Steel plate bend

[Brad]

I have a very nice AR 500 plate I got from another member. It has only been hit with handgun rounds. Likely 500 hits or more?
I was tearing things down after shooting this morning and noticed the plate has taken-on a dish shape. The curve is TOWARDS the impacts, not away from them.
I layer a straight edge across the plate and it is roughly 1/8” of curve.
May need to turn it around and see if I can shoot it flat again.

 

[RicinYakima]

More edge hits, than center hits?

 

[Snakeoil]

Not uncommon. Many of our silhouette targets are dished.

 

[Brad]

I expected some warping over time but I expected it to be dished in the center with the bottom of the bowl away from the impacts. This is bowing towards the impacts.

Ric, I am nowhere near good enough to hit the edge only! This one is pretty evenly distributed for hits.

 

[Pistolero]

The peening makes the impact side larger, so it bends towards the impacts. Yes, flipping it
over will stretch the other side very slowly, and will go flat. We had some mild steel plates
that we shot ONLY with handguns for decades in my indoor IPSC club. By the end of 30 years
some were easily 1/2" dished towards the shooter. But welded to an angle, which was welded to a
hinge, so not trivial to flip over, and didn't hurt anything, really.

Bending loads will tend to bend it inward, but these loads are nowhere near enough bending to
bend the plate. But there is a tiny local plastic (permanent) deformation made on the front, right
at the surface from each HV rifle hit. And this is pushing metal sideways, making the front face larger.
Pistol with lead only on AR500 will probably not bend it.

Bill

 

[waco]

Interesting. I have never had one warp or dish. Then again, I’m usually shooting them from several hundred yards.

 

[RicinYakima]

My reasoning is that the closer the hit is to the center, the more energy is adsorbed by the mass of the plate. The closer to the edge, more energy is absorbed not only pushing the plate to the rear but also rotating it on the chains. Curve should be towards the direction of impact.

 

[Pistolero]

RicinYak - that would be true of a thinner plate. In this case the bending loads that you are thinking of
are not great enough to put ANY permanent bend into the plate. But the tiny tip of a rifle bullet at high
velocity leaves a tiny ping in the plate. Look hard at an AR500 plate shot with a FMJ high vel rifle, that
tiny area is hammered JUST a bit, and squished outwards in all directions. Lots of these and slowly the
front fact is slightly bigger than it was, and the back 2/3 or 3/4 of the thickness is unchanged. Imagine
a bimetal disk, brass on front, steel on back. Heat it up, same thing will happen.

Even that dead center hit does not put enough bending force on the plate to do any permanent bend,
just an elastic bend, 100% bounces back. But those tiny pings are permanent dents. Like 1/16" diam and
maybe .002" deep or .005 or something. VERY little, but permanent. Ten or twenty do nothing noticable,
but a couple of hundred, you will see the bend.

Bill

 

[freebullet]

Geez Brad what the hell calibers you shooting these days. Some guys can find a way to break anything.

 

[Snakeoil]

Basically, you are shotpeening the plates with bullets.

 

[fiver]

I know we have seen this before.
but look at the edges of the metal on the shots close to a hole or the edge of the plate.

your actually moving the metal around inside the plate when you hit it.

 

[Ian]

Late to the party but Bill and Fiver finally described the mechanism of plate dishing accurately. The face surface of the hardened plate stretches the most, regardless of where it's hit. Over time, the expansion causes the dishing to be convex toward the shooter., as in the excellent "bi-metal disk" analogy. Even near edge hits put spreading stress on the impact face. If the plate were softer metal, the effect would be a little different with deeper deformation and a far different energy/vector profile, but with the hardened surface resisting being dimpled, the force is diverted radially at nearly 90° along the surface as the bullet mass dissipates.

Think about this in reverse, when we launch the bullet, to imagine how much force the base of the bullet experiences from the propellant.