The "pole gate" in Bozhou, Anhui Province is terrible! Junior high school physics knowledge helps you rebuild the "crime scene"

A video of a traffic accident in Bozhou, Anhui Province exploded the circle of friends. According to Bozhou Evening News, at 0: 50 on June, 5438+February, 65438+March, a traffic accident occurred at the intersection of Tongxiang Road and Du Yao Avenue in Qiaocheng District, Bozhou City, and five people were trapped in the car and seriously injured. After receiving the alarm, the fire department quickly rushed to the scene for rescue. After the car hit the telephone pole, the body was severely deformed and almost broke in half. Afterwards, firefighters demolished the accident vehicle, and the on-site doctor confirmed that two people had no signs of life, and rescued three injured people. At present, the cause of the accident is under investigation.

Watching the shocking video of the accident and the comments on vehicle safety in major forums reminded me of a major traffic accident at the "wedding gate" in Hangzhou, Zhejiang Province on June 9, 2005, with 3 deaths and 2 injuries. The car was more seriously damaged in the accident, and its body broke in two. At that time, when the accident came out, the public opinion that "there were serious defects in vehicle safety" and "the car factory cut corners" was rampant.

Later, after more than two months of investigation, the accident Quality Appraisal Report made by Zhejiang Institute of Quality and Technical Supervision and Inspection showed that it was not directly related to vehicle safety. "In order to avoid it, the driver suddenly swerved when the speed reached 1 12km/h, causing the vehicle to slip. The car moved about 36 meters to the left at the arc roundabout (sideslip). The left front wheel collides with the cement guard plate of the green isolation belt in the middle of the road and bounces off, then the right front wheel collides with the cement guard plate of the green isolation belt in the middle of the road and bounces off, and then the left and right front wheels cross the green isolation belt. The whole body is almost at right angles to the original driving direction. After sliding forward about 10 meter, it crashed into the rigid partition wall at a speed of 83.4km/h, that is, the right rear door of the car collided violently with the end face of the rigid partition wall, resulting in the body breaking under the impact force and reaction force. At the same time, the first half of the car body crashed into the ground of the opposite lane, and the second half of the car body was thrown to the other side of the opposite lane under the reaction force, causing three deaths and two injuries.

Pay attention to the three key words in the last paragraph: "at a speed of 83.4km/h", "the position of the right back door" and "the end face of the rigid separation wall". Look at the picture above, which is a commonly used picture of the expressway separation wall found on the Internet. The shape of the end face is probably trapezoidal at the bottom and rectangular at the top. The size of the bottom is 480mm, the size of the upper part is 150mm, the height of the upper rectangle is 450mm, the total height is 800 mm, and the estimated cross-sectional area is about 0.17m2.

According to the conservation theorem of momentum and kinetic energy in middle school physics knowledge, in an ideal state, assuming that the instantaneous collision time between the rear door and the end face of the car body is 0.065 seconds (the width of the car body is 1.5m, and the initial collision speed is 23. 17m/s), the speed of the two cars will be reduced by 80% after the collision (for the sake of simplifying calculation, it is actually not), then the weight of the car body is 60. Where 83.4km/h=23. 17m/s, and the interaction force during collision is f =1900 * 23.17 * 0.8/0.065 = 5465438+.

However, the collision is an accidental event. Obviously, it is impossible for the car body and the end face of the partition wall to contact evenly in front. The high probability is that the car body touches the end face at a certain angle. No matter how big the angle is, the contact between the car body and the end face is only the edge of the end face. Assuming that the sharp corner of the end edge is 2mm wide and 0.8m high, the contact area is only 0.00 16m2, which is only 0.94% of the uniform contact area of the front. According to the previous calculation method, it can be concluded that the impact force is 106.25 times of the previous one, reaching 5874.6t, and the pressure is as high as 338.9MPa.

This stress far exceeds the requirements of body material and design strength. The above picture shows the strength classification of Baidu 20 12 body steel. However, there are two points that need special emphasis. Steel plate has two parameters: yield strength (compressive strength) and tensile strength (tensile strength). At present, the tensile strength used in the car factory is because the tensile strength of steel is higher and looks safer!

The tensile strength of car body steel refers to the ultimate strength of steel being pulled off. According to the idealized rough calculation of physical knowledge at noon, the collision limit of the "wedding gate" accident in Hangzhou in 2005 reached 340MPa, while the reference value of body structure strength released by Baidu reached 320-420MPa. Ultra-high strength steel plates are only used in a small amount on both sides of the roof, the front of the car, the floor of the car body and the rear of the car. Hot-formed steel plates exceeding 1000MPa are only used on the steel beams on both sides of the roof, the B-pillar and the anti-collision steel beams on the lower side of the front door, and the whole broken underbody is high-strength steel plates of 180-240MPa, so the car body broke into two parts in the accident, which is also in line with our rough theoretical analysis.

In fact, the conclusion in the quality appraisal report of Zhejiang Institute of Quality and Technical Supervision and Inspection is: "No abnormality was found in the steering system of the accident vehicle; No abnormality is found in the braking system; The airbag pops up when it hits the end face of the cement partition wall, which is normal. The structure and manufacturing process of the broken parts of the car body meet the requirements of drawings and relevant standards. The reason for the fracture is the violent collision between the right side of the car body and the end face of the narrow rigid partition wall, and the collision strength exceeds the design strength of the car body structure itself. " Although the public opinion of this accident generally believes that the rigidity of the car body is not enough, science cannot tolerate any falsehood. No matter the official quality appraisal report or our rough calculation results, it is concluded that the collision strength exceeds the design strength of the car body structure.

Speaking of the traffic accident in Bozhou, Anhui Province, according to the video and picture analysis seen afterwards, the diameter of the circular lamp post in this collision is about 0.3m, the height of the car body is 1.5m, minus the ground distance is about 150mm, and the collision contact area is estimated to be 0.3 * (1.5-0.15. Although the collision contact area is larger, and the cylindrical lamp post increases the collision buffer time, which is more conducive to reducing the degree of collision damage, because the lamp post has no displacement, the vehicle speed becomes zero instantly after the collision. If the collision speed is the same as that in Hangzhou, the energy consumed in this collision will be even greater (it is also full of five people, and the weight of the car can be regarded as the same). Fortunately, the impact site of this car is near the B-pillar, which is the place with the highest steel strength in the whole car. Finally, the body twisted along the lamppost, forming an encirclement without breaking.

In fact, no matter how damaged the vehicles in the two accidents are, we can't and shouldn't judge the body safety of any vehicle. We can clearly understand how much the instantaneous pressure will be in the limit state of collision only through the physical knowledge that junior high school students understand, and we can also understand that it is too serious to judge the safety of vehicles by the damage in the limit state!

In the end, we have to wait for the investigation of authoritative organizations, and we should not follow the trend to spread false information. In fact, many people have many misunderstandings about vehicle safety: for example, cars with thin iron body are unsafe, cars with light weight are unsafe, and cars with serious deformation before and after collision are unsafe. ...

These views are extremely naive. Have you ever seen that the firmness of a skyscraper is determined by the thickness of the wall skin? The heavier vehicles have greater inertia when driving, and the braking distance in emergency is also greater, so the kinetic energy in collision is greater, and the heavier vehicles bring greater safety hazards than the lighter vehicles. In addition, when a traffic accident occurs, the space of the carriage is not deformed, which is the "amulet" to ensure the life safety of the drivers and passengers. So, instead of discussing the front and back of the vehicle after the traffic accident? Partial damage, it is better to look at the serious deformation of the carriage first. Finally, I want to say one thing: don't be superstitious about the safety of vehicles. I can tell you responsibly that no amount of materials, design and safety configuration are as safe as a driver with high safety awareness. In this world, there is no safest car, only the safest driving.

This article comes from car home, the author of the car manufacturer, and does not represent car home's position.