A snap-fit asks a rigid-feeling material to bend on purpose
A snap-fit arm has to flex enough to release, then return to its original shape reliably, which is a very different demand than the rigid strength most FDM parts are designed for. Some materials handle repeated elastic flexing well; others are prone to snapping (rather than flexing) at exactly the strain a snap-fit requires. Geometry matters just as much: a short, stubby arm concentrates strain into a small area and is more likely to crack than a longer, thinner one that spreads the same deflection over more material.
How the check works
Material is rated by general flex-friendliness, TPU and Nylon favorable, PETG and ABS/ASA moderate, PLA generally poor for repeated flexing. Arm geometry is checked as a length-to-thickness ratio, with a stubbier arm (a low ratio) flagged as concentrating strain. Repeated use combined with a less flex-friendly material raises fatigue concern specifically, since a single flex and a thousand flexes are very different tests of the same design.
Worked example
PETG, an 18 mm arm at 2 mm thickness (9:1 ratio), repeated use expected: PETG's moderate flex-friendliness combined with repeated use flags a fatigue concern, while the 9:1 slenderness ratio itself is not flagged as stubby, so the tool recommends printing a test coupon and cycling it by hand to see how it actually holds up before committing the full part.
A common mistake
A frequent error is testing a snap-fit arm only a handful of times by hand and declaring it good, when the real design goal is hundreds or thousands of cycles over the product's life. A few successful flexes prove the arm does not break immediately, not that it will not fatigue and crack after months of normal use; match your test cycle count to the real expected use, not just a quick sanity check.
Limitations
This is a qualitative feasibility check based on material flex-friendliness and arm slenderness, not a strain calculation. It does not predict how many cycles this specific design will survive; print and cycle a test coupon before committing a functional part.