Mechanism of sealed postcards is discussed based on contact mechanics, i.e., elastic adhesion theory. Their adhesive interface is assumed as a semi-infinite elastic body contacting on a rigid body which has a two-dimensional sinusoidal surface roughness. Their equilibrium state is obtained as a function of the external pressure and the surface roughness using material constants of the postcards. It is found that after the initial line contact the interface is snaped to the area contact. There exists a critical external pressure which snap the interface from the partial contact to the perfect contact. After the perfect contact, the interface can not return to the area contact, even if the tensile external stress is applied. The necessary condition for the function of the sealed postcard is expressed using the parameter of surface roughness and the material constants.

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In the context of the Global Technical Regulations (GTR) aimed at harmonizing safety standards for composite pressure vessels, discussions on safety margins are actively conducted. However, in order to control safety margins rationally, it is inevitable to incorporate probabilistic considerations. For the future establishment of safety factor settings based on rational justifications, it is considered essential to first establish a solid probabilistic foundation.
Particularly, in cases such as GTR standards where safety assessments for burst strength are conducted based on small samples, such as three or five samples, it is unavoidable to address the uncertainties associated with small-sample treatment. Specifically, when discussing safety factors based on sample data, consideration of both reliability and confidence levels is necessary¹⁾²⁾ . This perspective is crucial in the introduction of reliability design, and the author has already compiled introductory content and challenges in an explanatory article³⁾.
In this article, emphasis is placed on the treatment of probabilistic safety factors, and explanatory additions are made to contribute to the discussion, referencing the approach of G.W. Mair⁴⁾.

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