Showing papers by "Maik Gude published in 2019"

Numerical modelling and simulation of fatigue damage in carbon fibre reinforced plastics at different stress ratios

Abstract: A numerical study of damage behaviour in unidirectional Carbon Fibre Reinforced Plastics (CFRP) laminates at different stress ratios is self implemented. The study is conducted using the commercial finite element software ABAQUS employing a layer-based fatigue damage model (FDM) that is capable to regard typical fatigue phenomena like stress redistributions and load sequence effects. A description of the FDM model is provided. At first, the model is calibrated according to experimental findings. Next, a constant fatigue life (CFL) diagram is constructed using actual test data to generate S-N curves for different stress ratios. Finally, different damage behaviour in CFRP laminates is predicted under quasi-static and cyclic loading conditions. The numerical results are discussed and compared to experimental data. The results obtained show the applicability and accuracy of the calibrated FDM.

Experimental and Numerical Determination of the Local Fiber Volume Content of Unidirectional Non-Crimp Fabrics with Forming Effects

Abstract: Detailed knowledge of the local fiber orientation and the local fiber volume content within composite parts provides an opportunity to predict the structural behavior more reliably. Utilizing forming simulation methods of dry or pre-impregnated fabrics allows for predicting the local fiber orientation. Additionally, during the forming process, so-called draping effects like waviness, gapping or shear-induced transverse compression change the local fiber volume content. To reproduce and investigate such draping effects, different manufacturing tools have been developed in this work. The tools are used to create fabric samples with pre-defined deformation states, representing the different draping effects. The samples are evaluated regarding the resulting fiber volume content. The experimental results are compared with the predictions of an analytical solution and of a numerical solution based on draping simulation results. Furthermore, the interaction of the draping effects at arbitrary strain states is discussed regarding the resulting fiber volume content.

Damage mechanisms of tailored few-layer graphene modified CFRP cross-ply laminates

Abstract: A tailored nanoparticle modification approach for fibre reinforced polymers (FRP) is presented that allows exact analysis of the impact of a nanoparticle matrix modification in either 0°- or 90°-layers of cross-ply laminates on the mechanical properties and damage mechanisms. A modification of the 0°-layers unexpectedly increases the quasi-static tensile strength, although dominated by fibre properties. Positive crack stopping effects as well as negative effects, such as accelerated delamination growth, are discussed. The applicability of nanoparticle modified resin systems to improve the performance of FRP-laminates is sensitive to the loading case. For laminates only loaded in tension a modification appears to be advantageous, whereas in case of bending or compression loads the effect of nanoparticles is ambiguous and should be applied carefully.

Functional Properties of Poly(Trimethylene Terephthalate)-Block-Poly(Caprolactone) Based Nanocomposites Containing Graphene Oxide (GO) and Reduced Graphene Oxide (rGO)

Abstract: This work reports a study on the influence of graphene oxide (GO) and reduced graphene oxide (rGO) on the functional properties of poly(trimethylene terephthalate)-block-poly(caprolactone) (PTT-block-PCL-T) (75/25 wt.%/wt.%) copolymer, obtained from dimethyl terephthalate (DMT), 1,3-biopropanediol and polycaprolactone diol (PCL) via in situ polymerization. The article presents, if and how the reduction of graphene oxide, in comparison to the non-reduced one, can affect morphological, thermal, electrical and mechanical properties. SEM examination confirms/reveals the homogeneous distribution of GO/rGO nanoplatelets in the PTT-block-PCL-T copolymer matrix. More than threefold increase in the value of the tensile modulus is achieved by the addition of 1.0 wt.% of GO and rGO. Moreover, the thermal conductivity and thermal stability of the GO and rGO-based nanocomposites are also improved. The differential scanning calorimetry (DSC) measurement indicates that the incorporation of GO and rGO has a remarkable impact on the crystallinity of the nanocomposites (an increase of crystallization temperature up to 58 °C for nanocomposite containing 1.0 wt.% of GO is observed). Therefore, the high performances of the PTT-block-PCL-T-based nanocomposites are mainly attributed to the uniform dispersion of nanoplatelets in the polymer matrix and strong interfacial interactions between components.

Diffraction-grating-based in situ displacement, tilt, and strain measurements on high-speed composite rotors.

Abstract: Polymer composite rotors offer promising perspectives in high-speed applications such as turbomachinery. However, failure modeling is a challenge due to the material's anisotropy and heterogeneity, which makes high-speed in situ deformation measurements necessary. The challenge is to maintain precision and accuracy in the environment of fast rigid-body movement. A diffraction-grating-based sensor is used for spatio-temporally resolved displacement, tilt, and strain measurements at surface velocities up to 260 m/s with statistical strain uncertainties down to $16\,\,\unicode{x00B5}{\epsilon}$. As a line camera is used, vibrations in the kHz range are measurable in principle. Due to sensor calibration and the use of a novel scan-correlation analysis approach, the rigid-body-movement-induced uncertainties are reduced significantly. The measurement of strain fluctuations on a rotating composite disc show that the crack propagation can be tracked spatially resolved and as a function of the rotational speed, which makes an in situ quantification of the damage state of the rotor possible.

Experimental Strain Measurement for Fibre-Reinforced Finite Mulitlayered Composites with Cut-out Under Bending for Validating an Analytical Calculation Model

Abstract: For validating a newly developed analytical calculation method for the layer-by-layer stress-strain analysis of thin-walled notched multilayered composites with finite outer boundary, the strain field of specimen with cut-out is experimentally determined. For this reason, a unique flexural testing device is enhanced and adapted for composites undergoing large deflection. The strains and their gradients around the cut-out are measured using a digital image correlation technique. Beyond that, especially for the investigation of small measuring fields with a high local resolution and/or with a large relocation during the experiment, a new tracking method for the measurement system is developed and applied. In the case of specimen deflection caused by bending, that system keeps the relative distance and position between the measuring field on the specimen and the optical measuring system within the allowable range. By using the combination of enhanced flexural testing device and new tracking system, the 2D strain-field on the specimen surface is measured with a high local accuracy and shows a good correlation with the predicted strain-field calculated with the new analytical calculation method.

Optical strain measurements on fast moving fiber reinforced polymer rotors using diffraction gratings

Abstract: Abstract In-situ measurements of the deformation and of the structural dynamical behavior of moving composite structures, such as rotors made of glass fiber reinforced polymers (GFRP), are necessary in order to validate newly developed simulation models. Local methods like strain gauges and fiber Bragg gratings lack spatial resolution, while contactless optical methods like image correlation or speckle interferometry suffer from noise effects in the presence of fast rigid body movements. A novel compact sensor – based on the diffraction grating method – is introduced for spatially and temporally resolved strain measurement. The use of a line camera allows the measurement of vibrations up to several tens of kHz. With a scanning movement, strain fields at submillimeter resolution can be recorded. The use of two diffraction orders and an objective lens reduces cross sensitivities to rigid body movements on the strain measurement by two to three orders of magnitude. A validation on a GFRP probe was conducted in a quasi-static tensile test with an optical extensometer up to 14500 µϵ. Furthermore, a strain measurement on a moving rotor at surface speeds up to 75 m/s was performed and the results were compared with those of strain gauges as a gold standard. The statistical standard deviation was around 10 µϵ and independent of the rotational speed.