PSI - Issue 64

André Weber et al. / Procedia Structural Integrity 64 (2024) 1959–1966 A. Weber/ Structural Integrity Procedia 00 (2019) 000 – 000

1961

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The load is always redistributed from softer to stiffer and from faster creeping to slower creeping materials. For this reason in unidirectional pultruded bars with small imperfections under sustained load the following model of creep is proposed by Weber (2012).

Fig. 1 creep of rebars under high loads composed from micro defects The failure probability of a random fibre is influenced by the external load, the orientation of the fibres, the individual strength of the fibres, the diameter of the fibres, through local load concentrations, temperature, external load, and various other smaller influences. If a single fibre is broken, the neighbor fibres can transmit this additional load within some length if the matrix is still intact and these fibres are not overloaded. So multiple of such small defects can occur all releasing a local small additional strain. Seen from external this can be detected as strain increase under constant load, what is the definition of creep. If bigger areas are broken, the loads for the remaining section are increased and the probability of a failure for these areas is also increased. This leads to a progressive failure. In figure 1 these effects are seen. The higher the load, the higher the initial strain, the faster the creep and shorter the time to failure. It is important to repeat, that this kind of creep is not based on creep of the fibre itself. In so far members with straight fibres tend less to creep than members with less oriented fibres like stirrups or bars with a great distribution between fibre orientaions and lengths. An important issue is the durability of the bond between fibre and surrounding matrix. The weaker this bond, the larger the areas damaged by a fibre failure. 2.2. Combined actions for stress, alkaline attack and temperature Aim of the testing is to establish a safe and practical characteristic value of strength, taking into account most mechanical and environmental actions possible in a service life. This test procedure shall be on the one hand representative to real life situations on the other hand simple and safe. From first approaches decades ago, the alkaline attack has been identified as the most important action to the bar itself. Highly alkaline pore waters of freshly poured as well as hardened concrete are a source for the alkaline attack. In addition the use of susceptible E-glass fibres and the use of simple resins have underlined the need of such tests. For this kind of degradation harmful elements are transported into the laminate by different mechanisms. Diffusion, access through voids as well as fibre matrix interfaces need to be called here.