I've used polyethylene boards in the kitchen for a long while now, and have often marveled how they hold up to cuts, hacks and stabs. I have even on occasion, slipped one under a jumper when I've been "expecting some trouble" and helping out a friend in a possibly stabby situation, Ned Kelly style.
So to test this, I loaded up my very under-inspiring Zombie Outbreak Grunt plate carrier with one of the boards I typically use, a Legitim from Ikea, which at 500g, (1lbs2oz) and 8mm (1/4") thick, has always served me well in the kitchen.
Up against it, I pitched an assortment of blades.
- Schrade US Army knife
- KA-BAR Famine Tanto
- American Kami Super Colubris
- Boker Tomahook (front and back)
- United Cutlery M48 Tomahawk (front and back)
- Ontario Black Wind sword
- United Cutlery M48 spear
Here is the video we made of the testing.
I gave each stab or thrust a "I want to do you harm" amount of effort. Taking from my 16 years of kendo I have a fairly good idea of what these impacts would have on an armored target, as well as stabbing a bunch of things over the years for the hells of it. I was pretty happy with my strikes.
Here is the board once we removed it and married it up to the footage.
The two main "bleeder" shots were the back of the M48 tomahawk, the American Kami Super Colubris and the maybe M48 spear (if it actually punched that hole of its own accord).
So, in conclusion, unless someone is coming at you with a spear, or the pointy breaching end of an axe, you will probably be able to shrug off some stabs, and certainly all the slashes that land on a cutting board under your shirt. The good thing about polyethylene is that it is heat-labile, meaning you could mold it to be more chest-shaped.
Obviously there are commercially available, professionally made and certified stab proof inserts and garments you could use, if they were legally available to you, but for my purposes, they sem to fit exactly what I need from them. Won't stop a bullet, sure, but will turn a blade wielded by someone as strong as me.