Bonded joints problems

Water has proved to be the most harmful environment for bonded joints. Problems arise because water is universally found, and the polar groups which confer adhesive properties make the adhesives inherently hydrophilic the substrates or substrate surfaces themselves may also be hydrophilic. Experience has demonstrated that the main processes involved in the deterioration of joints subjected to the influence of moisture are (a) absorption of water by the adhesive (b) adsorption of water at the interface through displacement of adhesive (c) corrosion or deterioration of the substrate surface. 

For the production of bonded joints with metal materials, appropriate surface pretreatment is of priority. In the technical literature, various formulations of pickling solutions are to be found, their application, however, is limited for reasons of occupational safety and due to the disposal problem. Therefore, we refrain from describing them here. 

Experimental assessments of the effects of surface pretreatment are of limited value using mechanical tests unless environmental exposure is included. It is very sound policy to collect and examine information on joints loaded and exposed to natural weathering conditions rather than depend solely on laboratory experiments. It is clear that water is the substance which causes most problems in attaining environmental stability of bonded joints interfacial failure generally indicates that a better surface pretreatment would impart improved joint performance. 

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. 

The problem with bonded joints is that most of the load is transmitted through the edge zones, and it is these which come under environmental attack first. In fact, the load becomes progressively borne by the inner region of the joint but, nevertheless. 

Many problems associated with bonded joints at cryogenic temperatures are the result of stress concentrations and gradients developed within the bond. There are a number of causes of stress concentrations in adhesive joints, and a number of these causes are aggravated by cryogenic temperatures. Some of the principal causes are as follows ... 

In the bonded joint design the most basic problems are the unavoidable shear stress concentrations and the inherent eccentricity of the forces causing peel stresses both in the adhesive and in the adherends. At the ends of the overlap both the peel and shear stresses reach their maximum values, resulting in reduced load-bearing capacity of the joint, see Figure 5.28. 

In addition to the general principles which apply to bonded joints, there are certain manufacturing aspects specific to bonded insert joints. The insert is usually of metallic material and therefore it often requires a chemical surface treatment prior to the bonding. Once the insert is ready to be bonded, it has to be verified that the whole bond area is covered by the adhesive after the insert has been installed. This problem needs to be solved by developing production methods that guarantee the quality and consistency of the bonding. 

The finite element (FE) technique is an approximate numerical method for solving differential equations. Within the field of adhesive technology, it is most commonly used to determine the state of stress and strain within a bonded joint. It can also be used to determine moisture diffusion, natural frequencies of vibration and other field problems. Although this article will concentrate on the stress analysis, the same concepts can be applied to these other applications of finite element analysis. The basis of any finite element method is the discretization of the (irregular) region of interest into a number of... 

Other investigations [2], with aging times of 2l6. and 3 years under different artificial and natural conditions, show that on the PP side of bonded joints (in that case, PP steel joints) no remarkable changes in the adhesional area occur. So it can be stated that the durability of adhesive bonding of PP is no problem when the surface of the PP is treated as mentioned earlier. 

Composite materials as well as adhesively bonded joints are not exempt from some problems even though they have great advantages and applicability. Defects related to composite bonded joints can be classified into two main streams- manufacturing defects and in service defects. 

The application of these principles to an actual adhesively bonded joint is anything but straightforward. One problem is the lack of pertinent information on the performance of adhesive joints. Most test data generated by adhesive producers are only useful for comparison purposes and are of limited use to the design engineer. Also, there is only limited information available on the performance of bonded joints exposed to service environments, while subjected to static or dynamic stresses. Adequate adhesive characterization and prediction of joint durability remain goals for the future. 

Brittleness. Cyanoacrylate adhesives are known for their lack of toughness, especially in metal-to-metal bonds. This problem is less severe in plastic or rubber joints. The inclusion of adhesion promoters or tougheners in the adhesive can mitigate this weakness. 

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