Zum Tragverhalten von Gitterrosten aus glasfaserverstärktem Kunststoff
Bien, Jonas; Hoffmeister, Benno (Thesis advisor); Feldmann, Markus (Thesis advisor); Schröder, Kai-Uwe (Thesis advisor)
Aachen : RWTH Aachen University (2022, 2023)
Dissertation / PhD Thesis
Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022
Conventional pressure-locked or welded gratings made of metallic materials are increasingly replaced by handlaminated gratings made of glass fibre-reinforced plastics, especially where high corrosion resistance or electrical conductivity is required. The consequences arising from the material-related and structural differences on the loadbearing behaviour of such gratings are currently not covered by the applicable technical rules for the design of such components, nor have they been sufficiently investigated in a scientific context. The present study therefore aims to systematically investigate the load-bearing behaviour of hand-laminated gratings made of GFRP and to derive methods for the computational determination of limit states that are relevant for the design. In a first step, the load capacities of the gratings are analysed. For this purpose, resistances of individual grating bars under tension, torsion and combined bending/shear loading are determined experimentally. The experiments are then simulated using complex finite element models to identify relevant stress and damage states at the scale of a single layer. Subroutines are developed for ANSYS Mechanical and integrated via the USERMAT interface to account for nonlinear material behaviour related to microdamage and fracture (Puck's fracture hypoth-esis + degradation analysis) of unidirectionally reinforced and isotropic layers. The characteristics of each lamina, required for the detailed analysis, are esti-mated based on experimental investigations at the level of the base materials (fibres and plastic resin) in combination with micromechanical mixture rules.The simulations agree well with respect to the (nonlinear) stiffnesses and load-bearing capacities of individual bearing bars and allow a differentiated and accurate prediction of the relevant failure mechanisms. Under torsional loading, significant warping stresses in the intersection area between two orthogonal bearing bars are clearly identified. Furthermore, a significant influence of the nonlinear material behaviour as well as a strong warping restraint in the crossing areas is observed even under shear-dominated loading. These effects are comprehensively analysed and evaluated within parametric studies. In addition, the fracture be-haviour of individual grating bars under combined loading is investigated in a pa-rameterised manner in order to complement the experimental findings. Based on these investigations, e.g. a mutual interaction of bending and shear stresses on the load-bearing capacity can be excluded. In the second part of the thesis, the focus shifts to accurately predicting the stress of a bearing bar being part of an actual grating (highly statically indeterminate system) under typical, plate-like loading. For this purpose, the structural frame analysis program gridIT is developed in the MATLAB Appbuilder environment. Spatial beams with 7 degrees of freedom per node and first order shear deformation theory are used for discretisation. In addition to warping torsion, geometrically non-linear effects as well as non-linear support conditions can be considered in the calculation. To calculate the internal forces, equivalent, homogenized stiffnesses are adopted for the beam elements. Stresses on the lamina level, required for the failure analysis, are approximated with good accuracy in a subsequent step. In the absence of suitable solutions for the determination of equivalent torsional parameters of inhomogeneous and trapezoidal cross-sections, additional investigations are carried out using the finite difference method. To calibrate the employed modelling approach, the experimental and numerical investigations on the single bearing bar are used. Moreover, various tests are carried out on grating panels with different spans, widths as well as bearing bar cross-sections and recalculated with gridIT. A comparison of the results suggests that a full warping restraint shall be assumed in the crossings between orthogonal bearing bars, increasing the torsional stiffness of the bearing bars significantly for some grating types. In addition, the prediction of the flexural capacity in the intersection points can be significantly improved by introducing a partially dam-aged state of the cross-section. Finally, it can be shown that torsioninduced shear stresses do not contribute to the development of catastrophic interlaminar shear cracks and can thus be neglected with respect to the evaluation of the load-bearing behaviour of glass fibre reinforced gratings.The validated model is then used to investigate the influence of the material, laminate and cross-section dimensions and their scatter on the resulting equivalent crosssection parameters and capacities of a bearing bar by means of Monte Car-lo simulations. Furthermore, numerous calculations are performed on rectangular gratings under variation of the panel dimensions, grating type, support condi-tions, mesh sizes, loading positions and load cube dimensions as well as under consideration and neglection of warping torsion and geometric nonlinearity in order to systematically evaluate the influence of the different input variables on the resulting ultimate loads and failure modes.The holistic approach of the thesis, starting with investigating the base materials and progressing to the structural element, allows to properly study the effects of a wide range of parameters and their interactions with regards to the load-bearing behaviour of the final grating. The selected procedure can be easily adopted for future analysis of other grating types, material compositions and laminate structures in order to develop general rules for the structural design of GFRP gratings.
- Chair for Steel and Lightweight Metal Construction and Institute of Steel Construction