US the assumptions of limit state of collapse and in its 1st assumption it is clearly stated that "plane SECTION normal to the axis remain plane after bending".It means that the strain at any point in the cross-section is proportional to the distance from the neutral axis.So at the neutral axis, the distance is Zero, hence the strain becomes Zero.We can also see that through a diagramHere it is clearly SEEN that at the neutral axis the strain is Zero.In LSM strain diagram is linear having centre at the neutral axisBut STRESS diagram is parabolic below the 0.42 Xu and rectangular above 0.42 Xu [Xu is the depth of neutral axis]