In this research, we explore the mechanical stability and interfacial adhesion of aluminum-coated polyethylene terephthalate substrates under tensile strain, a structure commonly used in flexible sensor platforms. Utilizing physical vapor deposition, we applied thin aluminum films and subjected them to tensile strains of 10, 15, and 20%. The resulting mechanical deformations, such as shear-lag-induced cracks and Poisson-effect-induced lateral compressive forces, were analyzed by digital microscopy and scanning electron microscopy. A buckling-delamination model based on Euler’s critical load theory was used to quantify lateral compressive forces and predict interfacial failure. Results showed that regions with small buckling lengths and crack widths demonstrated high compressive forces per unit area, correlating with improved adhesion. The average compressive force per unit area was calculated as 2.7 × 10−13 N/μm2. These insights offer a predictive framework for assessing and enhancing the reliability of flexible sensor coatings under mechanical stress, crucial for the development of robust, stretchable, and wearable sensor technologies.