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

Structural batteries in electric road vehicles -When is it a good idea?

Structural batteries, SBs, are composites that can be used as structural elements in electrical vehicles to store energy while also decreasing their weight and, consequently, their energy consumption. However, research has shown that a transition to SBs does not automatically provide environmental benefits (Hermansson et al., 2021), and that efforts need to be made to assess when the use of SBs will in fact decrease the environmental impact of electrical vehicles. This presentation will include early prospective LCA results of SBs in vehicles and discuss when their use is a good idea, as well as potential improvement opportunities.\ua0ReferencesHermansson, F., Berg, I., Sandberg, K., Asp, L. E., Janssen, M., & Svanstr\uf6m, M. (2021). The environmental benefits and challenges of a composite car with structural battery materials. Paper presented at the Resource Efficient Vehicles Conference, Stockholm

Climate impact and energy use of structural battery composites in electrical vehicles—a comparative prospective life cycle assessment

Purpose: Structural battery composites (SBCs) are multifunctional carbon fibre composites that can be used as structural elements in battery electric vehicles to store energy. By decreasing the weight of the vehicle, energy consumption in the use phase can be reduced, something that could be counteracted by the energy-intensive carbon fibre production. The purpose of this study is to shed light on such life-cycle considerations.Method: Prospective life cycle assessment is used to compare the future cradle-to-grave climate impact and energy use of SBCs in battery electric vehicles to conventional metals and lithium-ion batteries. Additionally, the influences from differ- ent technology development routes, primarily related to the carbon fibre production, are assessed. The functional unit is the roof, hood, and doors of a battery electric vehicle with maintained flexural stiffness used for 200,000 km. To capture the multifunctionality of the material, the lithium-ion battery is also included in the functional unit.Results and discussion: Results show that SBCs have a large potential to decrease the life cycle climate impact and energy use of battery electric vehicles, especially following routes focusing on decreasing the use of fossil resources, both for raw materials and as energy sources. The comparative assessment of multifunctional or recycled materials to conventional mate- rials introduces several methodological challenges, such as defining the functional unit and choice of allocation approach for distributing burdens and benefits between life cycles in recycling. This study illustrates the importance of using both the cut-off and end-of-life recycling allocation approaches to capture extremes and to not provide biased results. This study also highlights the importance of considering the ease of repairability in comparative studies, as damages to car parts made from SBCs are likely more difficult to repair than those made from conventional materials.Conclusions: SBCs have the potential to reduce the life cycle climate impact and energy use for most scenarios compared to conventional materials. Three main methodological challenges were found: the comparison to a material with a well- established recycling system throughout its life cycle, the need for expanding the system boundaries to include the lithium-ion battery, and the difference in repairability of SBCs compared to the conventional material

Physically Based Engineering Models for NCF Composites

Non-Crimp Fabrics - NCF – are increasingly being used as reinforcements in high performance composite materials. NCF offer the manufacturing advantages from textile preforms in combination with excellent mechanical performance. This study concerns the mechanical performance of NCF composites. Through a combination of experimental work and theoretical studies the mechanisms controlling the mechanical behaviour are explained. Fractography is used as a tool to identify governing mechanisms and link these to the material internal structure. Based on the experimental findings, engineering models are suggested predicting the mechanical behaviour of NCF composite laminates. A simplified constitutive model is presented that accounts for the fibre tow out-of-plane waviness. The model is based on Timoshenko beam theory applied on curved beams representing wavy tows in a NCF composite lamina. The model calculates stiffness knock-down factors to be applied on lamina homogenised properties. Experiments show compressive failure of NCF composites to be governed by formation and growth of kink bands. For this reason, a failure criterion predicting kinking failure under multiaxial loading is proposed and validated for a NCF composite system. The criterion is to be used on lamina level in a multiaxial NCF laminate. A test method is proposed for extraction of strength parameters valid for the lamina material in a multiaxial laminate. Compression-after-impact (CAI) behaviour of NCF composite laminates, as monolithic skins and sandwich panel face sheets, is investigated. Fractographic studies show CAI failure to be controlled by formation of kink bands. The experimental studies reveal that kink bands form at relatively low loads and grow gradually during compressive loading. It is suggested that the notch effect from the gradually developing kink bands cause final catastrophic failure in sandwich panel skins. Finite element analyses, simplistically representing the damage with an idealised notch, are shown to predict panel residual strength with reasonable accuracy.QC 2011012

Damage tolerance analysis of NCF composite sandwich panels

International audienc

Fibre waviness induced bending in compression tests of unidirectional NCF composites

Compression testing of carbon fibre composites according to ASTM D6641/D3410 is limited by a maximum of 10% bending for a valid test. This allowable was exceeded in a preceding study, where all the laminates had a large out-of-plane waviness. The aim of this study is to quantify the contribution from the out-of-plane fibre waviness to bending. The fibre waviness was characterized on samples with high magnitudes of bending and the corresponding fibre misalignment angles were then mapped to a plane strain finite element model. This model represents the geometry through the thickness and in longitudinal direction. Virtual strain measurements and associated bending calculations could then be performed in Matlab from the strain field on the upper and lower surfaces, which resulted from compression loading. Virtual bending calculations have also been performed from strain measurements with an optical system (DIC). The numerical model confirms that the out-of-plane waviness has an effect on bending. The magnitudes are however lower than expected, and lower than the experimental values. Bending was in the order of 5 % with a strain gauge of 5 mm, which constitutes half of the allowed amount of bending. It was also confirmed that the length of the strain gauge has a significant effect on the measured bending and on the experimental error

Fibre waviness induced bending in compression tests of uniderectional NCF composites

Compression testing of carbon fibre composites according to ASTM D6641/D3410 is limited by a maximum of 10% bending for a valid test. This allowable was exceeded in a preceding study, where all the laminates had a large out-of-plane waviness. The aim of this study is to quantify the contribution from the out-of-plane fibre waviness to bending. The fibre waviness was characterized on samples with high magnitudes of bending and the corresponding fibre misalignment angles were then mapped to a plane strain finite element model. This model represents the geometry through the thickness and in longitudinal direction. Virtual strain measurements and associated bending calculations could then be performed in Matlab from the strain field on the upper and lower surfaces, which resulted from compression loading. Virtual bending calculations have also been performed from strain measurements with an optical system (DIC). The numerical model confirms that the out-of-plane waviness has an effect on bending. The magnitudes are however lower than expected, and lower than the experimental values. Bending was in the order of 5 % with a strain gauge of 5 mm, which constitutes half of the allowed amount of bending. It was also confirmed that the length of the strain gauge has a significant effect on the measured bending and on the experimental error

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 representativematerial volume. The test matrix consists of two different widths for two off-axis cases, 15\ub0 and 20\ub0 witha total sample size of 24. A two-sample T-test is performed for each off-axis angle to check if there is astatistically 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 teston a 5 % significance level. Neither of the cases may be rejected, i.e. there is no statistically significantdifference on the 5 % level. The 15\ub0 off-axis case returns a p-value of 7.4 % and the 20\ub0 off-axis casegives a p-value of 21.3 %. It can be concluded that the effect is small and not statistically significant. Itmeans that remaining off-axis testing in the larger context can proceed with the nominal width of 12 mm