break down vs crumble

break down and crumble both are verbs.

Word	Noun	Adjective	Verb	Adverb
break down	No	No	Yes	No
crumble	No	No	Yes	No

As verbs, break down and crumble are synonyms defined as:

Other synonyms of break down include collapse, crumple, tumble.

break down (verb)	crumble (verb)
collapse due to fatigue, an illness, or a sudden attack	break or fall apart into fragments
stop operating or functioning	fall into decay or ruin
separate (substances) into constituent elements or parts	fall apart
make a mathematical, chemical, or grammatical analysis of; break down into components or essential features
make ineffective
cause to fall or collapse
lose control of one's emotions
fall apart