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Fatigue testing of CFRP materials

Final Report Summary - FATIMA (Fatigue testing of CFRP materials)

Executive Summary:
The FATIMA project consortium worked on testing and fatigue prediction of fiber reinforced plastics (FRP). The challenging objective was to go beyond the state of the art in terms of approaches to account for humidity, multi-ply, and multi-axiality effects in organic laminate structures.
Objective of FATIMA project was the adaptation and expansion of a methodology of accelerated fatigue testing and predicting of the lifetime of organic fiber reinforced structures, which has been introduced and comprehensively been described by Miyano.
Successful feasibility study of the existing methodology was carried out with the material IS 400, a typical epoxy based material with woven fiber reinforcement. The advance of the methodology was achieved by applying dedicated tests, by using Digital Image Correlation (DIC) methods, and advanced simulation techniques in order to expand the existing methodology towards humidity influence, multi-ply and multi-axiality effects.
The material was supplied by the Clean Sky ED-ITD partners and the tests were carried out in WP1 with equipment capable of varying temperature, frequency, and load amplitude. Thereby the effect of temperature dependent static, transient, dynamic and fatigue contributions was analyzed. The methodology is performed based on three kinds of tests: viscoelastic dynamic mechanical analysis (DMA), constant strain rate (CSR), and cyclic fatigue tests at various temperatures, frequencies, and loading ratios. Modes of structural fatigue were analyzed and compared to the acceleration factors based on the viscoelastic time-temperature shift function. Thereby the new method was clearly revealing its great potential for predicting the critical number of cycles to failure and its dependency on temperature as well as the long term fatigue strength of fiber reinforces polymers including PCB materials.
The methodology enhancement with respect to multi-ply effects was developed and validated in WP2. After material characterization for the individual plies the visco-elastic properties of the epoxy matrix material were extracted and implemented in the FE simulation macros. Additionally, microscopic investigations delivered the necessary geometric parameters for simulation. After build-up of detailed 3-D finite element models tensile, bending and shear simulations were carried out. Calibration of 3-D numerical models were successfully accomplished and resulted in a good agreement of the effective Young’s modulus with measured data. Additionally a good agreement of storage and the loss modulus was reached between simulation and experimental results obtained by dynamic mechanical analysis (DMA). In accordance with the multi-ply expansion approach a material characterization was simulated using a layered model. In the final step a full FRP stack was modelled according to the respective ply scheme.
After material characterization in WP1 and development of a toolbox for modeling FRP laminates in WP2 the methodology was expanded in WP3 to complex loading situations with combined tension and bending load. Therefore a specially designed specimen holder was used introducing a bending moment in addition to in-axis tension. Based on the experimental tests numerical simulations were performed to understand the multi-axial fatigue loads and the complexity of this combined load configuration. Finally, a number of failure criteria have been developed and tested for analyzing, describing, and predicting damage in composite materials due to multi-axial loads.

Project Context and Objectives:
fatigue testing and predicting of the lifetime of organic fiber reinforced structures, which has been introduced and comprehensively been described by Miyano.
The development of the methodology was achieved by applying dedicated tests, by using Digital Image Correlation (DIC) methods, and advanced simulation techniques in order to expand the existing methodology towards humidity influence, multi-ply and multi-axiality effects.
Intensive work in the field resulted in a number of concepts for accelerated lifetime predictions for carbon fiber reinforced polymers. Within FATIMA, these concepts were adapted to the material provided by the partners of the Clean Sky consortium (IS400 and HexPly913). Thereby the field covered by the Miyano methodology was substantially extended towards fast and cost-efficient testing of airframe structures. Additionally expansions were approached addressing humidity effects, different stack-up structures, and combining loading (such as tension and bending or tension and torsion) on the lifetime of these composite materials.
First, the change in visco-elastic behavior of organic resins due to moisture adsorption had to be modeled by expanding the time/temperature shift function into a time/temperature/moisture shift function. Second, a tool box had to be developed for composing appropriate models of a specifically stacked laminate. Therefore all plies and structures within the stacked/woven laminate have to be considered and included in the final model. Finally, a number of failure criteria were developed for analyzing, describing, and predicting the damage in composite materials due to multi-axial loads. These had to be evaluated according to their applicability to the provided CFRP systems. Final goal was the definition of an appropriate failure criterion. Main challenge of the project was the adoption of these concepts and their integration into the methodology of fatigue testing.

The project was split into four work packages (WP): WP 1 - Humidity Expansion, WP2 - Multi-ply expansion, WP3 - Multi-axiality expansion, WP4 - Project management and leadership. WP1 was dominated by experiments while the other two packages WP2 and WP3 were dominated by a combination of simulation and experimental work.

1. WP Humidity Expansion
Humidity Expansion addressed the determination of the effect of moisture content within the polymer matrix on the mechanical behavior and on the lifetime of FRP.
Main objectives:
? Characterization of sorption behavior of given sample material (humidity adsorption and desorption over time and temperature)
? Definition and specification of test conditions
? Evaluation of material in reference state , visco, strain rate, fatigue
? Quantitative determination of the effect of different humidity levels on visco-elastic behavior of the resin (effects on visco-elastic master curve and TTSF)
? Quantitative determination of the effect of different humidity levels on constant strain rate test results
? Quantitative determination of the effect of different humidity levels on fatigue test results
? Including the moisture effects into the fatigue model by introducing and quantifying another factor to the shift function resulting in the time-temperature-moisture shift function (TTMSF)
At all experimental stages, the tests were accompanied by state of the art characterization techniques concerning the physical failure analysis (x-sectioning, microscopy, 3-D X-ray tomography etc.). Scanning electron microscopy (SEM) imaging of failed specimen at crack area was carried out for detection of failure mechanisms (fiber matrix interface, matrix cracking, cohesion or adhesion dominated).

2. WP Multi-ply Expansion
Multi-ply Expansion addressed the establishment of a tool box for composing arbitrary laminate structures based on the ply types and orientations they consist of.
Main objectives:
? Physical analysis of plies contributing to the stack of the laminate structure and modeling representative sections of these plies in detail by parametric 3-D finite element meshes
? Experimental determination of the visco-elastic properties, strength, and fatigue behavior of individual plies and adjusting the material as well as the damage parameters by simulating the tests – DMA, constant strain rate, fatigue
? Transferring engineering parameters to effective models of plies and validating the model behavior by independent experimental tests
? Adoption of network and laminate theory to compose the model of full laminate structures based on the individual ply models and validation of simulation results concerning visco-elastic, strength, and fatigue behavior by tests of full stack samples
A tool box for composing arbitrary laminate structures based on the ply types they consist of was established. Starting point was a detailed characterization of the individual plies. Applying experimental and simulation techniques, visco-elastic properties, strength, and fatigue behavior of the single plies were determined. The extracted engineering parameters were then transferred to effective models of individual plies. Finally, the real structures were composed out of the effective ply models by combining network and laminate theories. All models were created based on closed loop combinations of experimental tests, physical analyses, and numerical simulations. In the first step, the engineering parameters were identified and determined for a number of cases defined by a design of experiment (DoE) plan covering the design space according to the options of the material provided by the Clean Sky partners. Afterwards, additional tests were performed to validate the findings at different positions within the design space independently.

3. WP Multi-axiality Expansion
Multi-axiality Expansion addressed the expanding of the existing methodology to complex load situations simultaneously combining several contributions like tensile, shear, and torsion.
The existing methodology of predicting fatigue in fiber reinforces laminates can be applied to any type of uniaxial load situation such as bending or tension. However, real situations are typically characterized by multi-axial loads, in which a mixture of bending, twisting, and tension or compression acts on the structure simultaneously. Hence, the existing methodology shall be expanded in order to avoid compromises like choosing one dominating load direction and neglecting all others. This expansion marks the objective of this WP 3. It was based on known failure criteria such as maximum strain / stress, Hashin and Puck. The applicability, validity, and accuracy of the criteria were assessed in order to identify the best fitting or to derive an even more suitable new criterion. Assessment work was carried out by combining numerical simulations with experimental test and characterization techniques. The simulations utilized FE analysis based on continuous and fracture mechanics approaches. They covered a representative sample while being loaded in several stress and fatigue tests. The computational results were compared to experimental readings of exactly those tests. This allowed calibrating the models the simulation is based on.

Main objectives:
? Specification of uniaxial and well defined multi-axial stress and fatigue tests for CFRP laminates with various strain rates at different temperatures
? Performing the stress tests and recording the evolution of the forces and the 2-D deformation fields vs. time; generating dynamic strain maps from the surfaces of the specimen (from unidirectional plate)
? Assessing the stress tests by numerical simulation based on models calibrated, verified, and validated by experimental readings
? Identifying an appropriate failure criterion to be implemented into the existing methodology of predicting fatigue of the CFRP laminates or deriving an even more suitable criterion
? Performing fatigue tests; determining the experimental lifetime; comparing to lifetime estimation based on simulation results applying the expanded methodology according to the task before

4. WP Project Management and Leadership
The main objectives of the WP Project Management and Leadership are:
? Provide leadership and project management to the task and partners
? Adjust activities in various WPs, interact with WP leaders
? Act as intermediate between project partners and European Commission
? Identify potentially exploitable technologies arising from research and manage IPR issues
Project Management organized work and co-operation between the partners within the various. All reports (technical, financial, management) and deliverables required by the European Commission were delivered in time. Furthermore dissemination and exploitation of results and IPR issues was organized and carried out.

Project Results:
Main S & T results/foregrounds see: attached pdf document 2014_02_27_FinalReport_FATIMA.pdf
Potential Impact:
the project brought together experts in the field of materials testing and property evaluation with the aim of delivering a methodology for evaluating of the long-term fatigue life of CFRP structures.
Materials design and testing has been recognized as a key requirement by airframe manufacturers including all major players. According to the ECO-Design ITD the minimization of the consumption of non-renewable fossilized materials goes along with the need for weight reduction of aircraft structures.
Development of highly effective and weight saving composite materials for civil aircraft was identified as a key enabler in meeting the goals of the ACARE 2020 challenge.
The understanding of the long-term behavior of CFRP structures is a key for the implementation of new materials in the aircraft industry. It is necessary for a world-class capability in this field to exist in Europe. Consequently, the competitiveness of the knowledge-intensive European SMEs will be increased.
FATIMA project supports that goal by:
• Improving the predictive capabilities of fatigue testing and material evaluation
• Having a methodology available for dedicated design of CFRP stacks using a powerful tool box

By the dissemination action and conference-papers the methodology was published to the scientific and industrial community. The exploitation will be driven by the project partners. Relevant customers for the methodology were identified and contacted to realize a shorter time-to-market and competitiveness of newly developed CFRP systems.
Results of the project serve as the basis for future analysis services. The strength of the method is established by a combination of experimental and simulative methods which forms the toolbox for fatigue analysis.

List of Websites:
During the project a website was not provided
For dissemination of the project main results will be published under website of AMIC
including links to other partners.

Partner 1:

Dr.-Ing. Jürgen Keller

AMIC Angewandte Micro-Messtechnik GmbH
Volmerstraße 9B
D-12489 Berlin

phone: +49 30 6392 2540
fax: +49 30 6392 2544

Partner 2:

Dipl.-Ing. Bettina Seiler

Chemnitzer Werkstoffmechanik GmbH
Technologie-Campus 1
D-09126 Chemnitz

phone: +49 371 5347 960
fax: +49-371 5347 961

Partner 3:

Dr. Thomas Winkler

Berliner Nanotest und Design GmbH
Volmerstraße 9B
D-12489 Berlin

phone: +49 30 6392 3880
fax: +49 30 6392 3881