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Thread: Stress-Strain curve of composite steel component

  1. #1 Stress-Strain curve of composite steel component 
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    Apr 2010
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    Hello there everyone I'm actually working on a thesis subject and I've got some questions.

    The first part presents an article which I've written but I'm sure that I would need your suggestions and your point of view to confirm my ideas.

    Elasticity Limit




    This curve is a typical stress-strain curve obtained during a static breakage test of a composite steel component. The problem in this case is to define where exactly the elasticity limit is situated? And why there are two different peaks?

    It seems that metal containing steel have a tendency to show a linear stress-strain relationship up to the yield point. The yield point is the moment where a deformation of a product changes from the elastic region to the plastic region. The final breakage occurs only when the maximum value of the plastic region has been attained. For most metals the yield point does not fall sharply i.e. the elasticity limit is reached slowly up to the yield point and at the yield point it also declines slowly. The slope of the yield point downwards depends on a number of factors. The stress at which the yield occurs depends on the rate of deformation (strain rate) and also on the temperature at which the deformation occurs. The stress falls after the yield point because of the interaction of carbon atoms and dislocation of the stressed steel.

    Now let us question more about the fact why the stress strain curve falls after the yield point. The shape of the stress strain curve depends upon several factors influencing the grain structure of the steel. Grain alignment can be uniform and linear as in swords, or like a pile of noodles in raw steel. The type of alloy is a factor in grain size, shape, and bonding between grains. The density determines how many grains are stressed, and how much room they need to rearrange. After initial yield, dislocations don't only pile up at grain boundaries since grains now may have shifted and the strain has grown to a point that it is generating new dislocations. They build up again and again as they are generated. They may compete with other pile ups that may offset temporarily the generation of slip planes in adjacent grains. From the structural point of view of the material, these dislocations are caused due to the formation of a Cottrell atmosphere. Cottrell atmospheres occurs in body-centered cubic (BCC) materials, such as iron or nickel, with small impurity atoms, such as carbon or nitrogen. As these interstitial atoms distort the lattice slightly, there will be an associated residual stress field surrounding the interstitial. This stress field can be relaxed by the interstitial atom diffusing towards a dislocation, which contains a small gap at its core (as it is a more open structure), like in the figure below. Once the atom has diffused into the dislocation core the atom will stay. Typically only one interstitial atom is required per lattice plane of the dislocation.Once a dislocation has become pinned, a small extra force is required to unpin the dislocation prior the yielding, producing an observed upper yield point in a stress-strain graph. After unpinning, dislocations are free to move in the crystal, which results in a subsequent lower yield point, and the material will deform in a more plastic manner.Leaving the sample to age, by holding it at room temperature for a few hours, enables the carbon atoms to rediffuse back to dislocation cores, resulting in a return of the upper yield point.



    After the first yield point a neck forms where the local cross-sectional area decreases more quickly than the rest of the sample resulting in an increase in the true stress. Therefore, in the end we have a second yield point called the upper yield point after which the final breakage takes place.
    Now the only point which I canít explain is why this phenomenon does not occur always on the same kind of a sample part when repeated ? Therefore, sometimes there is only one yield point !

    I would be greatful if someone could help me out.
    Cheers


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