I have worn a bike helmet without fail starting in the early 80s. I have always understood that they are disposable after any hard hit. Visible or not, polystyrene compresses, cracks, crumbles, etc. The shell or skin of the helmet never seemed to matter much, so whether it splits or shreds, doesn't matter. Maybe I'll have a look for a source on this.
Yeah, it's a crumble zone that might add a few precious millimeters to the very short deceleration path of the brain if limbs and reflexes fail to do that job completely. Disintegration means that it's doing its job.
That's a completely different story from the primary task of the helmets for rock climbers, construction workers or soldiers, which is distributing a small, concentrated impact (a rock or a dropped tool or random debris) to a wider area.
Rock climbing helmets differ in design substantially. Loads of modern ones are more like bike helmets - recognising, I suggest, that most head injuries climbing are head hitting the crag rather than rocks falling, making it more like cycle impacts. That said, I've no real insight into what makes one design better over another.
