Engineering properties of the adhesive

To analyse the stresses in adhesive joints requires a knowledge of the basic engineering properties of the adhesive. Typically, the main properties required are the tensile, or Young s modulus (Ea), the shear modulus (Ga) and the yield 

The second approach to determining basic engineering properties of the adhesive has been to employ specific joint geometries, where ideally (i) the stress state is simple, (ii) no stress concentrations are present and (iii) the 

The napkin-ring test (see Table 6.1 and Section 6.4.8) more nearly 

Adhesivet Modulustt, Ga(GPa) Yield stress, Tay (MPa) Fracture stress, Taf(MPa) Fracture strain, 

Thickness of adhesive layer from adhesive/alumlnium alloy interface (mm) 

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An adhesive is a material capable of holding together solid materials by means of surface attachment. Adhesion is the physical attraction of the surface of one material for the surface of another. An adherend is the solid material to which the adhesive adheres and the adhesive bond or adhesive joint is the assembly made by joining adlierends together by means of an adhesive. Practical adhesion is the physical strength of an adhesive bond It primarily depends on the forces of adhesion, but its magnitude is determined by the physical properties of the adhesive and the adherend, as well as the engineering of the adhesive bond. 

To facilitate the inclusion of bonded joints within an engineering structure, several stages of suitability must be demonstrated. First the mechanical properties of the adhesive itself must be satisfactory and these are quantified by bulk materials tests. Once knowledge of the material itself is gained, then some idea of how well it performs in a bonded joint is required. This is most often achieved via standard test specimens such as the single or double lap joints. 

The reliability of die attach adhesive has been extensively investigated for the past three decades. A complete survey of these studies has been previously published [4]. The following sections underline the critical factors that have an impact on the electrical, mechanical, and thermal properties of the adhesive bonds. For process engineers, the main concern is the evaluation of die attach integrity in relation with the assembly technologies and after a series of accelerated ageing tests. 

The engineering properties of the substrate materials may usually be found from handbooks of mechanical properties, although in the case of the fibre-composite materials, which possess of course anisotropic properties, some of the data may have to be determined by the adhesives technologist ... 

However, there are still many areas where further work is required. There is a need to confirm that the basic engineering properties that are currently being measured by different methods are yielding similar values and that when tests on the bulk adhesive are undertaken that they give representative properties of the adhesive as a thin layer in a joint. There is also a need to model the complete stress distributions in complex joint designs more readily and accurately, and to be able to undertake conveniently parametric design and fracture prediction studies. In connection with this last requirement, the failure criteria necessary for predicting joint fracture have yet to be established firmly. 

When a designer is selecting an adhesive for a specific application, the engineering properties of the individual plastic will be considered carefully. All too often, however, the data supplied by the plastic manufacturer will include melting point, mould shrinkage, tensile modulus, hardness, dielectric properties, water absorption, density and thermal conductivity but almost never the surface energy of the plastic, which is one of the key properties required for the adhesive application engineer. 

For industrial applications to be successful, the engineer should be aware of the various types of adhesives available and of their properties, and also of the new design philosophy to be adopted in such bonded structures, and a portion of the book is aimed at highlighting the properties of the adhesives and their suitability and at assisting the engineer in his choice of the proper adhesive for the job at hand. 

Table 1 contains the metal-to-metal engineering property requirements for Boeing Material Specification (BMS) 5-101, a structural film adhesive for metal to metal and honeycomb sandwich use in areas with normal temperature exposure. The requirements are dominated by shear strength tests. Shear strength is the most critical engineering property for structural adhesives, at least for the simplistic joint analysis that is commonly used for metal-to-metal secondary structure on commercial aircraft. Adhesive Joints are purposefully loaded primarily in shear as opposed to tension or peel modes as adhesives are typically stronger in shear than in Mode I (load normal to the plane of the bond) loading. 

Parylene C film could be utilized in system approach interface engineering (SAIE) if the adhesion of Parylene C film to the substrate and the adhesion of a eoating, whieh is applied on the surface of Parylene C, could be improved. The seheme of utilizing Parylene C film in SAIE is depicted in Figure 30.7. Methods to improve its adhesion have been developed, but improvement is limited due to the lack of specific chemical interactions in the interfaee [13]. Low-temperature plasma deposition has proved to be a very effeetive proeess in improving the adhesion properties of materials while maintaining their desirable bulk properties. 

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