Under small deformation, because of weeny bending, the curvature of stretch flexural plane along the coordinate direction can approximatively be denoted by bending w o-w k=一 o-w k=-2 oy Thus the strain components can also be written as 人z kz 人.z 717 Under small deformation, because of weeny bending, the curvature of stretch flexural plane along the coordinate direction can approximatively be denoted by bending : w x y w k y w k x w k xy y x    = −   = −   = − 2 2 2 2 2 2 Thus the strain components can also be written as: k z k z k z xy xy y y x x = = =   