Issue 7

C. Colombo et alii, Frattura ed Integrità Strutturale, 7 (2009) 65-72; DOI: 10.3221/IGF-ESIS.07.05 65 High-cycle fatigue strength of a pultruded composite material C. Colombo, M.Guagliano, L. Vergani Dip.to di Meccanica, Politecnico di Milano, Via La Masa, 1 - 20156 Milano Italy R IASSUNTO . I compositi pultrusi sono tra i più attraenti materiali per applicazioni che richiedono produzione su grande scala. Per questo motivo il loro impiego nell’ambito di applicazioni strutturali è in continuo aumento, anche se ancora limitato da una incompleta conoscenza del loro comportamento a fatica. La maggior parte dei dati a disposizione si riferisce, infatti, a prove interrotte dopo 3 milioni di cicli. In questa memoria si considera un pultruso utilizzato in ambito strutturale e si caratterizza il suo comportamento statico e a fatica. I risultati hanno permesso di ricavare le curve S-N del materiale e di verificare l’esistenza del limite di fatica. Le osservazioni condotte al SEM hanno consentito di valutare i meccanismi di danneggiamento che si verificano durante il cedimento statico e a fatica. A BSTRACT . Dealing with composites in polymeric matrix, the pultruded ones are among the more suitable for large production rates and volumes. For this reason, their use is increasing also in structural applications in civil and mechanical engineering. However, their use is still limited by the partial knowledge of their fatigue behaviour; in many applications it is, indeed, required a duration of many millions of cycles, while most of the data that can be found in literature refer to a maximum number of cycles equal to 3 millions. In this paper a pultruded composite used for manufacturing structural beams is considered and its mechanical behaviour characterized by means of static and high-cycle fatigue tests. The results allowed to determine the S-N curve of the material and to assess the existence of a fatigue limit. Observations at the scanning electronic microscope (SEM) allowed to evaluate the damage mechanisms involved in the static and fatigue failure of the material. K EYWORDS . Delamination; pultruded composites; DGM-XGM failure; fatigue damage. I NTRODUCTION ultrusion is one of the most attractive technological process for obtaining polymer matrix composite parts to be manufactured with large production rates and volumes [1-3]. Due to this characteristic and to some peculiar aspect of their physical and mechanical behaviour, pultruded composite are getting more and more used in structural applications in civil infrastructures. This is due to the progress made in pultrusion technology, that allows the capability to manufacture low-cost large-scale load carrying structural profiles. If the fact that these materials do not need painting, that they do not conduct electricity (thus, not needing to be insulated) and that, thanks to lightness, they use allows to reduce transportation costs and environmental pollution, it seems clear why they are becoming a serious alternative to metal alloys for the construction of shaped beams, pedestrian bridge decks, post for railway noise barriers, floors of bus and other structural parts [4-6]. However, their application in structural engineering is still somewhat limited by the incomplete knowledge about the fatigue strength, that is to say that the behaviour of pultruded composite under time variable loads is not completely investigated and understood. In fact, there are few data that can be found in the references. In [7] a comparative study between the fatigue behaviour of GRP hand lay-up and pultruded phenolic composites is described, but the maximum number of cycle of interest is limited to 1 million, thus preventing the application of these data to longer life-span, typical of civil infrastructures. In [8] the long-term environmental fatigue behaviour of pultruded glass-fiber-reinforced P