Yield Stress and the ‘Elastic Band’ – Structural Steel
To address the title of this article and its relevance to steel fabrication, it must be understood how our engineers select structural steels in the design of supplied fabrications. It must also be advised that the ‘Elastic Band’ is generally known to be made of a rubberised material and is not ‘Elastic’ in its material but in its properties.
All materials have a degree of ‘strength’ to them. In fabrications, typically steel is used as it has more ‘strength’ than other materials such as timber or plastics, where a higher load capacity is needed and the structural integrity of the load to be contained would simply squash (Compress) or flatten (Destroy) other materials.
That said, although under required testing conditions, steel does have a recorded ‘strength’, there is a point at which that can be exceeded, which would result in failure of a steel if it is exposed to forces beyond which it is designed to resist.
Different sizes of steel are structurally different in their load capacities as the size and thickness of those materials when applied to their Yield Stress can support varying load capacities.
‘Sound Engineering Principles’ are an industry recognised method of concluding a solution to a material requirement, where experience, professional education and design knowledge are used to select suitable materials for a project, where it is known that those materials are acceptable for the designated design.
But what about the ‘Elastic Band’.
As advised, the ‘Rubber’ band is extensively recognised because of its properties rather than its material, however the interpretation of the engineering term ‘Elastic’ can be applied to steel.
An example of this property can be explained through the humble ‘ 30cm School Ruler’. A 30cm ruler when position half over a table edge and retained by one hand, can be quite satisfyingly ‘twanged’, giving a vibrating and resonant sound.
Then ruler then returns to its original shape, despite the loading of the free end. Its subsequent bending and its release, resulting in the twang and return to its straight form.
Similar to the ‘Elastic Band’, when an item or material is loaded or stretched by a load which would not seem to be ‘excessive’, the elastic nature of the material will return it to its original shape and size once the load is removed.
If the ‘School Ruler’ were to be loaded to excess, which is at a state where there is no more flexibility or elasticity of the ruler left, or if the rubber Band were stretched until it could stretch no more, then the ruler and the Rubber band would typically ‘snap’.
This is the extreme case of failure of the material, but if the load were to be removed at the moment before failure, it could be seen that the ruler would not be straight. There may be signs of structural stress in the middle of the ruler and it would most likely now retain a ‘bent’ or ‘curved’ profile.
The Rubber band similarly could display ‘thinning’ of the material, where it has been stretched or loaded past it’s ‘Elastic Limit’.
These materials now have experienced ‘PLASTIC’ deformation. This is the point at which a permanent and irreversible change of shape due to loading has occurred.
Similarly, where a Rubber Band is known as ‘Elastic’, chemically manufactured items which are easily shaped or moulded to a final irreversible shape can be understood to be ‘Plastic’.
The point at which the ‘Elastic Deformation’ is exceeded and the force as applied causes an irreversible and permanent change to the material to ‘Plastic Deformation’ is called the ‘YIELD’ of the material; and the engineered value of this point per material type for the purpose of calculations in know therefore are the Yield Stress.
When applied to the application of Fabrications, the use of Structural Steels when selected, are chosen so that the forces as applied to the Steel, are sufficiently distributed through design, manufacture and Engineering, so that the Yield of the material is not approximated and that although in any occasion a typical comment of ‘Flexing’ may be referenced, this is well within the materials ‘Elastic’ state, where under normal conditions, the fabrication would return to its original form.
Indeed, with normal use, this ‘flexing’ is rarely even visually observered and the Plastic deformation is not approached.
In more advanced Engineered Projects, Finite Element Analysis can be used to virtually calculate the loadings of a fabrication through 3D Design. Loads applied virtually to the structure can be increased by multiples to evaluate the Elastic and potential Plastic deformation of the design, if the Yield Stress were to be exceeded.
Material selection is never such that all 50mm Box Sections are equal. Different material thicknesses in different steels are calculated to give their Yield Value. As thickness is a variable in the strength calculation of the steel, different loads are able to be sustained through the correct material selection.
It would never be practicable to evaluate two equal designs, when the materials can be completely different in their selection and application. This would typically be observed with a large variance in fabrication cost, where visually similar products are under review, but on an Engineered basis, they are not equally designed, evaluated or fabricated.
To design or fabricate a frame so that the material selection would support the load, but repeatedly ‘touch’ upon it’s yield point, would result in the subsequent weakening, thinning and compromising of the materials, such that the thickness is reduced, the subsequent yield value is recalculated downwards and the effort to finally break the material would result.
Such as flexing a paperclip until it was straight, then applying repeated opposed bending to one point, until it finally snaps.
(As a side, if you ever ‘snap’ a paperclip [and observing if it is safe and practicable to do so] and touch the broken end on to the back of the hand, it can be observed that for a fraction of time, the break is hot as the structure of the metal has been ripped apart and the release of energy in the form of heat can be experienced from the plastic deformation and failure of the material, being converted from the kinetic load applied in breaking the paperclip).
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