Body armour should never be considered ‘bullet proof’. The best it can be is more or less bullet resistant. A body armour vest containing multiple layers of Kevlar should be able to stop conventional hand-gun bullets but if you want to stop a high powered rifle bullet (800 m/s or more), you will need something more than layers of Kevlar.
Kevlar's effectiveness relies not only on the strength of the fibres, but also on its ability to flex. So, even when the Kevlar vest can stop the bullet, that does not mean the wearer will avoid injury. Blunt trauma, in which the bullet presses the protective material back into the body before it is stopped, can break bones, disrupt organs and kill. The armour bounces back and the wearer is not 'holed' but the bullet can still stop them. The story of someone wearing a Kevlar vest who is shot several times from close range and is not (at least temporarily) incapacitated belongs in the movies.
The weaves of Kevlar are very varied, the tighter and more complex the weave, the more expensive it is but the more effective it is – so one vest can offer the same protection as another but be lighter. Du-Pont sell the fibre and it is woven in various places around the world. It is now a security restricted purchase in much of Europe, so most is bought without that hassle from South Africa or the Far East. Some of the Kevlar is then treated with some kind of polymer to hold the fibres together and give good stab resistance. I have not got much experience with that, but the polymer will be sure to have a limited life – almost certainly made shorter by any temperature extremes and the polymer treatment may alter the V50 because the fibres are less free to move.
Like all nylons, the life of Kevlar is adversely affected by Ultra Violet light and by extremes of heat, so how often should Kevlar based demining armour be replaced?
The life of body armour cannot be simply predicted. Some early body armour (combat as well as demining armour) was made using very loosely woven Kevlar (woven in large hanks) and it ‘dropped’ in the covers so that there was nothing left in the top of panels after a while. It had labels warning users to store it lying flat. I have not seen any like that recently but it is probably still out there. Any armour in which the panels have started to gather at the lowest point should be discarded.
Using tighter weaves, well made modern armour that is never overheated and has a quality ripstop and water-resistant cover should last for at least ten years. (Overheating generally means putting it next to a heat source but I would include leaving it exposed wherever ground temperatures exceed 50C.) If armour gets wet (immersion) it must be left to dry without applying heat. Simply take the cover off and it will drip dry slowly (the fibres 'wick' water so drying can be painfully slow). Wet Kevlar is not only heavy, it also offers lower protection (crudely, by 10%). If covers are damaged, store the Kevlar in the dark and replace the cover or sew a patch onto it. If you do not, the exposed Kevlar will bleach (some may be be white when made, so bleaching can be hard to see). It will also harden, so reducing protection (not by a lot because the top layer of Kevlar will protect those underneath).
Both heat and Ultra Violet (UV) light seriously degrade the material. Kevlar is basically a nylon and the ballistic aramid alternatives (cheaper but heaver) are also nylons. Both heat and UV harden the material and make it stiffer, so comparing the flex of an old armour with a new can be a guide, but cross stitching across the armour panel may make it seem stiffer and so be misleading. Du-Pont Kevlar is very clever and is heat and flame resistant – up to a point. No armour (not even cotton or cheap nylon covered) has actually caught fire in a recorded accident but that is probably because the TNT is greedy for the available oxygen. Fragments of mine casing (plastic) are often welded to the Kevlar if the mine detonates at 30cm and bits of mine casing or the ground penetrate the cover. Some have wormed through multiple layers of Kevlar (not in a direct line) before welding themselves in place. Irregularly shaped bits of plastic mine casing can travel inside the deminer in the same way, ending up in unexpected places.
Lots of demining groups have the rule that PPE is replaced after an accident or after any visible damage to the cover. Usually this is unnecessary, but if they want to, that’s fine by me. I would prefer they patched it and replaced their visors annually, of course (visors UV harden quickly and are very exposed to UV in use).
I have done enough tests to be confident that a NATO STANAG 2920 V50 of 450 m/s is not an appropriate test of demining armour and it is entirely coincidental that 450m/s Kevlar does provide way over the top blast protection. For an example of why the ability to stop fragments travelling at 450 m/s is not a good test, early armour made using Dyneema (a kind of puffed polythene) passed the NATO STANAG 2920 test and is light as a feather – but add heat to the fragment and it makes a hole the size of a golf-ball. Given that TNT detonates at 4000C+, the debris thrown out by the blast does tend to be a little warm. There is not much time for heat transfer – but the way the fragments weld themselves to armour and visors – and the way they leave necrotic tissue behind as they penetrate flesh – proves that they are pretty hot. Dyneema passed the STANAG 2920 requirement and was positively dangerous in demining (although might still be useful in a mix with Kevlar).