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The front shell structure resists the external load primarily through the medium surface stress that distributes uniformly across its entire thickness dimension, rather than relying on the bending stress that varies progressively along the thickness as found in conventional plate structures. This distinctive internal force distribution characteristic inherent to shell geometries enables the material to achieve more comprehensive utilization of its available strength properties compared to flat plate configurations, thereby resulting in substantially greater bearing capacity and load-carrying performance.

Shell theory functions as a specialized domain within applied elasticity mechanics, requiring the introduction of supplementary hypotheses that extend beyond the fundamental assumptions underlying classical elasticity theory.

Thin shell theory represents a foundational framework within structural mechanics and engineering analysis, serving as a classical approach to understanding the behavior of curved surfaces under loading conditions. This theoretical paradigm is fundamentally constructed upon the assumption of direct normals, which establishes a set of mathematically rigorous postulates that govern the mechanical behavior of shells across various industrial applications ranging from aerospace fuselage design to pressure vessel engineering and architectural dome structures. The first of these core assumptions addresses the strain behavior perpendicular to the midplane direction, positing that positive strain occurring in this transverse orientation is extremely small in magnitude and therefore can be reasonably neglected in subsequent calculations and stress analyses without introducing significant computational error. The second fundamental assumption concerning the middle normal establishes that it maintains its straight geometric configuration throughout the deformation process, and critically, the right angle relationship between the middle normal and its associated vertical line segment remains preserved and unchanged, which mathematically translates to the condition that shear strain in both orthogonal directions equals zero, a simplification that proves computationally advantageous in solving complex shell problems. The third assumption addresses normal stress distribution across sections parallel to the middle plane, specifically recognizing that the extrusion stress acting on these parallel sections is substantially less in magnitude than the normal stress developed on vertical planes, and consequently the influence of this relatively minor stress component on the overall deformation characteristics can be considered negligible for practical engineering purposes.