FLT INSTRUCTOR AND STUDENT NARROWLY AVOID RWY EXCURSION WHEN LEFT MAIN WHEEL ASSEMBLY MISALIGNS 90 DEGREES DUE TO STRUCTURAL FAILURE.

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I TOOK OFF WITH A STUDENT FROM ST AUGUSTINE (SGJ) FOR A TRAINING FLT. THE STUDENT WANTED TO PRACTICE TOUCH-AND-GOES AT AN UNFAMILIAR ARPT. I ELECTED TO GO N TO CRAIG DUE TO IMC AT MOST OF THE ARPTS TO THE W. THE TKOF; CLB AND CRUISE TO CRAIG WERE ALL NORMAL. I CONTACTED THE TWR 8 MI S OF THE ARPT; ADVISING THAT WE WERE INBOUND FOR PATTERN WORK. WE WERE TOLD TO RPT A 3 MI FINAL; AND AT 3 MI WE WERE GIVEN A TOUCH-AND-GO CLRNC. THE APCH TO THE RWY APPEARED NORMAL. UPON TOUCHDOWN THE AIRPLANE VEERED TO THE L AND THE BRAKES LOCKED UP. I TRIED TO STEER USING THE RUDDERS TO AVOID GOING OFF THE RWY; BUT LOST ALL DIRECTIONAL CTL OF THE ACFT. I WAS ABLE TO GET THE ACFT STOPPED ABOUT 6 INCHES FROM THE SIDE OF THE RWY. AFTER STOPPED; I INFORMED THE CTL TWR OF THE SITUATION. THE RWY WAS THEN CLOSED; WITH THE NUMEROUS ACFT ON FINAL SENT AROUND. AFTER ABOUT 30 MINS; A TUG WAS SENT TO TOW THE ACFT OFF THE RWY. IT WAS DETERMINED A PART ON THE L STRUT HAD SNAPPED IN HALF; WHICH HAD CAUSED THE ENTIRE WHEEL TO TURN ALMOST 90 DEGS TO THE L. ON THE PREFLT I HAD NOT NOTICED ANY DEFECTS IN THE LNDG GEAR/STRUT. I DO NOT BELIEVE THAT THERE WAS ANY WAY I COULD KNOW THAT THE LNDG GEAR STRUT WAS DEFECTIVE. IN THE FUTURE; I PLAN TO DO A THOROUGH PREFLT OF THE ACFT; AND NOT RELY SO MUCH ON THE STUDENT TO DO THE PREFLT.