Analysis: the core of automotive safety - the body

The safety of cars is mainly composed of two aspects: active safety and passive safety. Active safety can be summarized as "the ability of a vehicle to avoid collisions," which is related to the handling and driving stability of the vehicle itself. The passive safety of the vehicle is mainly determined by the structure of the vehicle and the restraining devices (such as seat belts) in the vehicle. The performance of airbags, and of which the body structure dominates.

The main body frame structure of a car is often referred to as a frame, and after a cover member such as a hood, a trunk lid, etc. is added, it constitutes a body of our usual meaning. Many times, because these parts of the car body do not bear the supporting force, they do not contribute much to the rigidity of the whole car body. Therefore, in many cases, the car body and the car frame mean the same thing, that is, the main frame that determines the rigidity of the car. structure.

Frame type

In general, the body of a car can be divided into non-bearing body and load-bearing body.

Non-loaded body mainly refers to the body with a ladder frame (LadderChassis), trapezoid frame is also called array frame, is the earliest form of the frame. As the name implies, the trapezoidal frame looks like a ladder lying on a flat surface. It consists of two longitudinal main beams, which are combined with many sub-beams of different sizes and thickness. In some cases, a diagonal beam is added for consolidation. The biggest feature of non-loaded bodywork is strong and durable, with high rigidity, so the ability to resist frontal crashes is also very strong. Moreover, due to the non-fixed connection of the frame and the shell, the vehicle's distortion is absorbed by the sturdy trapezoidal frame under the rough road conditions with a large drop, which prevents the twisting of the car shell. It was a very necessary design, and it was still used by most cars until the 1960s.

However, the trapezoidal frame also has shortcomings, which is the body weight, high center of gravity. Therefore, starting from the 50's, the car gradually uses a load-carrying body instead of a non-bearing body. The Monocoque made of steel in the late 40's and early 50's was a load-carrying body. The so-called one-piece frame, that is, the body structure is welded and combined into a rigid body shell by folding grooved steel beams and pressed sheet metal pieces, and the sheet metal pieces also jointly carry the body structure load. Current car bodies and urban SUVs use this body structure, and only a few hard-core off-road vehicles still use trapezoidal frames.

Lightweight and high-strength steel applications

When it comes to car body structure design, the highest guiding principle of automotive engineers is lightweight and high rigidity. Although the all-in-one frame has been greatly reduced in weight compared with the original ladder frame, the car has been undergoing a weight-reduction plan for decades since the beginning of the oil crisis of the 1970s, plus the related environmental protection standards for automobiles. The more stringent, automotive engineers are always looking for ways to reduce weight in order to achieve better fuel economy and reduce emissions. After the weight reduction efforts of the past three decades, the weight of a typical passenger car has been reduced by about 250 kilograms, while fuel consumption has also been reduced by about half.

While the weight of the frame continues to decrease, its rigidity continues to increase. In this regard, thanks to the increasingly rational design of the frame structure, it is also due to the application of high-strength steel. Although the same steel, but the intensity is high and low, the same steel, but if the carbon content of the composition of the alloy, or manufacturing, heat treatment is not the same way, the load bearing strength will vary greatly. The medium-strength low-alloy steel (MSLA), for example, has more phosphorus and manganese alloy components added to the melting process and is often used as a vehicle body shell. High-strength low-alloy steel is High-strengthlow. -alloysteel (HSLA) incorporates rare metals titanium and tantalum, which can be twice as strong as low-strength alloy steels. In addition, different ways of processing can also lead to changes in the strength of the steel during the forming process. For example, steels that are pressed with high water pressure are superior in strength to steels that are pressed by a high-weight molding machine.
However, no matter what the steel, its essence or main component is still iron, the strength can be several times worse, but the density is basically the same, so the same volume no matter what the steel, the weight is almost exactly the same. As mentioned earlier, the use of high-strength steels allows even the thinner and thinner structures to achieve the same strength, so the materials can be used less, the weight can be reduced, and the body rigidity can be higher.

In addition to advances in materials, advances in the structure of automotive engineering are striking. We can often hear that “the newly designed frame of a certain vehicle model has increased the stiffness of the previous generation by xx%”. It is reported that the progress of the body structure is better than the improvement of the material's rigidity. The common development has made the safety of the car progress at an alarming rate. However, the body structure itself is a more complex topic, we will talk about in detail later.

With high-strength steel, even thinner, thinner structures can achieve the same strength, so the material can be used less, the weight can be reduced, and the body rigidity can be higher.

Is strength just rigid?

The strength and rigidity of the structure seem to be no different from each other, but in reality they are two different things. The "strength" in English is "strength". The concept is that the material will be destroyed when it receives much external force. This is a data related to the material itself and has nothing to do with the structure. In the automotive structure, only about 20% of the structural parts take strength as the most important consideration, mainly the part of the body structure that handles the impact load, such as the impact beam, the reinforcement of the car body B pillar, etc. Usually we hope to bear in these major The impact of the parts using high-strength steel.

The rigid English is “stiffness”. The main concept is the size of the deformation when the structure is subjected to external forces. This is a data related to the texture of the material itself and the structure. Simply speaking, a large amount of deformation by the same external force means that the structural rigidity is poor, and the amount of deformation is small, indicating that the structural rigidity is good. When measuring the rigidity of the frame, we usually use "torsional stiffness" to indicate the rigidity of the frame, which is defined as the magnitude of the torque required to twist the frame once, in units of Nm/degree or kgm/degree.