Papers by Author: Peter Horst

Abstract: In this paper experiments on the fatigue behavior of flat and tubular Glass Fiber Reinforced Plastics (GFRP) specimens with three different layups ([0/90₅/0], [0/90₂/0_1/2]_s and [0/±45/0_1/2]_s) are presented. The experiments are conducted to study the mechanics under cyclic tension-tension loading (R=0.1) until crack saturation (CDS). Fatigue testing is performed below the critical static load level where first matrix cracks can be observed. Therefore Load Levels (LLs) are derived from crack evolution curves obtained by static tests under usage of transmitted light photography. The shear lag model of Berthelot [1] is applied to the two cross-ply specimens to predict crack evolution. The results show good agreement between the prediction and the experimental data. Deviations can be found in prediction of crack evolution in [0/90₂/0_1/2]_s-specimens. For predicting fatigue stiffness degradation the phenomenological model of Adden [2] is used. The results show good capabilities for predicting stiffness degradation after crack onset.

Abstract: This paper presents experimental and numerical results on the deformations and failureof integral composite T-joints subjected to a realistic combined tensile and bending (mixed mode)load case. For this reason, standard pull-off and mixed mode load cases are experimentally studiedby means of a novel test fixture which keeps the force angle constant to the T-joint’s base and allowsfor repositioning of the specimen in order to minimize constraining forces. Two types of specimenswith varying deltoid radius are investigated. Additionally, kinematically nonlinear numerical simulationsare performed to locate damage onset and deformations of the specimens. It is found that thesimulations are in good agreement with the experimental results.

Abstract: For a set of applications, thin metallic sheets are used in fibre composite materials to reach higher load introduction capacities. The paper deals with a simplified generic problem of this kind. The main subject is the comparison of results of three types of models, namely, a 1D-, 2D- and a 3D version. What can be shown is the fact that results are quite similar with respect to behaviour along the loading direction, but very different in the actual values.

Abstract: In this paper a new variable forming tool concept and associated numerical methods for calculating optimal actuator layout and estimating CFRP part quality are presented. The concept of the tool features a modular design and active control of the forming process to achieve the desired geometry. Initially the laminate is placed on the flat top layer of the forming tool. There it is fixed and compacted using vacuum bagging. After compacting, it is heated up to increase the performance of the forming process using water based tempering of the forming tools top layer. The heated laminate is then formed, pulling the tools top layer into the desired geometry using the actuators. Finally, the formed laminate is cooled and transferred into a mold for curing. The position of the forming tools actuators on the base plates is variable. Numerical optimization in combination with finite element (FE) technologies is utilized, to approximate the tool surface within given error margins, with as few actuators as possible. In addition, results of a numerical method for part quality estimation are shown. The influence of the forming process on mechanical properties due to fiber waviness is taking into account using a self-developed method that includes manufacturing characteristics in FE modeling of the part. The method is based on mathematical descriptions of fiber waviness, which are implemented into a FE model. Therefore a structure discretization assuming perfect fiber orientations is realized and the expected fiber waviness induced by the forming process is applied element-wise.

Abstract: This paper presents a 3D unit-cell approach which enables the estimation of bending, inplaneand coupling stiffness properties in the sense of the classical laminated plate theory for arbitrarilyheterogeneous plates. Periodic boundary conditions which simultaneously allow for in-plane as wellas for bending and twist deformation modes are introduced. Additionally, bending experiments of glassfibre/epoxy cross-ply laminates with cracked outer layer at the tension side are conducted and the stiffnessdegradation due to these transverse cracks is compared to numerical results.

Abstract: The fatigue crack growth behavior in integrally stiffened, welded panels is influenced by residual stresses caused by the welding process. The paper presents a semi-numerical method for the determination of stress intensity factors, taking into account the residual stresses in such a way that the relaxation of the residual stresses during the crack propagation phase is covered. This approach is different from the one presented in [1].

Abstract: The fatigue behavior of non-crimp-fabrics under different multi-axial loading conditions is presented, using a beneficial characteristic of the material, i.e. the fact that cracks may be detected by using an optical microscope. This allows to compare two different damage parameters, the crack density and the stiffness reduction over the life of the specimens.

Abstract: This work presents the enhancement of a pseudo-numerical tool for fatigue crack growth investigations on integrally stiffened metallic panels. The model is based on an analytical approach that demands compatibility of displacement between skin sheet and stiffener. Since the basis model was presented before, the focus of the present work is on the incorporation of residual stress effects in order to improve simulation results of welded panel configurations that are manufactured by laser beam welding or friction stir welding and exhibit a significant amount of residual stresses. The necessary input parameters for the developed residual stress module are determined from experimental residual stress field measurements. Simulation results using the presented approach are compared with results from finite element simulations on a two stringer panel which show the good accordance of the base model as well as the capability of the tool enhancements to account for the crack retarding effect caused by residual stresses.

Abstract: Multiple Site Damage (MSD) is a typical problem of aging aircraft structures. On the other hand a similar situation may occur, if damages in the vicinity of the crack tip of a large crack is investigated. In some preceding papers the author has shown the possibility to assess the criticality of a MSD-scenario by comparing certain data compression measures of the crack scenario in an early stage. All of these scenarios were linked to cracks in one row of fasteners of equal distance, and the method was used in conjunction with the Monte-Carlo Simulation. The idea of using such ways to assess multiple crack scenarios is extended now in the paper presented here. The investigations are extended now in the sense that, the scenarios are more complex as they include large cracks and small cracks in the vicinity of the crack, where no fixed distance of the cracks is foreseen. The attempt is made to relate such results to experimental data on residual strength and therefore, to relate them to sophisticated models like the Gurson model.

Abstract: Widespread Fatigue Damage (WFD) is a technical expression used in the certification of aircraft. The paper presents a set of issues which may influence the probability that a structural item exhibits WFD. The paper mainly follows the line of a stochastic approach to these subjects, and includes the influence of realistic parameters - like manufacturing - and tries to assess what may be the impact of new technologies.