On November 24, 2020, the much-anticipated Chang'e-5 entered space with the help of the Long March 5 carrier rocket, which lasted 112 hours into lunar orbit and finally landed safely on the far side of the moon. This feat opened a new chapter in the continent's space exploration.
Do you know? When the Chang'e-5 probe landed on the lunar surface, although the reverse engine greatly reduced its descent speed, it would still withstand about 4 times the gravitational acceleration of The Earth at the moment of landing. That is, the landing bracket bears an overload equivalent to 4 times the ground weight of the probe. At the moment of landing, once the landing bracket breaks due to the inability to withstand overload, the Chang'e-5 probe and many of the high-precision scientific research and detection equipment it carries with it may be damaged.
Therefore, how to make the landing bracket withstand high overload and protect the detector from injury is a major difficulty encountered by engineers.
The lunar surface is uneven, so the landing position of the probe, landing attitude, mass distribution, center of mass, vibration characteristics at landing, mechanism motion coordination and other factors are all places that need to be considered when designing. In addition, because the impact force of the landing moment will transmit highly destructive energy to the detector through the landing bracket, the landing bracket must not only be hard and stable, but also need to have the buffer protection ability to absorb the destructive performance efficiently.
Which material is up to the task? After a comprehensive screening, the engineers finally adopted a protective material called honeycomb, which was installed as a filling material in the hollow landing bracket of the Chang'e-5 probe.
Huh, honeycomb material? What does it have to do with a bee's hive?
That's right, honeycomb materials are inspired by nature's real hives!
Chang'e 5 applied a honeycomb buffer structure derived from nature, image source: homemade
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Honeycomb material from nature
Bees are magical social insects that often exhibit intelligence comparable to those of higher animals. Our common hexagonal mosaic hive is the unique secret (patentable) mastered by the bee family after millions of years of evolution.
Bees are nature's craftsmen, image credit: theconversation
The structure of the hive must be familiar to everyone: it is composed of many hives, some used to breed young bees, some to store nectar... At the same time, the cross-section of each hive is almost a standard hexagon, a unique shape that is so impressive that whenever you see a similar shape, you will think of bees and their homes. Therefore, this planar structure made of many hexagons is often referred to as a "honeycomb structure".
Engineers and artists have long used honeycomb materials in various industries, and we can find hexagonal honeycomb elements in various works of architecture, sculpture and painting.
Honeycomb style building, image credit: archcollege
A cellular-style commemorative coin issued by Cameroon, image source: powercoin
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Honeycomb materials contain the beauty of mathematics
Scientists are also very interested in honeycomb structure, and scholars such as Farrow in ancient Rome and Popes in ancient Greece have studied honeycombs. However, they do not give a definitive explanation for why each hive is hexagonal. Later scientists proposed a "cellular conjecture" that suggested that this cellular structure might have optimal space utilization. But for too long, this conjecture has not been confirmed.
In 1999, mathematician Thomas Hales used mathematical methods to prove the honeycomb conjecture: if a plane is to be divided into many areas of the same area, then when using the method of regular hexagon mosaic, the minimum line perimeter is required. In other words, the bee adopts this hypnical tightly inlaid hive design, which can maximize the use of space with the least amount of material, which is really smart!
Mathematician Thomas Hales, image source: Wikipedia
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Honeycomb materials are highly protective
The reason why honeycomb materials can land on the moon with Chang'e 5 is because it also has great application value in the field of buffer protection.
If we place honeycomb material between two thin layers of solid plates and glue them firmly together, we get a multilayer composite plate called "honeycomb sandwich board". The sandwich panel looks thick, and the porous honeycomb material core material occupies most of the volume, but its mass is very light. At the same time, the honeycomb sandwich panel also has a high resistance to bending deformation and cushioning protection, so it is very suitable for use in aircraft, rockets, satellites and other equipment with very high requirements on weight, load carrying efficiency and protection capacity. In 1915, engineer Hugo Junkers first applied honeycomb sandwich panels to the structural design of aircraft, thus opening the era of honeycomb materials flying into the blue sky.
Honeycomb sandwich panel, image source: see watermark (if infringing, please contact to delete)
How does honeycomb materials protect rear equipment in hazardous environments such as explosions and high-speed collisions? We can observe this by a simple compression destruction experiment.
Honeycomb materials are also called two-dimensional materials because they can be obtained by stretching vertically through hexagonal mosaic structures within a two-dimensional plane. It can be found that if the honeycomb material is squeezed in different directions, it must be destroyed in a different way. In order to study the compression damage of honeycomb materials in different compression directions, scientists generally define the X and Y directions in the two-dimensional plane as the in-plane direction, and the tensile direction (Z direction) as the out-of-plane direction.
Definition of the direction of honeycomb materials, image source: homemade
As shown in the figure below, the honeycomb material is an extra-surface compression experiment, with a rigid indenter above it and an important piece of equipment below. Its task is to protect the rear equipment as much as possible when the indenter is applied downwards.
After the compression test is initiated, the indenter begins to slowly move downwards and transfers the pressure backwards through the honeycomb material. According to Newton's third law and the equilibrium conditions of the force, the pressure applied by the indenter to the honeycomb material is equal to the pressure exerted by the honeycomb material on the rear equipment. When the pressure exceeds the withstand limit of the rear equipment, the equipment can be destroyed.
Compression experiment in the out-of-plane direction, image source: homemade
As compression progresses, the pressure between the indenter and the honeycomb material does increase rapidly, and it seems that it is about to exceed the limits of the rear equipment. But at this moment, the honeycomb material succumbed to the huge pressure in advance, and its sidewall began to bend and fold. This deformation of the sidewall is initially present only in a local area, and then expands to a wider area as the indenter continues to press down.
The changing law of stress, image source: homemade
Local bending and folding phenomenon of honeycomb materials, Image source: Literature[2]
Every time this local sidewall bending and folding breaks, a tiny gap will appear between the honeycomb material and the indenter, just like a boxer punching empty, which will cause the pressure to drop instantly. The pressure will not continue to rise until the indenter continues to press the honeycomb material tightly.
Throughout the compression process, a large number of sidewall bending and folding phenomena make it impossible for the pressure to continue to rise, always fluctuating up and down near a constant value. Only when the honeycomb material is completely compacted into a cake shape will the pressure continue to rise rapidly, posing a threat to the rear equipment.
The above is the case of out-of-plane compression. When the honeycomb material is subjected to compression in the direction of the surface, although its sidewall bending and folding law is different, the change law of the pressure curve is still similar, and it also has a more obvious platform area. Until the indenter completely compacts the honeycomb material, the pressure can never achieve an effective breakthrough, and naturally it cannot pose any threat to the protective object in the rear.
In-surface compression experiments on honeycomb materials can be seen that the side walls have also undergone bending and folding, image source: Literature[3]
All in all, honeycomb material is a self-sacrificing material, through its own "crushed bones" like a large crushing damage, the destructive external energy into its own internal energy, thereby ensuring the safety of the partner behind. As a result, honeycomb materials were installed by engineers as filler materials in Chang'e-5's landing brackets. It turns out that honeycomb materials have indeed successfully fulfilled its mission.
Scientists have also innovated the basic configuration of honeycomb materials, from common triangles, rectangles, and hexagons to uncommon concave hexagons, Kagome types, etc., these materials are collectively known as honeycomb materials, and each has many unique mechanical properties. This wonderful idea of being good at learning and having the courage to break through and innovate can really be described as "blue out of blue and better than blue"!
Various new honeycomb materials, image source: homemade
bibliography:
1.Hales T C . The Honeycomb Conjecture[J]. Discrete &Computational Geometry, 2001.
2.QIAO Jisen, KONG Haiyong, MIAO Hongli, LI Ming. Mechanical response of gradient aluminum alloy honeycomb sandwich panel composites[J].Chinese Journal of Plastic Engineering,2021,28(03):183-189.
3. Lorna J. Gibson, Michael F. Ashby; translated by Peisheng Liu. Structure and Properties of Porous Solids, 2nd Edition[M]. Beijing:Tsinghua University Press, 2003.11.
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