Joint temperature effect

For irradiation at 313 m/, the reactor consisted of a 6- X 2-inch o.d. cylindrical borosilicate glass tube fitted directly to the mass spectrometer by a 24/40 joint. A Hanovia analytical lamp, having an estimated output of 325 watts, was mounted externally alongside the tube. The radiation received by the samples inside the tube was filtered through borosilicate glass and was essentially 313 m/. The ambient temperature within the reactor during irradiation was 25 °C. Some comparative experiments were carried out at 70 °C. with additional heating. There was no detectable temperature effect between 25° and 70°C., and the results reported here are for 25 °C. 

Brewis et al. studied the effects of moisture and temperature on the properties of epoxy-aluminum joints by measuring changes in the mechanical strength properties of the soaked adhesive [90]. The Tg of the wet adhesive and relative strengths of wet and dry joints were evaluated for up to 2500 hours. They concluded that the joint weakening effect of water was due to plasticization of the adhesive that, in turn, was dependent on the rate of water diffusion within the adhesive. 

Ammonium chloride is a better hardener than hydrochloric acid, as the latter produces weaker joints. The effect of a fixed amount of ammonium chloride on the pH change and on the rate of resin curing as a function of time and temperature is shown in Fig. 2. 

D.L. Hunston, "Rate and Temperature Effects in the Failure of Adhesive Joints," ACS ORPL Preprints, 41, Washington, D.C., 9/1979. 

For a press-fit joint, the effect of thermal cycling, stress relaxation, and environmental conditioning must be carefully evaluated. Testing of the factory assembled parts under expected temperature cycles, or under any condition that can cause changes to the dimensions or modulus of the parts, is obviously indicated. Differences in coefficient of thermal expansion can result in reduced interference due either to one material shrinking or expanding away from the other, or it can cause thermal stresses as the temperature changes. 

F. Delale and F. Erdogan, Time-temperature effect in adhesively bonded joints, J. Compos. Mater. 5, 561-581 (1981). 

All of the above conclusions have been drawn for isothermal conditions without considering a thermal fluctuation under creep conditions but which in practice represent the usual case during the service life of structural adhesive joints. Considering temperature effects in creep-dependent lifetime, prediction can either follow a worst case scenario in which the definition of the load-dependent fracture envelope or the test for compKance with predefined strain limits is carried out at the maximum temperature to he expected during service life. This conservative approach is likely to lead to excessive contingency reserves. 

Snap-Fit and Press-FitJoints. Snap-fit joints offer the advantage that the strength of the joint does not diminish with time because of creep. Press-fit joints are simple and inexpensive, but lose hoi ding power. Creep and stress relaxation reduce the effective interference, as do temperature variations, particularly with materials with different thermal expansions. 

The movement-absorbing devices used in semirigid and nonrigid piping systems are usually called expansion joints. Based on the method by which the pressure seal is effected, joints are either of the packed type or the packless or bellows type. Selection depends not only on the required movement but also on the severity of service in terms of pressure and temperature, tolerabiUty of leakage, and the number of service cycles. 

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