1 LONG-TERM BEHAVIOUR OF GFRP PIPES: REDUCING THE PREDICTION TEST DURATION Hugo Faria, Rui Miranda Guedes Faculdade de Engenharia da Universidade do Porto Porto, Portugal ABSTRACT The certification of glass-fibre reinforced plastic (GFRP) piping systems is regulated by normative standards in which test series of 10000 hours are required to estimate the residual properties at the end of the expected life (normally, 50 years). In this paper, the possibility to reduce the tests duration, maintaining an equivalent prediction of long-term properties, is discussed. Experimental results from standard test procedures conducted on GFRP pipes of four different types and respective data analysis support this possibility. The estimation error when using only data from shorter tests is consistently less than 10% if compared to the standard methods. INTRODUCTION Glass-fibre reinforced plastic (GFRP) pipes have been increasingly introduced in piping systems. They find attractive applications in chemical industry, ducts, offshore, water supply and sewage systems. However, the lack of fully understanding the failure mechanisms and long-term materials performance necessarily leads to over-design, in-service prototype evaluations and, furthermore, inhibits greater utilization. Moreover, the existence of different types of GFRP pipes construction, namely filament wound, centrifugal cast and hybrid ones, makes this task even more difficult. Thus, the mechanical behaviour of GFRP pipes under ring deflection and/or internal pressure is assessed in experimental procedures. Two typical in-service load conditions are internal pressure and ring deflection. For these, empirical test methods have been developed and are described in the European Standards EN1447 [1] (based on ISO10471-2) and EN1227 [2] (based on ISO7509), respectively. The very long testing periods stated in these standards, in addition to the factors mentioned above, strongly discourage the industrial improvement and innovation of the products and also prevent the end users from performing confirmation tests. Shorter but reliable tests and/or better predictive models, capable of being standardized, have been required by the GFRP piping industry. The few research works conducted specifically on long- term properties of GFRP pipes between 1970 and 2000 gave relevant information but the conclusions could only be applied to classical filament wound GFRP pipes since all the experiments have been only performed on these [3-7]. More recently, several alternative short-term test methods for the various loading conditions and different GFRP pipes construction types have been studied in a co-normative European research project [8]. Ultimate elastic wall stress (UEWS) and strain at failure tests have been studied as alternative methods for estimation of long-term pressure and dynamic loading, UEWS and relaxation for the case of long- term ring deflection. In some of these procedures it was additionally considered a period of preconditioning under water previously to the tests. In fact, the slowness of the liquid diffusion at room temperature is a major aspect that is not taken into account in the existing standard test methods. This phenomenon has been recently investigated [9-10]. Other limitations of the existing standard methods are the implicit assumption that the mechanisms responsible for the long-term failure are the same at different levels of load and the non inclusion of material variability parameters that would damp the scatter typically observed in these tests. Although these problems are not explicitly considered in the standards, the experimental evidence of the whole failure phenomenon and the lack of adequate information was the main reason to extend the testing periods over 10000 hours. In a logarithmic time scale, this period is only 1.5 decades distant from 50 years and this makes the existing test and prediction methods seem reasonable. Any eventual phenomena occurring after 10000 hours contributing to unpredictable decrease in properties are, however, ignored in the actual standard analyses. Polymeric materials exhibit a time dependent behaviour whose degree of linearity or nonlinearity is, often, not clearly determined and several modelling approaches have been