I keep seeing comments on posts where people point out that PLA gets brittle when exposed to moisture or UV light, criticizing someone's design/use of PLA or whatever material for their intended application. Sometimes I see people correctly explain that this doesn't matter in that specific applications. For those who aren't sure, I thought I'd make a post trying to explain various loading types and failure mechanisms so people know when it matters and when it doesn't.
Tl;dr at the bottom
Brittle materials fail due to maximum normal stress (stress is aligned with a major axis of a load bearing geometry), whereas ductile materials fail due to shear stress (stress is misaligned/diagonal to a major axis of a load bearing geometry). Additionally brittle failure is caused by tension (pulling), not compression. If the compression is large enough, this can cause tension along one of the axes that is perpendicular to the compressive load, but generally, compression is not the issue for brittle materials.