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At elevated temperatures, preconditioned specimens tend to dry out during the test, although for static tests the effects are minimal. For fatigue testing, however, drying of the test component may be considerable. To maintain constant moisture levels, a humidity-controlled chamber is the preferred option, which is not affordable by most laboratories. Alternative approaches, include enclosing the specimen in a polythene bag containing a salt solution (specimen not in direct contact) appropriate to the humidity requirements of the test, or totally encapsulating the specimen with a sealant. For example, saturated potassium chlorate solution at 70 °C will give a relative humidity of 85%.
For encapsulated specimens, travellers are required to monitor the moisture content of the specimen throughout the duration of the test. Ideally, the traveller should be identical to the test specimen (i.e. same materials, dimensions and thermal history) thus ensuring accuracy in moisture content to within ±1%.
Moisture absorption results in a number of effects including swelling of the adhesive, which increases with moisture content and temperature. Swelling, if significant, may induce additional stresses within the bonded joint compromising the durability of the joint. The coefficient of moisture expansion is defined as the fractional increase in length per unit mass variation due to the moisture absorption or desorption. Determination of the coefficient of moisture expansion involves measuring the dimensional change of the material in the principal material directions as a function of moisture concentrations (i.e. moisture weight gains). Tests involve immersing pre-dried specimens in a temperature controlled water bath and measuring the swelling strains as a function of weight gain. Specimens are pre-dried in an oven at a moderately high temperature (70°C or less) until the specimen weight reaches a constant value.
Moisture expansion or swelling can be measured with a micrometer and/or vernier calliper, or a dilatometer (commercial units are available). Strain gauges are not particularly suited for this purpose due to environmental attack on the strain gauge adhesive. Moisture attack of an adhesive and strain gauges will occur from the top, edges and in the case of fibre-reinforced polymer composites beneath the gauge. A reduction in accuracy of the strain gauge measurements can be expected with exposure time. In addition, the presence of the strain gauge on the surface of the specimen may inhibit moisture absorption within the vicinity of the gauge. It is important to ensure the adhesive selected for bonding strain gauges remains unaffected for the entire duration of the test and that strain gauges and associated electrical wiring, is suitably encapsulated. Silicone rubber and foil have been placed over strain gauges in order to isolate the gauges during environmental conditioning. The process is only partially effective, as moisture will eventually penetrate the bond-line.
Embedding sensors, such as optic fibres (i.e. Fibre Bragg Grating), within the adhesive may prove more effective in measuring the moisture expansion, provided the introduction of the sensor does not cause any local disturbance, which may affect the process of moisture diffusion. The advantage of using embedded strain gauges is that swelling can be monitored throughout the duration of conditioning without the need to remove the specimen from the water bath.
Note: It is assumed that all absorbed moisture is translated into a change in resin volume. In fact, during the initial stages of conditioning, water may also be filling microvoids and cracks. A plot of swelling strain versus weight gain will show a change in gradient for high porosity materials.
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