Compressive failure of unidirectional NCF composites

With more people flying every year, new technologies are needed to reduce our impact on the environment. One option is to reduce the energy used in flight by introducing lighter materials such as carbon fibre reinforced polymers (CFRP). This type of material can be used in cold to moderately high temperature regions of the aero-engine and the current trend is to maximize its use. The current use of CFRP in aerospace is dominated by tape-based composites (pre-pregs), which are processed in an autoclave. This offers optimum performance but at high cost. The current research project is carried out in response to an industrial need of cost-effective CFRP components in load carrying parts of the engine. Composites based on dry textile\ua0 reinforcements such non-crimp fabric (NCF) offers potential cost savings over tape-based pre-preg materials, with good mechanical properties in general. One problem with the textile composites is their relatively low strength in longitudinal compression. This is due to the higher degree of fibre waviness generated by the textile architecture. The aim with this research projects is to develop failure criteria and computational methods needed for reliable and efficient design of aero-engine components from textile composites. When longitudinally arranged fibres in a composite are wavy, local misalignments are generated with respect to the load axis. These induce shear stresses, critical to compressive failure. The sensitivity to fibre misalignments is generally well known. Yet, no systematic measurements have previously been conducted of its spatial distribution. Existing models for strength prediction consider fibre misalignment representations as either, a scalar value, periodic or random. Our approach is instead based on measurements of fibre misalignment with high accuracy and high spatial resolution in a large number of samples. Misalignment data has been used for statistically based direct assessments of compressive strength. The misalignment data has also been used to calibrate models for strength prediction and for numerical studies to increase understanding. The diversity in studied fibre misalignments are not generated by artificial means, but instead reflect upon the relevant material architecture and processing principles. Many studies on compressive failure seek to model or understand details on kink-band formation. We have instead maintained a clear focus on failure initiation, relevant to aero-engine components. We have addressed the extreme sensitivity of kink-band initiation to fibre misalignment angle that subsequently lead to compressive failure within a ply. We conclude that kink-band initiation in practical fibre composites is a coordinated kinematic event. It requires studies of regions with real (measured) spatial distributions of fibre misalignment angles. These studies are preferably conducted on 2D micrographs parallel to the kink-plane

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

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

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

X-ray tomography data of compression tested unidirectional fibre composites with different off-axis angles

This data article contains lab-based micro-computed tomography (μCT) data of unidirectional (UD) non-crimp fabric (NCF) carbon fibre reinforced composite specimens that have been deformed by compression. The specimens contain UD fibres with off-axis angles of 0\ub0, 5\ub0, 10\ub0, 15\ub0 and 20\ub0 and the compression testing induces kink-band formation. This data formed the basis for the analysis of the influence of in-plane shear on kink-plane orientation as reported in Wilhelmsson et al. (Wilhelmsson et al., 2019)

Effects of ocean sprawl on ecological connectivity: impacts and solutions

The growing number of artificial structures in estuarine, coastal and marine environments is causing “ocean sprawl”. Artificial structures do not only modify marine and coastal ecosystems at the sites of their placement, but may also produce larger-scale impacts through their alteration of ecological connectivity - the movement of organisms, materials and energy between habitat units within seascapes. Despite the growing awareness of the capacity of ocean sprawl to influence ecological connectivity, we lack a comprehensive understanding of how artificial structures modify ecological connectivity in near- and off-shore environments, and when and where their effects on connectivity are greatest. We review the mechanisms by which ocean sprawl may modify ecological connectivity, including trophic connectivity associated with the flow of nutrients and resources. We also review demonstrated, inferred and likely ecological impacts of such changes to connectivity, at scales from genes to ecosystems, and potential strategies of management for mitigating these effects. Ocean sprawl may alter connectivity by: (1) creating barriers to the movement of some organisms and resources - by adding physical barriers or by modifying and fragmenting habitats; (2) introducing new structural material that acts as a conduit for the movement of other organisms or resources across the landscape; and (3) altering trophic connectivity. Changes to connectivity may, in turn, influence the genetic structure and size of populations, the distribution of species, and community structure and ecological functioning. Two main approaches to the assessment of ecological connectivity have been taken: (1) measurement of structural connectivity - the configuration of the landscape and habitat patches and their dynamics; and (2) measurement of functional connectivity - the response of organisms or particles to the landscape. Our review reveals the paucity of studies directly addressing the effects of artificial structures on ecological connectivity in the marine environment, particularly at large spatial and temporal scales. With the ongoing development of estuarine and marine environments, there is a pressing need for additional studies that quantify the effects of ocean sprawl on ecological connectivity. Understanding the mechanisms by which structures modify connectivity is essential if marine spatial planning and eco-engineering are to be effectively utilised to minimise impacts

On matrix-driven failure in unidirectional NCF composites - A theoretical and experimental study

With ever increasing traffic levels, the civil aircraft industry is in constant need of new technologies to make air travel environmentally sustainable. One such technology is light materials to reduce the energy consumption in flight. Currently, up to 53 weight percent of the Airbus A350XWB aircraft is made from carbon fibre reinforced polymer (CFRP) material, mainly in the fuselage and wings. There are other areas in the aircraft where components traditionally made from metals can be replaced with composite material, for instance the cold and moderately high temperature parts of the engines. CFRP fanblades were successively introduced in a civil aircraft engine (GE90) in 1994 and there is now an interest to increase the use of CFRP deeper into the engine. The current research project is carried out in response to an industrial need of cost-effective CFRP components in load carrying parts of the engine. The preferred route is an automated, out-of-autoclave manufacturing method using resin transfer moulding (RTM) with a non crimp fabric (NCF) as reinforcement. Carbon fibre/Epoxy composites reinforced with NCF offer potential cost savings over tape based prepreg materials, with good mechanical properties - close to that of prepreg type composites. For this reason, NCF-reinforced composites provide an interesting alternative to prepregs for the aerospace industry. One limiting factor for their use in primary structures is their relatively low strength in compression. The overall goal of this research project is to further understand the compressive behaviour of NCF composites and to develop a strength assessment method for the aerospace industry.In the first part of the project, we increase the fundamental understanding of the composite material on a meso-scale level, where the NCF have a specific architecture, consisting of fibre tows in various configurations. A new failure criterion has been developed to take into account the orthotropic properties in the transverse (2-3) plane. Local interaction between two bundles out-of-plane was simulated with a finite element model and it was proved to be one possible explanation for the reduced strength out-of-plane. In the second part of the thesis, we investigate the influence of intrinsic material variations on the performance of an NCF composite loaded in compression. These intrinsic variations to the material have been identified as potentially likely to occur in future aero-engine composite structures. In conjunction with the compression test campaign, a method to measure fibre misalignment angles out-of-plane has been developed based on microscopy and a Matlab script. The fibre waviness was found to have a strong adverse effect on the compressive stiffness and strength of the material. The measured compressive strength was reduced to half when the mean fibre misalignment angle was doubled

A high resolution method for characterisation of fibre misalignment angles in composites

In this paper a novel method to characterise fibre waviness in composites is presented and assessed. The proposed method referred to as the “high resolution misalignment analysis” (HRMA) method and is suitable for measurements with high spatial resolution. The HRMA method measures misalignment angles tracing individual fibres in detailed micrographs. Here, the method is evaluated using software-generated images with known statistics to mimic real micrographs. Results reveal that the HRMA method provides very accurate measurements on composites with high fibre waviness, outperforming existing methods, whereas it performs on par with existing methods for materials featuring medium fibre waviness. The HRMA method is capable of characterising a 2 cm2micrograph with a spatial resolution of 55 μm in approximately 1 min on a standard laptop computer. The HRMA code and software-generated images are supplied as supplementary material to this paper

Varför startar inte fler unga företag? : En studie i vilken grad faktorer i samhället utgör hinder för unga i valet att starta företag

I dagens samhälle får småföretagen allt mer fokus där entreprenören blir en viktig aktör. Entreprenören bidrar med ekonomisk tillväxt, arbetstillfällen, sociala- och ekonomiska strukturer. Sverige har få entreprenörer sett till resten av världen och den typiska entreprenören utgörs av en man i 35-54 års ålder. Den grupp där det finns minst entreprenörer är bland ungdomar, där bara 3 % av den yngre befolkningen är entreprenörer. Detta trots att flera undersökningar visar att yngre är mer intresserade av att starta företag än äldre individer. I denna studie har det använts enkäter för att undersöka vilka faktorer unga upplever som hindrande för att starta företag. Där ingick faktorer från både individens omgivning och personliga faktorer som individen själv besitter. Enkäterna besvarades av sistaårselever på gymnasiet. Tanken med studien är att öka förståelsen till varför inte fler unga startar företag idag. Resultatet av studien visade att större delen av ungdomarna upplevde de flesta av de undersökta faktorerna som hindrande men i varierande grad. Främst var det den personliga drivkraften och bristen på tid som var de största hindren varför de inte startade företag. Ungdomars omgivning kom däremot längst ner på deras lista över hindrande faktorer, där majoriteten av ungdomarna inte upplevde sin omgivning som speciellt hindrande