Omni-directional angular acceration reduction for protective headgear

Abstract

Protective headgear includes an outer shell rotatable relative to an inner shell via a thin layer of elastically and/or plastically yielding material disposed between and secured to the inner and outer shells. The yielding material deforms continuously but non-linearly at least in the tangential or shear direction to mitigate angular acceleration of the head during an impact. The yielding material prevents the inner and outer shells from separating or stopping suddenly to avoid imparting a large angular acceleration to the head. In one embodiment, a protective helmet includes an outer shell secured to an inner shell by a yielding material that elastically deforms continuously but non-linearly when subjected to an angular acceleration below a first threshold and plastically deforms when subjected to an angular acceleration above the first threshold.

1 . A helmet comprising:
 an inner shell;   an outer shell generally surrounding the inner shell;   a thin layer disposed between, and secured to, the inner shell and the outer shell, the thin layer having a non-linear rate-dependent shear characteristic such that the thin layer deforms during impact so the outer shell is displaced relative to the inner shell to reduce angular acceleration imparted to a head of a user.   
     
     
         2 . The helmet of  claim 1  wherein the inner shell comprises an energy absorbing material. 
     
     
         3 . The helmet of  claim 1  wherein the thin layer comprises an elastically deformable material. 
     
     
         4 . The helmet of  claim 1  wherein the thin layer comprises an elastically deformable material having a shear stiffness that increases non-linearly as a function of increasing strain/deformation. 
     
     
         5 . The helmet of  claim 1  wherein the thin layer deforms continuously but non-linearly with increasing stiffness as deformation increases. 
     
     
         6 . The helmet of  claim 1  wherein the thin layer is generally isotropic in the shear direction such that the outer shell rotates with respect to the inner shell regardless of the direction of impact or location of impact. 
     
     
         7 . The helmet of  claim 1  wherein the thin layer comprises a material having shear properties such that relative rotation between the inner and outer layers begins at a desired torque corresponding to a tightness of fit for a particular type of helmet. 
     
     
         8 . The helmet of  claim 7  wherein resistance to rotation increases as a function of extent of rotation. 
     
     
         9 . The helmet of  claim 1  wherein the thin layer comprises a deformable yielding material that deforms elastically when angular acceleration is below a first threshold, but plastically when angular acceleration exceeds the first threshold. 
     
     
         10 . The helmet of  claim 9  wherein the first threshold is about 6000 radians per second squared. 
     
     
         11 . The helmet of  claim 1  wherein the thin layer comprises a yielding material that prevents the inner and outer shells from stopping suddenly to avoid imparting a large angular acceleration to the head. 
     
     
         12 . The helmet of  claim 1  wherein the thin layer comprises a material having an isotropic rate-dependent shear stiffness that increases non-linearly as deformation approaches a separation limit of the inner and outer shells by relative rotation. 
     
     
         13 . A method for mitigating angular acceleration imparted through protective headgear to a user, the method comprising:
 securing an inner shell of the headgear to an intermediate layer;   securing an outer shell of the headgear to an opposite side of the intermediate layer;   wherein the intermediate layer is a yielding material having a shear stiffness that increases nonlinearly with increasing relative displacement of the inner and outer shells due to increasing deformation of the yielding material.   
     
     
         14 . The method of  claim 13  wherein the intermediate layer comprises a material having a rate-dependent shear stiffness. 
     
     
         15 . The method of  claim 13  wherein the intermediate layer comprises a yielding material that elastically deforms continuously but nonlinearly. 
     
     
         16 . The method of  claim 13  wherein the intermediate layer is a yielding material that elastically deforms continuously but nonlinearly when subjected to an angular acceleration above below a first threshold and plastically deforms when subjected to an angular acceleration above the first threshold. 
     
     
         17 . The method of  claim 16  wherein the first threshold is about 6000 radians per second squared. 
     
     
         18 . The method of  claim 13  wherein the intermediate layer is isotropic with respect to the shear stiffness. 
     
     
         19 . A helmet comprising:
 an inner shell;   an outer shell surrounding the inner shell;   an intermediate layer disposed between and connected to the inner shell and the outer shell, the intermediate layer comprised of a substantially isotropic yielding material that deforms continuously, nonlinearly, and with a rate dependent shear characteristic such that the intermediate layer deforms during impact to allow rotation of the inner shell relative to the outer shell to reduce angular acceleration imparted to a head of a user.   
     
     
         20 . The helmet of  claim 19  wherein the intermediate layer elastically deforms in response to an acceleration up to about 6000 radians per second squared and plastically deforms in response to an acceleration exceeding about 6000 radians per second squared.