Originally Posted by
mconlonx
Can anyone say, with studies to back it up, that cracked helmets have not deformed as intended, absorbing forces during a crush phase? Breakage does not necessarily imply failure; your failure is to infer failure on the part of cracked helmets.
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."