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Databases for fatigue analysis in composite materials
Databases for fatigue analysis in composite materials
4
K.A.M. Vallons KU Leuven, Leuven, Belgium
4.1
Introduction
Since the early emergence of composites, there has been active research into their fatigue performance. Hundreds of studies on a wide range of composite materials have been carried out over the past few decades, analysing the behaviour of these materials under repetitive cyclic loading conditions. The topic also remains of high interest in today’s composites community, as many of the current and envisaged applications of composite materials are situated in areas where fatigue is a very relevant factor, for example wind turbines, automotive, aeronautics, etc. The result of all these experimental studies into the fatigue performance of composites is that an enormous amount of fatigue data has been generated over the years. This type of data can be extremely useful for design purposes, or for the development or validation of analytical or finite element models for the prediction of fatigue life. Unfortunately, many of the fatigue studies have only been published as conference and journal papers, often even only showing the obtained fatigue data on a graph. In most cases, the explicit data sets are unavailable to the general public. Of course, through personal communication with the author(s), access to the data can be obtained, but this approach is far from ideal. To facilitate the use of the existing data, it would be much more convenient to publish these data sets with the paper, or at least make them easily available to the public in some way. Some systems, like ResearchGate (www.ResearchGate.net), already offer researchers the possibility to upload their data sets so that other members can access them, although this option is not yet used very frequently. A number of journal publishers is also aiming to expand the availability of research data. Elsevier, for example, is working on providing ‘reciprocal data linking’ capabilities, which allows for a two-way link to be created between a research paper published on its online platform and the corresponding data set, which is then located in a separate repository (Boersma, 2013). Although a system of reciprocal linking between research papers and research data greatly improves the accessibility and visibility of the material, one key issue remains, namely the individualistic nature of these records. Researchers or designers looking for data on a variety of materials often still have to ‘assemble’ manually separate data sets to obtain the desired data. A great effort to collect a multitude of fatigue data from literature and combine them was done by Quaresimin, Ricotta, and Susmel (2004), who used about 140 different data sets from literature, for various types of composites and testing conditions, to develop a phenomenological model for the prediction of Fatigue of Textile Composites. http://dx.doi.org/10.1016/B978-1-78242-281-5.00004-3 Copyright © 2015 Elsevier Ltd. All rights reserved.
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fatigue life. Unfortunately, though, the database built by these researchers was never made public. Some large-scale initiatives relating to the fatigue behaviour of composite materials have, however, been taken over the past few decades to bring extensive amounts of data together in publicly accessible databases. To the author’s knowledge, three of these databases exist: the FACT database, the OptiDat database, and the SNL/MSU/ DOE Fatigue of Composite Materials database. Not surprisingly, all three of these databases are connected to the wind turbine industry, where fatigue is a crucial factor. This chapter will treat these existing databases in some detail, explaining their history, discussing where to find them and giving an overview of which type of data one can expect to find in them.
4.2
FACT database
The FAtigue of Composites for wind Turbines (FACT) database was developed by De Smet and Bach (1994). This database was commissioned by the Netherlands Energy Research Foundation, and was presented at the Third IEA Symposium on Wind Turbine Fatigue in The Netherlands (De Smet & Bach, 1994). It mainly contains results collected from an extensive literature survey, and was published as an MS Excel type of file. The database contains around 1500 data points. Each row in the spreadsheet represents one data point. The material and production details and the test parameters are listed in the columns. Results in the database include data obtained for similar materials tested in different laboratories, under different conditions, and using different test standards. Most of the data are for glass fibre composites, although a few carboneglass hybrids are included. Epoxy, polyester and vinylester are the main matrix materials. The database also contains a number of data points for fatigue tests at temperatures different from room temperature, and for high humidity conditions. Other than the information included in the database itself, it is very hard to retrieve any information on the exact testing conditions. The data in the FACT database is limited to the raw test data, and very little reference is made to related test reports or publications. An important caveat is that it is very difficult to distinguish which reinforcement type the tested materials are built up of. In some cases, the stated production method discloses this information, for example in the case of a filament winding process. Most of the data included, however, originates from laminates produced by hand lay-up. These are very likely based on textiles, but, other than the areal density and fibre orientations, no data is shared on the nature of these fabrics. Although from the quoted fibre orientations in the textiles it can be suspected that many of these materials in the database are based on non-crimp fabric reinforcements, no additional information is available, for example on the stitching parameters. The FACT database was not actively made available on the internet since its publication, but upon the development of the OptiDat database, which is discussed in the next section, it was absorbed into the former, thereby also making these data more accessible.
Databases for fatigue analysis in composite materials
4.3
77
OptiDat database
In 2002, the OPTIMAT BLADES project was started by a consortium of 18 partners from eight European Union countries, including research institutes, manufacturers and certification bodies. The project was supported by the European Commission by means of a fifth framework research grant. The main objectives of the OPTIMAT BLADES project were to gain improved knowledge of the behaviour of wind turbine rotor blade materials under different conditions and interaction effects, and to provide a consistent and integral approach to the design of rotor blades and, as such, offer a basis for updated design recommendations for wind turbine blades (Wingerde et al., 2003). In the framework of this project, the OptiDat database was constructed. During the project, OptiDat was used to archive and exchange results between the partners, to track the progress and to aid in the development of material models. It has been made publicly accessible, however, so that the data collected during the research is now available to anyone, for example for the comparison of a new wind turbine composite material’s performance to that of the OPTIMAT-investigated laminates. It can also be used for the development and validation of models to predict fatigue life under various conditions, or material behaviour in complex stress states, extreme conditions, variable amplitude fatigue, etc. The OptiDat database can be downloaded as an Excel document from http://www.wmc.eu/optimatblades_optidat.php. The database was updated in 2011 with the results of the more recent UpWind project. OptiDat contains around 3500 data points from the OPTIMAT BLADES project, plus an additional 1300 from the UPWIND project (and the 1500 results from the original FACT database). As a consequence of the strong link to wind turbine blade materials, all of the data generated in the OPTIMAT BLADES project is for glass fibreeepoxy systems. The later-added UPWIND data do contain some very limited results on glass/carbon hybrid and carbon fibre composites. Like the FACT database, little reference is made in the database itself to the exact reinforcement architecture (non-crimp, woven, etc.) used for the tested laminates. The accompanying documents, however, make it clear that the majority of the data is for non-crimp fabric composites, which is not surprising, since they are heavily used in the wind turbine industry. The reference reinforcing materials for the OPTIMAT project consisted of a quasiunidirectional non-crimp fabric and a 45 non-crimp fabric, for the UPWIND project it was a quasi-unidirectional woven fabric with 5% off-axis reinforcement. The database contains data for a number of different lay-ups and laminate orientations, and results are also included for loading under extreme conditions, spectrum loading and biaxial loading conditions. There are data for different temperatures and humidity levels and for samples immersed in salt water. Static strength measurements are quoted, as well as post-fatigue residual strength data. The type of fatigue loading covered is very diverse, with R-ratio mainly 0.1, 10 and 1, but by no means limited to these values. Data for both constant amplitude loading and variable amplitude loading are included. The Excel file is structured with several sheets (tabs) in the document. The actual OptiDat, FACT and UPWIND data are each on an individual sheet, but a number of other sheets have been added to make the database more practical to use. There
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are separate sheets on the different specimen geometries, laminate characteristics and constituent materials, plate properties (fibre volume fraction, void content, DSC results, etc.), OPTIMAT coupon nomenclature, the different test types, etc. The main OptiDat database sheet itself is also very extensive, with about 100 columns providing detailed information on the exact test parameters and material/coupon properties. Because of the size of the database sheet, and the use of many codes for the testing conditions and material specifications, it can be difficult to locate the exact data of interest in the file. The developers of the database, however, have added a convenient tool (Excel macro) to facilitate retrieving relevant data and plotting of the data. This tool allows the user to show/hide the columns of interest, and to create plots or data excerpts from selected data. To further facilitate working with the database, a reference document for it has been made available (Nijssen, 2006). It can be downloaded from the same Website as the main database file. This document is intended to provide guidance to users of the database. It contains a detailed description of the layout of the database, and a short explanation is given for each of the spreadsheets in the document. It also contains an explanation of the OPTIMAT nomenclature for the tested specimens. A strong point of OptiDat is the fact that for many of the data, a specific reference is given in the database to a relevant report, discussing the related tests and test results in more detail. These documents often also contain more information on the exact material types used for that series of tests and an analysis of the results. The documents can be downloaded from http://www.wmc.eu/public_docs/. Several conference papers and other types of publications on the OPTIMAT BLADES project, based on or referring to the data contained in OptiDat, can also be found on that Website. The graph in Figure 4.1 is based on data from the OptiDat database. It shows the fatigue life for samples that have been exposed to different environmental conditions (elevated temperature and salt water exposure).
4.4
SNL/MSU/DOE database
The SNL/MSU/DOE Fatigue of Composite Materials database was generated under the SNL/MSU fatigue testing program. It is the result of a long-term joint effort between the Sandia National Laboratory (SNL), a division of the American Department of Energy (DOE), and Montana State University (MSU). The primary goal of the program, which was originally started in 1989, is to research and test a broad range of structural laminate materials that are of interest to wind turbine blade structures. As a consequence, and as was the case for OptiDat, the data in the database consists mostly of results for glass fibre composite systems. A number of the more recent results in the SNL/MSU/DOE database, however, are for carbon fibre or carbon/glass hybrid composites, as they have become of interest for use as blade materials in very large-scale wind turbines. The vast majority of data is based on stitched reinforcement textiles (non-crimp fabrics) with fibre orientations of 0 , 45 or a combination of these, common in the wind energy industry. Very few data on woven fabric composites are included in the earlier results.
Databases for fatigue analysis in composite materials
79
Maximum fatigue stress (MPa)
1000
100
Room temperature –40ºC +60ºC Immersed in salt water for 12 months 10 10
100
1000
10,000
100,000
1,000,000 10,000,000
Cycles to failure
Figure 4.1 Example of data extracted from the OptiDat database, illustrating the effect of extreme environmental conditions on the fatigue life of the tested glass fibre composite.
The SNL/MSU/DOE database can be downloaded from the Website of Montana State University (http://www.coe.montana.edu/composites/). It now includes more than 12,000 individual test results from over 250 different material systems. The materials used include laminates with a range of fibres and fabrics, resins, fibre contents and laminate constructions, as well as adhesives and core materials. A broad range of standard and specialised test methods and loading conditions were used, and environmental (temperature, water exposure) and processing effects were also assessed. In the past, the SNL/MSU/DOE database was only available as a very long PDF file, enumerating all the individual results one by one. This format was very cumbersome to work with, and there were no convenient search tools available to locate data of interest. In a recent update (24 June 2013), however, the database was released as an Excel file. This greatly enhances the possibilities for researchers and designers looking for relevant data, allowing for more convenient searching and sorting by materials and/ or properties. Like the OptiDat database, the current database document is structured by using several sheets (tabs) in the Excel file. The first tab contains an introduction, with some general information and notes, and references to related reports and publications that can be downloaded from the same Website as the database. The actual data from the fatigue tests is located in the following sheets. A distinction is made between ‘older’ data, that is data generated up to 2008, which is collected on one sheet called ‘earlier materials,’ and the more recent data, which has been split up into different sheets, based on the laminate fibre orientations (100% 45 , multidirectional, and 100% unidirectional). A separate sheet was added with fatigue results from variable amplitude loading tests. Apart from those, there are also several sheets with results from more specialised types of tests, some of them dynamic, but static test results are also included (3D tests, adhesive tests, etc.).
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On the data sheets, similar to the OptiDat database, each data point is on a separate row. In the columns are various material and laminate characteristics, and of course the test details and results. The used fabrics are only identified by their (manufacturers) code, not even quoting the type of fabric (woven, non-crimp, etc.), making identifying the exact type of reinforcement textile and certainly its specific characteristics still rather difficult, although the numerous reports and publications linked to the database data sometimes can give a bit more information or a picture of the fabrics. The creators of the database regularly present the obtained results in conferences and the contractor reports are also made public. These documents can be downloaded from the Website (http://www.coe.montana.edu/composites/); the most recent are Samborsky, Mandell, and Miller (2012, 2013) and Mandell et al. (2010). The in-depth discussion and analysis of test results treated in the records greatly enhance the value of the data in the database, as it provides the user with the necessary context and framework in which the data was collected. It is, however, not always easy to identify the exact publication that treats the data of interest, as, contrary to the OptiDat database, only a global list of publications is included with the database, and no direct references are made from individual (sets of) data to the related documents. Figure 4.2 shows an example of the use of the SNL/MSU/DOE database, where data for a series of similar glass fibre quasi-unidirectional composites with a range of fibre volume fractions was extracted from the database and analysed to show that an increasing fibre volume fraction can have a negative effect on the fatigue behaviour of this type of composite.
Normalised maximum fatigue stress
1.25 Vf = 0.333 Vf = 0.388 Vf = 0.492 Vf = 0.520
1
0.75
0.5
0.25
0 1.0E+00
1.0E+02
1.0E+04
1.0E+06
1.0E+08
Number of cycles to failure
Figure 4.2 Illustration of the use of data from the SNL/MSU/DOE database, to assess the influence of the fibre volume fraction on the fatigue life of quasi-unidirectional glass fibre composites. Reprinted with permission from Vallons, Adolphs, Lucas, Lomov, and Verpoest (2013).
Databases for fatigue analysis in composite materials
4.5
81
Concluding remarks
This chapter gave an overview of the available channels for finding existing composite fatigue data. Apart from individual study results, as published in journal papers, etc., there are only two currently active, publicly accessible databases containing fatigue data results for composite materials: the European OptiDat database, into which the older FACT database has been absorbed, and the American SNL/MSU/DOE database. Both databases have their origin in wind turbine blade materials research, which is reflected in the types of laminates that have been tested, as well as in the type of tests. Data in both resources is mainly for glass fibre composites, although very few data for carbon fibre or glass/carbon hybrid composites are included, since these materials have now become of interest for very-high-power wind turbines. Another consequence of the link with the wind industry is that the vast majority of data is for composites based on stitched (non-crimp) reinforcements. Very few results for other types of textiles are included. Fibre orientations in the laminates are usually 45 , 0 or a combination thereof, and fatigue loading ratios are usually 0.1, 1 or 10. A summarising overview of the three databases discussed in this chapter is provided in Table 4.1.
Summary table of the main features of the discussed databases
Table 4.1
FACT
OptiDat - UpWind
SNL/MSU/DOE
Fibres
Mainly glass, some glass/carbon hybrids
Mostly glass, some glass/carbon and carbon
Mostly glass, some glass/carbon and carbon
Resins
Epoxy, polyester, vinylester
Epoxy
Epoxy, polyester, vinylester, limited short carbon fibre thermoplastic
Production techniques
Hand lay-up, filament winding
Vacuum infusion, RTM (resin transfer moulding)
Vacuum infusion, RTM, prepreg, hand lay-up
R-ratios
Not listed
Environmental testing
Temperature, relative humidity
Temperature, humidity, salt water
Temperature, water exposure
Available from
Not active, has been incorporated in the OptiDat database
http://www.wmc.eu/ optimatblades_ optidat.php
http://www.coe. montana.edu/ composites/
Updated
No
Occasionally, last update: 2011
Regularly, last update: 2013
2,5; 1; 0,4; 0,1; 0,5; 0,9; 1; 1,1; 2; 10
2; 1; 0,5; 0,1; 0,5; 0,7; 10; 15
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Apart from these two databases linked to wind turbine blades, it is very likely that other, similar initiatives exist, probably linked to certain large organisations or companies. Unfortunately, although they almost certainly are out there, they are not open to the general public. This is regrettable, since making this kind of data available to researchers and designers could greatly improve the understanding of the fatigue process in composites and facilitate the difficult task of designing with these materials. Without access to essential experimental data, the successful widespread use of composite materials is slowed down, while researchers developing fatigue models and designers are obliged to run costly test series themselves, or rely on the limited available data.
References Boersma, H. (2013). Bringing data to life with data linking [Online]. Available http://www. elsevier.com/connect/bringing-data-to-life-with-data-linking Accessed 21.01.14. De Smet, B. J., & Bach, P. W. (1994). DATABASE FACT, fatigue of composites for wind turbines. 3d IEA symposium on wind turbine fatigue. The Netherlands: ECN Petten. Mandell, J. F., Samborsky, D. D., Agastra, P., Sears, A. T., Wilson, T. J., Ashwill, T., et al. (2010). Analysis of SNL/MSU/DOE fatigue database trends for wind turbine blade materials contractor report SAND2010-7052. Albuquerque, NM: Sandia National Laboratories. Nijssen, R. (2006). Optidat database reference document. Available http://www.wmc.eu/ optimatblades_optidat.php Accessed 24.01.14. Quaresimin, M., Ricotta, M., & Susmel, L. (2004). Fatigue life prediction of composite laminates. 11th European conference on composite materials (ECCM-11), May 31eJune 3 2004 Rhodes, Greece. Samborsky, D. D., Mandell, J. F., & Miller, D. (2012). The SNL/MSU/DOE fatigue of composite materials database: recent Trends. In 53rd AIAA/ASME/ASCE/AHS/ASC structures, structural dynamics, and materials conference, 23e26 April 2012. Honolulu, Hawaii. Samborsky, D. D., Mandell, J. F., & Miller, D. A. (2013). Creep/fatigue behavior of resin infused biaxial glass fabric laminates. Boston: AIAA SDM Wind Energy Session. Vallons, K., Adolphs, G., Lucas, P., Lomov, S. V., & Verpoest, I. (2013). Quasi-UD glass fibre NCF composites for wind energy applications: a review of requirements and existing fatigue data for blade materials. Mechanics and Industry, 14, 175e189. http://dx.doi.org/ 10.1051/meca/2013045. Wingerde, A. M. V., Nijssen, R. P. L., Delft, D. R. V. V., Janssen, L. G. J., Brøndsted, P., Dutton, A. G., et al. (2003). Introduction to the OPTIMAT BLADES project. In European wind Energy conference (EWEC) Madrid, Spain.
4
K.A.M. Vallons KU Leuven, Leuven, Belgium
4.1
Introduction
Since the early emergence of composites, there has been active research into their fatigue performance. Hundreds of studies on a wide range of composite materials have been carried out over the past few decades, analysing the behaviour of these materials under repetitive cyclic loading conditions. The topic also remains of high interest in today’s composites community, as many of the current and envisaged applications of composite materials are situated in areas where fatigue is a very relevant factor, for example wind turbines, automotive, aeronautics, etc. The result of all these experimental studies into the fatigue performance of composites is that an enormous amount of fatigue data has been generated over the years. This type of data can be extremely useful for design purposes, or for the development or validation of analytical or finite element models for the prediction of fatigue life. Unfortunately, many of the fatigue studies have only been published as conference and journal papers, often even only showing the obtained fatigue data on a graph. In most cases, the explicit data sets are unavailable to the general public. Of course, through personal communication with the author(s), access to the data can be obtained, but this approach is far from ideal. To facilitate the use of the existing data, it would be much more convenient to publish these data sets with the paper, or at least make them easily available to the public in some way. Some systems, like ResearchGate (www.ResearchGate.net), already offer researchers the possibility to upload their data sets so that other members can access them, although this option is not yet used very frequently. A number of journal publishers is also aiming to expand the availability of research data. Elsevier, for example, is working on providing ‘reciprocal data linking’ capabilities, which allows for a two-way link to be created between a research paper published on its online platform and the corresponding data set, which is then located in a separate repository (Boersma, 2013). Although a system of reciprocal linking between research papers and research data greatly improves the accessibility and visibility of the material, one key issue remains, namely the individualistic nature of these records. Researchers or designers looking for data on a variety of materials often still have to ‘assemble’ manually separate data sets to obtain the desired data. A great effort to collect a multitude of fatigue data from literature and combine them was done by Quaresimin, Ricotta, and Susmel (2004), who used about 140 different data sets from literature, for various types of composites and testing conditions, to develop a phenomenological model for the prediction of Fatigue of Textile Composites. http://dx.doi.org/10.1016/B978-1-78242-281-5.00004-3 Copyright © 2015 Elsevier Ltd. All rights reserved.
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Fatigue of Textile Composites
fatigue life. Unfortunately, though, the database built by these researchers was never made public. Some large-scale initiatives relating to the fatigue behaviour of composite materials have, however, been taken over the past few decades to bring extensive amounts of data together in publicly accessible databases. To the author’s knowledge, three of these databases exist: the FACT database, the OptiDat database, and the SNL/MSU/ DOE Fatigue of Composite Materials database. Not surprisingly, all three of these databases are connected to the wind turbine industry, where fatigue is a crucial factor. This chapter will treat these existing databases in some detail, explaining their history, discussing where to find them and giving an overview of which type of data one can expect to find in them.
4.2
FACT database
The FAtigue of Composites for wind Turbines (FACT) database was developed by De Smet and Bach (1994). This database was commissioned by the Netherlands Energy Research Foundation, and was presented at the Third IEA Symposium on Wind Turbine Fatigue in The Netherlands (De Smet & Bach, 1994). It mainly contains results collected from an extensive literature survey, and was published as an MS Excel type of file. The database contains around 1500 data points. Each row in the spreadsheet represents one data point. The material and production details and the test parameters are listed in the columns. Results in the database include data obtained for similar materials tested in different laboratories, under different conditions, and using different test standards. Most of the data are for glass fibre composites, although a few carboneglass hybrids are included. Epoxy, polyester and vinylester are the main matrix materials. The database also contains a number of data points for fatigue tests at temperatures different from room temperature, and for high humidity conditions. Other than the information included in the database itself, it is very hard to retrieve any information on the exact testing conditions. The data in the FACT database is limited to the raw test data, and very little reference is made to related test reports or publications. An important caveat is that it is very difficult to distinguish which reinforcement type the tested materials are built up of. In some cases, the stated production method discloses this information, for example in the case of a filament winding process. Most of the data included, however, originates from laminates produced by hand lay-up. These are very likely based on textiles, but, other than the areal density and fibre orientations, no data is shared on the nature of these fabrics. Although from the quoted fibre orientations in the textiles it can be suspected that many of these materials in the database are based on non-crimp fabric reinforcements, no additional information is available, for example on the stitching parameters. The FACT database was not actively made available on the internet since its publication, but upon the development of the OptiDat database, which is discussed in the next section, it was absorbed into the former, thereby also making these data more accessible.
Databases for fatigue analysis in composite materials
4.3
77
OptiDat database
In 2002, the OPTIMAT BLADES project was started by a consortium of 18 partners from eight European Union countries, including research institutes, manufacturers and certification bodies. The project was supported by the European Commission by means of a fifth framework research grant. The main objectives of the OPTIMAT BLADES project were to gain improved knowledge of the behaviour of wind turbine rotor blade materials under different conditions and interaction effects, and to provide a consistent and integral approach to the design of rotor blades and, as such, offer a basis for updated design recommendations for wind turbine blades (Wingerde et al., 2003). In the framework of this project, the OptiDat database was constructed. During the project, OptiDat was used to archive and exchange results between the partners, to track the progress and to aid in the development of material models. It has been made publicly accessible, however, so that the data collected during the research is now available to anyone, for example for the comparison of a new wind turbine composite material’s performance to that of the OPTIMAT-investigated laminates. It can also be used for the development and validation of models to predict fatigue life under various conditions, or material behaviour in complex stress states, extreme conditions, variable amplitude fatigue, etc. The OptiDat database can be downloaded as an Excel document from http://www.wmc.eu/optimatblades_optidat.php. The database was updated in 2011 with the results of the more recent UpWind project. OptiDat contains around 3500 data points from the OPTIMAT BLADES project, plus an additional 1300 from the UPWIND project (and the 1500 results from the original FACT database). As a consequence of the strong link to wind turbine blade materials, all of the data generated in the OPTIMAT BLADES project is for glass fibreeepoxy systems. The later-added UPWIND data do contain some very limited results on glass/carbon hybrid and carbon fibre composites. Like the FACT database, little reference is made in the database itself to the exact reinforcement architecture (non-crimp, woven, etc.) used for the tested laminates. The accompanying documents, however, make it clear that the majority of the data is for non-crimp fabric composites, which is not surprising, since they are heavily used in the wind turbine industry. The reference reinforcing materials for the OPTIMAT project consisted of a quasiunidirectional non-crimp fabric and a 45 non-crimp fabric, for the UPWIND project it was a quasi-unidirectional woven fabric with 5% off-axis reinforcement. The database contains data for a number of different lay-ups and laminate orientations, and results are also included for loading under extreme conditions, spectrum loading and biaxial loading conditions. There are data for different temperatures and humidity levels and for samples immersed in salt water. Static strength measurements are quoted, as well as post-fatigue residual strength data. The type of fatigue loading covered is very diverse, with R-ratio mainly 0.1, 10 and 1, but by no means limited to these values. Data for both constant amplitude loading and variable amplitude loading are included. The Excel file is structured with several sheets (tabs) in the document. The actual OptiDat, FACT and UPWIND data are each on an individual sheet, but a number of other sheets have been added to make the database more practical to use. There
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are separate sheets on the different specimen geometries, laminate characteristics and constituent materials, plate properties (fibre volume fraction, void content, DSC results, etc.), OPTIMAT coupon nomenclature, the different test types, etc. The main OptiDat database sheet itself is also very extensive, with about 100 columns providing detailed information on the exact test parameters and material/coupon properties. Because of the size of the database sheet, and the use of many codes for the testing conditions and material specifications, it can be difficult to locate the exact data of interest in the file. The developers of the database, however, have added a convenient tool (Excel macro) to facilitate retrieving relevant data and plotting of the data. This tool allows the user to show/hide the columns of interest, and to create plots or data excerpts from selected data. To further facilitate working with the database, a reference document for it has been made available (Nijssen, 2006). It can be downloaded from the same Website as the main database file. This document is intended to provide guidance to users of the database. It contains a detailed description of the layout of the database, and a short explanation is given for each of the spreadsheets in the document. It also contains an explanation of the OPTIMAT nomenclature for the tested specimens. A strong point of OptiDat is the fact that for many of the data, a specific reference is given in the database to a relevant report, discussing the related tests and test results in more detail. These documents often also contain more information on the exact material types used for that series of tests and an analysis of the results. The documents can be downloaded from http://www.wmc.eu/public_docs/. Several conference papers and other types of publications on the OPTIMAT BLADES project, based on or referring to the data contained in OptiDat, can also be found on that Website. The graph in Figure 4.1 is based on data from the OptiDat database. It shows the fatigue life for samples that have been exposed to different environmental conditions (elevated temperature and salt water exposure).
4.4
SNL/MSU/DOE database
The SNL/MSU/DOE Fatigue of Composite Materials database was generated under the SNL/MSU fatigue testing program. It is the result of a long-term joint effort between the Sandia National Laboratory (SNL), a division of the American Department of Energy (DOE), and Montana State University (MSU). The primary goal of the program, which was originally started in 1989, is to research and test a broad range of structural laminate materials that are of interest to wind turbine blade structures. As a consequence, and as was the case for OptiDat, the data in the database consists mostly of results for glass fibre composite systems. A number of the more recent results in the SNL/MSU/DOE database, however, are for carbon fibre or carbon/glass hybrid composites, as they have become of interest for use as blade materials in very large-scale wind turbines. The vast majority of data is based on stitched reinforcement textiles (non-crimp fabrics) with fibre orientations of 0 , 45 or a combination of these, common in the wind energy industry. Very few data on woven fabric composites are included in the earlier results.
Databases for fatigue analysis in composite materials
79
Maximum fatigue stress (MPa)
1000
100
Room temperature –40ºC +60ºC Immersed in salt water for 12 months 10 10
100
1000
10,000
100,000
1,000,000 10,000,000
Cycles to failure
Figure 4.1 Example of data extracted from the OptiDat database, illustrating the effect of extreme environmental conditions on the fatigue life of the tested glass fibre composite.
The SNL/MSU/DOE database can be downloaded from the Website of Montana State University (http://www.coe.montana.edu/composites/). It now includes more than 12,000 individual test results from over 250 different material systems. The materials used include laminates with a range of fibres and fabrics, resins, fibre contents and laminate constructions, as well as adhesives and core materials. A broad range of standard and specialised test methods and loading conditions were used, and environmental (temperature, water exposure) and processing effects were also assessed. In the past, the SNL/MSU/DOE database was only available as a very long PDF file, enumerating all the individual results one by one. This format was very cumbersome to work with, and there were no convenient search tools available to locate data of interest. In a recent update (24 June 2013), however, the database was released as an Excel file. This greatly enhances the possibilities for researchers and designers looking for relevant data, allowing for more convenient searching and sorting by materials and/ or properties. Like the OptiDat database, the current database document is structured by using several sheets (tabs) in the Excel file. The first tab contains an introduction, with some general information and notes, and references to related reports and publications that can be downloaded from the same Website as the database. The actual data from the fatigue tests is located in the following sheets. A distinction is made between ‘older’ data, that is data generated up to 2008, which is collected on one sheet called ‘earlier materials,’ and the more recent data, which has been split up into different sheets, based on the laminate fibre orientations (100% 45 , multidirectional, and 100% unidirectional). A separate sheet was added with fatigue results from variable amplitude loading tests. Apart from those, there are also several sheets with results from more specialised types of tests, some of them dynamic, but static test results are also included (3D tests, adhesive tests, etc.).
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Fatigue of Textile Composites
On the data sheets, similar to the OptiDat database, each data point is on a separate row. In the columns are various material and laminate characteristics, and of course the test details and results. The used fabrics are only identified by their (manufacturers) code, not even quoting the type of fabric (woven, non-crimp, etc.), making identifying the exact type of reinforcement textile and certainly its specific characteristics still rather difficult, although the numerous reports and publications linked to the database data sometimes can give a bit more information or a picture of the fabrics. The creators of the database regularly present the obtained results in conferences and the contractor reports are also made public. These documents can be downloaded from the Website (http://www.coe.montana.edu/composites/); the most recent are Samborsky, Mandell, and Miller (2012, 2013) and Mandell et al. (2010). The in-depth discussion and analysis of test results treated in the records greatly enhance the value of the data in the database, as it provides the user with the necessary context and framework in which the data was collected. It is, however, not always easy to identify the exact publication that treats the data of interest, as, contrary to the OptiDat database, only a global list of publications is included with the database, and no direct references are made from individual (sets of) data to the related documents. Figure 4.2 shows an example of the use of the SNL/MSU/DOE database, where data for a series of similar glass fibre quasi-unidirectional composites with a range of fibre volume fractions was extracted from the database and analysed to show that an increasing fibre volume fraction can have a negative effect on the fatigue behaviour of this type of composite.
Normalised maximum fatigue stress
1.25 Vf = 0.333 Vf = 0.388 Vf = 0.492 Vf = 0.520
1
0.75
0.5
0.25
0 1.0E+00
1.0E+02
1.0E+04
1.0E+06
1.0E+08
Number of cycles to failure
Figure 4.2 Illustration of the use of data from the SNL/MSU/DOE database, to assess the influence of the fibre volume fraction on the fatigue life of quasi-unidirectional glass fibre composites. Reprinted with permission from Vallons, Adolphs, Lucas, Lomov, and Verpoest (2013).
Databases for fatigue analysis in composite materials
4.5
81
Concluding remarks
This chapter gave an overview of the available channels for finding existing composite fatigue data. Apart from individual study results, as published in journal papers, etc., there are only two currently active, publicly accessible databases containing fatigue data results for composite materials: the European OptiDat database, into which the older FACT database has been absorbed, and the American SNL/MSU/DOE database. Both databases have their origin in wind turbine blade materials research, which is reflected in the types of laminates that have been tested, as well as in the type of tests. Data in both resources is mainly for glass fibre composites, although very few data for carbon fibre or glass/carbon hybrid composites are included, since these materials have now become of interest for very-high-power wind turbines. Another consequence of the link with the wind industry is that the vast majority of data is for composites based on stitched (non-crimp) reinforcements. Very few results for other types of textiles are included. Fibre orientations in the laminates are usually 45 , 0 or a combination thereof, and fatigue loading ratios are usually 0.1, 1 or 10. A summarising overview of the three databases discussed in this chapter is provided in Table 4.1.
Summary table of the main features of the discussed databases
Table 4.1
FACT
OptiDat - UpWind
SNL/MSU/DOE
Fibres
Mainly glass, some glass/carbon hybrids
Mostly glass, some glass/carbon and carbon
Mostly glass, some glass/carbon and carbon
Resins
Epoxy, polyester, vinylester
Epoxy
Epoxy, polyester, vinylester, limited short carbon fibre thermoplastic
Production techniques
Hand lay-up, filament winding
Vacuum infusion, RTM (resin transfer moulding)
Vacuum infusion, RTM, prepreg, hand lay-up
R-ratios
Not listed
Environmental testing
Temperature, relative humidity
Temperature, humidity, salt water
Temperature, water exposure
Available from
Not active, has been incorporated in the OptiDat database
http://www.wmc.eu/ optimatblades_ optidat.php
http://www.coe. montana.edu/ composites/
Updated
No
Occasionally, last update: 2011
Regularly, last update: 2013
2,5; 1; 0,4; 0,1; 0,5; 0,9; 1; 1,1; 2; 10
2; 1; 0,5; 0,1; 0,5; 0,7; 10; 15
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Fatigue of Textile Composites
Apart from these two databases linked to wind turbine blades, it is very likely that other, similar initiatives exist, probably linked to certain large organisations or companies. Unfortunately, although they almost certainly are out there, they are not open to the general public. This is regrettable, since making this kind of data available to researchers and designers could greatly improve the understanding of the fatigue process in composites and facilitate the difficult task of designing with these materials. Without access to essential experimental data, the successful widespread use of composite materials is slowed down, while researchers developing fatigue models and designers are obliged to run costly test series themselves, or rely on the limited available data.
References Boersma, H. (2013). Bringing data to life with data linking [Online]. Available http://www. elsevier.com/connect/bringing-data-to-life-with-data-linking Accessed 21.01.14. De Smet, B. J., & Bach, P. W. (1994). DATABASE FACT, fatigue of composites for wind turbines. 3d IEA symposium on wind turbine fatigue. The Netherlands: ECN Petten. Mandell, J. F., Samborsky, D. D., Agastra, P., Sears, A. T., Wilson, T. J., Ashwill, T., et al. (2010). Analysis of SNL/MSU/DOE fatigue database trends for wind turbine blade materials contractor report SAND2010-7052. Albuquerque, NM: Sandia National Laboratories. Nijssen, R. (2006). Optidat database reference document. Available http://www.wmc.eu/ optimatblades_optidat.php Accessed 24.01.14. Quaresimin, M., Ricotta, M., & Susmel, L. (2004). Fatigue life prediction of composite laminates. 11th European conference on composite materials (ECCM-11), May 31eJune 3 2004 Rhodes, Greece. Samborsky, D. D., Mandell, J. F., & Miller, D. (2012). The SNL/MSU/DOE fatigue of composite materials database: recent Trends. In 53rd AIAA/ASME/ASCE/AHS/ASC structures, structural dynamics, and materials conference, 23e26 April 2012. Honolulu, Hawaii. Samborsky, D. D., Mandell, J. F., & Miller, D. A. (2013). Creep/fatigue behavior of resin infused biaxial glass fabric laminates. Boston: AIAA SDM Wind Energy Session. Vallons, K., Adolphs, G., Lucas, P., Lomov, S. V., & Verpoest, I. (2013). Quasi-UD glass fibre NCF composites for wind energy applications: a review of requirements and existing fatigue data for blade materials. Mechanics and Industry, 14, 175e189. http://dx.doi.org/ 10.1051/meca/2013045. Wingerde, A. M. V., Nijssen, R. P. L., Delft, D. R. V. V., Janssen, L. G. J., Brøndsted, P., Dutton, A. G., et al. (2003). Introduction to the OPTIMAT BLADES project. In European wind Energy conference (EWEC) Madrid, Spain.