Cracking and durability of composites in a marine environment

Abstract

New renewable marine energy sources are increasingly being pursued as alternatives since they represent an important political and economic challenge for countries. Among these new energy sources, marine tidal turbines are growing considerably. Manufacturers used thick composite material to design most of the tidal turbine blades. To ensure the lifetime of the latter, it is necessary to develop damage models that take into account sea water, and analyse its effects on composite materials. This paper presents results from laboratory tests that were conducted to investigate the cracking of composite materials before sea water ageing. Two testing methods, image processing and acoustic emission (AE) techniques were used to evaluate the crack density within the material. Samples of the Infused and Pre-preg materials with a [02, 902]s stacking sequence were prepared and tested in tension on an electro-mechanical testing machine. Under these stresses, the material response results in a release of energy in the form of transient elastic waves that are recorded by AE sensors. By means of the AE technique, the monitoring of material damage lies in the ability to identify the most relevant descriptors of cracking mechanisms. The latter are identified by clustering the AE data. A K-means++ algorithm was used, and two AE features- peak frequency and number of counts-represent adequately the AE events clustering. This unsupervised classification allows the AE events that were generated by intra-laminar cracks to be identified. Results show a good correlation between normalized crack density evaluated by image processing, and the one monitored by means of AE cluster analysis.