ocw.metu.edu.tr/file.php/43/Tension_Test_-_ekme_Deneyi.PDF1Mechanical testing plays an important role in evaluating fundamental properties of engineering materials as well as in developing new materials and in controlling the quality of materials for use in design and construction. If a material is to be used as part of an engineering structure that will be subjected to a load, it is important to know that the material is strong enough and rigid enough to withstand the loads that it will experience in service. As a result engineers have developed a number of experimental techniques for mechanical testing of engineering materials subjected to tension, compression, bending or torsion loading.The most common type of test used to measure the mechanical properties of a material is the Tension Test. Tension test is widely used to provide basic design information on the strength of materials and is an acceptance test for the specification of materials. The major parameters that describe the stress-strain curve obtained during the tension test are the tensile strength (UTS), yield strength or yield point (y), elastic modulus (E), percent elongation (L%) and the reduction in area (RA%). Toughness, Resilience, Poissons ratio (v) can also be found by the use of this testing technique.In this test, a specimen is prepared suitable for gripping into the jaws of the testing machine type that will be used. The specimen used is approximately uniform over a gage length (the length within which elongation measurements are done).2Tensile specimens (a) rectangular, (b) roundTensile specimens are machined from the material to be tested in the desired orientation and according to the standards. The cross section of the specimen is usually round, square or rectangular. For metals, a piece of sufficient thickness can be obtained so that it can be easily machined; a round specimen is commonly used. For sheet and plate stock, a flat specimen is usually employed.The change in the gage length of the sample as pulling proceeds is measured from either the change in actuator position (stroke or overall change in length) or a sensor attached to the sample (called an extensometer). According to the loading type, there are two kinds of tensile testing machines;1 Screw Driven Testing Machine: During the experiment, elongation rateis kept constant.2 Hydraulic Testing Machine: Keeps the loading rate constant. The loading rate can be set depending on the desired time to fracture. A tensile load is applied to the specimen until it fractures. During the test, the load required to make a certain elongation on the material is recorded. A load elongation curve is plotted by an x-y recorder, so that the tensile behavior of the material can be obtained. An engineering stress-strain curve can be constructed from this load-elongation curve by making the required calculations. Then the mechanical parameters that we search for can be found by studying on this curve. Gage length (L0) do3A typical engineering stress-strain diagram and the significant parameters are shown on the figure in appendix. Engineering Stress is obtained by dividing the load by the original area of the cross section of the specimen. Stress = P/Ao ( Load/Initial cross-sectional area) Strain = e = Deltal/lo (Elongation/Initial gage length) Engineering stress and strain are independent of the geometry of the specimen.

Elastic Region: The part of the stress-strain curve up to the yielding point.Elastic deformation is recoverable. In the elastic region, stress and strain are related to each other linearly.Hookes Law: = Ee The linearity constant E is called the elastic modulus which is specific for each type of material. Plastic Region: The part of the stress-strain diagram after the yielding point.At the yielding point, the plastic deformation starts. Plastic deformation is permanent. At the maximum point of the stress-strain diagram (UTS), necking starts.Tensile Strength is the maximum stress that the material can support.UTS = Pmax/AoBecause the tensile strength is easy to determine and is a quite reproducible property, it is useful for the purposes of specifications and for quality control of a product. Extensive empirical correlations between tensile strength and properties such as hardness and fatigue strength are often quite useful. For brittle materials, the tensile strength is a valid criterion for design.4Yield Strength is the stress level at which plastic deformation starts. The beginning of first plastic deformation is called yielding. It is an important parameter in design.The stress at which plastic deformation or yielding is observed to begin depends on the sensitivity of the strain measurements. With most materials there is a gradual transition from elastic to plastic behavior, and the point at which plastic deformation begins is hard to define with precision. Various criteria for the initiation of yielding are used depending on the sensitivity of the strain measurements and the intended use of the data. 0,2% off-set method is a commonly used method to determine the yield stength. y(0.2%) is found by drawing a parallel line to the elastic region and the point at which this line intersects with the stress-strain curve is set as the yielding point. An illustration of 0,2% off-set method is shown in the appendix part.Ductility is the degree of plastic deformation that a material can withstand before fracture. A material that experiences very little or no plastic deformation upon fracture is termed brittle.In general, measurements of ductility are of interest in three ways:1. To indicate the extent to which a metal can be deformed without fracture in metalworking operations such as rolling and extrusion.2. To indicate to the designer, in a general way, the ability of the metal to flow plastically before fracture.3. To serve as an indicator of changes in impurity level or processing conditions. Ductility measurements may be specified to assess material quality even though no direct relationship exists between the ductility measurement and performance in service.Ductility can be expressed either in terms of percent elongation (z) or percent reduction in area (q) ;z = %l = [(lf-lo)/lo]*100q = %RA = [(Ao-Af)/Ao]*1005Resilience is the capacity of a material to absorb energy when it is deformed elastically.Toughness is a measure of energy required to cause fracture.Poissons Ratio is the lateral contraction per unit breadth divided by the longitudinal extension per unit length. =-( d/do)/(l/lo)OBJECTIVETension test is carried out; to obtain the stress-strain diagram, to determinethe tensile properties and hence to get valuable information about the mechanical behavior and the engineering performance of the material.TESTING SYSTEMThe testing system consists of a tensile testing machine, a load cell, a power supply and an x-y recorder.Testing Machine is of hydraulic type (Ala Universal Testing Machine). It isa load-controlled machine.Load Cell provides an electrical circuit for measuring the instantaneous loadalong the loading axis.Power Supply is connected to load cell. It feeds the load cell, amplifies the output signal and displays the load.Recorder plots the variation of load against time

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SPECIMENAs mentioned previously, tensile specimens are machined in the desired orientation and according to the standards. The central portion (gage portion) of the length is usually of smaller cross section than the end portions. This ensures the failure to occur at a section where the stresses are not affected by the gripping device. The gage length is marked and elongation is measured between these markings during the test.6PROCEDUREBefore the test1. Put gage marks on the specimen2. Measure the initial gage length and diameter3. Select a load scale to deform and fracture the specimen. Note that that tensile strength of the material type used has to be known approximately.During the test1. Record the maximum load2. Conduct the test until fracture.After the test1. Measure the final gage length and diameter. The diameter should be measured from the neck.

imechanica.org/files/handout3.pdf

TestingObjectiveThe primary objective of this investigation is to conduct a standard tensile test for determining the stress strain behavior of a material sample (mild steel or aluminum) and to analyze the results of the tensile test to find the mechanical/material properties of the sample.AbstractThe mechanical properties of a material are directly related to the response of the material when it's subjected to mechanical stresses. Since characteristic phenomena or behavior occur at discrete engineering stress and strain levels, the basic mechanical properties of a material are found by determining the stresses and corresponding strains for various critical occurrences. A wealth of information about a material's mechanical behavior can be determined by conducting a simple tensile test in which a cylindrical or flat specimen of uniform cross-section is pulled until it ruptures or fractures into separate pieces. The original cross sectional area, Ao, and gage length, lo, are measured prior to conducting the test and the applied load and gage displacement are continuously measured throughout the test using computer-based data acquisition. Based on the initial geometry of the sample, the engineering stress-strain behavior (stress-strain curve) can be easily generated from which numerous mechanical properties, such as yield strength and elastic modulus, can be determined. Universal testing machines, which can be hydraulic or screw based, are generally utilized to apply the test displacement/load in a continuously increasing (ramp) manner according to ASTM specifications.BackgroundThe mechanical properties of a material are related its' behavior when subjected to continuously increasing elongations up to rupture/fracture. A thorough understanding of a material's mechanical properties is required for engineers if catastrophic failures are to be avoided. The Tensile Test is a common standard test and is a valuable method of determining important mechanical properties of engineering materials. The procedural details of the test vary for different material types, but tensile tests are generally conducted at room temperature at relatively slow loading rates although various temperatures and loading rates may be required for the determination of material behavior under specific conditions.

The output of a standard tensile test is load versus displacement data. Since load-displacement characteristics are dependent on specimen size, for example it will require twice the load to produce the same elongation if the cross-sectional area of the specimen is doubled, load-displacement data is routinely converted to engineering stress-strain data. For axial loading, Engineering Stress, , is defined by the well known relationship where P is the Instantaneous Load applied perpendicular to the specimen cross section, in units of pounds force (lbf) or Newtons (N), and Ao is the Original Cross-Sectional Area of the specimen before any load is applied (in2 or m2). The units of stress are generally kips per square inch (ksi) or megapascals (MPa). Engineering Strain, , along the loading axis of an uniaxially loaded sample is defined according to in which is the original length before any load is applied and lo is the instantaneous length. Engineering strain is unit less, but inches per inch or meters per meter are often used; the value of strain is clearly independent of the units system applied! Strain may also be expressed as a percentage, in which case the strain value is simply multiplied by 100.In Tensile Testing, the test specimen is deformed, usually until complete rupture or fracture occurs, with a gradually applied increasing tensile load that is applied unaxially along the longitudinal axis of the specimen. Normally the test specimen is circular, but rectangular specimens can also be used. Each specimen is of a specific shape and dimensions that should be in accordance with ASTM (American Society for Testing and Materials) specifications for standardization. During testing, deformation is confined to the narrow center region which has a uniform cross section along its length.

DefinitionsFigure 1 - Typical Ductile Material Stress-Strain Diagram. [Flinn & Trojan, 1992]1. Proportional Limit (SPL) - The Proportional Limit is the maximum stress at which stress and strain remain directly proportional. The proportional limit is determined from the stress-strain diagram by drawing a straight-line tangent to the curve at the origin and noting the first deviation of the curve from linearity. Because the proportional limit depends on the precision of the measurement instrument it is not widely used in engineering calculations.2. Elastic Limit (SEL) - The Elastic Limit is the maximum stress that the material can withstand without causing permanent deformation. An exact determination of the elastic limit requires loading to successively higher stresses followed by unloading and measurements to detect permanent deformation. Its actual value is, like the proportional limit, dependent on instrument precision. Due to this and the difficulty in its determination, its engineering usefulness is limited.3. Yield Strength (SYS) - The Yield Strength is the stress at which a material exhibits a specified limiting permanent set. Below the elastic limit, the stress-strain relationships in loading and unloading are identical for practical purposes. Therefore, it is not necessary to unload a specimen in order to determine the yield strength. Rather, a line parallel to the initial straight-line portion of the curve is constructed. The construction line is displaced from the origin of the curve by an amount equal to the specified permanent set. The stress at the intersection of the parallel line with the stress-strain curve is the yield strength. The offset most commonly used is 0.2% strain or 0.002 in/in or mm/mm. The yield strength is a practical measure of the limit of elastic action. It is always greater than the elastic limit and is only minimally sensitive to measurement instrument precision.4. Ultimate or Tensile Strength (SUL) - The Ultimate Strength, also referred to as the Tensile Strength, is calculated by dividing the maximum load sustained by the specimen by the original cross-sectional area of the specimen.5. Fracture or Rupture Strength (SRU) - The Rupture Strength (ductile behavior), also referred to as theFracture Strength (brittle behavior), is determined by dividing the load sustained at rupture by the original cross-sectional area of the specimen. This load will be less than the maximum load because the cross section of the specimen is reduced drastically after the maximum load is reached. The reduction of cross-section produced has an "Hour-Glass" shape and is known as Necking or Necking Down.6. Modulus of Elasticity (E) - The Modulus of Elasticity is a measure of material stiffness and is termed Youngs Modulus for tensile loading. The Modulus of Elasticity, E, is the constant of proportionality between stress, , and strain, , at stresses below the proportional limit:E =stress/strain7. Modulus of Toughness (UT) - The Toughness of a material refers to the ability of the material to absorb energy up to the point of rupture. The Modulus of Toughness is determined by measuring the area under the stress-strain curve. This is not an exact indication of toughness because the specimen does not strain uniformly over its length, and hence does not absorb energy uniformly throughout its volume. The units of toughness are determined by multiplying stress by strain.8. Modulus of Resilience (Ur) - The Resilience of a material refers to the amount of elastic energy, which a material can absorb. The Modulus of Resilience is determined by measuring the area under the elastic portion of the stress-strain curve or by the expression:Ur=1Spl^2/2E9. Percent Elongation (%EL) - The Percent Elongation refers to the elongation at rupture and can be expressed as: %EL=100DL/ loWhere DLi represents total elongation and equals = lf - lo.10. Percent Reduction in Area (%RA) - The Reduction in Area refers to the reduction in cross-sectionalarea at rupture and can be expressed as: %RA=100(DAf/Ao)Where Af represents total reduction in area; Af = Ao - Af.11. Ductility - The Ductility of a material refers to the ability of a material to deform plastically before fracturing. Ductility is usually evaluated by considering values of Percent Elongation or Reduction in Area.12. True Strain* (TR) - True Strain is the change in length divided by the Instantaneous Length and can be simply determined as: ETR = ln (EENG + 1) (8)13. True Stress* (TR) - True Stress is the applied load divided by the Instantaneous cross-sectional Area.*True Strain & True Stress should be applied when SYS of a material is exceeded.Figure 2 - Schematic stress-strain curve illustrating the determination of the tangent and secant moduli. [Callister, 1991]For some materials (e.g., concrete, gray cast iron) the initial elastic portion of the stress-strain curve is non-linear as depicted in Figure 2 where it is impractical to determine a standard modulus of elasticity. Furthermore, the modulus of elasticity is restricted to the initial linear portion of a standard stress-strain diagram and is invalid beyond this region. For non-linear behavior, either the Tangent orSecant Modulus is generally utilized.14. Tangent Modulus (ET) - The Tangent Modulus is taken as the slope of the stress-strain curve at a specified stress level.15. Secant Modulus (Es)The Secant Modulus represents the slope of a secant drawn from the origin to a point at a specified stress level of the stress-strain curve.16. Strain Hardening - Following yielding, additional load may be applied which results in a stress-strain curve that continuously rises up to SUL indicating that the material is becoming stronger. When loaded beyond the yield point, ductile materials plastically deform and are subjected to cold working; this is referred to as Strain Hardening. In this range, the materials elastic region increases, but its ductility decreases.Strain hardening is generally modeled using the expressionTR = N (TR) n (10)Where N is the Strength Constant, andn is the Strain Hardening Coefficient.Taking the logarithm of both sides:ME 3701, Materials of Engineering, LSU 5ln(TR) = n[ln(TR)] + ln(N) (11)Note that this is the equation of a straight line! A simple ln(TR) versus ln(TR) should result in a straight line with a slope n (strain hardening exponent). The strength constant from the plastic region data is equivalent to the elastic modulus for the elastic region and has similar units.Alternatively, given the true stress and strain values for two data points within the plastic region of Stress-strain curve, the values can be plugged into Equation 11 generating two equations that can be solved simultaneously for the Strength Constant (N) and the Strain Hardening Coefficient (n).

ExampleRepresentative True Stress and True Strain values for low-carbon steel are given below. Determinethe Strength Constant and Strain Hardening Coefficient for this material:TR1 = 47,150 psi, TR1 = 0.0247 in/inTR2 = 67,100 psi, TR2 = 0.0953 in/inAns: n = 0.2613, N = 124,00017. Necking - When a specimen is loaded beyond its ultimate strength the cross-sectional area begins todecrease in a localized region instead of over its' entire length creating a so-called "neck" which rapidlyforms in this region as the specimen elongates. Since the cross-sectional area within this region iscontinually decreasing, the localized stress rapidly increases causing further localized elongation up torupture.ME 3701, Materials of Engineering, LSU 6Testing EquipmentIn Tensile Testing, loads are generally applied either mechanically with screw drives or hydraulically withpressurized oil in one of the two types of readily available Universal Testing Machines. The mechanicalmethod of applying loads has the advantage of providing a convenient means of accurately controlling therate of deformation. The hydraulic systems are generally preferred because of their higher load capacities and lower cost; furthermore, the hydraulic systems have been significantly improved with the introduction of digital control loops thus the accuracy of hydraulic systems are no longer vastly inferior to the mechanical systems.The general term which encompasses both machine types is "Universal Testing Machine"; the term is used because the machine can be adapted to test in tension, compression, flexure and bending.The 50 Kip Capacity Instron machine available for tensile testing at the mechanical engineering department at LSU is a servo hydraulic system where the load is applied by a hydraulic pump which controls the flow of oil into a cylinder thereby controlling the position of the piston within the cylinder. The cross-head can be moved between tests to accommodate specimens of various size, but the cross-head and table are fixed during testing. The machine is digitally controlled and can be operated in the Load, Position or Strain mode. Load,Position and/or Strain can all be simultaneously monitored by the system regardless of the control mode. A data acquisition system has been connected to the Instron controllers and readouts of Load from the Load Cell and Deflection (Strain) from the Extensometer can be continuously sampled during testing. The test specimen is held in grips which are attached to the cross-head and piston. The oil in the cylinder is controlled by means of coarse and fine - load and unload valves. In Tensile Testing, proper grip alignment eliminates bending loads and assures that the specimen is subjected to axial loads only. If bending loads are exerted on the test specimen then stresses will not be uniform across the thickness.A Load Cell is a device based on a strain-gaged beam with a mounted wheatstone bridge. The load signals are actually strain readings calibrated to appropriate loading levels. The LSU-ME Instron is capable of loads up to 50,000 pounds. Extensometers are devices that specifically measure deflection. Extensometers are also strain-gauge based and electronically transmit strain-gauge output which is calibrated based on deflection. During Tensile Testing, the Load Cell and mounted Extensometer will send analog electronic signals to the Instron control tower; both of these can be routinely sampled by the data acquisition system and loaded into a spreadsheet file. The data can be retrieved, converted to stress versus strain data, and the resulting stress train diagram can be easily generated. Various material properties can then be determined utilizing the resulting stress-strain diagram.

http://www.asminternational.org/documents/10192/3465262/05105G_Chapter_1.pdf/e13396e8-a327-490a-a414-9bd1d2bc2bb8

TENSILE TESTS are performed for several reasons. The results of tensile tests are used in selecting materials for engineering applications. Tensile properties frequently are included in material specifications to ensure quality. Tensile properties often are measured during development of new materials and processes, so that different materials and processes can be compared. Finally, tensile properties often are used to predict the behavior of a material under forms of loading other than uniaxial tension. The strength of a material often is the primary concern. The strength of interest may be measured in terms of either the stress necessary to cause appreciable plastic deformation or the maximum stress that the material can withstand.These measures of strength are used, with appropriate caution (in the form of safety factors), in engineering design. Also of interest is the materials ductility, which is a measure of how much it can be deformed before it fractures. Rarely is ductility incorporated directly in design; rather, it is included in material specifications to ensure quality and toughness. Low ductility in a tensile test often is accompanied by low resistance to fracture under other forms of loading. Elastic properties also may be of interest, but special techniques must be used to measure these properties during tensile testing, and more accurate measurements can be made by ultrasonic techniques.

Test procedureTest Procedure. The following general rules for test procedure may be applied to almost every tensile test. Load and strain ranges should be selected so that the test will fit the range. The maximum values to be recorded should be as close to the top of the selected scale as convenient without running the risk of going past full scale. Ranges may be selected using past experience for a particular test, or specification data for the material (if available). Note that many computer-based testing systems have automatic range selection and will capture data even if the range initially selected is too small. The identity of each specimen should be verified, and pertinent identification should be accurately recorded for the test records and report. The dimensions needed to calculate the cross-sectional area of the reduced section should be measured and recorded. These measurements should be repeated for every specimen; it should not be assumed that sample preparation is perfectly consistent.The load-indicator zero and the plot-load-axis zero, if applicable, should be set before the specimen is placed in the grips. Zeroes should never be reset after the specimen is in place.The specimen is placed in the grips and is secured by closing the grips. If preload is to be removed before the test is started, it should be physically unloaded by moving the loading mechanism. The zero adjustment should never be used for this purpose. Note that, in some cases, preload may be desirable and may be deliberately introduced. For materials for which the initial portion of the curve is linear, the strain zero may be corrected for preload by extending the initial straight portion of the stress-strain curve to zero load and measuring strain from that point. The strain valve at the zero-load intercept is commonly called the foot correction and is subtracted from readings taken from strain scale. When the extensometer, if applicable, is installed, the technician should be sure to set the mechanical zero correctly. The strain-readout zero should be set after the extensometer is in place on the specimen. The test procedure should be in conformance with the published test specification and should be repeated consistently for every test. It is important that the test specification be followed for speed of testing. Some materials are sensitive to test speed, and different speeds will give different results. Also, many testing machine load and strain-measuring instruments are not capable of responding fast enough for accurate recording of test results if an excessive test speed is used The technician should monitor the test closely and be alert for problems. One common sign of trouble is a load-versus-strain plot in which the initial portion of the curve is not straight. This may indicate off-center loading of the specimen, improper installation of the extensometer, or the specimen was not straight to begin with.Another potential trouble sign is a sharp drop in indicated load during the test. Such a drop may be characteristic of the material, but it also can indicate problems such as slippage between the specimen and the grips or stick-slip movement of the wedge grip inserts in the grip body. Slippage may be caused by worn inserts with dull teeth, particularly for hard, smooth specimens.

The stick-slip action in wedge grips is more common in testing of resilient materials, but it also can occur in testing of metals. Specimens cut from the wall of a pipe or tube may have curved tab ends that flatten with increasing force, allowing the inserts to move relative to the grip body. Short tab ends on round specimens also may be crushed by the wedge grips, with the same result. If the sliding faces are not lubricated, they may move in unpredictable steps accompanied by drops in the load reading.Dry-film molybdenum disulfide lubricants are effective in solving stick-slip problems in wedge grips, particularly when testing is done at elevated temperature. When wedge grips are used, the specimen must be installed so that the clamping force is contained within the grip body. Placing the specimen too near the open end of the grip body results in excessive stress on the grip body and inserts and is a common cause of grip failure. WARNING: Grip failures are dangerous and may cause injury to personnel and damage to equipment.

The tensile testing is carried out by applying longitudinal or axial load at a specific extension rate to a standard tensile specimen with known dimensions (gauge length and cross sectional area perpendicular to the load direction) till failure. The applied tensile load and extension are recorded during the test for the calculation of stress and strain. A range of universal standards provided byProfessional societies such as American Society of Testing and Materials (ASTM), British standard, JIS standard and DIN standard provides testing are selected based on preferential uses. Each standard may contain a variety of test standards suitable for different materials, dimensions and fabrication history. For instance, ASTM E8: is a standard test method for tension testing of metallic materials and ASTM B557 is standard test methods of tension testing wronght and cast aluminum and magnesium alloy products.A standard specimen is prepared in a round or a square section along the gauge length as shown in figures 1 a) and b) respectively, depending on the standard used. Both ends of the specimens should have sufficient length and a surface condition such that they are firmly gripped during testing. The initial gauge length Lo is standardized (in several countries) and varies with the diameter (Do) or the cross-sectional area (Ao) of the specimen as listed in table 1. This is because if the gauge length is too long, the % elongation might be underestimated in this case. Any heat treatments should be applied on to the specimen prior to machining to produce the final specimen readily for testing. This has been done to prevent surface oxide scales that might act as stress concentration which might subsequently affect the final tensile properties due to premature failure. There might be some exceptions, for examples, surface hardening or surface coating on the materials. These processes should be employed after specimen machining in order to obtain the tensile properties results which include the actual specimen surface conditions.

When a specimen is subjected to an external tensile loading, the metal will undergo elasticand plastic deformation. Initially, the metal will elastically deform giving a linear relationship of loadand extension. These two parameters are then used for the calculation of the engineering stress andengineering strain to give a relationship as illustrated in figure 3 using equations 1 and 2 as followsE= P/ = @(1)E=LF-LO/LO

where is the engineering stress is the engineering strainP is the external axial tensile loadAo is the original cross-sectional area of the specimenLo is the original length of the specimenLf is the final length of the specimenThe unit of the engineering stress is Pascal (Pa) or N/m2 according to the SI Metric Unitwhereas the unit of psi (pound per square inch) can also be used.

1.2.1 Young's modulus, EDuring elastic deformation, the engineering stress-strain relationship follows the Hook's Lawand the slope of the curve indicates the Young's modulus (E)stress/strainYoung's modulus is of importance where deflection of materials is critical for the required engineering applications. This is for examples: deflection in structural beams is considered to be crucial for the design in engineering components or structures such as bridges, building, ships, etc.The applications of tennis racket and golf club also require specific values of spring constants or Young's modulus values.

Yield strength, By considering the stress-strain curve beyond the elastic portion, if the tensile loading continues, yielding occurs at the beginning of plastic deformation. The yield stress, y, can be obtained by dividing the load at yielding (Py) by the original cross-sectional area of the specimen (Ao). The yield point elongation phenomenon shows the upper yield point followedby a sudden reduction in the stress or load till reaching the lower yield point. At the yield point elongation, the specimen continues to extend without a significant change in the stress level. Load increment is then followed with increasing strain. This yield point phenomenon is associated with a small amount of interstitial or substitutional atoms.

1.2.3 Ultimate Tensile Strength, TS Beyond yielding, continuous loading leads to an increase in the stress required to permanently deform the specimen as shown in the engineering stress-strain curve. At this stage, the specimen is strain hardened or work hardened. The degree of strain hardening depends on the nature of the deformed materials, crystal structure and chemical composition, which affects the dislocation motion. FCC structure materials having a high number of operating slip systems can easily slip and create a high density of dislocations. Tangling of these dislocations requires higher stress to uniformly and plastically deform the specimen, therefore resulting in strain hardening. If the load is continuously applied, the stress-strain curve will reach the maximum point, which is the ultimate tensile strength (UTS, TS). At this point, the specimen can withstand the highest stress before necking takes place.

Ts=pmax/ao1.2.4 Fracture Strength, After necking, plastic deformation is not uniform and the stress decreases accordingly until fracture. The fracture strength ( fracture) can be calculated from the load at fracture divided by theoriginal cross-sectional area, Ao1.2.6 Tensile ductilityTensile ductility of the specimen can be represented as % elongation or % reduction in area as expressed in the equations given Af is the cross-sectional area of specimen at fracture.The fracture strain of the specimen can be obtained by drawing a straight line starting at the fracture point of the stress-strain curve parallel to the slope in the linear relation. The interception of the parallel line at the x axis indicates the fracture strain of the specimen being tested.o

%elongation=deltal/l0x100 %RA=A0-AF/A0X100

http://eng.sut.ac.th/metal/images/stories/pdf/Lab_3Tensile_Eng.pdf

8. References

8.1 Hashemi, S. Foundations of materials science and engineering, 2006, 4th edition, McGraw-Hill, ISBN 007-125690-3.8.3 Norman E. Dowling, Mechanical Behavior of Materials, Prentice-Hall International, 1993.8.4 W.D. Callister, Fundamental of materials science and engineering/an interactive e. text,2001, John Willey & Sons, Inc., New York, ISBN 0-471-39551-x