Good for you to try a most difficult plant to grow successfully in our hot summer climate. Artichokes in Zone 8  should be treated as spring vegetables that give you  one or two chokes per plant and then quit when summer sets in. The main villain is the earwig, but aphids, snails and slugs can also be harmful.  Since there is some evidence of chewing in your photo, I would say that some insect (most likely an earwig) has gnawed away and the artichoke's flesh responded by turning the edge of the damage dark (i.e. dead tissue). Now that the heat has set in, it is unlikely that your plant will make any new chokes this year. Keep it  reasonably well-watered over summer;  before frost sets in, cut the stems one foot above ground and bend them over the plant base to protect it. A layer of mulch also helps. Good eating next spring!

There are several products on the market that kill earwigs or you can go the "old farmer traditional" route. Roll up several sheets of newspaper and dampen them. Place next to the plants and leave them overnight. Next morning, without unrolling the paper, put it in a plastic or paper bag, seal tightly and  dump it in the trash. Some folks put a sweet substance  in the roll, but it really isn't necessary. good luck with your chokes and don't forget that they are really seasonal in our