculty of Dentistry, Mansoura & Menoufia National University, Egypt Former Ass. professor in Dental Biomaterials Faculty of Dentistry, Jazan University, K.S.A.  2 Mechanical properties are defined as a group of properties that dealing with forces and their effect on dental materials. They are important because most restorative materials must withstand forces in service i.e. during fabrication or mastication.  3 This chapter introduces concepts of elastic, plastic, and viscoelastic deformation and mechanical quantities including force, stress, strain, strength, toughness, hardness, friction, and wear in terms of performance of materials in the oral environment. 12 October 2024 DR/ FAHIEM M. EL-SHAMY 4 I- Force: Is the external action hat produces or tends to produce motion of a body. Unit : Kg, Pound (Ib), the SI unit is Newton (N). A force is defined by three characteristics: 1- Magnitude (Amount), 500 Kg 2- Direction of application. 3- Point of application. Forces (load) may be static or dynamic Material Figure 1  12 October 2024 DR/ FAHIEM M. EL-SHAMY 5 II- Stress (σ): The internal reaction to the external force which is equal in intensity and opposite in direction. Unit. force per unit area (cross section area). stress =force/area σ = F/A stress Its unit can be; Kg/ cm 2 (kilogram per centimetre square). Or lb/ In2 (pound per inch square) Or N/mm2 (Newton per millimetre square) = mega Pascal MPa= 10-3GPa MN/m2 (Mega Newton per metre square) SI ( International System unit) units: it is common to report stress in units of mega Pascals (MPa).  Types of stresses a- Compressive stress Results when the body is subjected to two sets of forces acting towards each other in the same straight line. They tend to shorten the body. b- Tensile stress Results when the body is subjected to two sets of forces acting away from each other and in the same straight line. Tensile stresses tend to stretch or elongate the body. Types of stresses c- Shear stress Results when two sets of forces are directed parallel to each other i.e. not in the same line. tends to slide the atoms against each other and cause tearing. 8 Complex stress Results when all types of stresses develop in a structure as a result of application of load.  12 October 2024 DR/ FAHIEM M. EL-SHAMY 10 III- Deformation and strain (ε) Strain can be Defined as change in length per unit length of a body when a stress is applied. Unit less Deformation Strain ε = L Final - L Original / L Original Unit: it is dimensionless quantity (unit less).  11 If a load is applied to a wire with an original length of 2 mm resulting in a new length of 2.02 mm, it has deformed 0.02 mm and the strain is 0.02/2 = 0.01, or 1%. Strain is often reported as a percentage.  12 Types of strain 1. Elastic strain (temporary): The body returns to its original dimension With removal of stresses--→>> Deformation disappears on removal of the external force. 2. Plastic strain (permanent): The body doesn't return to its original dimensions The body demonstrates permanent amount of deformation (plastic deformation) On removal of stresses>>>Deformation doesn't disappear on removal of the external force. 12 October 2024 DR/ FAHIEM M. EL-SHAMY 13 IV- Stress -strain curve (relationship between stress and strain) Studied by measuring the load and defamation and then calculating the corresponding stress and strain. A plot of stress versus strain can be obtained. The machine used is called Universal Testing Machine  False and true curve 12 October 2024 DR/ FAHIEM M. EL-SHAMY 14 False or engineering curve: the cross-section area of the specimen is fixed (original cross- section) during testing. True curve : obtained if the cross-section area of the specimen is changed as the testing proceeds. Stress True Y.S. E.L. P.L F.S. False Strain False and true curves for a material in tension.  15 Relation between stress & strain Hook’s law Stress is directly proportional to strain , until certain stress known as proportional limit  16 TERMS OF STRESS-STRAIN CURVE I - Stress terms: 17 18  12 October 2024 DR/ FAHIEM M. EL-SHAMY 19 3- Yield Strength (YS): the stress at which the material start to deform& exhibits a specified limiting deviation from proportionality of stress to strain. importance of y.s.: Dental restorative material should have high Yield strength to withstand high stresses while in function without permanent deformation. Bridge materials should have high Y.S. than biting force to avoid permanent deformation.  20 4- Ultimate strength: The maximum stress a material can withstand before failure The yield strength is of greater importance than ultimate strength, since it is a measure of when a material will start to deform. Any restoration which undergoes permanent deformation in the mouth is considered a functional failure. 5- Fracture strength: Fracture strength or fracture stress is the stress at which a material fracture occurs. 12 October 2024 DR/ FAHIEM M. EL-SHAMY 21 6- Elastic Modulus (Young's modulus)= modulus of elasticity (E) Measures the stiffness or rigidity of a material within the elastic range. It is the ratio of stress- to its corresponding strain. The word modulus means ratio. Unit is as stress. Usually reported in MPa or GPa.  22 6- Elastic Modulus (Young's modulus) A materials such as elastomers and other polymers have low values for elastic modulus, whereas many metals and ceramics have much higher values  23 6- Elastic Modulus (Young's modulus) The stronger the interatomic or intermolecular forces, the greater the values of the elastic modulus and the more rigid or stiff the material. THE OPPOSITE IS flexible material The property is generally independent of any heat treatment or mechanical treatment that a metal or alloy has received, but is quite dependent on the composition of the material.  12 October 2024 DR/ FAHIEM M. EL-SHAMY 24 6- Elastic Modulus (Young's modulus) Importance: a) The rigidity of major connectors in partial dentures controls the stability of the whole design. b) The more rigid is the alloy, the thinner sections which can be used without risk of bending. c) Base under restoration should be rigid in order to increase the fracture resistance of the filling.  25 II- STRAIN TERMS 1- Flexibility: Stress P.L Represented by elastic strain portion opposite to linear portion of the curve Flexibility Ductility and Malleability When a material or structure demonstrates large elastic strain with slight stress it is described as flexible material. THE OPPOSITE IS STIFF OR RIGID MATERIAL 12 October 2024 DR/ FAHIEM M. EL-SHAMY 26 1- Flexibility: Importance: Flexibility is an important property in elastic impression materials, since it represents the ease by which the impression can be removed from the mouth. 12 October 2024 DR/ FAHIEM M. EL-SHAMY 27  2- Ductility and malleability. Ductility: is the ability of material to withstand tensile force Stress permanent deformation under P.L (be drawn into wire ) without fracture. A metal that can be drawn readily into a wire is said Flexibility Ductility and to be ductile. Malleability DR/ FAHIEM M. EL-SHAMY 28 Ductility is measured by 3 common methods: 1- By measuring the percentage elongation after fracture. (The deformation that results from the application of a tensile force is elongation) % Elongation = (Increase in length /Original length) × 100% 2- By measuring reduction in cross section area of fracture. 3- By using cold bend test DR/ FAHIEM M. EL-SHAMY 29 Malleability is the ability of material to withstand permanent deformation under Stress compressive force P.L (be hammered or rolled into thin sheets ) without fracture. Flexibility Ductility and Malleability 12 October 2024 DR/ FAHIEM M. EL-SHAMY 30  Importance of Ductility and malleability: In general, metals tend to be ductile, whereas ceramics tend to be brittle. Ductility related to the workability of a material in the mouth. Malleability represents the ability to be burnished (essential in inlay, onlay and crown). 31  P.L Stress 3- Brittleness: Brittle material is the opposite of ductile, i.e. the material fractures at or near its Stress-strain curve for a brittle material proportional limit. Showing no permanent deformation. Brittle materials are weak in tension Importance Many dental materials are brittle, e.g. porcelain, cements ,dental amalgam Dental amalgam has high compressive strength = 6 times tensile strength. 12 October 2024 DR/ FAHIEM M. EL-SHAMY 32 III- Energy terms 33 34  Resilient material: Tough material: The material has the the material has the ability to absorb most ability to absorb most of energy forces of energy forces without permanent without fracture. deformation. 35 Summary for terms of stress strain curve Remember (Stiff = rigid material) X (flexible material) (ductile material) X (brittle material) (strong material) X (weak material) Always ductile materials are tough material.  37 Different stress strain graphs  EVALUATION SHEET #3 1- Define, specify the units, and explain the different types of force, stress, and strain with illustrations? 2- Illustrate a stress-strain curve and label the associated stress, strain, and energy terms?“ 3- Define and explain the clinical significance of the following terms: yield strength, modulus of elasticity, flexibility, ductility& malleability, brittleness, and resilience. 4- Draw the stress-strain curve for the following dental materials: A. Stiff (rigid), strong, and brittle material. B. Stiff, weak, and brittle material. C. Flexible, strong, and ductile material. D. Flexible, strong, and tough material.  39 Draw stress strain curve for stiff (rigid) ,strong and brittle material ?  40 Draw stress strain curve for stiff, weak and brittle material ?  41 Draw stress strain curve for flexible , strong and ductile material ?  42 Draw stress strain curve for flexible , strong and tough material ?  Dr/ Fahiem M. El-Shamy 43 Other Mechanical Properties DR/ Fahiem M. El-Shamy Lecturer in Dental Biomaterials Faculty of Dentistry, Mansoura & Menoufia National University Former Ass. professor in Dental Biomaterials Faculty of Dentistry, Jazan University, K.S.A. 1- Diametral compression test for tension: used to determine the tensile strength of brittle materials. Applied to cements, amalgam and investments. P Technique: D T - A disc of the brittle material is compressed diametrically in a testing machine until it fractures. - The applied compressive stress introduces tensile stress in the material perpendicular to the plane of the force application P - Tensile stress is calculated from the formula: D → diameter T → thickness  45 Importance : compression test is performed on brittle materials because they are commonly used under compression loading  2- Transverse bending (Transverse strength, modulus of- rupture, three –point bending, or flexural strength). Obtained when a load is applied in the middle of a beam supported at each end. Such a test is called a three-point bending test (3PB). Different stress types will result - Compressive stress in the top of the beam - Tensile stress in its bottom. - Shear stress at the supports. Force Length L Thickness d Width b Clinical significance: Used to test denture base resin AND long span bridges  48 3- Fatigue: It is the failure of a structure subjected to repeated or cyclic stresses below its proportional limit. Fatigue tests Performed by subjecting a specimen Stress to alternating(cyclic) stress below the yield stress until fracture occurs Counting No. of cycles for failure under Endurance limit X each repeated loading stress Failure dependent on the magnitude of the load Number of cycles48 and the number of loading repetitions.  49 3- Fatigue: ❑S-N Curve: Stress A plot of the different stresses against the number of cycles for fracture. Endurance limit When the stress is high, the X material will fracture at a relatively low number of cycles. Number of cycles49 The fatigue limit or endurance limit is the stress that can be applied in infinite number of times without causing material failure.  Dr/ Fahiem M. El-Shamy 50 Importance many dental materials which are subjected to alternating forces during mastication or while in function, e.g. ✓Failure of amalgam restorations, implants. ✓Complete dentures. ✓Metal clasps of removable partial dentures, which are subjected to repeated loading during insertion and removed from the mouth (the clasps open and close under force)  4- Impact strength : Def: energy required to fracture a material under sudden force. A material may have reasonable high static strength values, such as compressive, tensile, and shear strengths, but may fail when loaded under impact. e.g. Complete dentures when dropped on a floor *Two types of impact testers are available: 1-Charpy testing machine. the specimen is supported horizontally at the two ends. Izod Charpy 2-Izod instrument, the material is clamped at one end and held vertically. Clinical Significance: dropping the complete denture on a floor may cause its fracture, for this reason high impact acrylic resin denture were developed.  Dr/ Fahiem M. El-Shamy 52 5- FRACTURE TOUGHNESS is The amount of energy required for fracture brittle material with crack or it is the resistance of brittle material to catastrophic propagation of flaws under applied stress.  Dr/ Fahiem M. El-Shamy 53 Clinical significance: 1. In composite resin, the presence of glass particles (fillers) increases the fracture toughness because the glass particles will stop crack propagation. 2. In porcelain, the addition of zirconia particles increase fracture toughness because Zirconia particles absorb the energy needed for crack propagation.  Dr/ Fahiem M. El-Shamy 54 6-Surface mechanical properties In this section, mechanical properties that are more a function of the surface condition of a material are presented. In particular, the concepts of hardness, wear, and friction are summarized. 1-Hardness 2-Wear 3-Friction  6.1 Hardness Definition: the resistance of the material to penetration, indentation, penetration, or scratching. It is a surface property which cannot be determined from stress- strain curve. Clinical significance: 1- Denture-wearing patient must take care not to be aggressive during cleaning of their denture buy using brushes with hard bristles. 2- Hardness is an important property to be considered for model and die materials on which crown and bridge wax patterns are made, because a soft surface may become scratched, affecting the accuracy of the final restoration 3- Hardness is an indicator of the easy of finishing of a structure and it’s resistance to scratching. 4- High hardness of co-cr denture-base material is Advantage: it remains its mirror surface (as it resists scratching) Disadvantage: difficult in finishing and polishing. 5- Natural teeth (KHN=340) shouldn't opposed by porcelain teeth (KHN=460 i.e. harder) to avoid their abrasion. Dr/ Fahiem M. El-Shamy 55  Clinical significance: 1- Denture-wearing patient must take care not to be aggressive during cleaning of their denture buy using brushes with hard bristles. 2- Hardness is an important property to be considered for model and die materials on which crown and bridge wax patterns are made, because a soft surface may become scratched, affecting the accuracy of the final restoration Removable Die with Waxed Inlay Dr/ Fahiem M. El-Shamy 56 3- Hardness is an indicator of the easy of finishing of a structure and it’s resistance to scratching. 4- High hardness of co-cr denture-base material is Advantage: it remains its mirror surface (as it resists scratching) Disadvantage: difficult in finishing and polishing. Dr/ Fahiem M. El-Shamy 57 5- Natural teeth (KHN=340) shouldn't opposed by porcelain teeth (KHN=460 i.e. harder) to avoid their abrasion. Dr/ Fahiem M. El-Shamy 58  2. Wear Wear : is the loss of material resulting from mechanical action. It is a material removal process that can occur when surfaces slide against each other. Wear is usually undesirable but during finishing and polishing procedures wear is highly desirable. Importance of wear in dental biomaterials ✓ Wear can produce biologically active particles, which can excite an inflammatory response. ✓ Wear process can also produce shape change that can affect function.  Causes and types of wear: Wear of tooth structure and restorative materials may result from: 1. Physiological Wear: Normal mastication may cause attrition of tooth structure. Tooth to tooth contact/ grinding Affect mainly occlusal AND Incisal surface.  Causes of wear: 2. Mechanical wear, Abrasive wear >>>Tooth abrasion >>>> Improper use of tooth brushing (aggressive) may cause an abrasive form of wear. Most noticeable near to gingival margins. TOOTH ABRASION  Causes of wear: 3. Pathological wear. >>ABFRACTION >>> Due to excessive stresses created in the cervical region under occlusal loading>>> PARAFUNCHIONAL HABITTS E.g. Bruxism  Causes of wear: 4- Chemical wear : Erosive wear >>>caused by intrinsic or extrinsic acids>>> Acidic drinks : Lemon juice (citric acid) & soft drinks like Pepsi and cola drinks (phosphoric acid) In a patient with gastric troubles “gastro-esophageal reflux” Tooth erosion  3. Friction Definition: It is the resistance to motion of one material over another. This resistance results from the molecules of two object, are bonding with their surfaces and in the same time are in close contact. Clinical significance: 1- Roughening of the surface of dental implant is important to reduce motion between implant and adjacent tissue. 2- Friction is also important in sliding mechanics between orthodontic wire and brackets. Dr/ Fahiem M. El-Shamy 64  EVALUATION SHEET #4 1. Define and discuss the clinical significance of the following: A. Diametral compression test B. Transverse strength C. Fracture toughness D. Fatigue E. Impact strength F. Friction 2. What does hardness mean, and what is its clinical relevance? 3. What is wear, and what are its causes and types? 65  Dr/ Fahiem M. El-Shamy 66 THANK YOU