Using 3-Axis Digital Accelerometer to Measure Rotational Acceleration in a Helmet?
In general, linear accelerometers are specifically designed to be insensitive to rotation. However, if you mount an accelerometer away from the axis of rotation, it will provide an indication. You can convert this indication into an angular acceleration measurement only if you know the exact geometry of the rotation. In a sports helmet application, this seems unlikely.

If you combine the accelerometer with a 3-axis rate gyro, you'll be able to combine the measurements in order to come up with a complete picture of the motion of the helmet, in terms of both translation and attitude (rotation). From this model, you will be able to determine both the angular acceleration(s) AND where the center of rotation is.