crumble vs break down

crumble and break down both are verbs.

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

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

Other synonyms of crumble include collapse, crumple, tumble.

crumble (verb)	break down (verb)
break or fall apart into fragments	collapse due to fatigue, an illness, or a sudden attack
fall into decay or ruin	stop operating or functioning
fall apart	separate (substances) into constituent elements or parts
	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