Assembly considerations structural joints

The paper is presented in three parts. First, the tests employed to determine the mixed mode fracture envelope of a glass fibre reinforced epoxy composite adhesively bonded with either a brittle or a ductile adhesive are briefly described. These include mode I (DCB), and mixed mode (MMB) with various mixed mode (I/II) ratios. In the second part of the paper different structural joints will be discussed. These include single and double lap shear and L-specimens. In a recent European thematic network lap shear and double lap shear composite joints were tested, and predictions of failure load were made by different academic and industrial partners [9,10]. It was apparent that considerable differences existed between different analytical predictions and FE analyses, and correlation with tests proved complex. In particular, the progressive damage development in assemblies bonded with a ductile adhesive was not treated adequately. A more detailed study of damage mechanisms was therefore undertaken, using image analysis combined with microscopy to examine the crack tip strain fields and measure adherend displacements. This is described below and correlation is made between predicted displacements and failure loads, based on the mixed mode envelope determined previously, and measured values. 

A snap joint is economical in two respects it allows the structural member to be molded simultaneously with the molded product, and it allows rationalizing the assembly, compared with such other joining processes as screws. Table 3-4 provides a comparison of its advantages and disadvantages. Some examples of the various types and their design considerations are shown in Figs. 3-23 to 3-25. 

Recycling and end-of-hfe management of marine structures are topical issues. Adhesively bonded joints assembled using thermosetting adhesives are, like welding, not easy to disassemble, and alternative thermoplastic adhesives may become more popular. There are also some bio-sourced adhesives available, but to date their properties, particularly after inuner-sion, are considerably lower than those of conventional adhesives. It should be noted that the quantities of adhesives used in marine structures, such as boats, are significantly smaller than the amounts of composites, and the recycling of the latter is currently the main priority. 

Load and mode of loading. Whatever the load, co-axial joints are usually best assembled with anaerobic adhesives unless special circumstances demand otherwise. By contrast, there are many candidates for a lap joint based assembly. A crude, but fairly effective, means of selecting which types may be used is based on the anticipated load. If only nominal loads are to be borne any adhesive may be used, but once the working load exceeds an average level of 3-5 MPa (500 psi) then only structural adhesives of the thermoset type should be used. This figure may appear to be very low but it has been derived from the considerations reviewed below (see Design ). Incidentally, the fact that thermally induced stresses can lift to quite high levels what would otherwise be nominal loads should not be overlooked. 

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