When I was a kid, I especially liked to play with those four-wheel drive cars, which were fast and could easily crash when they weren't on the track. I was naughty at that time, and often crashed the hull of the four-wheel drive car, and after the crash, I could only use glue to stick.
After growing up and touching more cars, I found that the original routine of using glue to stick and make cars with plastic was a routine operation in the automobile industry. At first I thought it was a toy for dumplings like the British Air Force Mosquito bomber in World War II, but a closer look revealed that there was a lot of room for it.
First of all, we must clarify a concept, the strength and rigidity that everyone often mentions are different. I'll use a relatively simple example to illustrate it.
When you were a child, you should have played with the ruler on the desk, the ruler was pressed down and bounced back, the main performance is rigidity, the same force pressed down the smaller the deformation of the ruler, the better the rigidity of the ruler.
And some friends with greater strength are easy to break the ruler, so the greater the force of the ruler breaking, we say that the higher the strength of the ruler. And the point that makes the ruler start to bend is what we call "yield strength."
Generally speaking, the characteristics of the car in normal driving, such as NVH, handling, etc., will be related to the rigidity of the body, and the characteristics shown during the collision will be related to the strength of the body. With this in mind, you can continue to read on.
Yes, cars are getting heavier now, but companies aren't sitting still, they're trying to make them as light as possible.
Components such as high-strength thermoformed steel and extruded aluminum profiles are gradually beginning to be applied to the main components of the body. However, the need for materials in different parts is not the same.
For three-beam and six-column (longitudinal beam, beam, side beam, ABC column) structural parts, in order to ensure the safety of the occupants in the car, high yield strength is very necessary.
The front and rear longitudinal beams are the most important energy-absorbing components in the collapse zone, and it is necessary to absorb the impact energy as much as possible before the A column is hit by the impact force, so as to preserve the A column and the side beam.
Other body plates and structural parts are responsible for fixing the body surface plate and part of the impact force dispersion. They, like the front and rear stringers, require a certain degree of ductility to absorb the impact energy.
As for the body covering version, as long as it has enough elasticity and is not easy to leave scratches.
After knowing the material requirements for each part of the body, you can see what materials will be used in these places in practice.
The first is the highest strength ABC column and side beam, generally will use 1000MPa or high-strength steel, for models with higher handling requirements, yield strength may be higher.
For the ABC column and the side beam, in the collision process must be kept in a state of elasticity to protect the safety of the occupants of the car, once it enters the plastic deformation stage, the result is a variety of bending of the ABC column. Therefore, the yield strength of the ABC column is generally the highest among the body structural parts.
Then there are the front and rear longitudinal beams, which are responsible for absorbing energy, and need to be plastically deformed to absorb enough energy in the event of a collision. So their stiffness will be slightly lower, and the yield strength will be about 700-800MPa. Body sheet yield strength in other locations will be lower. The outer covering is probably a softer steel plate or aluminum alloy plate.
The above is the normal operation of body materials, but in recent years, many organic materials have begun to be widely used on car bodies.
Recently, many models have begun to appear with a tailgate made of resin materials, which is more visible in hatchbacks and SUVs, such as the most uninteresting Highlander in the desert and the fun Qijun. Their resin tailgates are made of polypropylene containing glass fibers and are injection molded in one piece.
The advantage of the resin tailgate is undoubtedly lightweight and low cost, but the sound insulation will be relatively poor, in order to make up for the sound insulation performance and paste a variety of sound insulation materials, it is contrary to the original intention of lightweight. But the highlander's third row of seats is soundproof and quiet, and I think it's okay, without too much ambient noise coming from the rear of the car. Qijun is also fine.
And as a component visible to the naked eye of consumers, how to reduce the plastic feeling of the tailgate is also a big problem. The plastic feeling of the tailgates of the two vehicles from Highlander and Qijun alone is very small, and it will not feel that it is made of plastic without directly tapping with your hands.
Of course, models that are not bad for money will use more organic materials. For example, BMW, which likes to use carbon fiber the most in mainstream car companies, uses a carbon fiber roof in M6 (E63), and the body of the 7 series (G11/12) is composed of a mixture of steel and aluminum carbon fiber.
The most extreme is the i3 and i8, when BMW pioneered the Concept of LifeDrive, which separately designed the Life layer and the mechanical layer (Drive), which is very similar to the skateboard chassis I introduced earlier.
The living layer uses a carbon fiber monomer shell + screw fixing resin overlay, which takes into account the lightweight, strength, stiffness and ease of maintenance, which is what it does... It's expensive, and it's easy to scratch on the outside.
The newly launched iX largely inherits the tradition of its predecessors, although in addition to the large proportion of aluminum profiles, the carbon fiber outer panel covering the B-pillar and the lower beam provides enough strength to save the iX the beam in the B-pillar position on the roof.
With all sorts of body parts, how to put them together is the next problem.
The most commonly used method of component combination in body manufacturing is, of course, welding, and the most commonly used are resistance welding and laser welding. Resistance welding is the heat generated by the current passed on to the solder joint, and two or more steel plates are melted together.
Pull down the rubber strips of ABC columns to see the neatly arranged solder joints, which are also the most common basis for judging whether ABC columns have been repaired in used car trading. Laser welding is mostly used in long strip welding such as roofs.
Of course, there is another joint method commonly used in the automotive industry, that is, glue, or more accurately, adhesive.
Drop a drop of water in the middle of the two pieces of glass, and then sandwich them together, and the two pieces of glass will be difficult to separate in the vertical direction. This is the basic principle of adhesives.
Adhesives are used not only in the combination of steel plates and steel plates, but also in the combination of steel plates with different types of materials such as carbon fiber and aluminum.
Or BMW i3, the living layer uses a carbon fiber monomer shell which must use adhesives on a large scale. In order to join the monocoque shell and the resin suspension tower roof, BMW used the adhesive shown by the red arrow.
The fact that the adhesive is extruded in the gap between the joint surfaces is not a design defect, but rather that the two parts have been glued together with the adhesive, which is quite common in European car companies, but Japanese car companies feel that the joint surface without adhesive overflow will look better.
So don't think that the body glued out with the adhesive is cutting corners, where to use the adhesive, how much adhesive to use are scientifically analyzed.
In addition to the above, some unconventional joining methods will also be used in small-batch production models. For example, the all-aluminum alloy body of many sports cars likes to use rivets as a connection.
In the process of automobile design and production, the use of different materials and processes brings different characteristics to the vehicle. But in the end, which material to use at a certain location, which process, the decision-making process is very complicated.
So don't think that the A car uses aluminum alloy somewhere, the B car in this place is an ordinary steel plate and starts to hold a step, it is really unnecessary.
In the next article, I will focus on explaining the handling of the vehicle and the NVH from the structure of what the evaluation indicators have to do with the body.