In this blog post, we will explain the phenomenon of necking in materials and the destruction process it causes, and look at strain hardening and other technical approaches to prevent it.
Recently, South Korea has been heatedly debating the “silver spoon” controversy. The “silver spoon” craze, which began with the saying that children born into wealthy families are born with a silver spoon in their mouths, has sparked criticism that wealth is inherited in South Korea, with children from rich families being called “diamond spoons” and children from poor families being called “clay spoons.”
Children born into poor families have no choice but to live in poverty, and the same phenomenon occurs in nature. What if this phenomenon occurred with materials? Materials must have different properties depending on their use, but they generally have one thing in common: they must not break.
This is because if materials break, they cannot perform their functions. Let’s imagine a material empire where countless citizens, called atoms, form a country and live in a certain combination. Villains who want to destroy the empire pull on the materials, putting pressure on the material empire, which is called stress. This stress causes deformation in the bonds between the citizens, continuously shrinking the village. If stress continues to be applied, can the material empire be deformed indefinitely? No, it cannot. The empire of materials has a limit to how much it can deform, and eventually it will be destroyed, or broken. During this process, a phenomenon called necking occurs, in which the materials become thinner at the center.
At this point, the leaders of the Material Empire cannot just stand by and watch. They implement a policy called strain hardening to prevent necking from occurring. Strain hardening is a phenomenon in which a material becomes stronger as it is deformed. When deformation occurs, the positions of the atoms within the material change, increasing interference between them and giving the material the ability to resist deformation. In other words, it prevents the parts that have been deformed a lot from being deformed any further. The leaders of the Material Empire support the village and prevent the villains from attacking again to prevent the destruction of the materials. Let’s learn about the struggle for the survival of the Material Empire, necking, and strain hardening.
Simply put, necking is a phenomenon in which a rod made of a certain material does not stretch evenly when pulled, but instead stretches with a noticeable thinning in certain areas. You may have experienced this when pulling the ends of a piece of flour dough, which does not stretch to a uniform thickness like noodles, but instead breaks in the middle where it is thinner. This phenomenon occurs because the stresses exerted on the object are not uniform. The magnitude of the stress exerted on a material can be obtained by dividing the force by the cross-sectional area of the surface receiving the force. Therefore, stress is inversely proportional to the cross-sectional area of the surface receiving the force, which means that the smaller the cross-sectional area of the surface receiving the force, the greater the stress and the greater the deformation. No matter how evenly a stick is cut, it is impossible to make the cross-sectional area of all parts equal to the atomic level. Therefore, when force is applied to the material, the part with the smallest cross-sectional area undergoes the greatest deformation, causing the length of that part of the stick to increase and its thickness to decrease. As a result, when looking at the material as a whole, a relatively large difference in cross-sectional area can be seen.
If the force continues, the difference will become larger and larger, causing a narrowing called necking. If necking occurs as soon as the material is deformed, any material will break easily. However, when actual material is pulled, necking does not occur immediately. This is because the deformed part of the material gains the ability to resist deformation, which is called strain hardening.
The Material Empire adopted a policy of supporting shrinking villages so that they would not become smaller. The villains are not stupid, so they know that attacking villages supported by the empire is futile. In this case, the villains attack other villages, and the empire implements a system of providing priority support to the villages under attack. In this way, the parts of the material that have been slightly deformed are deformed first, resulting in uniform deformation of the entire material. The main reason for deformation hardening is the misalignment between atoms called dislocations, which prevent deformation by gathering in specific areas when deformation occurs. These dislocations can be considered the loyal support troops of the Material Empire.
The effect of strain hardening greatly contributes to increasing the overall strength of materials. For example, when making steel, the process of repeatedly hammering iron and heat treating it to increase its strength is an application of the principle of strain hardening. In addition, strain hardening plays an important role in improving the ductility of materials and reducing the possibility of destruction.
For the sake of simplicity, we have explained necking and strain hardening in that order, but in reality, when force is applied to a material, strain hardening occurs first, and necking begins when strain hardening stops. The strain hardening of the Material Empire can prevent the attacks of the villains to a certain extent, but if strong and continuous attacks continue, the hardening will reach its limit, and necking will occur. Just as the wealth gap is a problem in society, the gap in the quality of materials and necking are also undesirable phenomena in materials, as they can lead to destruction. Therefore, the strain hardening ability of materials is important as a measure of their resistance to destruction.
Recently, research is being conducted to improve strain hardening ability by utilizing ultra-fine grain materials and carbon fiber reinforced materials. Engineering continues to require strong materials. Therefore, the potential for research to enhance strain hardening ability is endless, and will continue indefinitely. Engineers are constantly striving to create stronger and more flexible materials by developing new materials and improving existing ones. This research and development will greatly contribute to improving the quality and safety of all the products we use.
