Numerical simulation of carbon nanotube energy absorption capability under ballistic impact

Senior Thesis for Master's degree

Abstract

Carbon nanotube can be considered the most amazing nanostructured material discovered so far. Unique features such as high strength, light weight, and high energy absorption capability have made this material recently attracted the attention of scientists in making bulletproof vests. In this thesis, a structural model in the Abaqus software environment is used to analyze the mechanical behavior of carbon nanotubes under ballistic impact. In this structural model, nonlinear interfaces are used to model the bond interactions of tension and torsion, as well as the axial nonlinear spring element to model the bond interaction of angle change. One of the advantages of this method is its implementation in the CAE space of Abaqus software. This means that there is no need for programming to analyze the mechanical behavior of carbon nanotubes under different boundary and loading conditions, and any capability that the software has in analyzing mechanical problems can be applied to nanotubes. In this thesis, the impact process is simulated by a rigid body with a certain mass as a bullet. The effect of various parameters such as the geometry, type and boundary conditions of carbon nanotube, the location of the bullet hitting the carbon nanotube and its impact angle on the mechanical behavior of carbon nanotube has been investigated. Also, the effect of various defects in carbon nanotubes on its energy absorption rate under impact has been investigated. In the end, considering that in the investigation of the mentioned parameters, the bullet was geometrically designed according to the previous studies, by changing the geometry of the bullet, the behavior of the nanotube under impact was investigated. The obtained results show that in the carbon nanotube with two fixed ends, the energy absorbed is maximum when the bullet hits the middle of the carbon nanotube; While in the case of an intruder, this value becomes maximum at the relative height of z=0.6. As the angle of the bullet increases with respect to the absorption horizon, the carbon nanotube energy decreases. In the defective carbon nanotube, the amount of energy absorption is reduced, and this reduction is more in the Stonewalls defect than in the excitation defects.

Key words: carbon nanotube, structural mechanics model, abacus, impact, energy

List of articles extracted from the thesis

Shariati Mahmoud, Farhadian Mohammad and Ipekchi Hamidreza, "Numerical simulation of carbon nanotube energy absorption capability under ballistic impact" Modares Mechanical Engineering Journal, (submitted for refereeing). Therefore, he has been wearing armor since thousands of years ago. In the fifth century BC in Iran and Greece, 14 layers of linen were used to make armor. 700 years after Christ, a kind of sleeveless armor consisting of steel or iron plates fastened on leather strips was made in China and Korea, which had special lightness and flexibility. With the development of weapons and the use of firearms, the attention of craftsmen was drawn to the production of armor that could protect the human body against bullets with the help of thicker steel plates and extra heavy plates. This makes the armor heavy, and its use is exhausting for the person wearing it. In the 1960s, engineers developed a bulletproof vest that was comfortable to wear, unlike traditional body armor. Lightweight bulletproof vests were not made of metal, but of fibrous fabrics that could be sewn onto vests and other light clothing. In 1965, the DuPont company produced Kevlar brand fiber (from the Aramid family) and produced fabric from it. At first, Kevlar was used in the rubber industry and then in various products such as ropes and washers and various parts of airplanes and boats. In 1971, Kevlar fibers were introduced as an alternative to nylon fibers in bulletproof vests. Currently, this fiber is one of the most important fibers used in the production of this type of clothing [1].

Today's bulletproof vests are divided into two types:

Hard bulletproof vests

Soft bulletproof vests

     Hard bulletproof vests are made of thick metal or ceramic plates and are resistant enough to deflect bullets and other weapons. The materials used in these vests push the bullets out with the same force that is going in. This way the vest will be impenetrable. Hard bulletproof vests provide more protection than soft bulletproof vests, but are more strenuous. Police officers and military personnel wear this type of protective equipment when the level of possible danger is high; But for everyday use, they generally use soft bulletproof vests that can be worn as a jacket or a regular t-shirt. Inside these vests is a bulletproof material, which is actually a very strong net. To understand how it works, consider a soccer goal net that is tied behind the goal. When the ball is shot at the goal, it has a lot of energy and when it hits the net, it pushes the net back at a certain point. Each string extends from one side of the pole to the other and distributes the force applied at that specific point to the end of the net. This force is spread due to the interweaving of the threads, and in this way, all parts of the net absorb the energy coming from the ball, and it does not matter which part of the net the bullet hit. Long strands of fibers woven together to form a dense net-like structure. Now, considering that a bullet moves much faster than the ball, so this net must be made of stronger material. The most famous material used in making bulletproof vests is a fiber called Kevlar [2]; A lightweight fiber that is 5 times stronger than a piece of steel of the same weight. When this material forms a dense net, it can absorb a lot of force. In order to prevent the bullet from reaching the surface of the body, the bulletproof vest must work against the direct impact of the bullet. Recently, the use of spider web fibers has become common in the production of vests. The strength of this thread is about 20 times higher than steel [3].

However, two important factors in bulletproof vests are their ability to absorb energy and their lightness. Therefore, it is important to use materials that have these properties to repel or deflect bullets. Vests that have been developed so far may prevent death, but they still cause bruising and damage to vital organs. Therefore, the research to find the best material for use in bulletproof vests is still ongoing. The latest research conducted in this field shows that carbon nanotube threads [3] are even more resistant than spider silk. Due to their high strength, low weight and high energy absorption capacity, carbon nanotubes are the best materials for making anti-impact devices, especially bulletproof vests. They are in the focus of researchers all over the world and a lot of work has been done in different fields about these materials. After the discovery of carbon nanotubes, researchers have turned to conducting experiments on this structure; But spending a lot of money to conduct these experiments led the researchers to use different computer methods to simulate the different behaviors of this material. Among the most important of these methods, we can refer to the methods of initiation [5] and molecular dynamics simulation [6]. Of course, it should be noted that the molecular dynamics method has a very high accuracy; But using it requires a lot of time and money, and it is not possible for everyone to use it. This caused the researchers to continue to look for a comprehensive and reliable method so that carbon nanotubes can be investigated under different loadings and boundary conditions. Therefore, in 2002, the use of structural properties for modeling nanostructures was proposed [5].