What are the three primary methods for testing the bond strength of adhesives? Shear, tensile, and peel strength.

Shear strength and tensile strength refer to the maximum load-bearing capacity of an adhesive joint when subjected to forces acting parallel or perpendicular to the bonded interface, respectively. While shear strength tests for adhesives are most commonly performed using a single-lap joint configuration, the double-lap shear test—as specified in the ASTM D3528 standard—allows for a more balanced distribution of forces, thereby minimizing the influence of peel and cleavage stresses often present in single-lap tests. In practice, the tensile and shear strengths of adhesives are typically evaluated together.
Peel strength measures an adhesive's resistance to a 180-degree peeling force, a test primarily applied to joints between flexible and rigid materials. Key factors influencing peel strength results include the distribution of stress, the peel angle, and the thickness of the adhesive layer. When determining the peel strength of adhesives used to bond metallic materials, a "non-uniform" (or uneven) tensile test method is generally employed.
Bond strength is critical in determining how reliably an adhesive can join two or more materials—whether they are identical or dissimilar. Shear strength, tensile strength, and peel strength serve as the three primary metrics for assessing bond strength, effectively characterizing the mechanical performance of an adhesive. Consequently, a thorough understanding of these three properties is highly beneficial for both the production and application of adhesives.