Rubber joints under stress

High peel strength The intrinsic properties of rubbers (high ability to produce high elongation under stress) impart adequate strength to the joints under peeling forces (see Peel tests). However, rubber polymers show poor resistance to shear stresses. 

Mixed modulus joints have been proposed in the past to improve the stress distribution and increase the joint strength of high modulus adhesives. The stiff, brittle adhesive should be in the middle of the overlap, while a low modulus adhesive is applied at the edges prone to stress concentrations. Sancaktar and Kumar (2000) used rubber particles to toughen the part of the adhesive located at the ends of the overlap and increase the joint strength. The concept was studied with the FE method and proved experimentally. Fires et al. (2003) also proved with an FE analysis and experimentally with two different adhesives that the mixed adhesive method gives an improvement in joint performance. Temiz (2006) used an FE analysis to study the influence of two adhesives in double-lap joints under bending and found that the technique... 

FIGURE 7.16 Joints for rubber under stress (a) equal modulus and (b) unequal modulus [11]. 

The theory has been examined by measuring the ratio K,c/Klcs as a function of j/g, as shown in Fig. 17. The theoretical lines have been fitted to the experimental points by choosing suitable values of the critical distance, c, which is the only fitting parameter. The agreement between theory and experiment has been found to be equally good for many different epoxy polymers cured with many different hardeners, both unmodified 44- 45,51), rubber-modified 45) and containing glass particles 22) and even, under certain circumstances, for structural adhesive joints S3). Values of critical stress, ct,c, and distance, c, for various epoxy materials, obtained from bulk and... 

Figure 7.15 shows several types of joints for rubber under tension. The horizontal white lines are equidistant when the joints are unstressed. It is obvious that the scarf joint is least subject to stress concentration with materials of equal modulus, and the double scarf joint is the best for materials of unequal modulus. These designs offer the best resistance to peel and, all other factors being equal, represent the best choices. ... 

Viscoelastic characteristics of polymers may be measured by either static or dynamic mechanical tests. The most common static methods are by measurement of creep, the time-dependent deformation of a polymer sample under constant load, or stress relaxation, the time-dependent load required to maintain a polymer sample at a constant extent of deformation. The results of such tests are expressed as the time-dependent parameters, creep compliance J t) (instantaneous strain/stress) and stress relaxation modulus Git) (instantaneous stress/strain) respectively. The more important of these, from the point of view of adhesive joints, is creep compliance (see also Pressure-sensitive adhesives - adhesion properties). Typical curves of creep and creep recovery for an uncross-Unked rubber (approximated by a three-parameter model) and a cross-linked rubber (approximated by a Voigt element) are shown in Fig. 2. 

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