FANROE IFREMER Infrastructure Access Report

8/16/2019 FANROE IFREMER Infrastructure Access Report

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Infrastructure Access Report

Infrastructure: Materials and Structures group, IFREMER

User-Project : FANROE#1

Fatigue Behavior of Natural Rubber for Ocean

Energy devices (Phase I)

TARRC

Marine Renewables Infrastructure Network

Status: Draft

Version: 01/20-08-13Date: 20-08-13

EC FP7 “Capacities” Specific Programme

Research Infrastructure Action


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ABOUT MARINET

MARINET (Marine Renewables Infrastructure Network for emerging Energy Technologies) is an EC-funded networkof research centres and organisations that are working together to accelerate the development of marine renewableenergy - wave, tidal & offshore-wind. The initiative is funded through the EC's Seventh Framework Programme

(FP7) and runs for four years until 2015. The network of 29 partners with 42 specialist marine research facilities isspread across 11 EU countries and 1 International Cooperation Partner Country (Brazil).

MARINET offers periods of free-of-charge access to test facilities at a range of world-class research centres.Companies and research groups can avail of this Transnational Access (TA) to test devices at any scale in areas suchas wave energy, tidal energy, offshore-wind energy and environmental data or to conduct tests on cross-cutting areassuch as power take-off systems, grid integration, materials or moorings. In total, over 700 weeks of access is availableto an estimated 300 projects and 800 external users, with at least four calls for access applications over the 4-yearinitiative.

MARINET partners are also working to implement common standards for testing in order to streamline thedevelopment process, conducting research to improve testing capabilities across the network, providing training at

various facilities in the network in order to enhance personnel expertise and organising industry networking events inorder to facilitate partnerships and knowledge exchange.

The aim of the initiative is to streamline the capabilities of test infrastructures in order to enhance their impact andaccelerate the commercialisation of marine renewable energy. See www.fp7-marinet.eu for more details.

Partners

Ireland

University College Cork, HMRC (UCC_HMRC)

Coordinator

Sustainable Energy Authority of Ireland (SEAI_OEDU)

DenmarkAalborg Universitet (AAU)

Danmarks Tekniske Universitet (RISOE)

France

Ecole Centrale de Nantes (ECN)

Institut Français de Recherche Pour l'Exploitation de

la Mer (IFREMER)

United Kingdom

National Renewable Energy Centre Ltd. (NAREC)

The University of Exeter (UNEXE)

European Marine Energy Centre Ltd. (EMEC)

University of Strathclyde (UNI_STRATH)

The University of Edinburgh (UEDIN)

Queen’s University Belfast (QUB)

Plymouth University(PU)

Spain

Ente Vasco de la Energía (EVE)

Tecnalia Research & Innovation Foundation

(TECNALIA)

Belgium

1-Tech (1_TECH)

Netherlands

Stichting Tidal Testing Centre (TTC)

Stichting Energieonderzoek Centrum Nederland

(ECNeth)

GermanyFraunhofer-Gesellschaft Zur Foerderung Der

Angewandten Forschung E.V (Fh_IWES)

Gottfried Wilhelm Leibniz Universität Hannover (LUH)

Universitaet Stuttgart (USTUTT)

Portugal

Wave Energy Centre – Centro de Energia das Ondas

(WavEC)

Italy

Università degli Studi di Firenze (UNIFI-CRIACIV)

Università degli Studi di Firenze (UNIFI-PIN)

Università degli Studi della Tuscia (UNI_TUS)

Consiglio Nazionale delle Ricerche (CNR-INSEAN)

Brazil

Instituto de Pesquisas Tecnológicas do Estado de São

Paulo S.A. (IPT)

Norway

Sintef Energi AS (SINTEF)

Norges Teknisk-Naturvitenskapelige Universitet

(NTNU)

http://www.fp7-marinet.eu/http://www.fp7-marinet.eu/http://www.fp7-marinet.eu/http://www.fp7-marinet.eu/


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DOCUMENT INFORMATION

Title

Distribution Public

Document Reference MARINET-TA1-FANRO#1

User-Group Leader, Lead

AuthorAlan Muhr TARRC

Brickendonbury, Hertford, SG13 8NL, Hertford

User-Group Members,

Contributing AuthorsRobert Picken, TARRC

Infrastructure Accessed: Materials and Structures Group, IFREMER

Infrastructure Manager(or Main Contact)

Pierre-Yves Le Gac

REVISION HISTORY

Rev. Date Description Prepared by

(Name)

Approved By

Infrastructure

Manager

Status

(Draft/Final)

01 20-08-13 Alan Muhr Draft


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ABOUT THIS REPORT

One of the requirements of the EC in enabling a user group to benefit from free-of-charge access to an infrastructure isthat the user group must be entitled to disseminate the foreground (information and results) that they have generatedunder the project in order to progress the state-of-the-art of the sector. Notwithstanding this, the EC also state thatdissemination activities shall be compatible with the protection of intellectual property rights, confidentiality

obligations and the legitimate interests of the owner(s) of the foreground.

The aim of this report is therefore to meet the first requirement of publicly disseminating the knowledge generatedthrough this MARINET infrastructure access project in an accessible format in order to:

progress the state-of-the-art

publicise resulting progress made for the technology/industry

provide evidence of progress made along the Structured Development Plan

provide due diligence material for potential future investment and financing

share lessons learned

avoid potential future replication by others

provide opportunities for future collaboration

etc.In some cases, the user group may wish to protect some of this information which they deem commercially sensitive,and so may choose to present results in a normalised (non-dimensional) format or withhold certain design data – thisis acceptable and allowed for in the second requirement outlined above.

ACKNOWLEDGEMENT

The work described in this publication has received support from MARINET, a European Community - Research

Infrastructure Action under the FP7 “Capacities” Specific Programme.

LEGAL DISCLAIMER The views expressed, and responsibility for the content of this publication, lie solely with the authors. The EuropeanCommission is not liable for any use that may be made of the information contained herein. This work may rely ondata from sources external to the MARINET project Consortium. Members of the Consortium do not accept liabilityfor loss or damage suffered by any third party as a result of errors or inaccuracies in such data. The information in thisdocument is provided “as is” and no guarantee or warranty is given that the in formation is fit for any particular purpose. The user thereof uses the information at its sole risk and neither the European Commission nor any memberof the MARINET Consortium is liable for any use that may be made of the information.


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EXECUTIVE SUMMARY

Natural rubber (NR) is widely used in marine applications, and its excellent survival after prolonged submersion in seawater is well documented, at least for static and mild fatigue conditions. The aim of this project is to provide anindication of the durability in sea water of a rubber formulation designed by TARRC to have excellent fatigue

properties in air for arduous mechanical loading cycles. The expectation of little effect of sea water is broadlyconfirmed except under non-relaxing duty cycles. In sea water the effect of non-relaxation conditions is less beneficialthan it is in air, and a sequel project is proposed to identify why this should be, and to seek methods of mitigating it.


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CONTENTS

1 INTRODUCTION & BACKGROUND ................................................................................................................. 7

1.1 I NTRODUCTION ................................................................................................................................................. 7

2 OUTLINE OF WORK CARRIED OUT................................................................................................................ 7

2.1 MARINE FOULING ................................................................................... ERROR ! BOOKMARK NOT DEFINED. 2.2 SEA WATER ABSORPTION ....................................................................... ERROR ! BOOKMARK NOT DEFINED. 2.3 FATIGUE LIFETIME IN SEA WATER .......................................................... ERROR ! BOOKMARK NOT DEFINED.

3 MAIN LEARNING OUTCOMES ........................................................................................................................ 11


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1 INTRODUCTION & BACKGROUND

1.1 INTRODUCTION Natural rubber (NR) is widely used in marine applications, and its excellent survival after prolonged submersion in seawater is well documented. However, several proposed wave energy converters call for very severe cyclical loading ofrubber (eg Anaconda) beyond what is customary even in air. Such service loadings, and the critical need for longevityand maintenance-free operation of devices deployed in the sea, raise the need for characterisation of behaviour outsidenormal working limits as well as for a large numbers of cycles, high strain amplitudes and coupling with sea water.

Ageing must be fully understood if reliable cost-benefit analyses are to be made and expensive failures avoided. Theaim of this project is to provide an indication of the durability in sea water of a rubber formulation designed by

TARRC to have excellent fatigue properties in air. The test facilities available at IFREMER have been specificallydesigned for such characterization studies and a limited test programme has been performed to provide data in order toassess whether there are issues which merit a more complete study.

2 OUTLINE OF WORK CARRIED OUT

2.1 MARINE FOULING Fouling of surfaces by animal or plant growth is an issue for marine devices, and it was confirmed in this project that

the NR material, filled with 15pphr N330 carbon black, submitted by TARRC for the programme of work was no

exception. Dumbbells (type 2 of BS ISO 37:2005; see Figure 1) were die-stamped from moulded 2mm rubber sheet

and assembled, unstrained, in test frames. The frames were immersed in the sea in Brest Bay, holding them fixed

relative to the sea bed, for periods of 3, 6, 9 and 12 months. This resulted in substantial marine growth, as seen in

Figure 2.

Tensile tests were carried out on the dumbbells after exposure to the sea, to find the effect this had on stress-strainbehaviour, elongation at break (EB) and tensile strength (TS). Typical stress-strain responses are shown in Figure 3.

After 6 months, the rubber had apparently softened significantly, and its EB and TS had fallen. With further exposure

the material apparently stiffens, but without much further change to the EB and TS. It might be suspected that the

changes in modulus are related to water absorption, leaching of antidegradants and oxidative ageing. The

combination of softening but drop in EB after 6 months exposure suggests significant weakening in addition to

softening, since normally EB is higher for softer materials. The weakening is possibly related to initiation of failure by

surface stress concentrations caused by the fouling. However, any such initiating effect does not seem to intensify in

the next 6 months. The softening may be caused by interaction of water with the non-rubbers, or by mild oxidation

of the NR. At longer times stiffening occurs, possibly due to the stiffness of the biomaterials adhering to the

dumbbell surfaces, or by further oxidation.

The effects of biofouling on tensile properties is of less concern than the direct additional effect of seawater on

fatigue life in non-relaxing conditions which is investigated below.

Figure 1 Dumbbell type 2 of BS ISO 37:2005


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Figure 2 NR dumbbells after exposure in seawater; A - 6 months; B - 9 months; C - 12 months


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Figure 3 Tensile tests to ultimate failure of dumbbells exposed to sea water for different periods of time

2.2 SEA WATER ABSORPTION Sea water absorption tests have been carried out on samples of rubber sheet (50mmx50mm x 5mm) at

temperatures of 5, 25, 40 and 60°C for the NR formulation using 3 replicates. Typical results are shown in Figure 4,

and are in accord with the literature: a few percent of water is absorbed, and at longer times the weight can

decrease, presumably because of leaching of ingredients with lower rates of diffusion than the water.

Figure 4 Water absorption plot at 5°C


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2.3 F ATIGUE LIFETIME IN SEA WATER Many factors are known to influence the mechanical fatigue life of rubber components but few studies have been

undertaken in the literature for a medium such as sea water. The main interest of the study is to see whether or not

sea water has an influence on the fatigue life of NR. That is why tests were realised in sea water tanks renewed at

4L/h. Taking into account that working either in air or in sea water signifies working in different thermal conditions,

temperature was controlled at 25°C in these two media. Oxidation is a phenomenon known to contribute to early

rupture in rubber. For this reason, to assure a medium filled with a maximum of oxygen, air bubbles were

incorporated, using a water pump in the tank. Also, to avoid any corrosion problems, mountings have been realised

in titanium. The fatigue tests were conducted by prescribing a sinusoidal displacement.

Dumbbells (type 2 of BS ISO 37:2005; see Figure 1) of the NR with and without antioxidant were used as testpieces,

with 7 replicates in each test (Figure 5). Fatigue experiments were carried out both in air and sea water, in order to

study the contribution of sea water during testing. Testing experiments were run in both media at 25°C and at a

frequency of 2 Hz. Previous studies have shown that self-heating is limited at this frequency.

The fatigue tests were conducted on a homemade fatigue machine developed at Ifremer for which a set of

specimens can be tested in parallel (Figure 6). The machine consists of an electrical displacement controlled jack

PRA3810S from Parker piloted by computer. Maximum force and frequency allowed during a cycle respectively are

1860 N and 7 Hz, although in this project 2Hz was used. Figure 6 presents one of the machines created at IFREMER.

To detect rupture of individual specimens, pictures were taken at regular time intervals using Logitech C310

cameras. Knowing date and hour at which a test was launched and the time at which it ended, it is possible to

determine the number of cycles to failure. Results were plotted on an equivalent of the Wöhler curve representing

the magnitude of a cyclic stretch ratio against the logarithmic scale of cycles to failure. The tests were done with a

ratio of minimum strain to maximum strain (R) of either 0 or – 0.2. At R = 0 the results were not significantly affected

by the medium, air or water. At R = 0.2, the service life was greatly increased in air for a given maximum cyclic stress

ratio compared to the results at R = 0. This beneficial effect of non-relaxing conditions was weaker for the tests in

sea water.

Figure 5 replicated dumbbells installed in the fatigue machine


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Figure 6 Fatigue test machine designed by the Materials & Structures Group, Ifremer

3 MAIN LEARNING OUTCOMESWe give here only a brief summary of the results, since we propose to publish them fully, along with those for asequel project aimed to identify the mechanisms for the observed effects.

1. biofouling and water absorption are much as expected from the literature, but now we have quantitativeand comprehensive data for specific materials of current interest.

2. for fully relaxing cycles, there is no significant effect on fatigue life of immersion in sea water, consistent

with such literature as exists. This holds true for both materials, ie with or without antioxidant.3. Both in air and in water the fatigue life of NR is better for the material with antioxidant, especially at

lower strain amplitudes, corresponding to longer fatigue lives. Again, this is consistent with the literature.4. For NR in air, the fatigue life is much greater for non-relaxing conditions (R = 0.2 was used in these

tests), as expected from the literature.

5. For NR in sea water, the benefit of non-relaxing load cycles on fatigue life is reduced, so that the fatiguelife is shorter in sea water than in air.

This last point is unexpected and of considerable significance, meriting thorough investigation in a sequel project.

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FANROE IFREMER Infrastructure Access Report