Tensile shear resistance

Adhesion to various substrates Tensile shear resistance Tensile resistance Peel strength... 

EN 1465 (1995) and ISO 4587 (1995) determination of tensile shear resistance of rigid-rigid assemblies, single lap samples,... 

Wood to wood bonds made with vinyl white glue are also tested with double lap shear test specimen (Fig. 32), in order to avoid cleavage effects at the ends of the joint. In this case the tensile shear resistance will be from 4 to 10 MPa and some wood fiber tear will occur. 

The test will be performed as for the tensile shear resistance but with the required configuration of the joints. The bonded parts should be aligned very carefully as is explained in the above standards, because if they are not there will be some cleavage effect and as the adhesives do not resist to cleavage the puU value will be low, and this is the reason why the bonded joints should never be loaded in pure tensile mode. For the same bonded area, the tensile value will be lower than the tensile shear value. 

In the past, for structural and semi-structural bonding, adhesives technologists favored the strong adhesives which had very high tensile shear resistance, for... 

In these tests, bonded specimens are subjected to alternate stresses of for instance 20% of the maximum tensile shear resistance, and this cycle may be repeated 100 000 or one million times, for instance, or until the bond breaks, and here again mechanical resistance is measured before and after the test. 

Although thermal performance is a principal property of thermal insulation (13—15), suitabiHty for temperature and environmental conditions compressive, flexure, shear, and tensile strengths resistance to moisture absorption dimensional stabiHty shock and vibration resistance chemical, environmental, and erosion resistance space limitations fire resistance health effects availabiHty and ease of appHcation and economics are also considerations. 

Urethanes are processed as rubber-like elastomers, cast systems, or thermoplastic elastomers. The elastomer form is mixed and processed on conventional mbber mills and internal mixers, and can be compression, transfer, or injection molded. The liquid prepolymers are cast using automatic metered casting machines, and the thermoplastic pellets are processed like all thermoplastic materials on traditional plastic equipment. The unique property of the urethanes is ultrahigh abrasion resistance in moderately high Shore A (75—95) durometers. In addition, tear, tensile, and resistance to many oils is very high. The main deficiencies of the urethanes are their resistance to heat over 100°C and that shear and sliding abrasion tend to make the polymers soft and gummy. 

Polysulfide resins combine with epoxy resins to provide adhesives and sealants with excellent flexibility and chemical resistance. These adhesives bond well to many different substrates. Tensile shear strength and elevated-temperature properties are low. However, resistance to peel forces and low temperatures is very good. Epoxy polysulfides have good adhesive properties down to -100°C, and they stay flexible to -65°C. The maximum service temperature is about 50 to 85°C depending on the epoxy concentration in the formulation. Temperature resistance increases with the epoxy content of the system. Resistance to solvents, oil and grease, and exterior weathering and aging is superior to that of most thermoplastic elastomers. 

These reactive hybrid epoxy-urethane adhesives were developed initially for bonding to oily cold-rolled steel, but they have given good results on other substrates as well. It is believed that the oil contaminant on the substrate is adsorbed into the uncured adhesive and acts as a plasticizer. Table 7.9 shows tensile shear values obtained after curing 20 min at 177°C on various untreated substrates. The epoxy-urethane adhesives resist hydrolysis and give useful bond strengths up to about 100°C. 

Although most epoxy adhesives have good weather resistance, optimum properties are generally achieved when the adhesive has a combination of good water resistance and thermal shock resistance. Figure 11.6 illustrates the retention of tensile shear strength of copper and aluminum strips bonded with an amidoamine cured epoxy after 2 years of weathering in a temperate climate. 

Epoxy-nylon adhesives show exceptionally high tensile shear and peel strengths however, they have poor resistance to moisture and elevated temperatures. These adhesives can absorb significant amounts of water from the ambient environment before and after cure. Table 7.5 gives tensile shear and peel strengths for a series of adhesives made by dissolving various ratios of nylon and epoxy resins in a alcohol-water mixtures. 

Fillers were found to significantly increase the room temperature tensile shear strength of BTDA cured epoxy adhesives. However, other modifiers such as phenolic or nylon resin did not significantly improve the peel strength, flexibility, or temperature resistance of the unmodified formulation. 

Chlorinated Polyether. This thermoplastic resists most solvents and is attacked only by nitric acid and fuming sulfuric acids. Thus, it is not capable of being solvent-cemented. Chlorinated polyether parts can be bonded with epoxy, polyurethane, and polysulfide-epoxy adhesives after treatment with a hot chromic acid solution. Tensile shear strength of 1270 psi has been achieved with an epoxy-polysulfide adhesive. 

When loading this test piece with force F it is noticed that, unlike in the tensile shear test, the force is not applied to an area A (= b x fu), but to a line X... X. The other area of the adhesive layer remains unstressed. Thus, in this case strength cannot be defined as force per area , but the force referring to a line is called peel resistance. If the test piece shown in Figure 10.5 is tom apart by means of force F, and the force over the peeled distance is recorded, the following peel diagram results (Figure 10.6). 

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