2 Strain:When the external force or load is applied to a material the phenomenon of strain occurs – this is a change in dimension of the material ( the change in length, or deformation per unit length )Deformation of lengthStrain =Length
4 Types of strain 1- temporary of elastic strain: Which disappears on removal of the external force. The material will return to its original shape.2- Permanent or plastic strain:Which will not disappear on removal of the external force. The material will not return to its original shape.
5 Stress:Associated with strain is the phenomenon of stress – this is an internal force/unit area in a material, equal and opposite to the applied load or force/unit area.ForceStress =Area
7 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.
8 2) Compressive stress:Compression results when the body is subjected to two sets of forces directed towards each other in the same straight line
9 3) Shear stress:Shear is the result of two sets of forces directed towards each other but not in the same straight line.
10 4) Complex stresses:A single type of stress is extremely difficult to induce in a structure so in practice the stresses within a material are complex. (complex stresses are produced by 3 point loading)compression sheartension
13 Stress – strain curve:A convenient means of comparing the mechanical properties of materials is to apply various forces to a material and to determine the corresponding values of stress and strain . A plot of the corresponding values of stress and strain is referred to as a stress- strain curve. Such a curve may be obtained in compression, tension, or shear.
14 From the stress strain curve, the following properties can be drawn: 1) Proportional limit (P.L):It is defined as the maximum stress that a material will withstand without deviation from the low of proportionality of stress to strain (it describes the relation between stress and strain)
15 2) Elastic limit (E.L):It is defined as the maximum stress that a material will withstand without permanent deformation resulting. (it describes the elastic behavior of the material)3) Yield strength (Y.S.):It is the stress at which the material begins to function in a plastic manner. (defined as the stress at which a material exhibits a specified limiting deviation from proportionality of stress to strain.
16 4) Ultimate strength (U.S.): If higher and higher forces are applied to a material, a stress will be reached at wich the material will fracture. If the fracture occurs from tensile stress, the property is called the tensile strength, and, if in compression, the compressive strength.The ultimate tensile strength is therefore defined as the maximum stress that a material can withstand before failure (fracture or rupture) in tension, whereas the ultimate compressive strength is the maximum stress a material can withstand in compression. It is calculated by dividing the load by the original cross-sectional area.
17 5) Modulus of elasticity or (Young’s Modulus) (E): It is the constant of proportionality between stress and strain. It represents the slope of the elastic portion of the stress – strain curve. It is a measure of rigidity or stiffnessMaterials with higher Young’s modulus value are said to be stiffer or more rigid than those of low Young’s modulus values because they require much more stresses to produce the same amount of strain.
18 Modulus of elasticity or (Young’s Modulus) (E): 2stress Kg/cmElastic modulus = = = Kg/cmstrain Cm/cm
19 Modulus of elasticity or (Young’s Modulus) (E):
20 6) Flexibility :Maximum flexibility is the strain resulting in the material when the stress reaches the elastic limit.This is very important for impression materials, which often must be severely deformed to be removed from undercuts, but must have the ability to spring back without suffering any permanent change in shape.
21 7) poisson’s ratio:The increase in length of a material under tension is associated with a decrease in cross-sectional area. The increase in length is known as axial strain and decrease in cross sectional area is know as lateral strain.lateral strainPoisson’s ratio =axial strain
22 8) Ductility and malleability: Ductility is the ability of a material to withstand plastic deformation under tensile stress without fracture. Malleability, is the ability of a material to withstand plastic deformation compressive stress without fracture.In other words, the malleability of a metal is its ability to be hammered in to thin sheets without fracturing, while, ductility is its ability to be drawn into wire without fracturing .ductility is measured by the percentage of elongation.
23 A material which has good ductility shows high elongation before fracturing. The percentage elongation represents the maximum amount of permanent deformation.increase in lengthPercentage elongation = X 100original length
26 9) Brittleness :If a material showed no or very little plastic deformation on application of load it is described as being brittle, in other words, a brittle material fractures at or near its proportional limit. More over ,brittle materials are weak in tension; for example, dental amalgam has compressive strength which is nearly six times higher than its tensile strength.
28 Ductile material Brittle material 1) Is the ability of a ) brittle materialMaterial to withstand fractures at or nearPlastic deformation its proportional limit.Under tensile stressWithout fracture.Fracture occur far Fracture occur at orAway from P.L near P.L
33 10) Resilience :The modulus of resilience is the maximum amount of energy a material can absorb without undergoing permanent deformation. It is represented by the area under the elastic portion of the stress – strain curve.Acrylic resin denture teeth are more resilient than porcelain teeth and consequently absorb most masticatory forces and transmitted less to the underlying bone, preserving it.
35 11)Toughness :It is the energy required to stress the material to the point of fracture. It is represented by the area under the elastic and plastic portion of the stress-strain curve. Therefore toughness of a material is the ability to absorb energy. The toughest materials are those which high proportional limits and good ductility. However two highly different materials can have the same toughness.
37 12) Fracture toughness :It is the ability of the material to resist fracture through its resistance to crack propagation. In general, high fracture toughness indicates good resistance to crack propagation
38 Cantilever bending:The pending properties of many materials are equally or more important than their tensile or compressive properties. The bending properties of wires, endodontic files and reamers are specially important.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.
39 As the force is increased and the sample is bent, corresponding values for the bending (angular deflection) and the bending movement (force x distance) are recorded.All instrument will be permanently bent if the bending angle exceeds the value at the end of he portion of the curve.Bending moment = force x distance
44 Diameter compression test (indirect tensile test) The diametral compression test or indirect tensile test 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.
45 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 Ptensile stress =DT תּ P = load, D = diameter , T = thickness
47 Transverse strength test In practice, the stresses within the material are complex. Thus if a beam is in tension, and the top is in compression. Shear stresses are also present. The transverse strength of a material is obtained by loading a bar which is supported at each end with the load applied in the middling. It is often described as the modulus of rupture or flexure strength.
51 Hardness and hardness test: Hardness : is the resistance of the material to scratching, indentation or penetration.It is a surface property not related directly to any other mechanical property i.e. strong or stiff materials are not necessary hard.Hardness can’t be seen or calculated from stress-strain curved but only by using one of the following: Brinel,Knoop,Vickers,Rockwell,and shore A hardness test.
52 Brinell hardness test: A steel ball is pressed into the surface of the material under a specified load. The load is divided by the area of the surface of the indentation. Thus, the smaller the indentation the larger the hardness number becomes, and the harder the material is. This test is used to determine the hardness of the metallic materials. it is expressed in B.H.N
55 Disadvantages: 1) It is difficult to measure the indentation area. 2) Not suitable for measuring hardness of brittle materials because the steel ball will fracture it.3) Not suitable for measuring hardness of elastic materials because the indentation is recovered on removal of the steel ball.
56 Rockwell hardness test: Rockwell hardness test is similar to Brinell test in that steel ball or cone is used. Instead of measuring the diameter of the indentation, the depth is measured directly by a dial gauge on the instrument.Advantage : it is a rapid and easy method for measuring hardness.Disadvantage: as for the Brinell test, Rockwell test is not suitable for brittle and elastic materials.
58 Vicker hardness test:Vicker hardness test a diamond square – based pyramid (cone) is used. The Vicker’s hardness number is determined by dividing the load by the area of indentation which is square and not round as in the Brinell test. This test is easy and suitable for brittle materials but not for elastic materials. It is expressed in V.H.N.
60 Knoop hardness test:Knoop hardness test uses a diamond cone designed to give an indentation having a long and a short diagonal(7 : 1). The load may be varied over a wide range, from one gm to more than a Kg, so that values for both hard and soft materials con be obtained. It is expressed in K.H.N.
63 Advantages: 1)Easy measuring of indentation depth. 2) Can test hardness of brittle materials without fracture.3) Can test hardness of elastic materials because when the indentation is made. The stresses are distributed in such a manner that only the dimensions of the short axis are subject to change by relaxation while the dimensions of the long axis remain unchanged.4) Hardness for both soft and hard materials can be measured.
64 Shore A hardness test:The hardness tests described previously cannot be used to determine the hardness of the rubbers, since the indentation disappears after the removal of the load. An instrument called a Shore A is used in the rubber industry to determine its hardness. The indenator is attached to a scale that is graduated form 0 to 100. if the indentor completely penetrates the sample, a reading of 0 is obtained, and if no penetration occurs, a reading of 100 results.
66 Clinical significance: 1) Denture – wearing patients must take care not to be aggressive during the cleaning of their dentures by using brushes with hard bristles.2)Hardness is an important property to consider 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.
68 Impact strength:It is describe to know the effects of the application of a sudden force to a material because under these condition materials are often more brittle.Fatigue strength:The repeated application of small stress (below the P.L) 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, ceramics can all fail by fatigue. Fatigue : is the fracture of a material when subjected to repeated (cyclic) small stresses below the P.L.