A CITABRIA ON LNDG HAD THE L MAIN GEAR COLLAPSE INCURRING DAMAGE TO THE PROP AND L WINGTIP. CAUSED BY A FAILED LIFE LIMITED BOLT.

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AFTER DROPPING THE ONLY BANNER FOR THE DAY AT THE E END OF THE RWY AT THE ARPT; I CIRCLED WITH A L-HAND PATTERN FOR LNDG TO THE W ON THE E/W PVT STRIP. THE WINDSOCK SHOWED VERY LITTLE MOVEMENT; BUT INDICATED APPROX 20 DEGS L OF THE RWY CTRLINE. I USED FULL FLAPS SINCE THE WINDS WERE SO FAVORABLE. AS USUAL; I EXTENDED THE FINAL AS NECESSARY TO CLR THE BANNER NOW LYING IN THE RWY. I FLARED FOR LNDG; ARRESTING A SINK RATE THAT WAS HIGHER THAN USUAL. THE TOUCHDOWN WAS IN A 3-POINT ATTITUDE AND MORE FIRM THAN I USUALLY DID. AFTER THE WHEELS CONTACTED; I HEARD A NOISE AND FELT A SHARP 'BUMP' IN THE L FLOOR OF THE ACFT (THE VICINITY OF THE L MAIN GEAR ATTACH POINT). THE ACFT BALLOONED UP; AND AS IT SETTLED I USED A COMBINATION OF R AILERON; R BRAKE; RUDDER; AND ELEVATOR TO HOLD THE L WING UP AS LONG AS PRACTICAL; AND TO KEEP THE ACFT MOVING RELATIVELY STRAIGHT. AS IT SLOWED AND SETTLED; THE NOSE BEGAN TO PITCH DOWN; STOPPING THE ENG AND THREATENING FLIPPING OVER ON ITS NOSE. I ADDED BACK ELEVATOR AND RELEASED THE R BRAKE. THE ACFT THEN PIVOTED ABOUT 45 DEGS TO THE L AS IT STOPPED AND SETTLED. IT NOW RESTED ON THE R MAIN AND TAILWHEEL; WITH THE L SIDE BEING SUPPORTED BY THE L WINGTIP; THE L WHEEL; AND THE INBOARD END OF THE L GEAR LEG; NOW BURIED IN THE SOFT GND. THERE WAS NO EVIDENCE OF FIRE; SO I SECURED ALL ELECTRICAL SWITCHES; MIXTURE CTL; AND FUEL SHUTOFF VALVE BEFORE EXITING NORMALLY THROUGH THE DOOR ON THE R. A SHORT WHILE LATER; I WAS CURIOUS AS TO WHAT BROKE; CAUSING THE PROB. I DUG UNDERNEATH THE L MAIN ATTACH POINT AND SAW THE BOLT HOLE AT THE INBOARD END OF THE GEAR LEG. I THEN WALKED BACK TO THE TOUCHDOWN POINT WHERE I PICKED UP THE PIECE OF THE BROKEN BOLT; MARKED 'MS 20007;' ASSUMED TO BE THE L MAIN INBOARD ATTACH BOLT. ON THE BROKEN END; 1/2 OF THE SHAFT WAS IRREGULAR AND JAGGED WITH NO DISCOLORATION IN THE METAL. THE OTHER HALF WAS MOSTLY FLAT; WITH CONCENTRIC SEMI CIRCLES RADIATING FROM A POINT ON THE EDGE OF THE SHAFT. THIS PART WAS ALSO DARKER; APPEARING TO BE SLIGHTLY CORRODED FROM A PREVIOUS CRACK. THE OTHER PART OF THE BOLT WAS LATER SEEN STILL FROZEN IN PLACE IN THE STEEL STRUCTURE OF THE FUSELAGE; HAVING BROKEN WHERE THE GEAR LEG AND FUSELAGE STRUCTURE MEET. POSSIBLE CONTRIBUTING FACTORS: 1) THE HARDER THAN NORMAL LNDG. THIS WAS OBVIOUSLY A CONTRIBUTING FACTOR. THE BOLT IN QUESTION REQUIRED A HIGHER THAN NORMAL STRESS TO CAUSE IT TO FAIL. IT JUST WAS NOT THAT HARD. 2) THE CRACKED BOLT: THIS WAS THE PRIMARY FACTOR. EVEN A PERSON UNKNOWLEDGEABLE ABOUT METALS WOULD AGREE THAT A BOLT CRACKED 1/2 WAY THROUGH WOULD ONLY HAVE 1/2 THE STRENGTH. BUT ANY ENGINEER WHO UNDERSTANDS THE EFFECTS OF CRACKS WOULD CONFIRM THAT THIS COMPROMISED PART HAD MUCH LESS THAN 1/2 OF ITS DESIGNED STRENGTH. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE LNDG COULD NOT BE CONSIDERED A HARD LNDG AND THE PRIMARY CAUSE OF THIS INCIDENT WAS THE FRACTURED L LNDG GEAR INBOARD BOLT. THE RPTR SAID THE MANUFACTURER ISSUED AN ALERT BULLETIN ON THIS BOLT AS BEING LIFE LIMITED TO 500 HRS. THE RPTR SAID A MAINT HISTORY SEARCH OF THE LOGBOOK INDICATES THE BOLT WAS REPLACED AT THE FIRST 500 HRS BUT NO RECORD OF SUBSEQUENT BOLT REPLACEMENTS. THE RPTR STATED THE AIRPLANE HAD 1300 HRS TOTAL TIME AND HAD EXCEEDED THE MANUFACTURER'S ADVISORY BY 300 HRS. THE RPTR SAID THE FAA WAS CONTACTED AND RULED THE EVENT AS AN INCIDENT.