Shot peening is a process of inducing RESIDUAL COMPRESSIVE STRESS on the surface layer of metal parts. When shooting particles are colliding on the surface of metal there is a compressive stress. My question is, how can resultant compressive stress exists after the force causing the compression is removed (i.e. after the shooting particle leaves the surface)? Normally we need a force to hold the object in compressive state. For residual stress, are there any parts in tension to balance this stress?