Joint design adhesive joints, peeling

When bonding elastic material, forces on the elastomer during cure should be carefully controlled, as too much pressure will cause residual stresses at the bond interface. Stress concentrations may also be minimized in rubber-to-metal joints by elimination of sharp comers and by the use of metal adherends sufficiently thick to prevent peel stresses that may arise with thinner-gauge metals. As with all joint designs, polymeric joints should avoid peel stresses. Figure 7.16 illustrates methods of bonding flexible substrates so that the adhesive will be stressed in its strongest direction. ... 

As with all joint designs, polymeric joints should avoid peel stress. Figure 7.23 illustrates methods of bonding flexible subsfrafes so that the adhesive will be stressed in its strongest direction. 

In order to place surface preparation in proper perspective, the adherend-to-organic material (i.e., adhesive) interface must be considered from design through fabrication. Joint design, adhesive selection, and processing must be considered. These factors are interdependent. The use of an optimum surface preparation is of little value when an unsuitable adhesive is used, the bond is not properly processed, or the joint design involves peel or cleavage stress. ... 

ASTM D 1144 provides a recommended practice for determining the rate of bond strength development for either tensile or lap shear specimens. However, peel and can-teliever tests can also be used effectively. Measured bond strength values of partially cured test specimens are compared with those of a reference (i.e., fully cured adhesive joint) to assess the extent of cure. This method may suit some applications, but it is limited in accuracy because it does not directly measure the degree of cure in the adhesive, and the effect on the joint design and substrates may override the effect of cure development. 

Joint Design The adhesive bond joint must be considered to provide compressive stress rather than tensile stress and reduce stress concentration and peeling in the bonded joints (Lees 1984). 

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. 

Types of Stress. To effectively design joints for adhesive bonding, it is necessary to understand the types of stress that are common to bonded structures. Four basic loading stresses are common to adhesive joints tensile, shear, cleavage, and peel. Any combination of these stresses, illustrated in Fig. 7.13, may be encountered in an adhesive application. 

For the best possible performance, joints should be specifically designed for adhesive bonding. In a few cases only can an adhesive be used on a joint not specifically designed for adhesives - mainly cylindrical joints. Bond stresses, materials, type of adhesive, surface preparation, methods of application and production requirements can then all be considered in relation to each other at the outset. The designer should consider especially the effect of shear, tension, cleavage and peel stresses upon the joint (Fig. 1) (see Joint design strength and fracture perspectives). 

Although adhesives offer a number of advantages, they suffer from a number of limitations, listed in Table V, which must also be considered when assessing the suitability of an adhesive for a particular application. Most structural adhesives are strong in shear and tensile loading, but weak when peel or cleavage stresses are present. Joint design is therefore necessary to eliminate, as far as possible, these undesirable stresses. 

This chapter will discuss the testing, analysis, and design of structural adhesive joints. Adhesive bond test techniques to be considered include tensile, shear, peel, impact, creep, and fatigue. Some considerations will also be given to the effect of environment and test rate. A continuum approach to the analysis of adhesive joints will discuss tensile, shear, and peel stresses which arise in various joint geometries. Classical theories by Volkersen, Goland and Reissner, and others will be included. References to finite element analysis will be made where appropriate throughout the chapter. 

It is necessary, therefore, to test an adhesive by many techniques to simulate the conditions that it may be subjected to in service. The three types of tests to be discussed are tensile, shear, and peel. These tests are the most common and result in information which is useful for reliable joint design. Response to dynamic testing such as fatigue, creep, and impact will also be introduced. 

Most adhesively bonded systems are designed to eliminate peel stresses. However, bending of adherends or nonuniform loading of the bonded joint can cause peel stresses. 

Keywords Adhesive modulus Adhesys expert system Co-axial joints Compression Concealed joints Creep Elastic limit Epoxy Epoxy composite Einite element analysis Glue line thickness Goland and Reissner Hart-Smith Heat exchanger Hooke s Law Joint designs Joint thickness Lap shear strength Peel Plastic behaviour Polyurethane Pipe bonding Shear stresses Shear modulus Stress distribution Thick adherend shear test Tubular joints Volkersen equation Young s modulus... 

The successful and cost-effective application of adhesive bonding technology is crucially dependent on correct joint design. The adhesive joint must be adequately dimensioned for the forces to transmit. Large static loads should be avoided wherever possible - especially where the joint is exposed to higher temperatures. In such cases, joints should be optimised to provide additional support. Similar measures should be taken to counteract peeling stresses (Fig. 24). Some examples are illustrated in the chapter Technical Characteristics in Volume 1. 

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