European research on composites in high speed vessels

EUROPEAN RESEARCH ON COMPOSITES IN HIGH

SPEED VESSELS

Brian Haymanl and Andreas T. Echtermeyer2

This paper describes the topics investigated in a large European coilaborativeproject. The overall objective of this project was to establish a sound technicai basis for the large-scale use of

composites in naval vessels. The project included in-depth studies of material performance and characterisation, structural performance under both static and extreme ajwamic loads (shock blast and impact), non-destructive inspection and evaluation, damage repair, jire performance, and effectiveness of various electromagnetic shielding methoak. It has been shown that modelling of composites to describe structural response is generally reliable and correlates well with experimental results. it is possible to inspect and repair composites. Materials were identljied that provide suitablefire and EMSperformance for naval and civilian vessels.

1 INTRODUCTION

A five-year European collaborative research project concerning the application of fibre composite materials in naval ships has recently been completed. The project, entitled “Composite Structures: Naval Application Technology”, was organised under the EUCLID Programme, which is a collaborative R&D prograrnme involving the European NATO nations. (“EUCLID’ is an acronym for “mropean

collaboration for the ~ong-terrn In Qefence”.) The project involved research institutions, material suppliers and yards from five nations, and had a total budget of 12.5 MECU (about 16 million USD).

A special feature of this project was the very close co-operation between all nations in all the technical aspects. This gave a very useful exchange of ideas and resulted in a common understanding of technical approaches, a unified way of testing, and common technical guidelines. This combination of different national approaches into a common European view by thorough technical investigations is a major success feature of the project.

The overall objective was to establish a sound technical basis for the large-scale use of composites in naval vessels. The project included in-depth studies of

● _{material performance and characterisation,} ● _{structural performance under both static and}

extreme dynamic loads (shock, blast and impact),

● _{non-destructive inspection and evaluation,}

. damage repair,

. fire performance, and

● _{effectiveness of various electromagnetic}

shielding methods.

Many of the topics covered in the project are highly relevant to high-speed craft for both military and civilian use. The main aim of this paper is to provide a broad overview of the relevant topics that were covered in the project. Detailed results are being reported in other conferences and scientific journals.

2 MATERIAL CHARACTERISATION

Consistent sets of mechanical property data were obtained for a group of materials. These included a conventional GRP laminate consisting of E-glass woven roving in an isophthalic polyester resin, a similar lay-up but with a rubber-modified vinylester resin, another laminate with a fire-retardant polyester resin, and laminates with multi-axial, non-woven E-glass reinforcements in iso-polyester. End-grain balsa and PVC foam were the main sandwich core materials considered. Other materials were considered to a limited extent.

Great attention was paid to ensuring that, as far as practically possible, all test samples of a given material used basic material from a single production batch. For some materials (e.g. PVC foam) this was not possible because a single production batch was ‘ Det NorskeVeritas,Departmentfor StrategicResearch,Veritasveien1, N-1322H@vik,Norway

insufficient to provide the needs of the project; in such cases consecutive production batches were used. Consistency of production of laminates and sandwich lay-ups was ensured by using clearly defined

production procedures. The following typical 2-D mechanical properties were measured for laminates:

El modulus in main fibre direction

E2 modulus in other fibre direction

Glz in plane shear modulus

V]2 major Poisson’s ratio

V21 minor Poisson’s ratio

x, tensile strength in 1 direction x, ‘ compressive strength in 1 direction X2 tensile strength in 2 direction x*’ compressive strength in 2 direction XY in plane shear strength (1,2 direction)

El tensile failure strain in 1 direction El‘ compressive failure strain in 1 direction &2 tensile failure strain in 2 direction &2‘ compressive failure strain in 2 direction 32 in plane failure shear strain ( 1,2 direction) In addition the following through thickness data were obtained:

modulus in through thickness direction shear modulus 1-3 direction

shear modulus 2-3 direction Poisson’s ratio 1-3 direction Poisson’s ratio 2-3 direction

tensile strength in through thickness direction compressive strength in through thickness direction

shear strength 1-3 direction shear strength 2-3 direction

tensile failure strain in through thickness direction

compressive failure strain in through thickness direction

shear failure strain 1-3 direction shear failure strain 2-3 direction

Such a thorough investigation has to the authors’ knowledge never been performed before on marine laminates. The results are comprehensive data sheets, suitable test methods, and a much improved

understanding of the inherent variations of material properties.

Sandwich materials were also characterised carefully, in particular PVC and balsa cores. Tests included measurements of the anisotropic and

non-linear properties of these materials. Effects of high strain rates, as can be experienced under slamming and shock loadings, were also measured.

Fracture toughness measurements were made for PVC core/skin interfaces in mode I and mode II. A mode I test is shown in Figure 1. For PVC cores, the crack propagates slightly below the interface inside the core and test results were in good agreement with results for the core material itself.

Figure 1 Mode I fracture test of core skin interface.

3 STRUCTURAL PERFORMANCE

Carefully instrumented tests were conducted on a series of structural elements with the prime aim of obtaining data for validating analysis methods for composite structures. Cases included:

● _{Foam-cored GRP sandwich panels under static}

pressure loading (see Figure 2)

. Stiffened, single-skin panels under in-plane compression and shear loading

. Single-skin and sandwich panels with underwater

shock loading

● Single-skin and sandwich panels under oblique

impact loading Zn Im ~~

J

Figure 2 Panel geometries for static pressure loading.

Additional analyses and tests were performed on composite-metal joints and on joints from shock-tested panels (to help understand the damage mechan-isms involved in the failure under shock loading). Analyses were also carried out on local buckling (wrinkling) of sandwich panels under in-plane shear loading, and on some panels with air-blast loading.

Since all tests were carried out with the same well-characterised batch of materials one could compare FE-modelling results and experiments, to assess the accuracy of modelling, to develop guidelines for good modelling practice, and to increase confidence in the prediction methods. Generally, agreement between test results and FE calculations was good. An example of a comparison between test results and predictions for a static loading case is given in Figure 3 (Hayman et al. 1998). o -1 \—

1

“’

~

Dlstence x from end of panel (mm)

Figure 3 Variation of vertical displacement along centre-line of a pressure-loaded sandwich panel: values from analyses with 2 types of shell elements and from test.

4 EVALUATION OF NON-DESTRUCTIVE

INSPECTION METHODS FOR

SINGLE-SKIN AND SANDWICH COMPOSITES The suitability of existing NDE methods for thick marine laminates and sandwich structures was evaluated. The most promising methods were tested experimentally to establish their capabilities to detect defects in a quantitative way. The main activities were:

. Laboratory benchmark testing of various NDT

methods on panels with artificial defects of various types. Test specimens are shown in Figure 4. An example of test results is given in Figure 5.

. Testing in shipyards to estimate speed and cost for these methods when used in the field.

Detailed results are reported by Weitzenbock et al. (1998, 1999) and Artiga-Dubois et al. (1999).

‘ES WITH; THINLAMINATES ~ _THERMOGRAPHULT&SOUND

“c’l-’”’mms - ‘:rMfxL(caNTAp) WEEQ,Si

~ %ll”’wnoNs

SANOWICH IMPACT

(PVC&BALSA] CORS/SKIN DESONOS_{CORE wLEs/cRAcKs}

Figure 4 Specimens and methods for non-destructive benchmark testing.

Pad Toldn.mbu MpWII#d8fad Awmd ,bwmy ~

Na C4hyw$ IOcatbn(In mm) * U-rid

1

UT* 60 ? Swa 0.1 $.1

lmtim

tow 0.8 2,3 1.1

Small depth m 1.’I . ad 0.0

. . m 60 15 lW ~ mm 1.6 lCW m_ found 200 UTS 60 20 1% IIGiM La~e depth +% 1 ~ m 80 150 m~g 100 Lmo 60 100 UTI1 24 150 lCQ

m .

Figure 5 Example of test results from benchmark test.

5 REPAIR METHODS FOR SKINS OF

SANDWICH STRUCTURES

Repair procedures were developed for composites, in particular sandwich structures. The durability of resins and adhesives was tested and the best candidate materials were evaluated further by repairing large beams. There is no ideal modelling approach for repaired structures, but knowing the strengths and weaknesses of each approach allows optimisation of repair geometries. The experiments were used to evaluate modelling methods and to determine how much strength can be regained in a repair. The following detailed studies were made:

●

●

●

Studies on alternative adhesives/resins for performing both “wet” and “dry” repairs to the skin of a sandwich structure. An example of a test specimen is shown in Figure 6. Typical results are shown in Figure 7.

Studies of alternative geometries (scarf and step repairs with various angles or step arrangements). Evaluation of associated modelling approaches (FEM, analytical).

Figure 6 Typical procedure for reconstruction of a rehuninated composite plate

12.0 Iin_,,.” [ ❑Resin A I 10.0

w

no egelng immersion Immersion 90 cycles 120cycles 2QO0 h 4003 h

Figure 7 Joint strength with* one standard deviation

6 FIRE PERFORMANCE OF COMPOSITE

STRUCTURES

Fire resistance and fire reaction of a large number of materials were evaluated by various small-scale test methods. Tests also included the combination of base materials with fue protection systems. A database of fwe reaction and thermo-mechanicd properties was built up. The small-scale data gave valuable information for selecting materials for different fue requirements.

Large-scale fwe tests were performed to

establish whether the selection procedures are correct. These tests were the room corner test and IMO deck and bulkhead structural panel tests. Results were evaluated with regard to both naval requirements and the IMO Code of Safety for High Speed Craft.

Modelling of both fire reaction and fwe resistance tests were carried out to enable prediction of behaviour in future cases. The database established tlom the small-scale tests was a valuable asset for performing rhe calculations.

Predictions and experimental results matched very well. However, better knowledge of mechanical material properties at high temperatures seems to be be needed to predict the collapse behaviour of a structure accurately, As it appears very difficult to predict failures of joints in the insulation material by

medelling alone, large scale testing will probably be required also in the t%ture to ensure the integrity of the fwe protection system. However, the modelling is of great help to identify promising base material -insulation combinations. The project succeeded in finding economic f~e protection solutions for exposure times between 10 minutes and 1 hour.

7 ELECTRO-MAGNETIC SHIELDING OF

COMPOSITE STRUCTURES

Because most fibre-reinforced composite materials are electrically non-conducting, EM shielding often has to be provided in order to prevent sensitive equipment on board tkom electrical interference from sources such as radar antenna$ or to prevent signals from enclosed equipment from escaping to the environment and, for example, being detected by an enemy. In this project a series of studies on EM shielding were performed

● _{“Round robin” tests of shielding effectiveness of}

a range of shielding materials added to GRP panels. The purpose was to compare test methods and also results from different facilities using the same test method

. Evaluation of resistance of shielding materials to mechanical loading and environmental exposure

● Evaluation of shielding solutions in regard to

ease of joining and repairability.

. Modelling studies.

The materials fell into three broad categories:

● _{conductive paints}

● _{embedded coated fibre mats}

● _{embedded metallic meshes.}

The studies resulted in a better understanding of the test capabilities and the comparisons that can be made. Satisfactory EMS solutions were found Modelling could qualitatively predict the performance of each solution.

8 IMPACT RESISTANCE OF COMPOSITE

HULL PANELS

Partly in the EUCLID project and partly in other projects, DNV has studied the question of oblique impacts on hull panels. The test rig is shown in Figure 8. The aims of these test programmed were

● _{to seek rational, functional acceptance criteria for}

impact in place of the rather prescriptive minimum thickness approach;

● _{to study the influence of the various test}

parameters, see for example Figure 9; this investigation was done to ensure that the ranking of the materials is not influenced by test details.

In the EUCLID project the emphasis was placed on generating basic reference data for aluminium, GRP single skin (with conventional combi-mat reinforcements) and foam-cored GRP sandwich panels with similar skins. Some results on sandwich panels are reported by Wiese et al. (1998).

composites to describe structural response is reliable and correlates well with experimental results. It is possible to inspect and repair composites, though not all defects can be reliably detected in sandwich structures. Solutions can be found that provide satisfactory fire and EMS performance for naval and civilian vessels. . .---- --- --- . . . . .. .. . . .— -.. . Guiding tube \ o + Imp actor

4

I

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Figure 8Schematic view of the impact test rig

RT

0 2 4 6 6 10 12

Psnelthickness(mm)

Figure 9 Variations in critical impact energy with thickness and span width for GRP single skin panels, (ct=35°)

9 SUMMARY AND CONCLUSIONS

This project has produced a large amount of valuable results for the use of composites in naval applications, The use of the same materials in investigations of many different aspects allowed good comparison of results. It has been shown that modelling of

ACKNOWLEDGEMENTS

This work was part of the EUCLID RTP3.8 project. The main project participants were: Det Norske Veritas (Norway, lead company), Direction des Constructions Navales (France), Defence Evaluation and Research Agency (UK), Fincantieri (Italy), TNO (The Netherlands). The support of the Ministries of Defence of the five participating nations is gratefully acknowledged. The authors wish also to thank Dr. J. Weitzenbock of DNV for his assistance in preparing the manuscript of this paper.

REFERENCES

ARTIGA-DUBOIS, F., PARMAR, M.,

ECHTERMEYER, A.T. and WEITZENBOCK, J.R., 1999 Nondestructive Testing of Composites for Naval Applications. 20th SAMPE Europe/JEC ’99 Int. Conf., Paris, France, 1999.

HAYMAN, B., WIESE, M., DAVIES P.,

CHOQUEUSE D., HOYNING, B. and MITUSCH, P. 1998 Foam-Cored Sandwich Panels Under Static Pressure Loading: Some New Tests and Analyses. 4th Int. Conf. on Sandwich Construction, Stockholm, Sweden, 9-11 June 1998.

WEITZENBOCK, J.R., ECHTERMEYER, A.T., ARTIGA-DUBOIS, F. and PARMAR, M. 1998 Nondestructive Inspection and Evaluation Methods for Sandwich Panels. 4th Int. Conf. on Sandwich Construction, Stockholm, Sweden, 9-11 June 1998. WEITZENBOCK, J.R., ECHTERMEYER, A.T., GRONLUND, P. K., ARTIGA-DUBOIS, F. and PARMAR, M. 1999 Nondestructive Inspection and Evaluation Methods for Composites Used in the Marine Industry. 12th Int. Conf. on Composite Materials (ICCM-12), Paris, France, 5-9 July 1999 WIESE, M., ECHTERMEYER, A.T., and

HAYMAN, B. 1998 Evaluation of Oblique Impact Damage on Sandwich Panels with PVC and Balsa Core Materials. 4th Int. Conf. on Sandwich