I was just reading the thread of Toyota Etios Liva getting a 4 star safety rating by NACP and found the link to this thread. Being a metallurgist, I think I can add some value to this post.
In principle, I do not agree that cars with thick metal sheet are safer than cars with thin metal sheet. Why I say so:
Firstly, there are several types of steel with different alloying elements and hence different properties. With present day technology, it is possible to make a very high strength steel, a 0.5 inch thickness sheet of which can have more strength than 2 mm thickness of another steel. So a light car might be using a costly high strength thin steel sheet while a heavier car might have a less expensive and less strong steel sheet but with a higher thickness. So overall strength of both metal sheets can be similar.
Secondly, a high strength metal sheet is not the ultimate required property for a car body. The most important property from a safety point of view that is required from a metal sheet for cars is “Toughness”. Toughness is the amount of energy absorbed by a metal before it fails. In the below diagram, toughness is the grey area under the curve.
To explain this: The above diagram shows the stress-strain curve for 3 kinds of steel. In simple terms, “Stress” is the applied force per unit area and “Strain” is the deformation due to the applied force. The grey area under the curves is the toughness of the steel.
As you see in high carbon steel, a lot of force is required to bring a small deformation so it is a very strong steel but it absorbs least energy before it fails (less grey area) so it has least toughness. We do not want this steel in our cars. A more intuitive way of looking at this is: Imagine a car made from 10 inch thick steel crashes, what do you think will happen to the car? The metal will not be deformed much but the passenger will be hurt badly due to the impact.
In medium and low carbon steel, the steel deforms significantly when force is applied and in the process of deformation, it absorbs a lot of energy (more grey area) thus minimizing the energy transfer to the cabin and the passengers. That said, the amount of energy absorbed by the metal sheet is quite less compared to the energy absorbed by the specially designed crumple zones and beams that are designed to fail at particular stresses to increase the safety of the passengers.
In cars, very low carbon steel is used since it has several advantages including easy to form into different contours and shapes, has good paint adhering and does not show bad deformations when a dent happens.
If I am right, why do the Germans use thicker steel? Here is my guess:
Germans started making cars several years ago when the technology to make thin steel sheets with high strength steel was not very developed. Just to maintain the structural integrity of the car, they had to use thick steel. As German cars became popular around the world, they became known for their solid build quality. This actually became one of their USP (Unique selling point).
I imagine metallurgists in these companies telling the management that they can use thinner steel and still achieve same strength and safety. But the marketing and branding guys telling them to shut up and not ruin the brand. A brand is made on the “feel” factor and we humans perceive the build quality of a car by its solid thick outer body. I am sure the solid thud sound of a closing door has been engineered to give that “feel” of solid build. This is their USP. They cannot give it up even if it makes the car lighter while keeping it at same safety level. Even I like the solid build “feel” of a Polo better than Etios in spite of knowing that the outer metal sheet does not really matter much.
This made a perfect case for the Japanese to come up with their own USP: fuel efficiency. They actually went a bit too far by using such thin steel that even leaning against a car can leave a small dent on the body.
As seen by NACP tests, both types of cars are equally safe if designed properly and have safety features like airbags, ABS etc. Now, it’s just a fight between the brands to convince people that they either have a better build quality or a better fuel efficiency.