SHORTLY AFTER TAKEOFF FROM ANCHORAGE, THE AIRPLANE FLEW INTO AN AREA OF SEVERE TURBULENCE, WHILE CLIMBING THROUGH AN ALTITUDE OF ABOUT 2000 FEET. THE NUMBER 2 ENGINE AND ENGINE PYLON SEPARATED FROM THE AIRPLANE. THE FLIGHTCREW DECLARED AN EMERGENCY AND THE FLIGHT RETURN TO ANCHORAGE, WHERE AN UNEVENTFUL LANDING WAS ACCOMPLISHED. THE INVESTIGATION REVEALED THAT A STRONG EASTERLY WIND INTERACTED WITH MOUNTAINS EAST OF ANCHORAGE, WHICH PRODUCED MOUNTAIN WAVE ACTIVITY. THE AIRCRAFT ENCOUNTERED SEVERE OR POSSIBLY EXTREME TURBULENCE. THERE WAS EVIDENCE THAT THIS RESULTED IN DYNAMIC MULTI-AXIS LATERAL LOADINGS THAT EXCEEDED THE ULTIMATE LATERAL LOAD-CARRYING CAPABILITY OF THE NUMBER 2 ENGINE PYLON, WHICH HAD ALREADY BEEN REDUCED BY THE PRESENCE OF A FATIGUE CRACK NEAR THE FORWARD END OF THE PYLON'S FORWARD FIREWALL WEB.