## 3 Results and Discussion
### Strain Contour Comparison  
The strain contours obtained from the finite element analysis can be directly compared with those from Digital Image Correlation (DIC) to validate the numerical model against experimental observations. In Figure 17a,b, the dashed black outline indicates the overlap region, which is the critical zone where the adhesive is bonding the two adherends. Although the 0.5 mm and 3 mm adherend configurations exhibit different absolute strain levels due to variations in stiffness and load distribution, their strain distributions remain almost the same because both show similar longitudinal gradients and regions of elevated strain near the overlap edges. Additionally, the same color bar scale has been applied across these plots to facilitate visual comparison; thus, greens and blues correspond to comparable  
Figure 16: Adhesive mid-thickness distribution for the (**a**) numerical peel; (**b**) numerical shear; (**c**) analytical peel; (**d**) analytical shear stress; for 3 mm thick adherend at a load of 2 kN in the elastic region. strain ranges for each thickness. As a result, while the thicker adherend case may show lower peak values than the thinner one, the overall pattern of increasing strain toward the overlap region is analogous in both figures, confirming that the single-row cohesive element model captures the fundamental deformation behavior in each scenario. From Figure 17a,b, a simple analytical estimate for strain (Load \(\times\) Length/Area \(\times\) Modulus) yields approximately 1.12 \(\times\) 10\({}^{-3}\) mm/mm, which can be seen across the adherend surface of the two different thicknesses. The same applies to (c) and (d), where the same calculation yields 2.4 \(\times\) 10\({}^{-3}\) mm/mm. Although these single-lap joints have an overlap region where the  
Figure 17: Strain contour comparison at the overlap (of 25 mm overlap adherend surface at (**a**) \(\delta\) = 0.12 mm for 0.5 mm thick adherend; (**b**) \(\delta\) = 0.12 mm for 3 mm thick adherend; (**c**) \(\delta\) = 0.3 mm for 0.5 mm thick adherend; (**d**) \(\delta\) = 0.3 mm for 0.5 mm thick adherend. The dashed black outline indicates the overlap region. adherennds are effectively double their thickness, this analytical expression assumes a uniform cross-sectional area along the axial direction. The total length of the joint is significantly larger than the overlap zone, and the local stiffening effect of the double-thickness region is relatively small in comparison to the overall specimen length; using the single-thickness cross section still provides a reasonable first-order approximation of the average strain. The surface variation is due to the eccentric load path, meaning lower strains can be noticed at the lower part of the overlap region. In contrast, high strain can be observed in the middle/upper region of the overlap area. Moreover, the color scale in these contour plots is normalized so that the green band corresponds approximately to the nominal strain predicted by the analytical estimate, making it easier to distinguish regions of higher strain than the expected elastic value. Collectively, these observations align with analytical considerations of single lap joint behavior, wherein the eccentricity amplifies local bending near the overlap edges, and any onset of yielding redistributes the load more evenly once plastic deformation sets in. Although all the strain contours presented in Figure 18 are derived from the finite element analysis, these numerical results can still guide and enhance the interpretation of DIC data from physical experiments. Since DIC primarily captures surface strains, it can miss important information within the adhesive layer or at the interface between the adhesive and the adherends. By contrast, the FEA enables virtual sectioning of the joints at different through-thickness locations, illuminating sub-surface strain distributions that remain hidden from optical measurement methods.