Albert Langkamp

Theoretical and experimental investigation of anisotropic damage in textile-reinforced composite structures

In the present work, a phenomenological plane-stress damage-mechanics-based model for textile-reinforced composites is presented and its predictive capability is evaluated by carrying out a series of experimental tests. Damage variables are introduced to describe the evolution of the damage state and, as a subsequence, the degradation of material stiffness. For calculating the nonlinear stress and strain distribution of complexly loaded composites with a textile reinforcement, a special emphasis has to be placed on the interaction between the fiber failure due to the stress in the fiber direction and the matrix failure due to the transverse and shear stresses. This demands the formulation of realistic failure criteria taking into account the microstructural material behavior and different fracture modes. The new failure criteria, like the fracture mode concepts, consider these fracture modes, as well as further fracture types, in the reinforcement plane. The failure criteria are based on equations for failure surfaces in the stress space and damage thresholds in determining the stiffness degradation of the composite. The model proposed was used to characterize the strength and the failure behavior of carbon-fiber-reinforced composites. For this purpose, several unidirectional and bidirectional tests were performed to determine the specific properties of the material. The specimens were investigated by using acoustic emission techniques and strain-controlled tension and torsion tests.

In situ integrity assessment of a smart structure based on the local material damping

Integration of functional elements into fibre-reinforced host structures provides the possibility for in situ monitoring of the structural integrity of critical components. In this study, a vibration-based monitoring function has been developed that allows the structural integrity identification of critical components. For this purpose, signal analysis algorithms were developed to enable the estimation of damage-dependent modal damping. The analysed smart structure was a carbon fibre–reinforced epoxy composite plate with an integrated actuating/sensing system. The local material damping is a parameter especially sensitive to different failure modes of composites. In order to characterise the changes of this parameter resulting from impact events, dynamical mechanical analysis on intact and damaged specimens made of the composite material was conducted. Based on the dynamical mechanical analysis results, a finite element model of the structure was developed. Then, modal damping ratios for different sizes and locations of damaged regions were numerically determined, and a relation between modal damping and damage-dependent local damping was identified. The deterministic decision trees describing the reverse relationship between online-measured modal damping and damage condition were determined. That was accomplished through the application of information entropy-based data-mining algorithms to the numerically generated learning dataset obtained using the developed finite element model.

CHAPTER 20 - Polymer composite bearings with engineered tribo-surfaces

Abstract This chapter presents the details of fabrication of surface-tailored composites based on commingled yarns of carbon and polyether-ether-ketone (PEEK), which is a novel method to eliminate some existing limitations in manufacturing bi-directionally reinforced composites. It further elaborates the need for surface tailoring of tribo-composites with solid lubricants rather than their inclusion in the bulk. The details of the process with various powdery lubricants, such as polytetrafluoroethylene (PTFE), MoS2, graphite, etc., are discussed and influence of these on mechanical properties is presented. Based on these studies, PTFE is proved as most promising lubricant for surface tailoring only on the top surface. PTFE in various forms, such as wool, short fibers of various lengths, long fibers, etc., was used to investigate benefits endowed by surface tailoring. Since the maximum performance enhancement (both mechanical and tribological) was observed due to the long PTFE fibers, bearing was fabricated by proper placement of PTFE fibers in commingled yarn composites of PEEK and carbon fibers (CF). Two bearings with and without surface modification were evaluated under different operating parameters. It was observed that CF at an angle of 45° led to the best combination of friction and wear performance. PTFE fiber inclusion removed the stick-slip problem associated with PEEK apart from reducing coefficient of friction from 0.6 to 0.12 and enhancing wear resistance approximately by 70 times. The bearing thus proved very effective for dry situations with very good combination of friction, wear and mechanical properties.

In-process, non-destructive multimodal dynamic testing of high-speed composite rotors

Fibre reinforced plastic (FRP) rotors are lightweight and offer great perspectives in high-speed applications such as turbo machinery. Currently, novel rotor structures and materials are investigated for the purpose of increasing machine efficiency, lifetime and loading limits. Due to complex rotor structures, high anisotropy and non-linear behavior of FRP under dynamic loads, an in-process measurement system is necessary to monitor and to investigate the evolution of damages under real operation conditions. A non-invasive, optical laser Doppler distance sensor measurement system is applied to determine the biaxial deformation of a bladed FRP rotor with micron uncertainty as well as the tangential blade vibrations at surface speeds above 300 m/s. The laser Doppler distance sensor is applicable under vacuum conditions. Measurements at varying loading conditions are used to determine elastic and plastic deformations. Furthermore they allow to determine hysteresis, fatigue, Eigenfrequency shifts and loading limits. The deformation measurements show a highly anisotropic and nonlinear behavior and offer a deeper understanding of the damage evolution in FRP rotors. The experimental results are used to validate and to calibrate a simulation model of the deformation. The simulation combines finite element analysis and a damage mechanics model. The combination of simulation and measurement system enables the monitoring and prediction of damage evolutions of FRP rotors in process.

On the Reduction of the Pre-Processing Effort and the Application of a Contact Meshing Approach for Complex Jet Engine Component Assemblies

A significant proportion of the work effort for finite element (FE) analysis is spent for pre-processing activities, especially for complex structural components and component assemblies. An exclusive use of hexahedron (hex) elements increases the meshing effort substantially compared to tetrahedral elements. An automated method to generate high quality hexahedral meshes for an arbitrary geometry does not exist. In this work, commercially available FE software tools for meshing were investigated with the focus on an advantageous pre-processing effort. The evaluation showed that the software package NX (Siemens PLM Software) offers robust advanced semiautomatic hex meshing capabilities.Furthermore, a Contact Meshing Approach (CMA) was elaborated to reduce the effort of the challenging and time-consuming geometry decomposition significantly. Using the example of an intermediate pressure compressor it can be shown that the pre-processing effort time can be reduced up to 75%. Due to the independent meshes, element transitions in the geometry become redundant. This results in lower total element numbers and higher mesh qualities and subsequently leads to more efficient calculations. Moreover, the increased element quality has positive effects on the result quality.Copyright

Characterising the thermoforming behaviour of glass fibre textile reinforced thermoplastic composite materials

Textile reinforced thermoplastic composites are predestined for highly automated medium- and high-volume production processes. The presented work focusses on experimental studies of different types of glass fibre reinforced polypropylene (GF-PP) semi-finished thermoplastic textiles to characterise the forming behaviour. The main deformation modes fabric shear, tension, thought-thickness compression and bending are investigated with special emphasis on the impact of the textile structure, the deformation temperature and rate dependency. The understanding of the fundamental forming behaviour is required to allow FEM based assessment and improvement of thermoforming process chains.Textile reinforced thermoplastic composites are predestined for highly automated medium- and high-volume production processes. The presented work focusses on experimental studies of different types of glass fibre reinforced polypropylene (GF-PP) semi-finished thermoplastic textiles to characterise the forming behaviour. The main deformation modes fabric shear, tension, thought-thickness compression and bending are investigated with special emphasis on the impact of the textile structure, the deformation temperature and rate dependency. The understanding of the fundamental forming behaviour is required to allow FEM based assessment and improvement of thermoforming process chains.

In-process deformation measurements of translucent high speed fibre-reinforced disc rotors

The high stiffness to weight ratio of glass fibre-reinforced polymers (GFRP) makes them an attractive material for rotors e.g. in the aerospace industry. We report on recent developments towards non-contact, in-situ deformation measurements with temporal resolution up to 200 µs and micron measurement uncertainty. We determine the starting point of damage evolution inside the rotor material through radial expansion measurements. This leads to a better understanding of dynamic material behaviour regarding damage evolution and the prediction of damage initiation and propagation. The measurements are conducted using a novel multi-sensor system consisting of four laser Doppler distance (LDD) sensors. The LDD sensor, a two-wavelength Mach-Zehnder interferometer was already successfully applied for dynamic deformation measurements at metallic rotors. While translucency of the GFRP rotor material limits the applicability of most optical measurement techniques due to speckles from both surface and volume of the rotor, the LDD profits from speckles and is not disturbed by backscattered laser light from the rotor volume. The LDD sensor evaluates only signals from the rotor surface. The anisotropic glass fibre-reinforcement results in a rotationally asymmetric dynamic deformation. A novel signal processing algorithm is applied for the combination of the single sensor signals to obtain the shape of the investigated rotors. In conclusion, the applied multi-sensor system allows high temporal resolution dynamic deformation measurements. First investigations regarding damage evolution inside GFRP are presented as an important step towards a fundamental understanding of the material behaviour and the prediction of damage initiation and propagation.

On the Development of Strategies for an Efficient Semi-Automated Hex-Meshing Process of Complex Jet Engine Component Assemblies

A significant proportion of the work effort for a whole engine analysis is spent for prep-processing tasks especially for component assemblies and complex structural components. With respect to the generation of a pure hexahedral mesh, the work effort increases due to the absence of an automatic method to generate high quality hexahedral meshes for an arbitrary geometry. In addition, the time-consuming hexahedral meshing process contains numerous, repetitive tasks for large and complex assemblies due to similar and identical components.In this work a modular strategy for hexahedral meshing of large and complex assemblies was explored with the aim to reduce and to simplify the development process due to a prospective semi-automation of time-consuming routines. The procedure bases on an initial identification and classification of each component of the whole assembly regarding e.g. overall meshing complexity. Meshing relevant parameters were identified for geometry preparation and hexahedral meshing itself. Furthermore, for semi-automation the software package NX (Siemens NX Software), in particular the incorporated automation tool Product Template Studio (PTS) was investigated which enables an automated re-meshing of the geometry model in case of design changes.Copyright

Advanced Models for the Simulation of Anisotropic Damage in Fibre and Textile Reinforced Ceramics

Abstract The high lightweight potential of fibre and in particular textile reinforced ceramics for high performance applications can only be used optimally, if the structural components as well as the composite material itself are designed according to the acting loads. The basic requirement for this purpose is the use of validated design and dimensioning concepts that primarily consider the failure and damage behaviour as well as the stress and deformation state. For many practical problems, the induced stresses can be calculated relatively well by means of existing analytical, numerical and experimental methods. In contrast, for the realistic description of the damage behaviour of ceramic composite structures, in particular for those with textile reinforcement, validated models barely exist. At the ILK, first physically reasonable failure criteria have been developed and successfully applied for fibre and textile reinforced ceramics. On the basis of extensive multi-axial fracture tests with unidirectional fibre reinforced ceramics, these novel fracture mode related failure criteria could be confirmed in the practically relevant (σ 2 , τ 21 )-stress-plane. Starting from the advanced fracture criteria, new approaches for the description of the failure behaviour of woven ceramic composites have been developed and verified. Thereby, it could be shown that such ceramic composites could be subdivided into two classes with respect to the failure behaviour. For the simulation of the anisotropic damage behaviour of textile reinforced ceramics, adapted material models have been developed at the ILK. These models realistically describe the different fracture modes as well as the anisotropic damage phenomena under consideration of the existing textile architecture, the matrix system and the fibre-matrix-interface.