THE AIRCREW RPRTD THAT WHEN THE LANDING GEAR WAS RETRACTED AFTER TAKEOFF, THE L MAIN GEAR INDICATED UNSAFE & THERE WAS AN UNSAFE DOOR INDICATION. THEY RECYCLED THE GEAR, BUT OBSERVED THE SAME INDICATIONS. THE GEAR HANDLE WAS THEN PUT IN THE NEUTRAL PSN & THE L MAIN GEAR EXTENDED TO A DOWN & LOCKED PSN. THE CAPT DECIDED TO RETURN TO MEMPHIS & MAKE A PRECAUTIONARY LANDING. AFTER THE GEAR EXTENSION CYCLE, ALL GEAR INDICATED DOWN & SAFE. ALSO, TWR PSNL CONFIRMED THE GEARAPPEARED TO BE DOWN. HOWEVER, AFTER TOUCHDOWN, THE L WING BEGAN SETTLING, BUT THE AIRCREW KEPT THE ACFT ON THE RWY. AN EMERG EVACUATION WAS INITIATED, BUT THE SLIDE AT THE L FWD DOOR DID NOT INFLATE. AN INVESTIGATION REVEALED THE L GEAR OUTER CYLINDER, PN 65-57901-19, HAD FAILED FROM FATIGUE. A STRESS CORROSION CRACK HAD INITIATED ON THE OUTER DIAMETER OFTHE STRUT CYLINDER WHERE A STRAP HAD OBSCURED THE CORROSION. THERE WAS NO HISTORY OF A HARD LANDING. NO PRESSURE WAS FND IN THE INFLATION BOTTLE FOR THE L FWD DOOR SLIDE. NO REASON WAS FND FOR A LOSS OF PRESSURE.