If you're talking about ritual suicide to avoid the dishonour of being captured by an enemy, then no. Not in the middle ages at least. Knights were mostly happy enough to be captured alive and be ransomed back to their families.

In Feudal Japan, it was considered honourable to commit suicide rather that be defeated but Christian Europeans viewed most forms of suicide (apart from altruistic self sacrifice) as a mortal sin one could go to hell for. As a result, it was better for a knight to die fighting, run away or get captured. Peasant soldiers would usually just be killed as there wasn't much to be gained from ransom.

Military officers in later periods from the 1600s to the 1900s have been known to commit suicide after defeat by an enemy usually to redeem the shame of losing but there was no standard method or ritual around it. Many would just blow there brains out with a pistol or kill themselves by more conventional means. Cavalry officers might stab themselves with their sabers as well but again as an impulsive method of suicide where the victim just happens to have a weapon handy.