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Aligning Fibre Break Models for Composites with the Observable Micro-Scale Material Behaviour

Boek - Dissertatie

Continuous fibre reinforced polymer composites offer tremendous opportunities to address the need for strong yet lightweight structural parts for a wide variety of applications. Due to their intrinsic multi-phase makeup, their failure is not straightforward to predict as the behaviour of each of its constituents needs to be considered. While multiple different damage modes exist in laminates made of plies with different fibre orientations, their final failure is often associated with the failure of the load bearing 0° plies. This thesis focuses precisely on this aspect by targeting the experimental and virtual characterisation of the longitudinal tensile failure of unidirectional composites. In this scenario the consecutive failure of individual fibres slowly but steadily accumulates microscale damage called fibre breaks. Through the rapid exponential increase of the number of fibre breaks, the expanding stress concentrations in the remaining intact fibre sections eventually result in a sudden local fracture of the whole composite. A critical state is reached when the material is saturated with individual fibre breaks and small groups of them called clusters of fibre breaks, which triggers a domino effect. Even though this idea has been widely accepted in the literature and many different models exist to describe the development of fibre breaks, prediction results do not align well with experimental observations. Often models overpredict the strength of composite material systems, preventing practical adaptation of this type of models in industry. To understand where observed mismatches between models and reality originate, reliable comparisons based on the measured input data for the constituent behaviour of matrix and fibres are needed. For these comparisons the development of fibre break density is a common choice, which has been used in earlier studies to validate model predictions. To obtain this development, state-of-the-art in-situ ultrafast synchrotron computed tomography is used in this thesis to repeatedly scan a small representative bundle of several thousand fibres up to failure. This technique has the advantage of minimising the test time, which allows the use of realistic continuous loading speeds, as well as to obtain a 3D image of the micro-structure near the failure load in very high resolution. Unfortunately, the analysis of the resulting large 4D image datasets is a tedious manual task. Therefore, a new image analysis approach to automate the identification and annotation process of fibre breaks by using segmented fibre trajectories was developed. The segmented trajectories allow a wide range of different micro-structural analysis and comparisons that helped to identify differences between fibres with individual fibre breaks and other fibres developing clusters of fibre breaks. By analysing changes of the orientation distributions obtained from individual fibres, realignment under load was assessed. For the overall distribution of angles, the mean value of in- and out-of-plane angle remains nearly constant. This indicates that realignment, if present, is a minor local effect that does not affect the bulk of the fibres. Model predictions obtained from the KU Leuven fibre break model are compared to the experimental fibre break development as well as to the experimentally obtained strength of macroscopic specimens. The used type of macroscopic specimen used a hybrid material layup were considered as the most accurate way to characterise the tensile strength. As a result, the accuracy of the fibre strength input parameters was evaluated as they are seen as most influential for the development of fibre breaks. The difference between the development of the density of clusters of fibre breaks in model predictions compared to in experiments provoked an analysis of local stress concentrations on the fibre surface, which can exceed average stress concentrations by a large factor. Clusters of fibre breaks are often judged detrimental for the composite strength. To correctly account for their effect, another study was performed focused on the simulation of their surrounding stress field. For directly neighbouring coplanar fibre breaks the ineffective length increases, which is detrimental for the strength of the material. Non-coplanar fibre breaks can have an opposite effect if placed in an intermediate distance. The stress concentrations follow a superposition of the expected values from single breaks. The results from this study will help to advance future models by taking into account the correct stress concentrations. Given the importance of stress relaxation and creep in practical applications, the time-dependent behaviour of epoxy resin system was characterised through a combination of compressive tests under constant strain rate and compressive creep tests. From the obtained input parameters an advanced version of the KU Leuven model was created able to simulate the time-dependent strength of fibre bundles. The simulation of the time-dependent strength of a fibre bundle under constant strain (stress relaxation) proved the increase of the fibre break density over time. By comparing the time-dependent failure of bundles at high strain levels below the expected mean failure strain, the effect of stress relaxation on the material strength was assessed. As a result of the conducted doctoral research, a series of models and methodologies were developed to aid the future development of fibre break models by providing the necessary data on currently neglected effects and facilitating fast model validation against experiments. With these outcomes a significant step forward has been achieved towards the virtual prediction of the longitudinal tensile strength of unidirectional composites.
Jaar van publicatie:2021
Toegankelijkheid:Open