Myth #1: You should replace a dropped carabiner because of undetectable "Micro-Fractures".
This is the biggest myth we know of, and was probably created by unscrupulous salesmen to get you to buy more carabiners.
Truth: In a test by Steve Nagode, an engineer at the REI quality
assurance laboratory, 30 carabiner bodies (half ovals, half D’s) were each dropped six times onto a concrete floor from a height of 33 feet. Following the drops, their open-gate strength was measured and compared to 30 control samples from the same production batch and which had not been dropped. The statistical result showed "no loss
of strength.” Inspect any piece of dropped equipment carefully, checking for proper function. Cast metal products are most
vulnerable to damage, fractures and cracks. To my personal knowledge, this happened once to a gray cast metal Jumar ascender in the 1970's. To my extensive knowledge: Drop forged carabiners (and similar gear) have not exhibited this problem.
Note: OEM Petzl says: 1mm of wear or gouge is serious enough wear or damage to require replacement.