Necking and strain hardening, the process of deformation and failure of a material, shows how stresses are concentrated in a specific part of the material, weakening it and eventually damaging the entire structure. This phenomenon can be used as an analogy to explain the recent ‘cutlery’ controversy and social inequality in South Korea.
Recently, South Korea has been hotly debated over the “cutlery” controversy. The “cutlery” craze, which began with the idea that children from wealthy families are born with golden cutlery depending on their parents’ financial status, has led to a critical consciousness of wealth inheritance in Korea, with children from rich families being called diamond cutlery and children from poor families being called earthen cutlery. What if the same phenomenon, where a child born into a poor family is bound to live in poverty, also occurs with materials in nature?
Materials have different properties depending on their purpose, but they generally have one thing in common: they must be unbreakable. If a material breaks, it cannot fulfill its function. Imagine a material empire with a large number of citizens called atoms that live in a certain bond. Villains who want to destroy the empire pull on the material, putting pressure on the material empire called stress, which deforms the bonds between citizens, causing the town to shrink. Can a material empire be infinitely deformable if the stress is constantly applied? Not really. The material empire has a limit to how much it can deform, and as it deforms, it will eventually break, which is called material necking.
At this point, the heads of the material empire can’t 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 deforms, and when it deforms, the atoms in the material change their positions inside the material, increasing their interference with each other, giving it the ability to resist deformation. In other words, the more deformed parts of the material are no longer deformed. The leaders of the Material Empire want to prevent the destruction of the material by supporting the village and making sure the bad guys can’t attack it anymore. Let’s take a look at necking and strain hardening, the two battles that are at stake in the Material Empire.
In a nutshell, when you pull on a stick made of a certain material, instead of thinning and stretching evenly, the stick becomes noticeably thinner and stretches in certain areas. You may have experienced that when you pull on both ends of a piece of flour dough, it doesn’t stretch to a uniform thickness like a noodle, but instead breaks in the middle as it thins. This happens because the stresses inside an object are uneven in magnitude. The amount of stress a material is subjected to can be found by dividing the force by the cross-sectional area of the area subjected to it. The stress is therefore inversely proportional to the cross-sectional area of the area receiving the force, which means that the smaller the cross-sectional area of the area receiving the force, the greater the stress and the greater the deformation. No matter how evenly a stick is shaved, it is impossible to make every part of it have the same cross-sectional area, down to the atom, so when a force is applied to a material, the smallest cross-sectional area in the material will experience the greatest deformation, which is the lengthening and thinning of that part of the stick. This results in a relatively larger difference in cross-sectional area compared to the material as a whole. As the force continues, the difference becomes larger and larger, resulting in a knockout shape.
If necking occurs as soon as the material is deformed, any material will break easily. However, when you pull on the actual material, necking does not happen immediately. This is because the deformed part of the material has the ability to resist deformation, which is called strain hardening. The Material Empire has chosen a policy of supporting shrunken villages so that they don’t become even smaller. Villains are not stupid, and they know that attacking a town that has the Empire’s support is futile. This will cause the villain to attack other villages, and the empire will prioritize support for the attacked villages. In this way, the material deforms before the more heavily deformed parts, until it eventually deforms evenly throughout. The main reason for strain hardening is the dislocations between atoms, called dislocations, which, when deformed, are attracted to certain areas and impede deformation. These dislocations are the faithful soldiers of the material empire.
The effect of strain hardening contributes significantly to the overall strength of a material. For example, when making steel, the process of hammering and heat-treating iron to increase its strength is an application of the principle of strain hardening. Strain hardening also plays an important role in improving the ductility of materials, reducing the likelihood of fracture.
For simplicity’s sake, we’ve described necking and strain hardening in this order, but in reality, when a force is applied to a material, strain hardening occurs first, and necking begins to occur when strain hardening is no longer possible. The strain hardening of a material empire can protect it from a villain’s attack to a certain extent, but if the attack is strong and continuous, the hardening will reach its limit and necking will occur. Just as wealth disparity is a problem in real life, material disparity and necking are not pleasant phenomena for materials because they cause destruction. Therefore, the strain hardening ability of a material is important as it is a measure to prevent material destruction.
In recent years, research has been conducted to increase the strain hardening ability by using ultra-fine grain materials or carbon fiber reinforced materials. Engineering continues to require stronger materials. Therefore, the possibilities for research to improve strain hardening are endless and will continue to be explored. Engineers are constantly working to create stronger and more flexible materials by developing new materials and improving existing ones. This research and development will go a long way toward improving the quality and safety of all the products we use.
