In reply to Smuge91:
I've had a sirocco for the last 18 months or so and if it fits you it's a great helmet, it weighs very little, is well ventilated and has taken the minor knocks I've given it pretty well (usually whacking my head on roofs etc). It does look pretty god awful, all of my mates take the piss out of me when I'm wearing it but at the same time I don't need to look at it really so it's not a massive concern for me. With regards TobyA's point about dropping foam helmets, if there is nothing in the helmet then its kinetic energy at impact will be really small (at most 10 J from a 2m drop) compared to the amount of kinetic energy which must be dissipated in an impact while it is being worn. In low energy impacts the foam should be able to elastically deform like conventional materials so knocking your head on a roof or dropping an empty helmet should not cause it to be damaged. The stuff about retiring it after an impact is because you can get fractures developing beneath the surface of the foam which compromise its energy dissipation abilities, however I don't think this is likely to occur as a result of a low energy impact. The Sirocco is manufactured from EPP which has a different energy absorption mechanism to EPS (elastic deformation rather than fracture) so it should be capable of taking multiple impacts.
The outer shell on the EPS style helmets is needed to increase the area over which the energy of the impact is dissipated (think of a bouldering mat's structure, they are very similar concepts) and to improve protection against piercing impacts. If you are concerned about the abrasion resistance of a climbing helmet you are probably doing it wrong, I can't think of any situation where you are likely to drag a helmet along a rough surface with a high load on it at any great speed in normal use but would be happy to be corrected on this.
In response to Beardy Mike's comment I have seen a similar failure to what you describe, it may compromise the impact protection of the helmet in an impact on the affected face of the helmet, however I think it is unlikely to have an enormous effect on the energy dissipation in a top down impact. On the flip side one of my friends treats his like a football, I've seen it be dribbled and booted off trees on multiple occasions and the only bit which he has broken is the fancy magnetic buckle.
I was planning on doing my final year project on the multiple impact performance relating to climbing helmets this year, however none of the helmet manufacturers were willing/ able to give me the material samples to do the project.
