Abstract: An improved knife blade apparatus adapted to be installed in a rupture disk assembly whereby upon failure of the rupture disk in the assembly, the disk is severed by the knife blade apparatus. The apparatus includes an annular support member adapted to be installed in the assembly adjacent the rupture disk having an annular groove and at least two longitudinal grooves intersecting the annular groove disposed interiorly therein. Knife blade means are disposed interiorly of the support member including exterior portions which extend into the grooves in the support member. The exterior portions of the knife blade means are moved by way of the longitudinal grooves in the support member into the annular groove thereof and then within the annular groove to positions away from the longitudinal grooves to thereby removably attach the knife blade means to the support member.