Lec. 2 Mechanical Properties PDF

Lec. 2 Noor Bader Noor Bader C- Mechanical Properties Describe the ability of the material to resist forces and their effects on the bodies. Examples of mechanical properties are stress, strain, strength, and stiffness One of the...

Lec. 2 Noor Bader Noor Bader C- Mechanical Properties Describe the ability of the material to resist forces and their effects on the bodies. Examples of mechanical properties are stress, strain, strength, and stiffness One of the most important properties of dental material is its ability to withstand the various mechanical forces placed during its use as a restoration, impression, model appliance, and tool. The mechanical properties measure the responses of material under an applied force, and they are important in understanding and predicting the behavior of a material under load. Materials used for restoration must be able to withstand forces during fabrication or mastication.. 1- Stress: is the force per unit area induced in a body in response to some externally applied force. Several types of stress may result when a force is applied to the material, these forces are compressive, tensile, shear, twisting movement, and bending movement (flexure). The unit of stress is the unit of force {Newton (N)} divided by a unit of area and is commonly expressed as Pascal (1 Pa = 1 N/m² = 1 MN/mm²). Types of Stress: There are different types of stresses according to the direction of the applied force. A-Tensile stress: results from two sets of forces directed away from each other in the same straight line or when one end is constrained and the other end is subjected to a force directed away from the constraint; it is accompanied by tensile strain. Examples: enamel: 10 Mpa, dentin: 106 Mpa, amalgam: 32 Mpa. Lec. 2 Noor Bader B- Compressive stress: It results from two sets of forces directed toward each other in the same straight line, also when one surface is constrained and the other is subjected to a force directed toward the constraint, its accompanied by compressive strain. Investment materials, restorative materials, and models should have high compressive strength. Examples: enamel: 384 Mpa, dentin: 297 Mpa, amalgam: 388Mpa. C- Shear stress: Shear is the result of two sets of forces directed parallel to each other (not along the same straight line) which is applied to one part of the body in one direction, and the rest is being pushed in the opposite direction. The result is the sliding of the molecules over each other. It is accompanied by shear strain. Examples: enamel: 90 Mpa, dentin: 138 Mpa, amalgam: 188 Mpa. Shear force is the force which causes tearing a paper or a card. D-Flexural stress (bending stress): it is the force per unit area of a material that is subjected to flexural loading. It results from an applied bending moment. Usually, three types of stresses occur at the same time If a piece of metal is being bending it will exhibit tensile stress on the outer surface, compressive on the inner and shear stress in the middle. Lec. 2 Noor Bader E- Torsion stress: Force per unit area of a material that is subjected to twisting of a body. Each type of stress is capable of producing a corresponding deformation in the body. The deformation from tensile force is an elongation, whereas a compressive force causes compression or shortening of the body in the axis of loading. Every stress is accompanied by a strain of the same type. Strain (Ɛ): is the change in length (dimension) or deformation per unit length (dimension) caused by externally applied force. Strain is often reported as a percentage, for example: acrylic: l.5, Co/Cr: 4%, stainless steel: 35%. 𝐟𝐢𝐧𝐚𝐥 𝐥𝐞𝐧𝐠𝐭𝐡 (𝐋) − 𝐨𝐫𝐢𝐠𝐢𝐧𝐚𝐥 𝐥𝐞𝐧𝐠𝐭𝐡 (𝐋₀) ∆𝑳 Strain = = 𝐎𝐫𝐢𝐠𝐢𝐧𝐚𝐥 𝐥𝐞𝐧𝐠𝐭𝐡 (𝐋₀) 𝑳₀ Strain under tensile stress is an elongation in the direction of loading. Lec. 2 Noor Bader Strain under compression is a shortening of the body in the direction of loading. Elongation: The deformation that results from the application of tensile stress. An alloy with high percent of elongation can be bent or adjusted without danger of fracture 𝑖𝑛𝑐𝑟𝑒𝑎𝑠𝑒 𝑖𝑛 𝑙𝑒𝑛𝑔𝑡ℎ 𝐸𝑙𝑜𝑛𝑔𝑎𝑡𝑖𝑜𝑛 = 𝑥 100. 𝑜𝑟𝑖𝑔𝑖𝑛𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ Types of the strain 1. Temporary of elastic or recoverable strain: the material is returned to its original length after removal of the applied force. 2. Permanent or plastic or unrecoverable strain: the material is not returned to its original length after removal of the applied force. The material may remain deformed Stress-Strain Curve (s-s) o A stress-strain curve is a graphical representation of the relationship between stress and strain in a material. o It is a convenient method of comparing the mechanical properties of materials by applying various forces of the material to determine the corresponding values of stress and strain. o The stress is plotted vertically and the strain is plotted horizontally. Lec. 2 Noor Bader The (s-s) curve description:- Proportional limit (point A): The greatest stress that a material will sustain without a deviation from the proportionality. Elastic limit (point A): The maximum stress that a material will sustain without permanent deformation. Elastic limit deals only with the elasticity of the material, but proportional limit deals with stress and strain proportionality Theoretically, the values will be the same. The region of the s-s curve before the proportional limit is called the “elastic region” (from 0 to A). The region of the curve beyond this proportional limit is called as “plastic region” (from A to D). If the stress is increased beyond the elastic limit or the proportional limit (A-D) , the material will deform, and if we remove the stress the material will not return to its dimension. This is called plastic deformation. If the stress is increased more and more, the material will break. Lec. 2 Noor Bader Ultimate Strength (point C): the maximum stress that a material can withstand before failure. Examples: acrylic: 8000 PSI, Co/Cr:100000 PSI, stainless steel: 15000 PSI. Fracture Strength (point D): The stress at which a material fracture. The fracture strength is not necessarily the ultimate stress at which the material will fracture. Mechanical properties A-Modulus of Elasticity (Elastic Modulus): represents the stiffness or rigidity of a material within the elastic range, it is the constant of proportionality. It can be determined from a stress-strain curve by calculating the ratio between the stress and strain on the slope of the linear region from the following equation: 𝒔𝒕𝒓𝒆𝒔𝒔 Modulus of elasticity = 𝒔𝒕𝒓𝒂𝒊𝒏 The unit of the elastic modulus is Pascal. Examples: enamel: 84Gpa, dentin: 17Gpa. Lec. 2 Noor Bader Application: The metal frame of the metal-ceramic bridge should have high stiffness. If the metal flexes, the porcelain veneer on it might crack or separate. B-Flexibility: The higher strain that occurs when the material is stressed to its proportional limit (the amount of strain up to the elastic limit), So that flexibility is the total amount of elastic strain in a material. Application: It is useful to know the flexibility of elastic impression materials to determine how easily they may be withdrawn over undercuts in the mouth. C-Ductility: (only in tensile stress): It is the ability of the material to withstand permanent deformation under tensile stress without fracture. It's the ability of the material to be drawn into a fine wire. examples: gold: most ductile. It's the ability of the material to be drawn into a fine wire. Examples: gold: most ductile. Lec. 2 Noor Bader D-Malleability: Malleability (only in compressive stress): It is the ability of the material to withstand permanent deformation under compressive stress without fracture. It's the ability of the material to be drawn into a sheet. Examples: gold: most malleable Elastic strain = flexibility. Plastic strain = ductility or malleability. E-Brittleness: It is the opposite of ductility; it requires a lack of plasticity. Application: Many dental materials are brittle, e.g. porcelain, acrylic, cement, and gypsum products. F- Resilience: The amount of energy absorbed by a structure when it is stressed within the proportional limit. Or it is the energy needed to deform the material to its proportional limit. Lec. 2 Noor Bader Application: Resilience has particular importance in the evaluation of orthodontic wires. An example: The amount of work expected from a spring to move a tooth. G-Toughness: It is the energy required to fracture a material. It is also measured as the total area under the stress strain curve (elastic and plastic portions of stress strain curve). Lec. 2 Noor Bader properties of Stress-Strain Curves The shape of a stress-strain curve and the magnitude of stress and strain allow the classification of materials with regard to their properties e.g. weak, strong, flexible, stiff, ductile, brittle, resilient, and tough. Strength: is the measure of the resistance of the material to the externally applied force. Fatigue strength: the fracture of a material when subjected to repeated (cyclic) small stresses below the Proportion limit. It is when the material is constantly subjected to change in shape due to frequent application of force. The repeated application of small stress (below the Proportion limit) to an object causes tiny (very small) cracks to be generated within its structure. These tiny cracks do not cause failure immediately, with each application of stress, the cracks grow until the material breaks. Metal and ceramics can all fail to fatigue. Lec. 2 Noor Bader Transverse strength: It is the strength of the middle of a beam, which is supported only at its ends. It is important in dental bridges. Examples: composite: 139Mpa, amalgam: 124Mpa. Impact strength: It is the ability of the material to break on sudden impact. Low impact strength means brittle material, like the dropping of the denture. Hardness: It is the resistance of the material to deformation caused by penetrating or scratching the surface. It is done either by using steal ball (Brinell or Rocwell test) or using a diamond (Vickers and Knoop test). The higher the number, the harder the material. Examples: Brienell hardness number: acrylic: 22, dentin:65, gold: 250. Knoop hardness number: enamel: 343, dentin:68, Co/Cr: 391.Kg/mm2