Poor designs, adhesive joints

Were it not for this peel-stress relief, the bonded splices would be prevented from developing their full potential shear strength. Some scholars would recommend not incurring the quantifiable cost of this tapering and have advocated instead the development of far more complicated analyses in which the adhesive shear and peel stresses are interacted and failure at a much lower applied load is predicted. This may be a realistic way to analyze a poorly designed bonded joint, but it is hardly the best way to encourage the use of more adhesively bonded... 

It is highly desirable to have a uniformly thin (2- to 10-mil) adhesive bond line. Starved adhesive joints, however, will yield exceptionally poor properties. Three basic methods are used to control adhesive thickness. The first method is to use mechanical shims or stops, which can be removed after the curing operation. Sometimes it is possible to design stops into the joint. 

As in adhesive bonding or welding, special considerations must be given to mechanical fastening because of the nature of the plastic material. Care must be taken to avoid overstressing the parts. Mechanical creep can result in loss of preload in poorly designed systems. Reliable mechanically fastened plastic joints require ... 

Thermoplastic adhesives are not ordinarily recommended for use at above 66°C, although they can be used up to 90°C in some applications. These materials have poor creep resistance and fair peel strength. They are used mostly in stressed joints and designs with caps, overlaps, and stiffeners. The materials most commonly bonded are non-metallic material, especially wood, leather, plastics, and paper. With the exception of some hot-melt adhesives, thermoplastic adhesives are not generally used for structural applications. Examples of thermoplastic adhesives are shown in Table 4.2. ... 

By comparison with their poor performance in peel and cleavage, adhesives can support compression loads extremely well. Indeed, the stronger materials are not destroyed until these exceed 350 MPa. Thus, wherever possible, a structural joint should always be designed to distribute imposed loads within the adhesive layer as a combination of shear and compressive forces. 

Many of the adhesive problems that develop are not due to a poor choice of material or joint design but are directly related to faulty production techniques. The adhesive user must obtain the proper processing instructions from the manufacturer and follow them closely and consistently to ensure acceptable results. Adhesive production involves four basic steps. 

Thermoplastic adhesives soften and melt when heated (provided that they do not decompose). In other words, they do not cross-link during cure. These adhesives are single-component and harden upon cooling by evaporation of a solvent or water vehicle. They have poor creep resistance, fair peel strength, and are used mostly in unstressed joints and designs with caps, overlaps, and stiffeners. The most common application is to bond nonmetallic materials such as wood, leather, cork, and paper.In general, thermoplastic adhesives are not used for structural applications or at temperatures above 66°C (150°F). Thermoplastic adhesives includeit t ... 

Strength and durability of the adhesive under various stress modes (tension, shear, creep, impact, cyclic, etc.) and stress levels (low, medium, high, etc.) in the intended joint design and when under the influence of the intended service environment. Although we may speak of the strength and durability of the adhesive, it must be emphasized that it is the serviceability and reliability of the joint or bonded assembly that we are ultimately interested in. It is not at all difflcult to take an adhesive with excellent potential and use it in an improper application or with poor bonding technique and make a very poor joint. 

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. 

If the adherends are rigid and a moment or offset load is applied to this joint then the adhesive will be subjected to quite a severe cleavage or peel load (Figure 5.20) and as such the butt joint is generally regarded as a poor joint design. 

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