Bernd Grüber

Experimental Investigation of the Strain Rate Dependent Behaviour of 2D Biaxially and Triaxially Reinforced Braided Composites

The performance of 2D biaxially and triaxially reinforced braided carbon fibre composites under dynamic loading is evaluated in the presented study. The accurate manufacturing of tensile specimen made of braided sleeves is explained particularly with regard to efficiency and reproducibility. In order to determine reliable strain rate dependent properties, the high-speed testing procedure is discussed. Using five materials, the parameter identification is described and relevant material data is provided. The measured stiffnesses and strengths are used to predict the non-linear stress-strain behaviour with an earlier proposed phenomenological damage model for textile composites. The gained orthotropic property-profile provides the input parameters for a numerical analysis of braided composite components using the calibrated model.

Novel Testing Device for the Experimental Stress Concentration Analysis of Multilayer Textile Composites under In-Plane Compressive Load

An analytically based calculation tool for the stress-strain analysis of multilayer composite plate structures with cut-outs is developed which offers high potential to be used in a sense as an analytical sub-model in combination with FE-Analysis Systems. For its validation extensive numerical and experimental investigations have to be carried out. Especially the examination of structures loaded by in-plane compression does not seem to be realizable by conventional testing techniques. That is why a novel testing device for the experimental stress concentration analysis of notched thin plate structures under in-plane compressive load is developed and its application in combination with an adapted optical measurement system is demonstrated.

Calculation method for the determination of stress concentrations in fibre-reinforced multilayered composites due to metallic interference-fit bolt:

Sophisticated analytical solution methods for the stress concentration problem in multilayered composites with interference-fit bolts have been developed on the basis of layer-related solutions. The model is based on a circular or elliptical isotropic bolt integrated in a fibre- or textile-reinforced multilayered composite plate. For the simulation of the interference between bolt and composite plate a modelling strategy has been proposed featuring the application of a defined temperature load to a metallic elastic inclusion. For analysing the stress concentration effects in the surrounding plate, complex-valued displacement functions in combination with the method of conformal mappings for the interface between inclusion and plate and a combination of boundary collocation and least squares method for the outer boundary are used. For the verification of the developed calculation methods, a number of experimental and numerical studies have been carried out and the decay behaviour of the distortions have been compared for different radians. For all combinations of multilayered composite plate and interference-fit bolt investigated so far, a good correlation of the analytically calculated and the numerically determined results can be observed.

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

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.