I've had that happen before too - was it the new one that cracked the second time or the other? Those eyes are susceptible to a lot of cycle loading and prone to fatigue at the weld. Dye crack detection methods are a good idea...or just spend the $30 to replace them both yearly.

With the dye, you spray on the dye, let it sit for a bit, then clean the part off (rag / towel / etc). Then you hit it with a solvent / developer and if you see dye appear in an area, there was a tiny crack it was hiding in. It's tough to use on a weld because the weld surface is usually pretty irregular and hard to get all the dye off to begin with. The better method is magnaflux - but it doesn't work on non-ferrous materials (stainless).