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An integrated NDT approach for damage assessment of CFRP composites under complex static and fatigue loads
Boek - Dissertatie
Carbon fiber polymer composite materials are considered one of the most promising lightweight candidates for load bearing applications in many engineering fields. Damage during loading of composite materials can appear under many different modes, like for instance matrix cracking, interlaminar delaminations and fiber breakage. Due to the anisotropy of composites, leading to complex stress states in the individual plies, the damage sequence is not unique and depends on many different parameters. Commonly applied damage criteria based only on calculations of stress components are not applicable in most of the cases and energy-based failure theories have to be developed. For this to happen, dedicated experimental data are of paramount importance. This PhD thesis contributes to this direction through an elaborate experimental damage characterization campaign on carbon/epoxy composite laminates in order to enrich the existing database in literature. In order to take into account the anisotropy of composite materials, angle-ply flat laminates having different off-axis plies were tested with the aim to develop different multiaxial stresses in their laminas and consequently different damage processes. Considering the fact that in the majority of real applications loads are dynamic, both static and fatigue experiments were conducted. The impact of different fatigue parameters on the mechanical response and the damage sequence of the material was examined. Detailed through-thickness damage observations were performed by using in-situ optical microscopy and precise correlations of the damage state with the stresses developed within the composite laminates were obtained. Apart from the different multiaxiality, a comparison between balanced and unbalanced laminates was performed, inspired by the shortage of experimental evidence regarding unbalanced configurations, even if they can lead to improved performance in applications requiring certain design parameters. Moreover, the impact of the relative number of off-axis layers with respect to the number of unidirectional plies on the overall mechanical deterioration was considered. In order to obtain detailed damage assessments, a Non-Destructive Testing (NDT) approach was established, based on Digital Image Correlation (DIC), Acoustic Emission (AE) and ultrasonic measurements. The potential of these techniques for damage identification in the composite material under multiaxial stress states was exploited. Classification of acoustic signals based on different damage modes was performed by selecting the most sensitive AE descriptors. Correlations of AE signatures with the stress state from early loading stages were established during static, incremental and fatigue loading. Finally, an upscaling of this approach was attempted by testing carbon/epoxy composite sub-components of a larger scale with a potential implementation in automotive applications. V-shape and hollow beam specimens were therefore examined in a progressive manner. Detailed damage observations and the precise location of fracture could be obtained by using a combination of DIC and AE assessments.