The bricks are the actual strength in a chimney. The mortar within the bricks acts more like a glue to hold the bricks together. Mortar in itself varies in strength anywhere between 750psi to 2500 psi. The strength of average concrete bricks however, varies between 4,000psi to 6,000psi, and clay bricks vary from 8,000psi to 10,000 psi.
When Vertical Cracking is found through bricks, it means that the structure split. When repairing, most masons grind open the cracks (which only accelerates the damage caused by freeze-thaw) and fill the crack with mortar. This is an incorrect method, because the repair does nothing for the strength of the chimney, and will crack and delaminate within just a few years.
Stitching is the best repair method in this situation. Stitching is similar to when we have a wound that requires stitches. Stitching consists of grinding out the horizontal mortar joints every couple of courses of bricks depending on the size of the crack. Steel rods are then embedded in horizontal joints within the pointing mortar, to help reinforce the structure and hold everything together.