analytical test methods. As an example, a long term ageing study was conducted on a glass fibre reinforced thermoset intended for offshore usage and subjected to seawater. In this case, the material was again immersed in the test fluid at temperatures above its normal operating conditions, i.e. 60, 80 and 100 C (140, 176 and 212 F) for up to 35 weeks. It is evident from Fig. 37.3, that temperature has an effect on fluid absorption. It appears that fluid uptake is faster and greater as temperature increases. Concentrating now on the highest temperature 100 C (212 F), after having reached saturation, the material exhibits a loss in mass from around 6 weeks (6.52 days). This could be due to several factors such as leachables escaping the composite, the dissolution of the resin or of the fibres. This behaviour is not visible at the two other temperatures. Does this mean that only the reaction rate of the processes at play increases or could other ageing mechanisms take place at the highest temperatures? The samples were assessed visually for changes with exposure time and temperature. Following removal after 245 days, the samples displayed a slight increase in surface roughness accompanied by discoloration. Both were more noticeable as temperature increased (Fig. 37.4). The physical changes observed could be linked to the fact that some of the resin may have been dissolving over the course of the immersion test, which can also explain the loss in mass sustained by the samples having undergone exposure to the fluid at the highest temperature. Three point bend (ASTM D790M) and short-beam shear (ASTM D2344M) testing was conducted following exposure, to monitor changes to the material properties with time in the test conditions. The test fluid does not seem to plasticise the material as maximum flexural strength decreases between dry and wet state at all temperatures (Fig. 37.5). As highlighted before, the loss in flexural properties could be due to residual stresses being relieved by micro-cracking. It appears that after the initial changes sustained due to fluid absorption, virtually no changes are seen over the 35 weeks of testing at the highest test temperature. The fibre dominated property—strain at maximum stress—does not exhibit much variation at 100 C (212 F). An overall decrease in maximum stress, strain at maximum stress and modulus is seen over the exposure period at both 60 and 80 C (140 and 176 F). This could mean that full saturation of the material may not have been reached by the time the first measurement is carried out after exposure. The fact that the flexural properties at the highest temperature are relatively stable seems to show that the material does not age. 300 350 400 450 500 550 600 650 700 750 800 0 200 400 600 800 1000 Maximum Stress [MPa] Exposure Time [Hours] Thermoplastic/Carbon Fibre 180°C 190°C 200°C 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 200 400 600 800 1000 Strain at Maximum Stress [%] Exposure Time [Hours] Thermoplastic/Carbon Fibre 180°C 190°C 200°C 30 32 34 36 38 40 42 44 0 200 400 600 800 1000 Flexural Modulus [GPa] Exposure Time [Hours] Thermoplastic/Carbon Fibre 180°C 190°C 200°C Fig. 37.2 Evolution of the flexural properties of a carbon fibre reinforced thermoplastic composite with exposure time and temperature in 380 bar (ca. 5500 psi) pressure sour fluid 37 Laboratory Testing on Composites to Replicate Oil and Gas Service 323
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