The main goal of this doctoral thesis was to investigate the preform compaction behaviour of RTM (Resin Transfer Moulding) laminates, focusing particularly on their fatigue behaviour. Within this thesis, the compaction behaviour of different preforms is investigated in order to define the various parameters for the RTM process. Using these results, it will be possible in the future to estimate the required compaction force for non-bindered, bindered and hybrid fabrics. This will support future design performance and curing mould concepts. The influence of different compaction methods and stitching threads on the permeability of the preforms has also been investigated. The results of these measurements have shown that the stitching has a positive influence on permeability if compared to preforms without stitches. Of the main variables, the stacking sequence was found to be the most effective means of influencing permeability. Stacking several layers into the curing mould yields higher permeability than using preform packages in which both constructions have the same number of layers. During this project, the fatigue tests were conducted under cyclic tension and compression loading. The measured data are presented in SN diagrams and hysteresis plots, with isocyclic stress-strain diagrams (ISSDs) for RTM laminates of variable compaction status and different quality levels. It was found that different preform compaction methods have only little influence on tensile fatigue behaviour under tensile loading up to 60% of the static tensile strength level. Nevertheless, it seems that if the stitching density exceeds a certain limit the fatigue behaviour is influenced in a negative way. Under cyclic compression loads, however, material degradation was dominated from the very beginning of the tests by visco-elastic creep, and it was not possible to identify an exact starting point for the degradation. The best performance with regards to fatigue behaviour was observed in the toughened RTM system. Isocyclic stress-strain diagrams supply a deformation analysis as a function of cycle numbers, permitting statements to be made about expected moduli as a function of load level and a conservative estimation of failure stresses, and thus can be used as a material law for component design, whereby consideration is given to the effects of loading rates and loading frequencies, respectively. Moreover, the resulting deformation contributions of visco-elastic effects and of cumulated damage during the fatigue tests could be assessed and compared. The results of this doctoral thesis could form the basis for development of new degradation criteria for composite materials. Non Destructive Test (NDT) methods such as thermography or Acoustic Emission (AE) were used to investigate the accumulation of defects within the RTM laminates during fatigue testing. An algorithm based on AE results was developed for the purposes of estimating fatigue behaviour. It was shown that Pulse Thermography as well as the Acoustic Emission Technique are capable of detecting and monitoring the defects and their accumulation during fatigue testing. In addition to applying the newly developed algorithm, it will be possible to calculate a reference frequency used for lifetime prediction of fatigue-loaded RTM structures derived from the rate of acoustic emission.
|Translated title of the contribution||Compaction and Fatigue Behavior of RTM-laminates|
|Publication status||Published - 2012|
Bibliographical noteembargoed until null
- isocyclic stress-strain diagrams
- degradation criteria
- Acoustic Emission