Mechanical Properties PDF 2024-2025

 Presented by: Dr./ Amr Sharawy Assoc. professor of dental materials Force: What is force? It is the action applied on an object to change its position of rest or motion Force is defined by four characteristics: Speed determine if the force is static or dynamic...

 Presented by: Dr./ Amr Sharawy Assoc. professor of dental materials Force: What is force? It is the action applied on an object to change its position of rest or motion Force is defined by four characteristics: Speed determine if the force is static or dynamic Magnitude Point of application Direction determine if the force along the long axis or parallel to it  Units: N, kg, Ib  Stress (σ): Def:-Internal reaction of a structure to externally applied load; external load and internal stress are equal in magnitude and opposite in direction. σ = force / area Units: MN/m2 = MPa (M = 1000) Types:  1-Compressive stress 2-Tensile stress 3-Shear stress The body is subjected to 2 sets of The body is subjected to 2 sets of The body is subjected to 2 sets of forces: forces: forces: On the same straight line. On the same straight line. Parallel to each other. Directed toward each other. Directed away from each other. Directed toward or away from each other.  Complex stress if we stretch a wire, the observed stress will be tensile but the cross section of the wire will decrease indicating the presence of compressive stresses.  STRAIN (ε) : Def:-Change in length per unit length STRAIN (ε) = Lf – Lo Unit: dimensionless Lo Deformation: Change in length ( L final - L original ) Unit: cm, m Types of strain:- 1-Elastic strain (temporary ) Totally disappears upon removal of external load. 2-Plastic strain (permenant) Doesn’t disappear upon removal of external load.  Relation between stress & strain The stress-strain relationship of a dental material is studied by applying a load, measuring the deformation and calculating the corresponding stress and strain. Stress 1 2 4 8 16 / 18 20 24 32 Strain 0.5 1 2 4 8 / 8.1 8.5 9.1 10 Hook’s law: Stress is directly proportional to strain , until certain stress known as proportional limit Proportional limit Yield strength And Elastic limit Ultimate strength 400 – Stress (MPa) - 300 – Fracture strength - 200 – - 100 – - - - - - – – – – – – 0.0 0.01 0.02 0.03 0.04 0.05 - Elastic Plastic 0– Strain Strain Strain  The stress-strain curve consists of two portions Elastic portion (Linear) Plastic Portion (Non-linear) - It obeys Hook's law -It does not obey the Hook's law -The strain is directly proportional to the - The strain isn’t directly applied stress i.e. doubling the stress will proportional to the applied stress. double the strain. -When the stress is removed the original size -When the stress is removed, and shape is recovered. the original size and shape isn’t recovered.  Properties derived from the stress strain curve 1-Proportional limit (σPL) 2-Elastic limit (σEL) 3-Yield strength (σY) 4-Ultimate strength 5-Fracture strength (σF) It’s the maximum stress that It’s the maximum stress It is the stress at which the material begins It’s the maximum stress that a It’s the stress at which a a material can withstand that a material will withstand to function in a plastic manner material can withstand before material fracture without deviation from the without permenant faracture.. proportionality of stress to deformation It’s difficult to be measured than PL and strain. EL. Importance in dentistry: NB: Below PL, strain is -Yield stress is more elastic. Above PL, strain is important than the ultimate both elastic and plastic. stress, GRF because yield stress deformation. represents the clinical failure (functional failure). - N.B. Both proportional limit Dental consideration: Proportional limit Yield strength and elastic limit represent the And same stress. However they NB: It’s the most important point on the Elastic limit differ in fundamental concept, curve because beginning of undesired Ultimate strength in that permanent deformation is considered the beginning of functional failure of the 400 – the proportional limit deals material, with the proportionality of. - stress to strain in the structure Destructive deformation: permanent Stress (MPa) deformation of dental appliance under 300 – whereas masticatory forces Fracture strength the elastic limit describes the - elastic behavior of the Constructive deformation: material. -Shaping of orthodontic wires 200 – - 100 – - - - - - – – – – – – 0.0 0.01 0.02 0.03 0.04 0.05 -burnishing of crown margins - Elastic Plastic 0– Strain Strain Strain 6-Modulus of elasticity (E): (Elastic modulus, Young’s modulus) Def.: the constant of proportionality between stress and strain It measures the stiffness or rigidity of a material. It’s calculated from the equation: A σ B C Units: N/m2, MN/m2 (MPa) or kg/cm2 - It represented by slope of the linear portion of the curve ε -From the figure, the order of rigidity (stiffness) is A > B > C -It depends on the strength of interatomic bonds in a material, so it’s affected by the composition of the material. i.e.: Elastomers and polymers low modulus flexible material Metals and ceramics High modulus Stiff material Clinical Significance: 1-Denture base should be constructed of a rigid material To be used in thinner sections and allow proper stress distribution without the risk of bending to reduce the rate of bone resorption. 2-Rigid base should be used under restorative filling material to increase the fracture resistance of the filling. 7-Flexibility: Def:- is the strain which occurs in the material when the stress reaches the elastic limit. Units:-dimensionless -From the figure, the order of flexibility is C > B > A Clinical Significance: a-Endodontic files and reamers where considerable amount of elastic bending may take place with little stress b-Large strain or deformation may be needed with a moderate or slight stress as in elastic impression material, since flexibility represents the ease by which the impression can be removed from the mouth Stress Max Flexibility Strain  8- Ductility and malleability Ductility Malleability It’s the ability of a material to undergo plastic It’s the ability of a material to undergo plastic deformation under tensile force without rupture. deformation under compressive force without fracture. i.e. drawn into wires i.e. humored into sheets Both are represented on the stress – strain curve by the plastic strain portion. From the figure, material A has higher amount of plastic deformation the material B, so material A is said to be more ductile (malleable) than material B. Material C has no plastic deformation, i.e. fracture occurred at or just beyond PL of the material. These materials are described as brittle materials Dental application: Burnishability of the crown margins. Percentage elongation is the measure of ductility. σ A -Ductility & Malleability are demonstrated in the stress- strain curve by long plastic portion of the curve B -From the figure, the order of ductility is A > B > C C ε 9-Brittleness: -Brittle material : material showed no or very little plastic deformation on application of load -In other words, a brittle material fractures at or near its proportional limit. -This fracture occurs by crack propagation -Brittle materials are not tough and have low % elongation -Moreover, brittle materials are weak in tension than compression e.g dental amalgam has a compressive strength which is nearly six times higher than its tensile strength. Fracture toughness: Def.: the amount of energy required to fracture a material with crack. -Fracture toughness gives a relative value of a brittle material's ability to resist crack propagation. -Brittle materials have lower fracture toughness than ductile materials. Importance in dentistry: -The addition of 50% zirconia to porcelain and the presence of fillers in polymers increase their fracture toughness because they deflect the crack or obliterate the crack by this more energy will be needed to propagate the crack leading to higher fracture toughness. Fillers Polymer 10- Resilience and toughness: Resilience Toughness (Spring back action) Def: It represents the amount of energy absorbed by a material It is the energy required to stress the material to the point of fracture. when it’s stressed to the proportional limit. Opposite: Not resilient Not tough On the stress It represents the area under the linear portion of the curve. It represents the entire area under the curve. strain curve: σ σ PL PL ε ε Affected by: PL. and modulus of elasticity. Strength and ductility Units: mMN/m3 mMN/m3 (energy per unit volume) (energy per unit volume) Measurement: By calculating the area of the triangle under the elastic By calculating the area under the elastic and plastic portion of the stress-strain curve portion of the stress-strain curve. Dental -It represents energy stored in an orthodontic wire to be -Brittle materials are not tough. application: released during teeth movement over extended period of time.  Properties and Stress-Strain Curves Stress Stress Strain Strain Strong = P.L. Weak = P.L. Stress Stress Strain Strain Stiff = E Flexible = E Stress Stress Strain Strain Ductile = Plastic deformation Brittle = Plastic deformation Stress Stress Strain Strain Resilient = area of the Not Resilient = area of triangle below elastic the triangle below elastic slope slope Stress Stress Strain Strain Tough = area under the Not tough = area under the elastic and plastic (curve) elastic and plastic (curve) Other mechanical tests: 1- Diametral compression test for tension: Advantages: P It’s used to determine the tensile strength of brittle materials. Technique: A disk of the brittle material is compressed diametrically in a testing machine until it fractures. D T The applied compressive stress introduces tensile stress in the material perpendicular to the plane of the force application Tensile stress is calculated from the formula: D → diameter P T → thickness P → load 2- Transverse strength: (flexural strength, three point bending;3PB, Modulus of rupture) -The transverse strength of a material is obtained when a simple beam, supported at each end, is loaded with a load applied in the middle. Such a test is called a three point bending test. 3 PL 3 X load X length = or Stress = 2 bd2 2 X width X thichness2 Dental significance: 1) Denture base materials are subjected to such type of stress during mastication. 2) In long span bridges both the length and the thickness of the span are critical as evident from the equation for deformation. The deformation varies as the cube of these two dimensions. 3- Fatigue: Def:It’s the failure of the material under repeated cyclic loading below its P.L. Endurance lim Endurance limit: it’s maximum stress below which a material can withstand an infinite number of cyclic loading without failure by fatigue. -From such curve [ S-N ] we can see that; i)when the stress is high, the material will fracture at a low number of cycles. ii) As the stress is reduced, the fracture occurs at a high number of S-N curve: cycles -Therefore, Failure under cyclic loading dependent on: a. the magnitude of the load b. the number of loading repetitions. Mechanism of fatigue: Cyclic loading promotes crack propagation, Fatigue fracture occurs is by the developing of small microcracks, which coalesce to form macrocracks, which will propagate through the material. Clinical significance Structures such as complete dentures, implant and metal clasps of removable partial dentures, which are placed in the mouth by forcing the clasps over the teeth, are examples of restorations that undergo repeated loading, and may fracture by fatigue. 4- Impact strength test: Def: energy required to fracture a material under sudden force. *Two types of impact testers are available: 1-Charpy testing machine. the specimen is h supported horizontally at the two ends. h’ 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. Izod Charpy Surface mechanical properties: 1) Hardness: Def:- is the resistance of the material to permenat indentation or penetration or scratching. Complete penetration i.e soft material No penetration i.e hard material -The most common methods for testing the hardness are the Brinell, Knoop, Vickers, Rockwell, and shore A -All methods used to measure the hardness, depend on the penetration of small indenter into the surface of the material. The smaller the indentation the higher is the number, the harder is the material and vice versa. Clinical Significance of surface hardness 1-Hardness is an important property to consider in order to avoid scratching structures like teeth or restorations e.g. Natural teeth should not be opposed by harder materials like porcelain. 2- Restorations made of hard material like cobalt chromium is very difficult to finish and polish but once it is polished it maintains its polished surface with No scratches. 2) Wear Def:- is the loss of material resulting from mechanical action. Wear is usually undesirable but during finishing and polishing procedures wear is highly desirable Causes of wear 1- Physiological e.g normal mastication 2- Pathological e.g bruxism. 3- Mechanical e.g improper use of tooth brushing Rheological Properties  Rheology: is the science which deals with flow and deformation of matter. Dentists are subjected to manipulate materials that flow or deform when subjected to stresses. Rheological properties includes: 1-Viscoelasticity 2-Creep  1-Viscoelasticity: Def: It describes materials that exhibit characteristics of both elastic solids & viscous fluids. -Viscoelastic materials are strain-rate dependent materials dependent on how fast they are stressed. -Increasing the rate of loading produces the higher value of their mechanical properties. At the beginning after 1 min. after 2 min. after 3 min. -Viscoelastic materials are combination of elastic, viscous and anelastic behaviors e.g. elastic impression materials, amalgam, dentin, gingiva and waxes. Mechanical models of viscoelasticity: 1-Ideal 2-Ideal viscous 3-Ideal elastic anelastic When load is Immediate strain Gradual strain Gradual strain applied The strain remains constant the strain increases linearly non linear (gradually) increase in with time. (gradually) with time. the strain with time When load is Immediate recovery No recovery gradual but complete recovery removed strain is time strain is time Strain is time independent dependent dependent Strain Strain Strain t0 t1 t0 t1 t0 t1 Time Time Time Viscoelastic behavior: It is a combination of elastic, anelastic and viscous behavior. This combination strain or viscoelastic strain is time dependent. -Upon load application i) Immediate strain will occur due to elastic portion and then followed by ii) gradual non linear increase in strain due to both viscous and anelastic parts. -Upon load removal i)The elastic strain is immediately recovered ii)The anelastic strain is gradually recovered. Strain iii)Viscous strain is not recovered which resultsin some permanent deformation from (1% -3%). t0 t2 t1 Time N.B : As viscoelastic strain is time dependent so rapid rate of loading (less time) will result in less permanent deformation Clinical Significance: Elastic impression materials must be removed rapidly from the mouth (snap removal) i.e. less time = high rate of loading) in order to: 1-Minimize the amount of permanent deformation in the impression 2-Increase the tear strength of the impression -On removal from the mouth they should be given time to recover before a model can be poured (to give time for gradual recovery of the anelastic part) Impression Tray Impression A-Before removal b-After removal) c-Slow removal d-sharp snap removal of the of the impression of the impression(ideal) of the impression impression(less permanent deformation) 2-Creep: Creep is: Time dependent permanent deformation occurs at stresses below the proportional limit and at temperature near the softening point of a material. wax wax At stress below proportional limit At stress below proportional limit and after certain time at elevated temperature No permanent deformation permanent deformation occurs -Since the softening temperature of most metals and ceramics is far above mouth temperatures, they do not creep in dental application. However, many polymers such as waxes and rubbers have softening point near mouth temperatures and can creep considerably. Clinical Significance: -In dental amalgam restorations, they contain components with melting temperature slightly above mouth temperature, thus they undergo creep which can be destructive to the restoration. N.B: -Flow: is the creep for amorphous materials, such as waxes. -Sag: is the creep at higher temperatures , such as metals.  Self assessment questions I-Give reasons for: 1-Amalgam and wax restorations undergo creep in patient mouth. 2-Structures subjected to bending fail on the surface that is increasing in convexity. 3-The yield strength is of greater importance than ultimate strength. 4-he high modulus of elasticity of ceramic solids. 5-The ability of pure gold to be easily (burnished) shaped into wires and sheets. II- Choose the correct answer: 1-The energy required to fracture a material under sudden force is …. a-Toughness b-Ultimate strength c-Resilience d-Impact strength 2- The greatest stress to which a material can be subjected and return to its original dimensions when the load is released is the: a. Elastic limit of the material b. Proportional limit of the material c. Compressive strength of the material d. Shear stress 3- The total work or energy required to rupture a material is: a-Resilience b-Toughness c-Brittleness d-Ultimate strength III. Draw and label the stress-strain curve of: 1-A stiff, ductile, strong and tough material. 2-A flexible, brittle and weak material. 3-Draw and label a strain-time relationship for a viscoelastic materials showing different behavior during and after load application