The skin of a thin aluminum can is soft and soft. If you press it from the inside, it will bend easily. However, if you roll it into a cylinder and press it with the same force, it is difficult to flatten it. If you seal it up and down to form a complete empty aluminum can, it can even withstand the weight of an adult like me. What is going on?
The carrying capacity of an object must be closely related to the material. The same material, tofu and steel are not the same, so we have to know what material cans are made of. Early aluminum cans were made of stainless steel, which was very strong but a bit heavy. Later, with the advancement of technology, aluminum gradually became the mainstream material for cans. Compared with stainless steel, the density of aluminum is about 1/3 of that of stainless steel, so the original 60g stainless steel cans became 15-21g aluminum cans. The weight is greatly reduced, and it is also enough to withstand the air pressure in the can. Now, all kinds of cans are made of pure aluminum except for the tinplate on the can lid.
So, how strong is pure aluminum? Students all know that aluminum alloys can improve their mechanical properties by doping with different materials, so there are many types of aluminum alloys, and their mechanical properties vary greatly. Although pure aluminum is called pure aluminum, different production methods and different treatment methods will also have different mechanical properties. The figure shows the stress-strain curve of 1060 industrial pure aluminum under different heat treatment processes. You can ignore these 7 or 8 passes, which are all a treatment method. Please pay attention to the highest point of this different curve, and you can see that it is about 220MPa. Although this data is still a little different from ordinary steel, the overall performance is already close.
In the thin sheet state, it cannot withstand a lot of pressure at all, and some strange phenomena will occur, which is called "buckling" in mechanics. The aluminum skin of the aluminum can is so thin, about 0.3mm, and it becomes a plane after unfolding, and both sides are under pressure. This is different from tension on both sides. Compression is an unstable situation. When tension is applied on both sides, even if there is a lateral force interference, the thin sheet will not produce a big change in state and can easily rebalance automatically. Just like a boulder in a col, even if it is pushed away, it will return to its original position under the influence of its own weight. However, when compression is applied on both sides, any slight lateral disturbance can break the original balance and it cannot automatically find the balance. Just like a boulder on the top of a mountain, it will roll down at the slightest disturbance and will never return to the top of the mountain. Under compression, once there is a slight bend somewhere, it will bend more severely under the action of pressure. At this time, the corresponding external load is far from reaching the load value corresponding to the ultimate stress of 220MPa. Therefore, the compressive capacity of the thin sheet is very weak.
Since the reason why the thin sheet has a weak compressive capacity comes from its instability, how to improve its stability? Here we have to mention containers. Think about it, the containers are stacked layer by layer at the dock, and its side panels are under pressure, so it is also easy to lose stability. If you look closely at the side panel of the container, you will find that it is not a flat side panel, but has some longitudinal wrinkles. It is these wrinkles that increase the equivalent thickness of the side panel, thereby improving its ability to resist instability. Putting the cans in a circle can also achieve the purpose of increasing the "equivalent" thickness. In fact, it improves its bending resistance, because buckling is the effect of bending. If the upper and lower sides are sealed again to form a whole, it is equivalent to having additional constraints near the seal, so that only the middle part is easier to bend, and the pressure-bearing capacity can be improved again.