Kritische Betrachtung der Versuchsmöglichkeiten zur Bestimmung der Elastizitätsgrößen, Festigkeits- und Reibungswerte für den transversal-isotropen Werkstoff

Der Bericht behandelt die Bestimmung von Kennwerten der transversal-isotropen Einzelschicht zur Anwendung des Cuntze Failure-Mode-Concepts

Holistic determination of physical fracture toughness values and numerical parameters for delamination analysis considering multidirectional-interfaces

Delaminations are a concern for the structural integrity of fibre composite structures. The decisive material parameter for delaminations is fracture toughness. Structures usually have a multidirectional ply stacking. However, interface orientation dependant fracture toughness values and R-curve effects are neglected in numerical delamination analysis. This work investigates interface orientation specific fracture toughness values and considers R-curve effects in mode I to improve simulation accuracy. Therefore, mode I, mode II and mixed mode fracture toughness values of different interface ply orientations are determined experimentally and verified using numerical analysis of the characterisation specimens with cohesive zone modelling. In this way, an engineering methodology is provided for the experimental characterisation and comprehensive numerical modelling of delaminations in mesoscale progressive damage analysis of multidirectional composite structures. In addition, a full parameter set for the simulation of four different interfaces of M21-T700GC prepreg material is given. It can be shown that the use of standard 0°//0°-values leads to very conservative results. The use of interface specific values increases the accuracy. Ultrasonic scans of the DCB specimens are used to compare the crack front shapes for validation. Not only the load displacement curves of the characterisation specimens are well captured, but also the crack front shapes. This demonstrates that by smearing the microscale effects, the material behaviour can be captured phenomenologically correct by mesoscale modelling suitable for industrial use

Potential of fibre metal laminates in root joints of wind energy turbine rotor blades

The length of rotor blades is showing continuous growth for future wind energy turbines leading to high bending moments, which must be transferred to the hub by the root section. As the growth of the root diameter is limited by factors such as transportability, motivation to improve the load carrying capacity without changing the geometry is high. Hybridisation with metals shows a possibility to intrinsically increase the bearing strength of fibre-reinforced plastics. This publication presents experimental investigations into hybrid laminates to be used in so-called T-joints for connecting rotor blades to the hub of the nacelle of a wind energy turbine. An overview is given about the bearing strength of several material combinations hybridising glass- and carbon fibre-reinforced plastics (GFRP, CFRP) with aluminium, titanium and steel alloys. A GFRP-steel-hybrid can be identified as a material with a high reinforcing effect even for low amounts of steel. A hybrid T-joint demonstrator is manufactured by resin infusion and tested under static tension. In comparison with a GFRP reference, a joining strength increase of about 33% is achieved for a steel content of 3%. Further coupon level tests reveal a weak spot in the transition zone between the monolithic GFRP region and full hybrid region as the static and fatigue resistance clearly decreases in comparison with monolithic GFRP and full hybrid references

How virtual testing can improve the experimental material property determination

For a beneficial use of composites in lightweight structures the correct determination of material properties is an important issue. Countless static tests are available, which should provide mechanical properties like elasticities, strengths, fracture toughness or internal friction. It can be ascertained that not every test leads to the true mechanical value required for the simulation of composite structures. For the understanding of the different test setups and their applicability for material characterization, modelling the test setup leads to great insights. Especially the knowledge about strain and stress distributions is of high importance. However, additional measurement techniques like digital image correlation or ultrasonic inspection can lead to similar findings and should therefore, if possible, be taken into account. The presentation contains different examples how the use of finite element simulation -containing linear or nonlinear material behavior- helps to decide, if a specific test is suitable to lead to the required material property value. Therefore, the specific test setups are modelled considering the determined material properties as input parameters. From the coincidence of the structural behavior of test and simulation, consequently the validity of the test method can be confirmed and the obtained material value can be used for further simulations