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Steel - Behavior and importance


First question comes to anyone's mind is, why the hell do we use steel as a construction material? What is so important about it? Well, the answer lies beneath the ground. Iron is one of the most abundant metal present on the planet. It is not very difficult to purify it and mold it in different shapes. Apart from it we can recycle the material very easily. This was the answer regarding it's availability. If you consider it from a strength point of view, well it is very strong and flexible. It is ductile too while helps in saving lives of thousands. It has got not strong but decent resistance against fire. Apart from it, the construction carried out using steel is pretty fast. As compared to a normal concrete structure, steel structure will take very less time to stand tall. 

This was all about advantage of steel. Now lets talk about the behavior of the material. Why? Because to build any structure it is important to understand the behavior of the material. It is the material on which tests are conducted and equations are drafted. It is the material which passes through different kind of rigorous experiments and then applied to the industry. Let's take an example, suppose if you are given a rope and you are told to tie it from one end at the building top and the other end to any part of your body. Now if I tell you to jump from the top of the building, will you jump? No..!! This is obvious because you don't know the strength and behavior of rope. So what you do is, you test the strength of the rope either by pulling or handing a very heavy object using it. This is the importance of testing a material, you get some result which you feed in so that you can get the best out of it.

Before moving forward, I will like to introduce you about stress and strain.

1. Stress or Normal stress
Let's take the most common definition of stress: Stress is the axial force acting on a cross section of a material per unit area. But what is axial force? Axial force is the pull or push that you apply on a material in the direction of it's longitudinal, as per newton's second law it is mass times acceleration, force is the weight of the body, etc.

Figure describing the force on the bar

Figure describing stress acting on the bar.

Well as shown in figure the force applied is equal in magnitude but opposite in direction. This effect is known as pulling of bar. If the magnitude of the two opposite forces will not be equal then the body will start moving towards the direction of force with higher magnitude. The other picture shows the stress developed in the material, as represented by small arrows distributed uniformly over the section or the green colored cross section of the bar. This is known as stress developed inside the member. Force per unit area and called resistance against the loading offered by the bar.

2. Strain
Strain means the amount of stretch any body experiences under a tension or compression. Tension causes the length of the body to increase while strain tries to reduce the body length.

Figure describing strain on the body.

Let the length of the bar shown in figure be L. The green color shows the deformed shape of bar in tension. While the red color shows the deformed shape of the bar in compression. (Trying pulling a long eraser with your hands you will get a similar shape).

Linear Strain is defined as: Change in length to the original length.
Surface strains is defined as: Change in area to the original area.
Volumetric strain is defined as: change in volume to it's original volume.

Here we are dealing with linear strain, i.e. the change in length to it's original length. Suppose the change in length is A, while the original length is L, so stain of the rod will be A/L. 

From our last blog on ductility and elasticity you might have to came to know that steel is an elastic as well as a ductile material.


Figure showing: Stress-Strain Relationship of steel.

Figure reflects the stress v/s strain relationship of steel. The test is carried out in tension. You can see that the relation between the stress and stain remains linear up to the point A, called upper yield point of the steel. This it the point till where steel behaves as an elastic material, i.e. it has the tendency to come back to it's original shape when you release the forces. Yield point is the limiting point of the material after which the material starts stretching itself without a significant increase in the force. The slope of the graph, i.e. change in stress to the change in strain is known as Young's modulus of the elastic member (E).

E = Change in stress / Change in stain...... (1)
or
E = Stress / strain ......(2)

Equation 2 can be used when the forces were started applying when the material was not deformed or there were no internal forces present in the body, a perfect condition. But in all other cases equation 1 comes into the picture. 

The point B is known as lower yield point of the material. Well, the graph shows that the strength of steel reduces to some extent. Why this happens? The reason behind this is, even if we consider steel to be a very homogeneous material still there are some voids and certain deformities present in the material. So when you pull it very hard, the small particles get aligned in a single direction and this consumes some energy,which causes the reduction in the stress.

After reaching the lower yield point, the material becomes very dense because of it's perfect alignment. Now it becomes extremely hard to break down the material. It's like a unity is formed between all the particles of the steel and it says that no matter what happens we are not going to leave each other's hand. So this unity in the material causes the hardening behavior and you require greater amount of load to pull the object. The initial stage of this behavior is known as plastic stage of the material because here the stress is not proportional to stain and the material behaves like a plastic. Take any plastic band and stretch it, you will feel the similarity between the band and the behavior of steel over here.

Then comes the stage of necking. When the load is at extreme, and the material now cannot take any more amount of load, but still the material is ductile so it can stretch itself. But again it is not possible for the complete bar to take load uniformly. So instead of getting itself stretched, a localized deformation will start taking place and the cross section of the material will go on reducing. This effect is called the necking of the material, which only happens in case of a ductile material. Here even though we have applied axial load there is a secondary shear force that is generated which causes the failure of the material.

Figure showing the failure pattern of ductile steel

The figure shows how the failure progresses in the bar from unloaded condition to the full load state. This happens every time you take a ductile steel, not a brittle steel.

Fun Fact: Even though when you load the bar then at the failure point the load is decreasing while it already took the force with a higher value, why the stress dips down near the fracture? Basically to calculate stress we take force by area. And this area is the original cross section of the bar, but because of necking the cross section area of the bar is reduced. If you will calculate the stress with this reduced area you will find out that the stresses were much more than any other point on the graph.

Importance of ductility of steel

Attacks on World Trade Center

The image above shows the attack on the world trade center. The two towers were attacked by aircraft and were badly damaged. But still one tower fell down after 55 minutes and the other after 100 minutes. Can anyone guess why? Yes, it's because of the ductile behavior of steel. If it wouldn't have been ductile, it wouldn't have survived for so long and would have crippled in less duration after the attack resulting in greater life loss. So always check the importance of the material.

Next time we will discuss on the various structural elements used in structural engineering.

Always ask questions in the comment box below. We will reply to your doubts.

Stay tuned and have a nice day.