UKC

I have found BD half domes and Petzl Elios very disappointing in the past 2 years with the moulded polystyrene coming loose from the hardshell in more than half of them and bits of it snapping off, particularly down the sides. 

Anybody else had similar?

No! My Mammut El Cap is as good as new structurally speaking, a good four years on. It’s taken a fair bit of abuse too, from head jams to falling rocks to head first crashes in to walls.

My Elios only disintegrated when it rolled 50M down a rocky gully strapped to the back of my rucksack full of climbing and bivi gear - the shell stayed in one piece but the polystyrene was quite broken up.  I thought it was pretty durable.  

My current Mammut Wall Rider would have been toast in that situation - but it is very light

Bits snapping off? No sounds like things poked at it.

The foam letting coming loose from the shell; yes happened to my (old model) half dome too. I think it's the glue (but it is hard to have good glue that works well in all temperature). 

I have an Elios, the polystyrene detached from the shell after about a year. Been meaning to stick it back with double sided tape but it would be fiddly. I'd have taken it back but I bought it abroad, in Calpé I think.

I use half domes at work, we have several dozen of them for use by clients. The polystyrene has come loose in almost every one, and it's quite common to find the foam around the side snapped off- I think I chucked about 10 or so because of it during the last inspection. All of them ate under 5 years. They do see near daily use though!

Sikaflex 291 polyurethane marine adhesive. It's so good that boatbuilding books warn you that if you ever have to separate whatever you've glued together one or both the items will break in the process.

Well i'm not sure using a better adhesive is good for the helmet properties. 

A; Both the shell and the foam are used to absorb the impact; but they do this in a different way. The shell deforms and possibly cracks, the foam collapses. So different properties; having them separate on impact or have a certain elasticity of the adhesive might be part of the absorption properties.  Just gluing them together with something that will hold, can affect the properties. 

B; Lacking a data sheet of all materials involved; no idea what chemical effect they have on each other

C: with regard to ChrisH89's situation - dodgy DIY user "repairs" of safety equipment rather clearly out of the question