Metal memory. Straightening frames, removing door dings, etc....

[HardDrive]

In another thread, I posted about a C3 Corvette for sale near me. Everyone agreed that it had seen some hard knocks. So that got me thinking about having a frame straightened.

Now, in a slightly different arena, I have noticed a couple dings on my my 997. Whats odd about these dings is that they don't appear to be fresh. The paint in the area is 100 perfect. When I got the car, it was 100 clean, and it almost appears that dings that were taken out prior to me buying the car are coming back. Is that possible?

And to tie these two things together, is it really possible to 100% straighten either the frame of the car, or the body of the car if its damaged? Will the metal always drift back towards the damaged position?

[Willem Fick]

Not likely.

When a small force (bending stretching etc.) is applied, metal first reacts elastically, and will return to its original state if the force goes away. If the initial force is great enough the metal will deform in a plastic manner which implies that the shape change will remain even once the force is removed.

In repairing dents and straightening frames the applied force has to be substantial enough push the metal past the elastic phase and into the plastic phase otherwise the metal would simply revert back to the unwanted shape immediately. In the instance of straightening something as substantial as a frame the results will be absolutely permanent.

In the case of body dents it may well be that some residual stress remains in the repaired metal that in turn may cause the metal to again deform away from the repaired state, especially with the addition of an external factor such a the heat of the sun, a particularly cold morning and the like. The likelihood is however very small, and even if it were to occur the resulting deformation will be very small.

Straightening is a relative thing. Once straitened the mounting points etc. may well be back in their original places, despite some kinks and dents remaining in the metal. This comes down to the fact that the straightening forces are big enough to overcome the elasticity of the frame (i.e. stretch it) overall, without it being enough to straigten out some of the damage.

The main problem that comes with this plastic deformation of the metal is that it may actually weaken the structure, so there is a limit beyond which you really do not want to stretch metal.

[vash]

Work hardening.


Typos and bad grammar sponsored by iPhone!

[Willem Fick]

Valid point but will (should) not cause dents to return or straightening to become undone. Work hardening is when working (shaping/machining) the metal causes the crystal structure to change, thus hardening the steel, leading to a likelihood of cracking in sheet metal in particular. The sheet-metal (and its chemical make-up) used in automotive applications typically is chosen in such a manner that work hardening will not easily occur.