Influence of adherend properties on the strength of adhesively bonded joints
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troduction Adhesively bonded joints are increasingly becoming an alternative to mechanical joints in engineering applications and provide many advantages over conventional mechanical fasteners. Among these are lower structure weight, lower fabrication cost, and improved damage tolerance.1–3 Design flexibility and joining of dissimilar or new advanced materials are also benefits of adhesive joints. Most of today’s industrial structures consist of combinations of different materials (e.g., steel, aluminum, magnesium, fiber-reinforced plastics, and sandwich structures) in order to provide the best performance. Adhesives can be used to join metals, polymers, ceramics, cork, rubber, and combinations of any of these materials.4 However, these materials need to be joined according to their specific characteristics. For example, metal–plastic combinations are used in structures to combine the high specific stiffness and strength of metals with the high degree of design freedom of polymer composites. Different materials and joint geometries give rise to distinct stress states in the adhesive layer, and the performance of adhesive joints depends on several factors (e.g., the joint geometry and the mechanical properties of the adhesive and adherends). There are a wide variety of joints available to the designer. Common joint configurations used in structures include single-lap joints (SLJs), double-lap joints, scarf joints, and stepped-lap joints (Figure 1). Many other configurations exist, with specific advantages, such as peel joints,
the joggle-lap joints and L-section joints among others.1 However, the SLJ is the most common joint used and studied today mainly due to its simplicity and efficiency. The SLJ (Figure 2) consists of two rectangular adherends bonded together by an adhesive. End (alignment) tabs, cut from the same material as the adherends, are often adhesively bonded to the specimens to reduce (not eliminate) the eccentricity of the load path. Thus, due to the load misalignment, which occurs even when alignment tabs are used, and due to the differential adhesive straining effect,5 the adhesive is subjected to a state of nonuniform shear and peel stresses (Figure 2b–c). Tension stresses arise in the adhesive due to peel, cleavage, or tensile loading of joints and can also occur in shear loaded joints due to bending moments resulting from the eccentricity of the loading, illustrated for single-lap joints in Figure 2c. These stresses result in failure of the adhesive before the shear stresses are fully developed so that the theoretical maximum joint strength is not attained. In addition, the bending moments and tension stresses may result in yielding in metallic and thermoplastic substrates, which may also limit joint strength. Peel strength can be described as a material’s ability to resist forces that can pull or peel it apart at a predetermined angle and rate. The ASTM D1002 standard recommends reporting the results of SLJ tests as the average shear stress at failure (maximum load divided by bond area).6 This ar
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