Flexible peel joints

The thickness of the substrate when no contribution from plastic yielding results, i.e. if/p 0, occurs when the thickness, d, of the substrate exceeds a critical thickness, dc, where dcis given by  

Finally, it should be noted that similar arguments will be applicable to the case of rigid, structural adhesives bonding metallic substrates being assessed via a peel test, as discussed in Section 6.4.10. 

As mentioned above the thickness, /la, of the adhesive layer does not usually affect the measured value of the adhesive fracture energy for relatively brittle adhesives but for tougher adhesives this parameter may significantly affect the measured value of Gcof the joint. 

Values of 2ryccalculated from Equation 7.56 are shown in Table 7.3 together with those of which were experimentally ascertained, and the agreement is very good. 

It is interesting to note that, because of the constraint effect, the value of Gic for adhesive joints prepared using these tough adhesives may under many conditions be greater than the fracture energy of the bulk adhesive determined from tests upon cast sheets of the material, and may also be less temperature dependent, again as indicated in Table 7.3. 

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The low shrinkage and good durability of epoxies also fits them for capable jointing compounds. Here a measure of flexibility is desirable. The cured article may need to be worked on from time to time, so systems that can be cut or peeled off may be required. Usable lives of 15 min to 1 h are the norm. Components should be selected for good water resistance (Section VI.E). 

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. ... 

Epoxies provide strong joints and their excellent creep properties make them particularly suitable for structural applications, but the unmodified epoxies have only moderate peel and low impact strength. These properties can be improved by modifying the resin, to produce more flexible materials which have an improved resistance to brittle fracture. These adhesives include combinations such as epoxy-nylon, epoxy polyamide. 

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. 

Flexibility The resilience of rubber helps to absorb the stresses applied to the joints. Therefore, these adhesives properly resist impact, shear, elongation, vibration and peel stresses. 

Cleavage and peel stresses are undesirable. Cleavage occurs when forces at one end of a rigid, bonded assembly act to split the adherends apart. Peel stress is similar to cleavage but applies to a joint where one or both of the adherends are flexible. Joints loaded in peel or cleavage provide much lower strength than joints loaded in shear because the... 

Corner joints for relatively flexible adherends such as sheet metal should be designed with reinforcements for support. With very thin adherends, angle joints offer low strengths because of high peel concentrations. A design... 

Clearly, a joint should be designed to avoid peel. However, if peel and cleavage forces must be borne, then some means of distributing the load must be found, for example by restraining movement at the end of a flat, flexible joint with a rivet or spot-weld (Figure 2.4). If such remedies are not practicable, then complete re-design may be required. 

This basic joint configuration can cope very adequately with a wide variety of adherend types and adhesives. While the major limitation is poor peel and cleavage resistance, it performs well with appropriate materials - for example, flexible, compliant rubber bonded to a second, stiffer adherend. The stress induced at the joint s edge by distortion will be dissipated over a greater area than usual by the bending of the rubber and the more closely matching cohesive strengths of the adhesive and the rubbery material - compared with those of an adhesive and metal - could well mean that the joint will ultimately fail only because the rubber tears at extreme load. 

Designs deliberately incorporating flexible rubbery inserts between two stiff adherends are well known. For example, cyanoacrylate adhesives are successfully used to bond spectacle lenses to frames through an intermediate rubber layer. Without the rubber to dissipate peel and cleavage loads, the joint between the lens and the metal frame would be readily over-stressed, resulting in premature failure. 

Peel Peel is the force applied to a joint in which one or both of the adherends is flexible and in which the force is concentrated at a boundary line. ... 

The climbing-drum peel specimen is described in ASTM D 1781. This test method is intended for determining peel strength of thin metal facings on honeycomb cores, al-thongh it can be generally used for joints where at least one member is flexible. 

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