Mechanical Properties PDF

Sinai University Faculty of Oral and Dental Medicine Dental Biomaterial Department MECHANICAL PROPERTIES By The End Of The Lecture, Students Will Be Able To: 1-Identify Mechanical Properties . 2- Define Stress And Strain And Their Types 3-Know The Stress- Strain Curve And Identify Different Poin...

Sinai University Faculty of Oral and Dental Medicine Dental Biomaterial Department MECHANICAL PROPERTIES By The End Of The Lecture, Students Will Be Able To: 1-Identify Mechanical Properties . 2- Define Stress And Strain And Their Types 3-Know The Stress- Strain Curve And Identify Different Points On The Curve. 4- Understand The Clinical Significance Of Mechanical Properties. MECHANIAL PROPERTIES Mechanical properties: are a group of physical properties that describe the behavior of materials under a force or a load The properties of a solid under applied load Are determined by the nature and or strength of its atomic binding forces. Biting force The average biting force of a person with, natural teeth is approximately 77 kg on the posterior part of the mouth. It represents approximately 1925kg /cm2 (188 Mn M2) on a single cusp of molar tooth. Force It is the external action that produces the motion of a body The units for measuring a force are the pound, kilogram and mega Newton Stress It is the internal relation to the external applied force; It is equal in intensity and opposite in direction to the applied force. External, force it, has a direction and magnitude. Stress: the force per unit area stress = (force / area) The stress in a structure varies directly with the external force and inversely with the area over which it is applied this is very important from the dental point of view since in dental restorations the areas over which the forces are applied are often extremely small Types of stress 1-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. F F Tensile stress When a load tends to cause elongation or stretching is applied to a body it induces a force in this body which resists the deformation this internal resisting force. Is called tensile stress it is always accompanied by tensile strain. 2- Compressive stress When the body is subjected to two sets of forces directed towards each other in the same straight line. When load tends to compress or shorten is applied to a body it which resist such compression. This internal resisting force is called compressive; it is always accompanied by compressive strain F F shear stress Stress When two sets of forces are directed to each other but not on the same straight line a motion that would cause twisting or sliding of one portion of the body over another will be resisted by stress tensile stress called shear it is accompanied by shearing strain F mental stress F Complex stresses and strain A single stress, pure, unmixed with other types of stresses is extremely difficult to induce in a structure. F F compression Shear Tension F F F F Types of strain Temporary or elastic strain which disappears on removal of the external force The material will return to its original shape. Permanent or plastic strain which will not disappear on removal of the external force . The material will not return to its original shape. Modulus of elasticity There is constant relation between stress and strain termed the modulus of elasticity. If any stress value equal to or less than the proportional limit is divided by its corresponding strain value we get a constant This constant of proportionality is termed the modulus of elasticity. It is a measure of rigidity or stiffness Young’s Modulus ,Shear modulus or Modulus of rigidity If a wire is difficult to bend, this means that great stress (force) must be induced before a notable strain (deformation) results this wire therefore posses a high modulus of elasticity since a high stress is required to deform it .If it attained (1) With compressive or tensile stress is called young’s modulus. (2) With shear stress it is sometimes called shear modulus or modulus of rigidity .It is a measure of stiffness of a material STRESS MODULUS OF ELASTICITY= LOAD = STRAIN = FORCE /IT/AREA DEFORMATION Materials / Elastic Modulus GPA Cobalt-chromium P.D. Alloy Gold Type Iv Gold Type IV Field Spathic Porcelain Amalgam Composite Resin Acrylic Denture Silicone Rubber 218.2 99.3 69.0 27.0 16.6 2.65 0.002 Values of Elastic Modulus of Some Restorative Dental Materials (GPA (GIGA PASCAL) = 103MPA Proportional Limit (P.L.) The greatest stress that a material can sustain without deviation from the proportionality of stress to strain below the proportional limit. When the stress is removed the structure will return to its original dimension. If the material is stressed to value below the Proportional limit an elastic or reversible strain will occur The elastic limit (E.L) The maximum stress that a material will withstand without permanent deformation resulting . The observed stress will be predominantly tensile but the cross section of the wire will decrease indicating the presence of compressive stresses . Units of stress are : Ib/in2 ( p.s.i) or kg/cm2 or mn/m2 Mpa ) mega bascal/ kg Kg/cm2 14.22 p.s.i = l45 MN/m2 Clinical Importance The importance of strain in dentistry is that a restorative materials. Such as a clasp or an orthodontic wire which can withstand a large amount of strain before failure can be bent and adjusted with less chance of fracturing. Deformation 6(L-L0) Strain (E) = Original length L0 L = L0 Yield strength (Y.S.) The point at which the material starts to fail in regaining its original shape and dimensions. Called the yield point, and is often used to describe the stress at which the material begins to function in a plastic manner. The amount of permanent strain may be indicated as 0.1% 0.2% or 0.5% permanent strain Ultimate Strength (U.S.) It is the greatest stress which can be induced in a body before or during rupture or the maximum stress that a material can withstand before failure it is determined by dividing the maximum load in tension or compression by the original cross sectional area of the test sample. Ultimate Strength (U.S.) The Ultimate Tensile Strength The maximum stress that a material can withstand before failure in tension. (fracture or rupture). Flexibility Is the total amount of elastic strain in a material. In dental restorations and appliances we require high elastic limit for their materials so as the material will return to its original shape after each stress. or The maximum flexibility is the strain which occur when the material is stressed to its proportional limit. Clinical importance This is very important for impression materials which often subjected to deformation to be removed from undercuts, but must have the ability to spring back without suffering from any permanent change in shape. Flexibility= EM = PL E Proportional limit Modulus of elasticity Poisson’s Ratio Under tensile loading : ( Axial loading) As a material elongates in the direction of force, there is a reduction in cross section . Under compressive loading There is an increase in the cross section within the elastic range, (Lateral loading) within the elastic range, the strain of the lateral strain to the axial strain is called Poisson’s ratio The ratio of the poisons = lateral strain Axial strain Ductility The amount of plastic strain produced in the specimen before fracture is called the ductility of the material or The ability of a material to withstand permanent deformation under tensile load without rupture. Malleability Ductility is dependent upon plasticity and tensile strength The ability of a material to withstand permanent deformation without rupture under compression. Is dependent on plasticity but not on strength. (Al) increased in length Percentage of elongation x 100 (l) Original length Brittleness If a material showed no or very little plastic deformation on application of load. A brittle material fractures at or very near to its proportional limit. Brittle materials are weak in tension. Example: Dental amalgam has a compressive strength six times higher than its tensile strength. Comparison between ductile and brittle materials: a) Fracture occur far away from the P.L.(Ductile materials). b) Fracture occur at or near the P.L. (Brittle materials). A) Necking tacks place before fracture. 1 2 3 4 B) No necking, but crack propagation tacks till fracture. 3-Examples: Ductile materials gold alloys, and nickel chromium. Brittle materials, Dental amalgam, porcelains and composites. Resilience Is the resistance of a material to permanent deformation? It is measured by the area under the elastic portion of the stress strain curve. For example acrylic resin denture teeth are more resilient than porcelain teeth and absorb most masticatory forces and transmit less force to the underlying bone, preserving it. Stress (Pa) A Strain Toughness It's the amount of energy necessary to cause fracture. Toughness requires strength and plasticity. The total amount of energy that a material can absorb it before fracture is a measure of the toughness of the material and is indication by the total area under stress strain curve it is also expressed in terms of J.m. (1joule =1nm ) an energy per unit volume. Resilience & Toughness Resilience Toughness Stress Stres s P.L P.L Strain Strain U.S Fracture toughness Its the amount of energy required to break it. It is the ability of the material to resist fracture through its resistance to crack propagation. The ability to plastic deformation without fracture is another meaning for the fracture toughness. Clinical Significance In composites The presence of glass particles increase the fracture toughness because glass will stop crack propagation. In porcelain The addition of zirconia’s particles increase the fracture toughness because zirconium particles absorb the energy needed for crack propagation Cantilever Bending The bending properties are sometimes more important than the tensile or compressive properties. The bending properties of wires, Endodontic files and reamers and hypodermic needle are especially important. It is measured by clamping a sample at one end and applying a force at a fixed distance from the face of the clamping. As the force is increased and the sample is bent, corresponding values for the bending (angular deflection) and the bending moment (force x distance) are recorded. A plot of bending moment versus the angle of bending can be obtained which is similar in appearance to stress strain-curve. Bending moment =Force x distance Cantilever bending tests Mechanical tests An alternative method of testing brittle materials in which the ultimate tensile strength of a brittle material is determined through compressive testing. (1) The diametral compression test – for tension the Brazilian test or indirect tensile test . 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 tensile is calculated by The diametral compression test Transverse strength test When a load is applied in the middle tested of simple beam. Which is supported at each is called a three – point. Bending test Flexural strength or the modulus of rupture Useful in (1) Comparing dental base material during application of biting forces (mastication) . (2) In long span bridge in which the biting stress may be sever Resulting deformation in such a bridge. Flexural strength or the modulus of rupture Impact strength and impact test Is the amount of energy absorbed by the material when subjected to sudden? Force. A charpy – type impact tester & Izod impact tester A pendulum is released which swing down to fracture the specimen. The energy lost by pendulum during fracture of the specimen can be determined by compression of the length of its swing after the impact with its free swing when no impact occurs. The units are joules. Foot / pounds, inch / pounds Fatigue strength and fatigue test Is the fracture of a material when subjected to repeated cyclic small stresses below the proportional limit? Failure under repeated or cyclic loading is dependent on the magnitude of the load and the number of loading repetitions. The repeated application of small stress to an object causes tiny cracks to be generated within its structure with application of stress the crack grows until the material breaks. fatigue test Useful in Determination of fatigue properties for certain types of dental restoration such as complete dentures, partial .dentures. Clasps, implants. Creep And Creep Test The time dependent plastic deformation that occurs in an object subjected to a small load below its E.L.(P.L.) Is the slow flow causing permanent deformation of materials held for long periods of time at stresses below their elastic limit . Usually this mechanism occurs only at temperatures near soft point of a material. Most metallic and ceramic restorations do not creep in the oral environment. An exception is dental amalgam under occlusal stress An amalgam filling may creep and thereby lose its marginal adaptation. Creep curve Showing the four stages of creep for long duration and high temperature A typical creep curve is show Four stags of elongation that can be identified 1-Intial elongation due to application of the load . 2-Transient or primary creep which tends by large effect. 3-Steady state (secondary creep) 4-Tertiary creep. Time Hardness Test Is the resistance of the material to scratching, indentation or penetration? It provides an indication of the resistance of the material to scratching or abrasion. The test involves the use of an indenter .which can be in the sharp of a ball (Brinell) a pyramid (Vickers or knoop) or cone (Rockwell) which of course must be harder then the material being tested. The choice of hardness tester depends to some extend on the nature of material being tested. Brinell hardness test A hardened steel ball of special diameter is pressed into a polished surface of a material under a specific load. The area of the surface of the indentation is measured and divided of the magnitude of the load. This proportion is called the (B. H. N) it is an index of hardness. Index of hardness BHN = Load Area of indentation Disadvantages: You cannot get a rounded circumference. In case of elastic material as soon as you remove the ball it will go back recover and you get a smaller indentation . Rock well hardness test Is similar to brinell test in that steel ball or cone is used the depth is measure directly by a dial gauge on the instrument. Advantages: It is rapid and easy method for measuring. Disadvantage Is not suitable for brittle and elastic materials. Vickers hardness test Diamond square –based pyramid cone Useful in : Measuring the hardness of small areas and very hard materials. E–g Gold castings Brittle material Tooth structure Not for elastic materials Knop hardness test It was developed to fulfill the need for a microindentation test. a load is applied to a carefully prepared diamond indenting tool with a pyramid shape. And the length of The diagonals of the resulting indentation in the material are measured Length Of Long diagonal cone test relating to projected area of the indent of the indentation . Advantages: 1-Easy measuring: can test hardness of brittle materials without fracture. 2-Can test hardness of elastic material. Disadvantages. The need of a highly polished and flat test sample and the time required to complete test operation. Shore hardness test Is used in the rubber industry to determine the relative hardness of Elastomers. The instrument consists of a blunt – point indenter 0.8mm diameter, that taper to a cylinder 1.6mm,the indenter is attached by a lever to a scale that is graduated from 0-to 100 units. If indenter completely penetrates the sample. reading of O is obtained and if no penetration occurs. Reading of 100 units results. The usual method is press down firmly and quickly on the indenter and record the maximum reading. Used In : Evaluate soft denture liners , mouth protectors and maxillofacial Elastomers. Stress compressive stress tensile stress mental stress © M.Lenhard References: 1-Van Noort R, Michele B., 2013. Introduction to Dental Materials (fourth edition); Elsevier Health. 2-Callister WD, Rethwisch DG., 2009. Materials science and engineering: Anintroduction (eight edition) Wiley, USA