Shinai can sometimes break right away even if you buy one brand new.
Did you know that there is a way to pick out a durable shinai?
If you look carefully at the surface of a shinai, you can see lots of lines of fiber.
Try looking for a shinai with all the lines going straight towards the 'kensaki' (tip of the shinai).
Those shinais' bamboos can diverse the impact evenly. So they are more durable compared to the one with curving fiber lines.
Try checking your shinais!
**Shinai with fiber lines that are not straight does not mean it is defected.
**Shinai may still break depending on the humidity and/or practice intensity.