Rupert's tears, the most car-scrapping tears in the world, can hold bullets and hammers. Why does it break when pinched?

Glass products always give people the impression of being hard and brittle. Its hardness is as high as 6.5-7 Mohs hardness. The file commonly used for filing metal can only leave white marks, but it is very fragile and will break with a hammer!

However, there is a kind of glass product in the world, the manufacturing process is very simple, but it can resist the bullet bombardment and will not break even under the hydraulic press of 10 ton. It is the famous tears of Rupert, but this indestructible product has a fatal weakness, and it will fall apart when pinched!

How did Rupert's tears come from?

The manufacturing process of Rupert's tears is very simple, which can be obtained by dropping molten glass droplets into cold water. Hot glass droplets are rapidly cooled by water, which is a bit like a quenching process, forming a transparent glass product with a long tail.

Its most peculiar feature is that the head of the droplet can be smashed with a hammer without breaking or even moving, but at its slender tail, it is the "cover door" of the whole glass product, which will be directly blown to pieces with a gentle pinch.

This glass product first appeared in Holland in17th century, and it spread to mecklenburg in northern Germany in 1625, so Rupert's tears in Europe began to spread. 1660, Prince Rupert brought this glass product to England. He presented this glass droplet product to King charles ii, who gave it to the Royal Society for research.

Robert hooke studied this and explained the reason why this droplet broke from the tail. Margaret cavendish, Duchess of Newcastle, the Netherlands, came to an interesting conclusion. She thinks it is strong and easy to break because there is a little liquid in it. Because of its peculiar characteristics, it was also brought to the court to make fun of those pedantic and uneducated old nobles.

How hard are Rupert's tears?

Its hardness has been paved, so I won't go into details here. Let's take some pictures to see how exaggerated it is!

The hammer won't break.

The hammer can't break a small glass product, which is beyond everyone's imagination.

The bullet broke in response.

This material is too NB, which can resist the bombardment of bullets.

Crushed by hydraulic press

However, please note that it was broken under the pressure of nearly 20 tons, which has far exceeded the strength of half of glass products.

Crushed by bullets

But I don't know if you have noticed that Rupert's tears were not shattered, but the impact of the bullet caused the tail to break because of the excessive impact, which quickly spread to Rupert's tears and directly smashed into powder.

Why do Rupert's tears have such strange characteristics?

After robert hooke got Rupert's Tears in the17th century, he carefully studied the structure of this glass product through micro-optics, and the cross-section texture and appearance were carefully depicted. It is not easy to think about scientists in those days. In addition to theoretical research, he must have at least one basic skill of sketching. The hand-painted works like printed matter in the picture below are admired or admired.

Conclusion from robert hooke: The principle of Rupert's tears has been glimpsed in the failure of brittle materials due to crack propagation. However, due to the conditions at that time, this study did not go further. It was not until the work of engineer Allen Arnold Griffith in the 20th century that the characteristics of Rupert's tears were widely known.

Griffith is famous for his research on metal stress and fracture, especially the soap bubble invented in 19 17, which stretches between several lines representing the edge of the research object. This simple way of coloring movies to express stress patterns is still widely used until now. It is easy to understand why Rupert's tears have such characteristics.

During the period of 1994, Srinivasan Chandrasekar, an engineering professor at Purdue University, and Munawar Chaudhri, the head of the materials group at Cambridge University, observed the tearing process of Rupert's tears by using ultra-high-speed photography technology, and found that the surface of the droplet was subjected to high-pressure stress and the inside was subjected to high tension, which led to it being in an unbalanced state and forming an easily broken tail.

In 20 17, Sheila Weng Ruidi of the team used a transmission polarizer to construct the stress distribution of the whole droplet when it passed through the glass droplet. Under the pressure of 700 MPa, the surface compressive stress of the droplet head is much higher than previously thought. The compressive layer is very thin, only 10% of the diameter, but it has extremely high strength. Only the internal cracks introduced from the tail tension extension zone can enter the head tension zone, so they can only be broken from the tail, and the breaking speed is as high as1450–1900 meters per second.

Further reading: tempered glass

The by-product of Rupert's tears is tempered glass. The study of Rupert's tears found that after heat treatment, the strength of glass can be 4~6 times stronger than that of annealed glass. After the tempered glass is treated, slight cracks will be squeezed by stress, and the possibility of cracks in the inner layer is relatively low.

Of course, the shattering of tempered glass is somewhat similar to Rupert's tears. It is either not broken or broken together, and there is no big piece when broken, which is relatively safe, but it also has a disadvantage that it may suddenly burst under certain conditions, such as sudden drop in temperature or stabbing by sharp objects.