178 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES h n=t/2 h k Ply k h k-1 midplane Ply 2 Ply 1 h o=-t/2 Fig.6.5 Ply coordinates in the thickness direction,plies numbered from the bottom surface. and N,(Figure 6.6);the N are generally termed stress resultants and have the dimension"force per unit length."Elementary equilibrium considerations give M=2a-山N=2o物-山 心=2h-) (6.11) Ny Nx Fig.6.6 Stress resultants.178 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES h n=t/2 hk h k-I hi h 0=-if2 Ply k ,.- y midplane Ply 2 Ply 1 T t 1 Fig. 6.5 Ply coordinates in the thickness direction, plies numbered from the bottom surface. and Ns, (Figure 6.6); the N are generally termed stress resultants and have the dimension "force per unit length." Elementary equilibrium considerations give n Nx = ~ O'x(k)(hk - hk-1), k=l Ns = ~ Zxy(k)(hk - hk- 1) (6.11) k=l Ny = ~ O'y(k)(hk - hk-1), k=l 't_ X Ny l ~ Nxy -1 Fig. 6.6 Stress resultants. Nx