Engaged in mechanical design of reading a doctorate is useful, engaged in what research direction is good

It depends on your own choice. If you have the conditions, you should go to read. Mechanical professional academic research direction is not the general impression of the traditional mechanical. There are doing mechanics, nanomaterials, MEMS, sensors, non-destructive testing, composite materials, explosion mechanics, optics-related (really do not know what they do), Vibration and Dynamic Control (vibration and control?) etc. They also do fluids and combustion related stuff.