The terrific EADS/Astrium press kits contain the following description of the H175 EPC.
"Type C main cryogenic stage:
The EPC is over 30m high. It has a diameter of 5.4m and an empty mass of only 14.1
metric tons. It essentially comprises:
- large aluminium alloy tank,
-thrust frame transmitting engine thrust to the stage,
- forward skirt connecting the EPC to the upper composite, and transmitting the thrust
generated by the two solid propellant boosters."
Note that the forward thrust frame (the "JAVE") won't need to be as robust for Liberty, suggesting mass savings. Offsetting this would be structural strengthening of, probably, the LH2 tank.
Everything I've seen tells me these are aluminum tanks covered in spray on foam insulation.
Here is one page from one of the best detailed references on EPC construction. It tells us that Aluminum alloy 2219 is used for the tank walls. The thrust frame is mostly aluminum, but also includes some composite elements.http://www.capcomespace.net/dossiers/espace_europeen/ariane/ariane5/production_ariane5_EPC.htm
- Ed Kyle