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.
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%). 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.
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.
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...
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