mechanical prop mahi .pdf

MECHANICAL PROPERTIES Presented by : Dr. Mahi Mansour  INTRODUCTION What are mechanical properties? They are a group of properties that describe the behavior of materials under forces or loads It is necessary to know the mechanical behavior of the material in...

MECHANICAL PROPERTIES Presented by : Dr. Mahi Mansour  INTRODUCTION What are mechanical properties? They are a group of properties that describe the behavior of materials under forces or loads It is necessary to know the mechanical behavior of the material in order to make a proper design such that any resulting deformation will not be excessive, and to avoid fracture.  FORCE The result of an applied force on a body is a change in its position of rest or motion. If the body remains at rest , the forces will cause its deformation. Forces can cause one or all the following : Displacement Acceleration Deformation  A force is defined by the following characteristics: Speed Magnitude Point of application Direction  EQUILIBRIUM Generally, anybody is in equilibrium External Internal equilibrium equilibrium An internal reaction balances the external force. This internal reaction is known as STRESS WHAT IS STRESS ? Stress is the internal reaction to the external applied force. It is equal in magnitude and opposite in direction to the external force. The external force and the internal resistance are distributed over a given area of the body. Therefore, stress can also be defined as force per unit area. TYPES OF STRESS Normal or axial Tangential  TENSILE STRESS Tension results in a body when it is subjected to two sets of forces directed away from each other in the same straight line. Tensile stress = Elongation  COMPRESSIVE STRESS Compression results when the body is subjected to two sets of forces directed towards each other on the same straight line. Compressive stress = Shortening  SHEAR STRESS Shear is the result of two sets of forces directed towards each other but not in the same straight line Shear stress Tearing or sliding  COMPLEX STRESSES A single type of stress is extremely difficult to be induced in a structure. Materials in a stressed component often have stresses acting in two or more directions at the same time. This is a complex stress situation.  IMPORTANCE OF STRESS IN DENTISTRY Dental restorations are subjected to extremely great stresses because the area over which the forces are applied is extremely small. The forces applied to a dental restoration are resolved as a combination of compressive, tensile, and shear stresses ( complex stresses) rather than a pure single stress.  STRAIN Strain is the change in length or deformation per unit length, when a material is subjected to a force.  TYPES OF STRAIN Temporary or Permanent or elastic strain plastic strain Which disappears on Which will not disappear removal of the external on removal of the external force. The material will force. The material will not return to its original shape return to its original shape STRESS- STRAIN CURVE  STRESS- STRAIN CURVE A stress–strain curve for a material gives the relationship between stress and strain. It is obtained by gradually applying the load to a material and measuring the deformation , from which the stress and strain can be determined. These curves reveal many of the properties of a material, such as the : Young's modulus, the yield strength and the ultimate tensile strength.  Stress strain curve Elastic Plastic behaviour behavior  HOOK’S LAW Hook's law states that the strain of the material is proportional to the applied stress within the elastic limit of that material, until a stress value known as proportional limit When the elastic materials are stretched, the atoms and molecules deform until stress is applied, and when the stress is removed, they return to their initial state. Therefore, the elastic portion of the curve obeys Hook’s law  ELASTIC PORTION (LINEAR PORTION) It obeys Hook’s law. It is a non-permanent deformation, when the applied load is removed, the body returns to its original shape and size. This behavior occurs till a point known as proportional limit “the highest stress at which the strain developed in the material is temporary”  Proportional limit➔ is considered departure from linearity. Elastic limit➔ The greatest stress to which the material can return to its original dimensions when the force is released.  YIELD STRENGTH The stress level at which plastic deformation begins. The yield strength for a metal is a measure of its resistance to plastic deformation.  ULTIMATE STRENGTH Ultimate strength is the maximum strength that any solid material can withstand before fracture.  From your point of view … Which is more important the yield strength or the ultimate strength ???? And why ? ‫سؤال ب عشرة جنيه‬  FRACTURE STRESS The stress at which the material will fracture  MODULUS OF ELASTICITY (YOUNG’S MODULUS) It is the constant of proportionality between the stress and strain. It represents the slope of the elastic straight-line portion of the stress- strain curve.  MODULUS OF ELASTICITY (YOUNG’S MODULUS) It is a measure of the rigidity or stiffness of a material within the elastic range.  Materials with higher modulus of elasticity are stiffer or more rigid than those with lower modulus of elasticity. Because they need more stress to produce the same amount of strain. The modulus of elasticity doesn’t change either tested in compression or tension. It is a fundamental property of the material that mainly depends on the inter- atomic or inter- molecular forces of a material. The stronger the inter-atomic forces, the greater the values of the elastic modulus and thus the more rigid the material.  CLINICAL SIGNIFICANCE OF MODULUS OF ELASTICITY 1. The denture base should be constructed of a rigid material in order to : 1. Allow load distribution on the whole design 2. Be used in thinner sections without the risk of bending. Where thinner sections gives comfort to the patient 2. In case of long span bridges, high modulus of elasticity is required to allow proper stress distribution.  FLEXIBILITY Maximum flexibility is the strain resulting in the material when the stress reaches the elastic limit For example, in an orthodontic appliance, a spring is often extended a considerable distance under the influence of a small stress. In such a case, the structure is said to be flexible and possesses the property of flexibility.  CLINICAL SIGNIFICANCE OF FLEXIBILITY Clasps are flexed during mastication; therefore, it is necessary to fabricate them with an alloy of high flexibility Flexibility is also an important property in elastic impression materials, since it represents the ease by which the impression can be removed from the tooth  DUCTILITY AND MALLEABILITY They are two significant properties of metals and alloys , which indicate their workability Malleability It is the ability of the material to be plastically deformed under compression without fracture ( hammered into thin sections) Ductility It is the ability of the material to be plastically deformed under tension without fracture ( hammered into a wire) PERCENTAGE ELONGATION IS THE MEASURE OF DUCTILITY  CLINICAL SIGNIFICANCE OF DUCTILITY 1. It indicates the degree to which a structure will deform plastically before fracture. 2. It specifies the degree of allowable deformation during fabrication operations. We sometimes refer to relatively ductile materials as being “forgiving,” in the sense that they may experience local deformation without fracture. Ex: Clasps can be adjusted , orthodontic appliances can be prepared, crowns can be burnished if they are prepared from alloys of high ductility  BRITTLENESS A brittle material is a material that on application of load is unable to deform plastically before it fractures. In other words, a brittle material fractures at or near its proportional limit  ‫سؤال ب خمسة جنيه‬ Give an example of a brittle material and a ductile material ? RESILIENCE The Tico jump  RESILIENCE Resilience is the ability of a material to absorb energy when it is deformed elastically and release that energy upon unloading. When the load is released complete recovery of the material will occur. It is measured by the area under the straight portion of the stress- strain curve  CLINICAL SIGNIFICANCE OF RESILIENCE Resiliency is highly important when evaluating orthodontic wires, since the amount of work expected from a spring wire for moving a tooth is fundamental. High resilient orthodontic wires increase the working range of the wire.  TOUGHNESS It measures the amount of energy a material can absorb up to fracture. It is represented by the area under the elastic and plastic portion of the stress- strain curve.  ‫سؤال ب عشرة جنيه‬ True or false ???? For a material to be tough, it must display both strength and ductility?  FRACTURE TOUGHNESS It is the amount of energy needed to fracture a sample with a crack. (It’s the measure of a material’s resistance to fracture when a crack is present). The fracture strengths for most materials are significantly lower than those predicted by theoretical calculations. This discrepancy is explained by the presence of very microscopic flaws or cracks that act as stress raisers. Stress raisers  Cracks may arise naturally in a material; any defect usually weakens the material and sudden fractures may happen at stresses below the yield stress ‫سؤال ب حاجة حلوة‬ COMPARE BETWEEN BRITTLE AND DUCTILE FRACTURE ?  Brittle fracture Ductile fracture - Fracture at or near the -Fracture away from the proportional limit. proportional limit. - No or limited plastic -Plastic deformation before deformation before fracture fracture. - Fracture by crack -Fracture by necking propagation - Brittle materials are -Ductile materials are tough not tough (see Toughness) - Brittle material has low - Ductile material has high % % Elongation Elongation Which can be considered sudden or catastrophic fracture ???? Brittle Ductile  AFTER STUDYING THE MECHANICAL PROPERTIES It is worth mentioning that having a material with sufficient strength to resist plastic deformation or fracture is NOT the only consideration for material selection. However, there are instances where elastic and plastic deformation are very critical. ARE YOU STILL LISTENING ? BENDING PROPERTIES  CANTILEVER BENDING The bending properties of many materials are equally or more important than their tensile or compressive properties. Cantilever bending properties are usually measured by clamping a sample at one end and applying a force at a fixed distance from the face of the clamping.  Clinical significance The bending properties of wires, endodontic files and reamers and hypodermic needles are especially important. As these instruments may be permanently bent if the bending angle exceeds the value at the end of the linear portion of the curve.  TRANSVERSE BENDING 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. In practice, the stresses within in a material are complex. Thus, if a beam is bent; the lower portion of the beam is in tension, and the top is in compression. Shear stresses are also present. This test determines not only the strength of the material () , but also, the amount of deformation expected. Stress or the transverse strength is calculated from the equation: Clinical Significance: The transverse strength test and the accompanying deformation are very important in comparing: 1) Denture base materials & 2) Long span bridges. DYNAMIC MECHANICAL TESTS  DIAMETRAL COMPRESSION TEST The diametral compression test or the indirect tensile test is used to measure the tensile strength of brittle materials. These brittle materials include dental amalgam, cements, ceramics and gypsum products. These materials are much weaker in tension than in compression thus this contributes to their failure in service.  In this test a disk of the brittle material is compressed diametrically in a testing machine until fracture occurs, the compressive stress applied to the specimen introduces tensile stress in the material, the tensile stress is calculated by: 2P Tensile stress =  DT Where P = Load D = Diameter T = Thickness  IMPACT STRENGTH A material may have high static strength values such as compressive, tensile and shear strengths but may fracture when loaded under impact i.e. Subjected to dynamic loading Impact strength: "Is the amount of energy absorbed by the material when subjected to sudden force". The Impact strength is measured by clamping a specimen of known dimensions firmly in position and breaking it with a swinging pendulum. Two types of impact testers are available: Charpy tester, and Izod.  Importance in dentistry: 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.  FATIGUE STRENGTH The fatigue strength is the stress at which a material fractures under repeated loading below the yield strength. - Fatigue tests are performed by subjecting a specimen to cyclic stress application below the yield stress until fracture occurs. A curve representing the stress at which a material will fail as a function of the number of loading cycles will be plotted ( S-N curve ).  CURVE FOR FATIGUE STRENGTH From such curve [ S-N ] we can see that; i) when the stress is high, the material will fracture at a relatively low number of cycles. ii) As the stress is reduced, the number of cycles required to cause failure increases. iii) Therefore, Failure under cyclic loading not only dependent on the magnitude of the load but also on the number of loading The fatigue limit or endurance limit is the stress repetitions. that can be applied an infinite number of times without causing material failure.  Clinical significance The determination of fatigue properties is of considerable importance for certain types of dental restorations subjected to alternating forces during mastication. Structures such as complete dentures, implants, 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. Surface properties  HARDNESS Hardness is the resistance of the material to permanent indentation or penetration or scratching. It is surface property which can not be determined from stress strain curve. 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.  Hardness is measured as a force per unit area of indentation. - Some of the most common methods of testing the hardness of restorative materials are the Brinell, Knoop, Vickers, and Rockwell. Brinell Knoop Rockwell Vickers  Importance in dentistry: Hardness is an important property to consider in order to avoid scratching structures like teeth or restorations eg: Natural teeth should not be opposed by harder materials like porcelain. Avoid scratching of soft materials (model and die materials) because it decreases their accuracy. Restorations made of hard material are Very difficult to finish and polish. Once they are polished it maintains its polished surface with no scratches. Therefore, hard materials is considered both an advantage and disadvantage.  SURFACE WEAR Wear is the loss of material resulting from different actions. Wear of tooth structure and restorative materials may result from Mechanical conditions Pathological conditions Physiological conditions Physiological form of wear like when normal mastication may cause attrition of tooth structure Pathological form of wear that can be caused by bruxism Mechanical (abrasive) form of wear that can be caused by improper use of tooth brushing RHEOLOGICAL PROPERTIES  VISCOELASTIC MATERIALS Viscoelastic materials are strain rate sensitive materials dependent on how fast they are stressed (relationship between stress and strain depends on time). Increasing the rate of loading produces higher value of their mechanical properties. Viscoelastic materials are a combination of elastic, viscous and anelastic behaviors. e.g. elastic impression materials, amalgam and waxes.  STRAIN TIME RELATIONSHIPS 1. Ideal elastic material If a material behaves as an ideal elastic solid When stress is applied below the proportional limit Strain immediate amount of strain will result and the strain remains constant with time. - When the load is removed (at the time t1) the strain immediately decreases to zero in no time. Therefore, the strain is independent of the rate of Time loading or time in which the load was applied.  2. Ideal viscous material Strain If a material behaves as an ideal viscous solid When stress is applied below the proportional limit at (T0 Time) the strain increases uniformly until the stress is removed at (T1 Time) Time The Strain Will Not Recover After Stress Removal. Therefore, the strain is directly proportional to the time of load application  Anelastic material (delayed elasticity ) When stress is applied at (t0 time) Non linear increase of the strain with time On load removal → strain will non linear decrease to zero with time (gradual but complete recovery) Therefore, the strain is time dependent  Viscoelastic material It is a combination of elastic, anelastic and viscous behavior. This viscoelastic strain is time dependent. The elastic and anelastic portions are recovered but the purely viscous components are not. Upon load application Immediate strain will occur due to elastic portion and then followed by gradual non linear increase in strain due to both viscous and anelastic parts. Upon release of stress ?? i) the elastic strain is immediately recovered and ii) the anelastic strain is gradually recovered. iii) viscous strain is not recovered which results in some permanent deformation (1-3%). ‫سؤال ب خمسة جنيه‬ Is viscoelastic strain time dependent ?  Since viscoelastic strain is time dependent therefore rapid rate of loading 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; a. Minimize the permanent deformation as a result of viscous deformation during removal. b. Increases tear strength i.e. less chance to tear. Furthermore, 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)  CREEP Time dependent plastic deformation under static loads at stresses below their yield strength at temperatures near the softening point  Clinical significance Metals tend to creep (slowly deform) when stressed near its melting temperature. In dental amalgam restorations, they contain components with melting temperature slightly above room temperature. Therefore, they undergo creep.