Effects of Collision on Car Body/Frame

[moriah]

TL;DR

I want to know what happens to the lattice of metals used for car bodies/frames when impacted by collision. I’m guessing that most cars today are made of steel and aluminum. I’m also guessing that the microscopic structure of these metals incur permanent fissures due to collision. If so, is there a way to fill these voids?

Do medium to high level car collisions create cracks/fissures in the frame and body of the car?

 

[Baluncore]

Cars are made by pressing sheet metal into the required shape panels. The sheet metal used is selected and heat treated to have properties that make it easy to stretch in the press. Stretching reduces the thickness of the metal rather than creating voids or cracks.

At the instant of impact, deformation changes the metal, which then bends in ways that could never be done slowly, or undone later. Bending metal tends to stretch the outside of the bend. Metal shrinking is more of a challenge than is replacing an entire panel.

Cars are now designed to crumple when in a collision. There is really no way of bending the original material back into place. The geometry of the car has been lost in many ways that are difficult to identify. The void must be filled with another car.

 

[Lnewqban]

...
Do medium to high level car collisions create cracks/fissures in the frame and body of the car?

Depending on the magnitude of the impact forces, the deformation could be temporary or permanent.
Cracks and fisures happen when the stress of the material in a specific location reaches certain values.

Please, see:
https://en.m.wikipedia.org/wiki/Yield_(engineering)

 

[jack action]

With metals, you get elastic or plastic deformations. In either case, cracks will appear (even if they are microscopic). With elastic deformations (metal comes back to its original shape), the cracks will grow slightly with each deformation. This is the case even with the car bouncing on the road. It is called metal fatigue and it is inevitable.

With a good design, it is usually without consequences, especially with steel.

If so, is there a way to fill these voids?

Some repair methods for serious cases of fatigue:

Repair​

1. Stop drill Fatigue cracks that have begun to propagate can sometimes be stopped by drilling holes, called drill stops, at the tip of the crack. The possibility remains of a new crack starting in the side of the hole.
2. Blend. Small cracks can be blended away and the surface cold worked or shot peened.
3. Oversize holes. Holes with cracks growing from them can be drilled out to a larger hole to remove cracking and bushed to restore the original hole. Bushes can be cold shrink Interference fit bushes to induce beneficial compressive residual stresses. The oversized hole can also be cold worked by drawing an oversized mandrel through the hole.
4. Patch. Cracks may be repaired by installing a patch or repair fitting. Composite patches have been used to restore the strength of aircraft wings after cracks have been detected or to lower the stress prior to cracking in order to improve the fatigue life. Patches may restrict the ability to monitor fatigue cracks and may need to be removed and replaced for inspections.

 

[moriah]

Cars are made by pressing sheet metal into the required shape panels. The sheet metal used is selected and heat treated to have properties that make it easy to stretch in the press. Stretching reduces the thickness of the metal rather than creating voids or cracks.

At the instant of impact, deformation changes the metal, which then bends in ways that could never be done slowly, or undone later. Bending metal tends to stretch the outside of the bend. Metal shrinking is more of a challenge than is replacing an entire panel.

Cars are now designed to crumple when in a collision. There is really no way of bending the original material back into place. The geometry of the car has been lost in many ways that are difficult to identify. The void must be filled with another car.

Once again, you’ve given me a great answer.

 

[moriah]

With metals, you get elastic or plastic deformations. In either case, cracks will appear (even if they are microscopic). With elastic deformations (metal comes back to its original shape), the cracks will grow slightly with each deformation. This is the case even with the car bouncing on the road. It is called metal fatigue and it is inevitable.

With a good design, it is usually without consequences, especially with steel.
Some repair methods for serious cases of fatigue:

Thank you. I can tell you have expertise. The patch solution is the most intriguing to me. I don’t know what “shot peen” is but I can Google it.

 

[anorlunda]

https://en.wikipedia.org/wiki/Shot_peening
Shot peening is a cold working process used to produce a compressive residual stress layer and modify the mechanical properties of metals and composites. It entails striking a surface with shot (round metallic, glass, or ceramic particles) with force sufficient to create plastic deformation.

You have probably seen a ball peen hammer also. The round end of the hammer is used to manually "peen" the surface of metal

Definition of peen​

: to draw, bend, or flatten by or as if by hammering with a peen

 

[moriah]

Got it. Thanks.

 

[Jodo]

If the vehicles frame has been damaged to the point where metal has been upset you should scrap the vehicle.
Welding on vehicle frames is highly discouraged unless you can stress relieve the whole frame after welding.

As far as voids in the metal I am unsure. Individual grains will stretch with a tensile load. Past the point of elastic deformation they will retain this deformed shape. Keep on stretching and they will tear.

Grains can also be compressed past a point where they will retain the compressed shape after the load is removed.
If the frame is bent the inside of the bend is under a higher then normal compressive stress. The outside of the bend will be under higher then normal tensile stress.
Shot peening does cause permanent surface damage but is a great stress relief technique ( for small parts ).

I prefer ultrasonic stress relief personally. It causes no plastic deformation of the substrate.
Junk the vehicle or change frames. I have welded since 1985 and have yet to weld a vehicle frame because I simply refuse to.

Just my 2 cents.

 

[moriah]

To Jodo:
thanks for your expertise. Would you mind telling me more about ultrasonic stress relief?

 

[Baluncore]

https://en.wikipedia.org/wiki/Vibratory_stress_relief