Assembly considerations, adhesive joint

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. 

Assembly techniques by dowel insertion, with or without the use of adhesives, are commonly apphed for joining soUd wood in furniture and wood joinery industries. High-speed rotation-induced wood dowel welding has been shown to rapidly yield wood joints of considerable strength for European wood species [1-3],... 

Figure 2.16 compares the relative co-axial shear strength (measured on the Collar and Pin test specimens described in MoD DTD 5628, Method H) of a number of adhesives with a resistance to disassembly similar to related press-fitted collar and pin components. The strength of the bonded joints is considerably in excess of the resistance to movement displayed by friction- fitted assemblies. 

In a study of nine commercially available acrylic adhesives, Wilkinson and Tyler found a considerable variation in the resistance to environmental exposure. The tests employed were 1,000-h water soak, 30 days at 125°F/100% RH, and 1,000-h 5% salt spray at 95°F aluminum adherends were used. The nature of the alloy was also a factor in determining joint durability. The authors concluded that excellent durability can be obtained with the proper selection of adhesive and alloy. A similar study by Zalucha using recently developed high-performance acrylic adhesives also indicates excellent performance for many acrylic-bonded assemblies. Minford has conducted weathering tests on both the early formulations and the more recently introduced second generation acrylics." He found that the durability of the earlier products was generally poor in both water soak and... 

This type of joint has the considerable advantage that peel/cleaving forces are reduced to an absolute minimum. The challenge for the designer is usually to ensure that the joint is completely filled with adhesive. The hurdle to be overcome in assembling two tubes is shown in Fig. 47. The radial modifications at the tube ends reduce stress concentrations and assist in efficient joint closure. 

Processing and quality control are usually the final considerations in the design of an adhesive-bonding system. These decisions are very important, however, because they alone may (1) restrict the degrees of freedom in designing the end product, (2) determine the types and number of adhesives that can be considered, (3) affect the quahty and reproduc-ibihty of the joint, and (4) affect the total assembly cost. 

Also, as discussed by Lees [1], practical joint designs must take other factors into consideration. They must allow cost-effective assembly and the design must allow the adhesive to be applied in an appropriate pattern stripes and crosses are preferred so that upon placing the substrates together air entrapment does not occur, which is possible with a closed-loop pattern. Liquid or paste form adhesives may be more accurately placed if they are deposited in a... 

As already stated, the durability or permanence of a bonded assembly is dependent on the intended use and service conditions to which the bond will be exposed. However, the joint design, choice of substrates, adhesive selection, substrate preparation, and primer selection, where appropriate, plus the method of application and assembly all have significant impact on the service life of adhesively bonded materials. Most or all of these considerations are interdependent, for example, the joint design and substrates chosen will limit the range of suitable adhesives that can be employed. In a similar way, the durability of a sealed joint is only as good as the adhesion of the sealant (and primer) to the surfaces forming the joint. Primers and/or sealants will adhere to surfaces only if those surfaces are properly prepared. A very large proportion of all sealant joint failures result from poor or inadequate surface preparation. 

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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