Karabiners and Maillon Links
From time to time these do have structural failures and as you can imagine, depending on when this failure occurs, the severity of the resulting situation could be catastrophic. If you are on the ground you are fine and if you are airborne you are probably in big shit!
Because these links are made of metal we feel they are solid therefore require no thought or maintenance. Actually this is the wrong approach to take. Just like all your other equipment you should inspect them frequently and look for signs of wear and tear, dings or deep scratches. If you can see any form of defect then change them.
We also recommend that you buy a new pair every two years and throw the old ones away (perhaps get them tested to destruction to satisfy your curiosity).
Remember that these are the only link you have to your wing; if something goes wrong with them it could be the last time. They do not cost much and are well worth the expense to gain that extra security.