fatigue crack vs fissure

fatigue crack and fissure both are nouns.

Word	Noun	Adjective	Verb	Adverb
fatigue crack	Yes	No	No	No
fissure	Yes	No	Yes	No

As nouns, fissure is a hypernym of fatigue crack; that is, fissure is a word with a broader meaning than fatigue crack:

Other hypernyms of fatigue crack include cleft, crack, crevice, scissure.

fatigue crack (noun)	fissure (noun)
a crack in metal resulting from metal fatigue	(anatomy) a long narrow slit or groove that divides an organ into lobes
	a long narrow cleft
	a long narrow depression in a surface

fatigue crack (verb)	fissure (verb)
	break into fissures or fine cracks