ABSTARCT: Tensile tests are fundamental for understanding properties of different materials, and how they will behave under load. These properties can be used for design and analysis of engineering structures, and for developing new Materials that better suit a specified use. This lab tested two materials mild steel and cast iron. The data from each test was used to determine valuable material properties such as ultimate tensile strength, modulus of elasticity, and yield strength. Other calculated properties included true fracture strength, percent reduction of area, and percent elongation. These material properties were used to define the material as brittle or ductile.
INTRODUCTION: Mechanical 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 a 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 (σy), elastic modulus (E), percent elongation (∆L%) and the reduction in area (RA%).
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Toughness, Resilience, Poisson’s ratio(ν ) 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).
Tensile 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).
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.
Engineering Stress is obtained by dividing the load by the original area of the cross section of the specimen. Stress σ = P/Ao ( Load/Initial cross-sectional area) Strain = e = ∆l/lo (Elongation/Initial gage length) Engineering stress and strain are independent of the geometry of the specimen.
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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.
Hooke’s Law: σ = Ee The linearity constant E is called the elastic modulus which is specific foreach type of material.
Yield 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 thevinitiation 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. σy(0.2%) is found by drawing a parallel line to the elastic region and the point at which this line intersects with the stressstrain curve is set as the yielding point. An illustration of 0,2% off-set method is shown in the appendix part.
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 (σ UTS), necking starts.
Tensile Strength is the maximum stress that the material can support. σUTS = Pmax/Ao Because 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.
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.
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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
