Optimized experimental methods for the description of the delamination and failure behavior of high performance composites and joints

Steffen Stelzer

Research output: ThesisDoctoral Thesis

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This thesis deals with the characterization of the delamination behavior of endless fiber reinforced polymer (FRP) composites and with through-the-thickness reinforcements that suppress the evolution of such delaminations in composites. Delamination is a common problem occurring in FRP composites because of their layered structure. Yet, there are no standards dealing with the fatigue delamination growth in composite materials. One part of this thesis deals with the characterization of the fatigue delamination behavior of composites in mode I and mode II. Round robin exercises were carried out to evaluate the potential of fatigue delamination tests for standardization. Test campaigns were conducted within subcommittee D30.06 of the American Society for Testing and Materials (ASTM), and committee TC4 within the European Structural Integrity Society (ESIS). The tests were carried out on four carbon fiber reinforced epoxy composites, on one glass fiber reinforced epoxy composite and one carbon fiber reinforced poly-ether-ether-ketone. They showed the reproducibility and in- and inter-laboratory scatter and also highlighted the limits of fatigue delamination tests. Round robin activities within ESIS TC4 and ASTM D30.06 emphasized the need to find new ways for data presentation in order to make the data accessible for design purposes. The three major questions are: (1) is it possible to reduce the scatter of the fatigue crack growth curves, (2) how can the slope of the Paris law curve be reduced and (3) is the detection of a threshold value feasible in composite materials? These questions are faced in this thesis by introducing a new way of data presentation. A Hartman-Schijve based approach, where the crack growth rate is dependent on the amount by which the strain energy release rate exceeds the threshold value, seems to be a reliable way to receive Paris like fatigue crack growth curves with slope values around 2. This is significantly less than slope values of around 10 seen in classical Paris law data presentations. With small slope values the errors in predicted crack growth rate are reduced when considering certain load cases in a composite component. Further, fatigue delamination tests were carried out on carbon fiber reinforced epoxy composites, which were produced via different manufacturing routes. These tests revealed that fatigue delamination tests can even pick up small changes in the fiber distribution, which are caused by changes in the preforming process. Another focus of this work was put on suppression of delaminations in composites and composite-composite joining. A novel joining technology based on 3D shaped metallic pins, which are produced via cold metal transfer (CMT) welding, is presented in this thesis. An integral part of this technology is the form fit connection between the metal and the surrounding CFRP, which is provided by the 3D shape of the CMT pin. The CMT pins are welded onto thin metal sheets, which act as carrier elements and ease the positioning of the pins in the joint. Composite-composite joints were reinforced with CMT pins with the aim to suppress/delay the initiation and growth of delaminations in the joint area by making use of the pins’ form-fit connection and to increase the damage tolerance of the joint by plastic deformation of the CMT pins. In a first test campaign, tests were carried out on co-cured specimens without CMT pin reinforcement. Numerical simulations of these tests helped to get a basic understanding of the failure behavior of co-cured single lap shear (SLS) specimens and to find optimum positions for through-the-thickness reinforcements. Detailed investigations of quasi-static tensile tests on CMT pin reinforced SLS specimens with different pretreatment yielded a failure model for the complex loading behavior of CMT pin reinforced SLS specimens. In a next step, the knowledge gained from this first test campaign was transferred to other types of pin reinforcements.
Translated title of the contributionOptimierte experimentelle Methoden zur Beschreibung des Delaminations- und Versagensverhaltens von Hochleistungs-Faserverbundwerkstoffen und deren Verbindungen
Original languageEnglish
  • Schledjewski, Ralf, Assessor A (internal)
  • Pinter, Gerald, Supervisor (internal)
Publication statusPublished - 2014

Bibliographical note

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  • delamination
  • fatigue
  • fiber reinforced polymer composites

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