Originally Posted by
closetbiker
There's a study by the Australian Transportation Safety Bureau by Morgan and Szabo, July 2001, entitled, "Improved shock absorbing liner for helmets" which said,
"In an impact situation involving a bicycle helmet, cracking through the thickness of the foam liner (slabcracking) is undesirable as it renders the foam liner of the helmet useless in its ability to further absorb an impact force. As a result the foam is unable to distribute the focal impact over a larger area and to decelerate the blow at the point of impact...
The majority of cracking displayed by samples was in the shape of an arc outlining the spherical headform on impact. Arc-cracking has minimal effect, as it is part of the crushing process. However, cracks developing partly or fully through the thickness of the foam-slab renders it useless in crushing and absorbing impact forces."
I also exchanged emails with a professor of mechanical engineering about helmets cracking along with compression of foam and he said,
"compression of a material in one direction must lead to tension in other directions. (
AFAIK, the only exception is one theoretical loading condition.)
Therefore, a helmet that has cracked may have actually provided protection."
when asked about claims of a helmet working when cracking happens without compression, he told me,
"Fundamentally, these things are supposed to work in compression... if it didn't compress significantly, it did not work as designed."
So it looks like learned consensus is that helmets work as designed, absorbing impact by crushing before cracking. While there might be those helmets out there that are defective and crack without crush deformation or impacts of such violence that the result in the same, it certainly sounds like those who should know on a professional basis seem to think that helmets work as designed.
On the consumer end, most will report that a helmet worked and back that up with the observation that it cracked. People here are quick to jump on such anecdotal reports, claiming that because the helmet cracked, they did not work as designed. Which is apparently a very incorrect inference.
My hypothesis is that breakage is reported because it is the most immediately recognizable damage to the helmet by the consumer. Crush damage is much harder to detect or measure, since I really doubt anyone takes too much time examining the 3d curvature of the inside of their helmets before they crash, to compare to deformation after a crash. After a crash, crush damage might be observable, but the head (round, non-uniform curvature) leaving an impression in the inside of a helmet (round, non-uniform curvature) means that such damage would be hard to observe.
So please, all of you with a penchant for dumping on anecdotal reports of cracked helmets as evidence that helmets did not work as designed, stop it. There's a better than even chance you are wrong.