A crack line may not justify for replacement... it depends on whether the performance has been affected.
If, during a stipulated round, a player’s club is damaged in the normal course of play, he may:
(i) use the club in its damaged state for the remainder of the stipulated round; or
(ii) without unduly delaying play, repair it or have it repaired; or
(iii) as an additional option available only if the club is unfit for play, replace the damaged club with any club. The replacement of a club must not unduly delay play and must not be made by borrowing any club selected for play by any other person playing on the course.
PENALTY FOR BREACH OF RULE 4-3a
See Penalty Statements for Rule 4-4a or b, and Rule 4-4c.
Note: A club is unfit for play if it is substantially damaged, e.g. the shaft is dented, significantly bent or breaks into pieces; the clubhead becomes loose, detached or significantly deformed; or the grip becomes loose. A club is not unfit for play solely because the club’s lie or loft has been altered, or the clubhead is scratched.