Nowadays it is a common practice for different clubs, departments, businesses, colleges, and schools to have their own customize t-shirts. Having a custom t-shirt for a group is one way of identifying if a person studying/working in a particular group or is a member of any specific organization.
A customized t-shirt talks a lot regarding your character, your job position and reflects the fashion style in you. You can find many online stores where you can make the customized t-shirt design and you can make a purchase.
These are the few decent t-shirt designs related to mechanical engineering communities, I have found while surfing the web.