Joint width adhesive

The cured sealant gives a tough, elastic, rubber-like seal and gives excellent adhesion to concrete and masonry, glass, aluminium and stainless steel. The shore A hardness ranges from 15-35. The movement accommodation factor is 25% in butt joints and 50% in lap joints. These sealants have the capacity to accommodate continuous and pronounced cyclic movements. They are suitable for joints where the joint width may range between 5 mm and 50 mm. Joints which are expected to experience cyclic movements should have a width depth ratio of 2 1. Minimum sealant depths recommended for different environments are mentioned in Table 7.8. Primers... 

Besides durability, premium sealants are judged by special properties as shown in Table 4. The ability to take on greater elongation and compression is measured by movement capability in terms of joint width. The stability to UV exposure is important for those glazing and insulation compounds used in modern high-rise structures. Thermal stability is in demand for solar collectors, or for other structural materials. On the basis of these evaluations, we can foresee future trends of sealants as shown in Table 4. Silicones appear to out-perform others. In the meantime, technical advances will provide low-modulus polysulfides, and better movement ability for both polysulfides and polyurethanes. Their cure time will be decreased and the UV stability will be improved to match or compete with silicones. All three will be developed for better adhesion under the un-primed conditions. 

Movement relative to Adhesive layer thickness and sealing joint width Thermal movement (e.g. very slow quasi-static) Loading and unloading Slight accident, infrequent incidents (e.g. derailment) Normal service operation (dynamic, fast)... 

Czamocki and Piekarski s) used a nonlinear elastic stress-strain law for three-dimensional failure analysis of a symmetric lap joint. Taking into account the variation of Poisson s ratio with strain within the adhesive, the authors concluded that the failure of the adhesive layer originates in the central plane of a joint (at the front edge). It was also observed that the joint width did not have any effect on the stress peaks in the central plane and that the application of a weaker but more flexible adhesive resulted in higher load-carrying capacity and lower stress concentrations in the adherends. 

To illustrate the use of the finite element method, a series of single lap joints was analysed using the following combinations of adherend titanium-titanium, titanium-composite, composite-composite, and aluminium-aluminium. For each adherend combination, both an epoxy and a bismaleimide adhesive were analysed and each joint was subjected to four different test temperatures —55°C, 20°C, 130° and 180°C. In all, therefore, 32 different cases of single lap joints were considered. The thicknesses of the adherends were 2 mm for the composite, 1.6 mm for the aluminium and 1.2 mm for the titanium. The joint width was 25 mm, the length was 12.5 mm, and the bondline thickness was 0.1 mm. 

If the joint movement amounts to 15—35% of the total joint width, a shouldow sealant depth in a wide joint will minimize stress on the sealant and on its adhesive bond to the substrate (this apphes to expansion, butt, capping, and some floor, lap, and comer joints). 

Kinloch and Shaw [106] have investigated the effect of joint width by employing a TDCB specimen consisting of mild-steel substrates bonded with a simple brittle epoxy or a rubber-toughened epoxy adhesive. For the former, low toughness adhesive no effect was observed. However, for the tough adhesive the relationships between Gic and adhesive layer thickness, /la,... 

Figure 7.16 The effect of joint width upon the maximum adhesive fracture energy, Clem, for a rubber-toughened epoxy adhesive bonding steel substrates using a TDCB geometry [106].

Finally, the above experiments on the effects of adhesive thickness and joint width were all conducted using through-thickness cracks. From some initial observations on cracks which were embedded in an adhesive layer the severe constraint imposed by the surrounding elastic material appears to restrict... 

Failure load vs. overlap length (Pgy is the failure load of the adhesive due to global yielding, Ty is the yield strength of the adhesive, b is the joint width, / is the overlap length, t is the adherend thickness, Py is the maximum load that can be carried which just creates adherend yield, and oy is the yield strength of the adherend)... 

This is because the joint starts to break at the stress peak at the end of the overlap where the adhesion or cohesive strength of the adhesive is exceeded. By increasing the width of the joint, the shear stress distribution is not changed and so the failure load of lap joints increases in the same proportion as the joint width increases (Figure 5.6). 

As with other adhesives peel strengths are considerably lower than direct tensile or tensile shear values so joint design is important. A peel strength of about 4 Newtons per millimetre joint width is the best that can be achieved to date with a toughened... 

Coating of one joint member with an adhesive strip of controlled thickness and width... 

Tensile shear strength (adhesive strength), in the sense of this standard, is defined as the maximum force Fmax at the break of the bonded joint in relation to adherend surface A. The adherend surface A results from the test piece width h (25 mm) and the overlap length lu (12.5 mm) ... 

As depicted in Pigs. 9.11 and 9.12, the BM investigations reveal broad, mechanically stiffened interphases in the EP adjacent to all the metal films. The v level of these interphases is almost the same but the position of the peak maximum and the half-width at half-maximum (HWHM) depend on the kind of metal. The EP interphases on aluminum and copper are almost twice as wide as on magnesium and gold. In contrast to the polished metals considered above, the adhesion strength is good enough to maintain the EP-metal joint in the mechanical test. The samples fail between the metal film and the silicon wafer. 

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