Bonded joints selection

ISO 9142 2003 Adhesives - Guide to the selection of standard laboratory ageing conditions for testing bonded joints... 

The best approach to achieving good performance in bonded joints is to select the proper adhesive, design the joint properly for the specific application, and maintain rigid process control. One disadvantage in the use of adhesives is the absence of good non-destructive tests to determine the strength of adhesive-bonded joints in a wide variety of ord applications... 

In selecting a pretreatment process for aluminum or any other substrate, both the initial strength and the permanence in a specific operating environment must be considered. Mechanical abrasion is a useful pretreatment in that it removes the oxide and exposes bare aluminum. When this is done, however, many of the benefits of the protective oxide layer are lost. For example, if bare abraded aluminum is bonded, the reactive metal at the joint interface can potentially become hydrolyzed and oxidized, which will displace the adhesive. Hence, this bonded joint may initially be much stronger than one made with unabraded metal, but it will deteriorate rapidly when exposed to a harsh environment such... 

Note In the case of metal bonded joints, the resistance to moisture and climate can decisively be influenced by the kind of surface pretreatment apart from the adhesive selection, especially in the areas adjacent to the adhesive surface (e.g., primer, sealing of gluelines). 

To avoid inner tensions through thermal stress, it is recommended to use only adhesives that cure at room temperature. The adhesive selection is limited by the fact that many applications require an invisible glueline. In such cases, adhesives with fillers are excluded, cyanoacrylates and in particular radiationcuring products (Section 9.3.3) are the suitable choice. If the visual appearance of the bonded joint is not important, two-component reactive adhesives based on expoxides, polyurethanes, methacrylates, contact adhesives and, if required, adhesive tapes are recommended. 

Figure 4.50i l indicates the adhesion of latex-modified mortars to ordinary cement mortar as a substrate, measured by four types of test methods. CJenerally, the adhesions in tension, flexure, and direct compressive shear of the latex-modified mortars to ordinary cement mortar increase with a rise in the polymer-cement ratio regardless of the type of polymer and test method. The adhesion in slant (indirect) compressive shear of the latex-modified mortars attains a maximum at a polymer-cement ratio of 5 or 10%, and is extremely large compared to the adhesions determined by other test methods irrespective of the polymer type and polymer-cement ratio. The reasons for diis may be due to the effects of the combined shear and compressive stresses and their relaxation by the polymer films formed on the bonding joints. Considering the above adhesion data, it is most important to select the best test methods to successfully reproduce service conditions in the applications of the latex-modifled mortars. 

Thus, it is impossible to obtain a strong adhesive-bonded joint when cementing in liquid media without supplying mechanical work to the adhesive-substrate interface, even when thermodynamic conditions are appropriate for selective wetting of the substrate by the adhesive. Practically, the mechanical work can be supplied by mutual displacement of the adhesive and the cemented surfaces, for example, by pumping the adhesive into the gap between the surfaces, by pressing the surfaces together so that the adhesive is expelled from the gap, by ultrasonic treatment of the adhesive layer, and so on. 

Kinloch(4) observed that the selection of appropriate failure criteria for the prediction of joint strength by conventional analysis is fraught with difficulty. The problem is in understanding the mechanisms of failure of bonded joints, and in assigning the relevant adhesive mechanical properties. Current practice is to use the maximum shear-strain or maximum shear-strain energy as the appropriate failure criterion. However, the failure of practical joints occurs by modes including, or other than, shear failure of the adhesive. This difficulty has led to the application of fracture mechanics to joint failure. 

Because of the large number of factors that influence the durability of adhesive-bonded joints, a durability test should be conducted on all systems before they are selected for any particular application. This test should include the adherends, surface preparation, adhesive, and cure parameters needed for each application. ... 

The physical and chemical properties of both the solidified adhesive and the plastic substrate affect the quality of the bonded joint. Major elements of concern in selecting an adhesive for plastic parts are the thermal expansion coefficient and glass transition temperature of the substrate relative to the adhesive. Special consideration is also required of polymeric surfaces that can change during normal aging or exposure to operating environments. 

B. Chen and D. A. Dillard, Crack path selection in adhesively bonded joints. Adhesion Science and Engineering -1, the Mechanics of Adhesion, D. A. Dillard and A. V. Pocius, Eds., Elsevier, 389-442, 2002. 

Sometimes bonded joints must form a positive seal. Some adhesives readily lend themselves to this dual role while others, for various reasons, do not. The selection procedure given in Section 5.2 highlights likely adhesives. 

Several chapters are devoted to the Materials to be bonded (metals, plastics, wood, composites). These chapters explain the important properties of adhesives and sealants used for these materials, the method of designing the joints, selecting the adapted adhesives and sealants, and also provide many practical examples of bonding in all industries,... 

The properties of the adhesive bonded joint depend upon the characteristics of the material, the cohesive strength of the adhesives, and the design of the joint. Unfortunately, there is no one adhesive system which will bond all GRTP materials. Each bonding operation must be reviewed to select the appropriate adhesive for ... 

The ply layer adjacent to most bonded joints should not be perpendicular to the direction of loading. Reduce composite section thickness in the joint area, soften the composite by adding fiberglass or angle pUes, and select the highest-strain-capability... 

Crack path selection in adhesively bonded joints... 

Together, the mode mixity and T-stress play critical roles in determining crack path selection and locus of failure in adhesively bonded joints. Hie tendencies induced by these two mechanics principles, combined with the spatial variation in mechanical properties within the adherends, adhesive, and interphase region, determine the ultimate failure mode. For bonded systems involving large spatial variations in mechanical properties, debonds may favor a weak interface rather than obey the tendencies imposed by the mechanics principles outlined in these two sections. For systems having reasonably adequate mechanical properties throughout the bond, however, these mechanics principles may control the failure event. 

To demonstrate the T-stress effect and to understand other factors affecting crack path selection, the authors and their coworkers [13,25,31,32] carried out a series of experimental studies with adhesively bonded joints to determine the effects of T-stress, specimen geometry, external loading conditions, surface pretreatment. 

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