Investigations on the high cycle fatigue strength of short glass fiber reinforced polyamide 66

  • The growth of composite materials is closely linked to the demands of the transportation industry for efficient and resource-saving solutions. The beginnings of modern lightweight construction lie in the aviation industry, which constantly strives to reduce weight in order to achieve greater travel distances and reduce fuel consumption. For this purpose, continuous fiber-reinforced plastics are used, that are characterized in particular by their outstanding specific mechanical properties and therefore meet the high requirements for load-bearing capacity in aviation. In the automotive sector, the focus is similar and increasingly trends towards weight-optimized solutions in order to meet the growing requirements to reduce CO2 emissions. Substituting metallic components with composite materials can be a solution for these challenges. However, the requirements for the materials used in the automotive sector are different from those in the aerospace sector. In contrast to the rather low production volumes in the aerospace industry, vehicle components are manufactured in large quantities and therefore require short cycle times and cost-efficient production. One group of materials that meet these requirements are short-fiber-reinforced thermoplastics (SFRTs). They are characterized in particular by their low-cost manufacturing, which can be achieved through the injection molding process and therefore enables the production of components in high volumes. Glass fibers are mainly used for reinforcement, with the fiber length limited to 1 mm due to the injection molding process. Despite this short fiber length, the strength and stiffness of the pure polymer can be significantly increased with short fiber reinforcement. However, the reinforcement effect is strongly influenced by the fiber orientation resulting from the injection molding process. Layers with different fiber orientation are formed, which are accompanied by a pronounced anisotropic material behavior. Due to the thermoplastic matrix, the mechanical properties of SFRT additionally depend on environmental conditions such as humidity and temperature. Consequently, characterizing the material and thereby covering all influencing factors requires effortful and lengthy testing. Especially when determining fatigue properties for a detailed service life analysis, long testing times combined with high costs must be taken into account. In this case, a quasi-static strength analysis is usually performed for the dimensioning of components, which is often also used with empirical reduction factors for the fatigue analysis. Considering the above mentioned context, the present work investigates the behavior of short glass fiber-reinforced polyamide 66 in the range of very high cycle fatigue (> 106 load cycles) and addresses the question as to whether a fatigue limit exists for the aforementioned material or not. For this particular purpose, various experimental methods are used, which all have in common, that they generate large amounts of data requiring automated processing. The experiments are carried out with test specimens longitudinally and transversely to the injection molding direction in order to observe a possible influence of the fiber orientation on the material’s fatigue behavior. In the first part of this experimental work, the stiffness and the hysteresis data are studied during fatigue tests with different maximum stress levels. Subsequently, the stiffness degradation and the dissipative energy are parameterized to correlate this data with the applied maximum stresses. The data analysis method identifies characteristic stress levels at which fatigue behavior changes. The changes occur in three fatigue life ranges of low cycle, high cycle and very high cycle fatigue. The research shows that a cyclic load with 105 cycles is sufficient to estimate the three mentioned fatigue ranges. In the second part of this experimental work, residual strength tests with acoustic emission (AE) analysis are performed on the cyclically preloaded specimens. AE uses sensors to detect acoustic signals generated by crack initiation and crack growth under mechanical load. Accordingly, in the residual strength tests, only the damage that has not already occurred under cyclic preloading can be recorded. Based on the "acoustic fingerprint", characteristic stress levels were identified at which a change in damage behavior occurred. As long as this "acoustic fingerprint" differs from that of a non-preloaded specimen, it can be concluded that damage was initiated under the applied cyclic load. Finally, a digital twin was generated to investigate the underlying micromechanical mechanisms at the experimentally identified characteristic stress levels. X-ray microscope scans of the specimens were imported into the commercial software GeoDict®. This voxel-based software uses numerically efficient Fast Fourier Transforms (FFT) to analyze the microstructure and simulate the experiments on the real 3D microstructure. The simulations show that the fiber-matrix interface significantly influences the damage behavior in the very high cycle fatigue range. By analyzing the matrix plasticization rate, the stress levels associated with high cycle fatigue can be estimated. Fiber fractures, on the other hand, are only relevant in the low cycle fatigue range. Thus, both the experimental methods and the simulation of a digital twin have been proven suitable to estimate the different fatigue ranges in a time-efficient manner. In addition, the analyses of stiffness degradation in fatigue tests and acoustic emissions in residual strength tests indicate that damage occurs at a loading above stresses, which correspond to a fatigue life of 1011 cycles. This in turn implies that no fatigue limit exists before 1011 load cycles for the short-fiber-reinforced polyamide 66 under investigation.

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Author:Janna Krummenacker
Publisher:BoD - Books on Demand
Place of publication:Norderstedt
Advisor:Joachim Hausmann
Document Type:Doctoral Thesis
Language of publication:English
Publication Date:2022/07/28
Date of Publication:2022/07/28
Publishing Institute:Technische Universität Kaiserslautern
Granting Institute:Technische Universität Kaiserslautern
Acceptance Date of the Thesis:2022/06/02
Date of the Publication (Server):2022/07/28
Number of page:XI, 127
Faculties / Organisational entities:Kaiserslautern - Fachbereich Maschinenbau und Verfahrenstechnik
DDC-Cassification:6 Technik, Medizin, angewandte Wissenschaften / 620 Ingenieurwissenschaften und Maschinenbau
Licence (German):Creative Commons 4.0 - Namensnennung, nicht kommerziell, keine Bearbeitung (CC BY-NC-ND 4.0)