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    Methods for the elastic and plastic evaluation of steel structures

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    Hi, I have to provide the different methods of elastic and plastic evaluation of steel structures.

    I must talk about the differences between the methods based on the cross section capacity including the cost effectiveness of the different methods.

    Any information and help provided would be much appreciated.

    © BrainMass Inc. brainmass.com December 24, 2021, 10:35 pm ad1c9bdddf

    SOLUTION This solution is FREE courtesy of BrainMass!

    Hi Student,

    There are various methods for Elastic & Plastic evaluation of simple & complex steel structures.

    The most common tests are as following:
    1. Tensile Test: A specimen of steel material (which is used for construction of structures) will be taken for tensile test. The standard tensile test specimen in US is circular of d=1.25cm diameter and l =5 cm gage length, so that l / d = 4 or, l = 4.51 √ A, where A = π d² / 4. The length of the cylindrical portion of the specimen is always somewhat greater than l and is usually l+d. The ends of the specimen are generally having larger cross section in order to prevent the specimen from breaking in the grips of the tesing machine due to severe stress conditions due to local irregularities in stress distribution.

    Tensile stress machines are usually provided with computer plotting automatically stress strain diagram of subject specimen. If, for example, there are 4 samples of carbon steel with various % of carbon content, there would be 4 such series of stress-strain diagram for all 4 samples. This will clearly help one to Evaluate the steel type based on Yield Strength at Elastic Zone end and as well as to observe its behaviour in plastic zone till breaking strength. Above test comparison will show that with increase in carbon content in steel, the fracture decreases Ultimate strength of steel increases, but at the same time elogation before since with increase in carbon % the steel specimen
    actually loses its ductility to an extent.

    In the tensile test there are 2 distinct zone tests usually taken place, e.g:
    a) Yield Point Determination
    b) Streching Beyond Yield Point

    2. Compression Test: The compression test is usually carried out for brittle materials like cast Iron. The specimens used in this test are either in cubic or cylindrical shape. The compression test actually determines the behaviour of materials under crushing loads. Now the question may arise one's mind why such compression test is required? In fact it is an effective procedure to measure the plastic flow behaviour and ductile fracture limits of a material.The axial compression test is also useful for measurement of elastic and compressive fracture properties of brittle materials of brittle or low ductility materials.

    Beyond the proportional limit the deformation increases at a faster rate relative to the load, as given in the following diagram. The equation for the curve is explained by Bach as: ε = σ^ח / E, in which n is the number depending on the properties of materials.

    3. Tests of materials under Combined Stresses: The combined stresses actually replicates complex real life situations.
    Tensile tests of various steels combined with lateral pressure showed the pressure had a great effect on the shape of the neck and on the reduction in area at the minimum cross section of test specimen. Subjecting a tube to axial tension combined with internal pressure, the Yield point stress for various ratios of the two principal stresses was
    established for many metals like steel, copper & nickel.In practical applications not only yield point stress but also ductility and strain hardening are of great importance in the cases of combines stresses. In fact, unusual cases of failures like explosion in large storage tanks actually brought the importance of combined stresses in the picture.

    4. Impact Tests: Impact tests are used to study the toughness of materials, i.e., the ability of the materials to absorb energy during plastic deformation. The steel which is having high toughness it should have high strength and most of the time high ductility. Whereas, for brittle
    materials it is low toughness represented by their small deformation before the fracture. The use of such steel in structural application is dangerous, as fracture might take place suddenly without any noticeable deformation.

    5. Creep & Rupture: Many materials will creep to some an extent and eventually fail under sustained stress lesser than the short time ultimate strength. After a short exposure to load the initial creep & strain hardening ceases and a steady state creep prevails. This is also called as viscous creep. The creep is temperature dependent.

    Overall Discussion: Looking at various kinds of fractures it was found that same material may behave as a brittle materia, entirely depending on the external conditions. As for example, for mild steel, under normal tensile stress in normal temperature, it would have large plastic deformation. However, under sub zero temperature it will act as brittle material.

    One more important aspect is the Fatigue of structural steel under dynamic & cyclic load conditions. Fatigue combined with combined stress gives rise to very complex real life situation. As for example, engine crankshaft, where the steel is selected based on fatigue resistance under combined stress condition.

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