Understanding and modelling failure of laminated composites

In this thesis, experimental investigations together with analytical and numerical work on the understanding and modelling of failure in laminated composites are presented. Failure of carbon fibre reinforced plastics is investigated using acoustic emission. Signals are collected for various test configurations which give rise to specific failure modes. The signals are then analysed using pattern recognition techniques and the fast Fourier transform. An identi cation of the failure modes with their acoustic signatures is proposed using the fast Fourier transform, which was found to be the most suitable technique. The failure modes in longitudinal compression are then studied using microscopy techniques and finite element modelling. Experimental observations show that failure results from an interaction between shear-driven compressive failure and kinkband formation. Micromechanical finite element analyses are used to explain the experimental observations. The interaction of shear-driven compressive failure and kinking captured by the model is used to explain the variation in characteristics typically measured in failure envelopes for combined longitudinal compression vs. in-plane shear. Based on the experimental and the numerical results, a failure criterion for fibre kinking and splitting is developed and used to predict failure envelopes for combined longitudinal compression vs. in-plane shear. The model correlates well with the numerical predictions and experimental results. The R-curve effect observed in mode I intralaminar matrix crack growth and its specimen-dependence are then investigated. Relationships between crack extension and crack opening displacement are obtained for the Double Cantilever Beam (DCB) and Compact Tension (CT) specimens. Measured R-curves are used with the previous relationships to define a trilinear cohesive law. The cohesive law is implemented in finite element models and the load versus displacement curves predicted for the DCB and CT specimens show that the R-curve effect is numerically well captured

Compressive failure and kink-band formation modeling

To increase the use of polymeric structural composites, a major issue is to properly account for intra-laminar failure mechanisms, such as fiber kinking induced under compression. We propose a new continuum damage model that can predict the fiber kinking response at the ply level. The model is based on a previous structure tensor-based model for the response of UD-plies. A novel feature is that the compressive UD-ply response at the macroscale includes the effect of the fiber misalignment shaped as a kink-band that is resolved at the sub-scale. Concepts of computational homogenization are used to include the fiber-shear of the kink-band at the sub-scale. The model calibration is adapted to account for either kink-band formation or shear-splitting depending on the off-axis loading. Finally, the model is validated at the laminate level against experimental data for OHC-tests available in the literature. A good agreement is found for predicted strength values and observed fracture patterns of the laminates. The size effect experienced when different hole sizes are tested is also addressed

Orthotropic criteria for transverse failure of non-crimp fabric-reinforced composites

In this paper, a set of failure criteria for transverse failure in non-crimp fabric-reinforced composites is presented. The proposed failure criteria are physically based and take into account the orthotropic character of non-crimp fabric composites addressing the observed lack of transverse isotropy. Experimental data for transverse loading out-of-plane in combination with in-plane loads are scarce. Therefore, to validate the developed criteria, experimental data are complemented with numerical data from a representative volume element model using a meso-micromechanical approach. The representative volume element model also provides a deeper understanding of how failure occurs in non-crimp fabric composites. Strength predictions from the developed set of failure criteria show good agreement with the experimental and numerical data

Effect of specimen width on strength in off-axis compression tests

Compression tests have been performed according to ASTM D6641 to check whether 12 mm is a sufficient width for off-axis tests of a unidirectional Non Crimp Fabric (NCF) reinforced carbon-fibre composite. Various off-axis angles are tested in a larger context and it is important to establish a representative material volume. The test matrix consists of two different widths for two off-axis cases, 15° and 20° with a total sample size of 24. A two-sample T-test is performed for each off-axis angle to check if there is a statistically significant difference of the compressive strength between specimens with different widths. The null hypothesis, that there is no difference between the mean values is tested with a double-tailed test on a 5 % significance level. Neither of the cases may be rejected, i.e. there is no statistically significant difference on the 5 % level. The 15° off-axis case returns a p-value of 7.4 % and the 20° off-axis case gives a p-value of 21.3 %. It can be concluded that the effect is small and not statistically significant. It means that remaining off-axis testing in the larger context can proceed with the nominal width of 12 mm

Interaction of Delaminations and Matrix Cracks in a CFRP Plate, Part II: Simulation Using an Enriched Shell Finite Element Model

Numerical simulations are presented of a recently developed test which creates multiple delaminations in a CFRP laminate specimen that grow and interact via transverse matrix cracks. A novel shell element enriched with the Floating Node Method, and a damage algorithm based on the Virtual Crack Closure Technique, were used to successfully simulate the tests. Additionally, a 3D high mesh fidelity model based on cohesive zones and continuum damage mechanics was used to simulate the tests and act as a representative of other similar state-of-the-art high mesh fidelity modeling techniques to compare to the enriched shell element. The enriched shell and high mesh fidelity models had similar levels of accuracy and generally matched the experimental data. With runtimes of 36 minutes for the shell model and 55 hours for the high mesh fidelity model, the shell model is 92 times faster than the high- fidelity simulation

The transition from out-of-plane to in-plane kinking due to off-axis loading

A comprehensive test campaign has been performed on coupon level to gain fundamental understanding of compressive failure in unidirectional NCF composites for aerospace applications. A subset of this study is focusing on the effect of off-axis loading, where a number of laminates have been tested with fibres oriented in off-axis angles in the interval 0-20\ub0 in steps of 5\ub0. Our hypothesis is that 0\ub0 laminates fail by kinking out-of-plane and as the off-axis angle is increased, there is a shift to in-plane kinking as the in-plane shear component increases. The contribution from this shear component on kinking will have little effect on the compressive strength until in-plane kinking becomes "dominant" over out-of-plane kinking. Preliminary results indicate a transition from out-of-plane to in-plane governed kinking to occur at an off-axis angle between 10\ub0 and 15\ub0

Verification of hot-spot in complex composite structures using detailed FEA

Motivation Current shell-element based design tools used in the automotive industry do not allow for failure prediction in complex composite structures. An automated method to identify and analyse structural hot-spots for all potential failure modes is therefore needed

Thermo-mechanical variability of post-industrial and post-consumer recyclate PC-ABS

The aim of this work is to investigate the performance variability of post-industrial (PIR) and post-consumer recycled (PCR) polycarbonate acrylonitrile-butadiene-styrene (PC-ABS). In addition, necessary testing methodology for understanding polymer variation in recycled polymers are established. The thermal expansion behaviour of all tested material were found to be similar and FT-IR testing revealed no conclusive evidence of oxidative degradation. Both PIR and PCR exhibited similar levels of variation in mechanical properties compared with prime samples, with the exception of elongation at break and quasi-static impact behaviour. In these two tests, prime polymers showed lower variation and superior performance to both recycled polymers. The presence of defects and changes in molecular weight were determined to be leading causes of the reduced deformability. Our work contributes by identifying key areas where recycled PC-ABS show good potential as replacements for neat PC-ABS. Furthermore, the work demonstrates methods for material testing against performance criteria to pave way for effective replacement of neat PC-ABS with its recycled counterparts

Implementation of failure criteria for orthotropic non crimp fabric composite structures

In this paper a set of failure criteria for Non-Crimp Fabric (NCF) reinforced composites are implemented in a Finite Element (FE) software. The criteria are valid for transverse failure of NCF reinforced composites with their inherent orthotropic properties and are used at ply level. Validation of the failure criteria is performed on coupons with features typically found in automotive structures

Efficient screening of composite structures using the extended 2D FEM approach in Meta together with a state of the art failure initiation criterion

In order to assess complex composite structures, the full 3D stress tensor is needed to ensure that all possible failure modes can be captured. Conventional shell elements only give accurate results for the in-plane stress components, while the out-of-plane components are neglected. The implementation of the Extended 2D FEM approach [1] into Metapost [2] makes it possible to get the full 3D stress tensor from second order shell elements. With the full stress tensor, state of the art set of failure initiation criteria can be used to evaluate the component. Two different cases are used to show the applicability of the procedure. The first example is a simply supported plate from the literature [3] is in particular used to demonstrate the visualisation capabilities in Metapost. The second example is a set-up used to measure\ua0 the out-of-plane strength of composite materials [4], it is here analysed using both the Extended 2D FEM approach and a state of the art set of failure initiation criteria, LaRC05 [5]