A Study of Structural Composite Tee Joints in Small Boats

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DOI: 10.1177/002199839002400604

1990 24: 644Journal of Composite MaterialsR.A. Shenoi and F.L.M. Violette

A Study of Structural Composite Tee Joints in Small Boats

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A Study of Structural Composite TeeJoints in Small Boats

R. A. SHENOI* AND F. L. M. VIOLETTE

Department of Ship ScienceUniversity of SouthamptonSouthampton, S09 5NH

England(Received February 22, 1989)

(Revised August 2, 1989)

ABSTRACT: The paper examines the influence of joint geometry on the ability to transferout-of-plane loads for a hull bulkhead joint in small boats. The hull bulkhead assemblieshave been designed on the basis of sandwich beam and laminated plate theories. Ananalytical solution has also been developed to predict the failure load. Experimental speci-mens have been produced and tested to failure in order to be compared with the analyticalmethod. Finally, a design tool based on the analytical model and the factors affecting costand weight efficiency of the joint has been produced for practical engineering purposes.

KEY WORDS: Boat structures, hull bulkhead joint design, FRP, structural synthesis,sandwich beam theory, composite laminate analysis.

1. INTRODUCTION

IBRE REINFORCED COMPOSITES are some of the most important materialsavailable to the designer of high performance structures. They provide a level

of flexibility and freedom which neither wood, aluminum, steel nor concrete canmatch; fibres such as glass, kevlar or carbon have outstanding strength, stiffnessand low specific gravity.The structural mechanics of sandwich composites is a relatively new science

with most of the theoretical work having been performed since the 1940’s. Atpresent, many boats are still designed through the use of empirical procedures.Such procedures are satisfactory when traditional building material is used butcannot cope with the new breed of lightweight, high strength polymers currentlybeing produced. For designers using these materials, it is essential that soundtheoretical principles are developed so that the behaviour of hulls under static anddynamic loadings can be predicted accurately. Such theory has been applied suc-cessfully in tailoring the in-plane strength and stiffness of plate panels [1]. How-

*Author/address for correspondence.

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ever, out-of-plane behaviour is difficult to adjust and is severely limited by theproperties of the matrix phase. The low strength and stiffness in the matrixdominated laminate directions poses a significant problem for the designer.

Out-of-plane load carrying members such as girders and bulkheads contributeto the overall rigidity of the hull and deck assembly and allow the hull to retainits shape after loading. These are of particular importance in high speed craft orsailing yachts, as illustrated in Figure 1. The &dquo;success&dquo; of the load carrying capa-bility of such members depends upon the efficiency of the T-joint. Early designsof G.R.P minehunters adopted the use of mechanically fastened joints [2]. How-ever, these are not suitable for small craft because they require penetrationthrough the original laminate. Such holes significantly reduce the load carryingcapability of the laminate and may also cause more rapid deterioration of lami-nate properties when the structure is subjected to a harsh environment. For smallboats, good guidance has been available from design manuals [3] though, in

terms of new materials and production technology, there is scope for more work.For these reasons, the need for investigation of out-of-plane joints utilising no

mechanical fasteners is of great importance. In particular there is a requirementfor laying down a logical design procedure for tee-joints based on first principles.This paper attempts to develop such a procedure. This theoretical procedure hasbeen validated by a series of experiments on models of typical tee-joints found insmall high performance craft. Finally the procedure has been extended to coverperformance indices for comparing different varieties of joints.

2. DESIGN PHASE

2.1 LoadingThe hull bottom is subjected to wave impact pressures especially for high speed

~ ~ ~~~ ~~~ . ~~.

Figure 1. Load configuration in small boats.



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Figure 2. Design pressure versus panel span.

planning hulls, mast and keel loads for a sailing yacht. The bulkhead transfers theload from the deck support and from the hull panel.The loads of interest for this study are the hull pressure loads and slamming

pressure loads. The design limit pressure currently used in structural design onyachts is based on American Bureau of Shipping (ABS) rules for offshore racingyachts [4]. These design pressures are empirically derived and are largely in-fluenced by statistical data of known failures. A more scientific approach basedupon theoretical and experimental work [5] is also widely used. It is evident fromFigure 2, which compares the two approaches, that ABS rules produce moreoptimistic values for the loadings. Because they are among the certifyingauthorities for boats it has been decided to adopt the ABS criteria for calculatingloads.

2.2 Structural ResponseThe hull bulkhead composite joint has been modelled as a section with the

bulkhead at mid-span as illustrated in Figure 3. It has been assumed that the panelis subjected to a slamming load which is to be transferred to the out-of-plane loadcarrying members, i.e. to the bulkhead via the joint. Because the bulkhead trans-fers the load to the floors, deck and other structural components, it could beassumed that the deck provides a very stiff dividing support. Thus when the panelis loaded symmetrically about the bulkhead, there is no rotation at the edges ofthe panel (i.e. at mid-span between two bulkheads). The synthesis of the hullbulkhead assembly has been based on the following theoretical criteria and tasks.



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a) A mathematical model applicable for sandwich beams has been developedfrom simple beam theory considerations [6]. This is used to investigate deflec-tions, stresses and failure of the panel.

b) Because of the anisotropic nature of composite laminates, laminated platetheory [7,8] has been used to acquire the various engineering properties. Thishas then been extended to cover failure loads as well.

c) Since the bulkhead is subjected to a compressive load, buckling characteris-tics of the panel have also been analysed. Particular attention has beenfocussed on local instability and wrinkling phenomena [6,9].

These analyses have been implemented as a comprehensive suite of computeralgorithms [10]. The analytical model has been validated using the data availablefrom an experimental and finite element analysis of a composite wing span [11,12]which has the same overall geometry as the hull bulkhead assembly as modelledin Figure 4. Table 1 lists the results for the joint: the failure load predicted hereis 1595 N. Compared with the 1651 N quoted in Reference [11], this indicates a3.4% error on the conservative side.

2.3 Synthesis of the Hull Bulkhead Assembly

Three typical joint geometries which are used in boatbuilding are shown inFigure 5. The choice of type for a particular boat or application is dependentupon several factors including strength requirement and the level of technologyinvolved in the boat construction yard. However, principal consideration are thefailing load, stiffness, strength-to-weight and strength-to-cost ratios.

Five different joint are considered:. filler fillet, radius 10 mm. filler fillet, radius 25 mm. filler fillet, radius 40 mm· foam pad. triangular foam insert

In all the five cases the scantlings of the hull and bulkhead panels have beenmaintained uniform in order to achieve a reasonable comparison.The design synthesis of the assemblies has been based on the structural analy-

sis of each component using the theoretical criteria mentioned in the previous

Figure 3. Hull bulkhead tee joint.



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section. In order to achieve skin compression failure rather than core shear fail-ure, a minimum span of 550 mm is desired. Hence, a panel span of 1000 mm hasbeen chosen as it satisfies these requirements and also because it reflects currentpractice in small boat design. The final panel details have been arrived throughthe use of an iterative procedure with compressive strength having been equatedto wrinkling stress.The basic hull laminate resulting from the considerations is:

~ unidirectional E-glass, 500 g/m2 at 0°~ woven roving E-glass, 166 g/m2 at 45 °~ PVC foam core, 80 kg/m3, 20 mm~ woven roving E-glass, 166 g/m2 at 45 °~ unidirectional E-glass, 500 g/mz at 0°

Figure 4. Modelling of properties.



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The bulkhead laminate details are:

woven roving E-glass, 292 g/mz at + 45 0woven roving E-glass, 292 g/M2 at - 45 0PVC foam core, 80 kg/m3, 20 mmwoven roving E-glass, 292 g/M2 at -45° 0woven roving E-glass, 292 g/M2 at + 45 0

The resin base is a low viscosity epoxy laminating system with a relatively lowpost cure temperature, typically 45° to 50°C to enhance the mechanical proper-ties.Due to the complex geometry of the bonding tape, a simple framework analysis

has been done to model the triangular foam joint and the foam pad joint. On thebasis of such analysis [10], the bonding tape laminate has been chosen for allspecimens as 3 layers of woven roving E-glass, 292 g/m2 at d= 45 ° with an overlapof 20 mm.The details of these assemblies have been fed into the analytical computer

model with a view to estimate failure loads. Table 2 lists the loads for the five

joint types.

3. ANALYSIS OF EXPERIMENTAL RESULTS

3.1 Experimentation Results

The five different joint configurations have been fabricated in laboratory condi-tions using vacuum bag technique and post cured in an oven. All experimentshave been performed on a JJ instruments M30K testing machine, with the com-pressive load being applied on the web of the tee (i.e. the bulkhead) and with thetwo extremities of the flange (i.e. the hull panel) being simply supported at theends. The loading was maintained until failure in each case.Loads and central deflections have been recorded in each case and the graphs

are linear up to the point of failure. Failure loads, maximum deflections andslopes of the graphs are summarised in Table 3.

3.2 Triangular Foam Insert

This failed at a load lower by 15 % than the value predicted by the analytical

Table 2. Failure loads-theoretical prediction.



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Table 3. Experimental results.

solution -see Tables 2 and 3. Because the failure was instantaneous, it has been

very difficult to analyse the failure mechanism even with the help of the videofilm in slow motion. As had been predicted in the analytical model [10], wrin-kling failure occurred at the edge of the bonding tape on the hull panel where thebending stress is maximum. Local buckling of the face resulted in debonding ofthe face from the core. While this process was spreading under the foam insert,the bonding tape on the face of the foam insert had become inefficient in transfer-ring the load. The bonding tape on the opposite side had to carry most of the loadand this resulted in local crushing of the face and core material at the connectionof the bonding tape to the hull panel. The result of this failure mechanism isshown in Plate 1.

3.3 Foam Pad

The foam pad joint failed at a load greater by 2.8 % than the value predicted bythe analytical solution. The failure mechanism was similar to the previous caseas shown in Plate 2. Local crushing of the face and core material was more severe.The extent of the face which debonded from the core was of the same order of

length despite the fact that the bonding tape on the panel did not extend as farfrom the bulkhead.

3.4 Filler Fillet-Radius 10 mm

This failed at a load greater by 8 % than the value predicted by the computermodel. Here too, wrinkling failure occurred at the edge of the bonding tape, thewrinkling stress being critical in a region of 10 mm from the bonding tape. At theedge of the bonding tape, debonding of the skin panel from the core presenteddifferent characteristics. Beyond the bonding tape edge, the glass bubble corebond remained on the core, while under the bonding tape, the glass bubble corebond was still bonded to the panel face and the core was pulled away with thebond. The buckled wave form of the face propagated towards the bulkhead up to



653

~__,;.,,... ,.,........ ,......... ,., ...,..., ’&dquo; ’-&dquo;’-’ . &dquo;I,--&dquo; -’:-...-..’-’.;.~,--&dquo;&dquo;:&dquo;:’{.’.&dquo;&dquo;’:-,&dquo;’Y->.

Plate 1. Triangular foam insert joint failure

. - _ , ___ 1.~,~.111-1-11, ~... ~1.1--l- &dquo;... ~.11,11-1&dquo;.~..,.&dquo; ... ~Ill’-&dquo;, ............11 ~1-1.~ - - ,, I - I ~ - -11 1 - - ~, I - I ~- I - , ’-&dquo; -&dquo;’<&dquo;&dquo;:&dquo;:&dquo;&dquo;’&dquo;’’M.’’’’’-’’-’’-’’-’’-’’’’’’’’’’-’’~’w~~,,<

Plate 2. Foam pad joint failure.



654

the opposite face of the bulkhead. At this location. the load was then being car-ried efficiently only by one face of the bulkhead. The panel skin and core wereseverely indented resulting in the fracture of the filler fillet. The bonding tapethen debonded from the panel skin to an extent equal to one lap length on the sidewhere wrinkling was induced and to nearly two lap lengths on the other side.Plate 3 illustrates the result of this failure mechanism.

3.5 Filler Fillet-Radius 25 mum

The filler fillet radius 25 mm joint failed at a load lower by 8.4% than the loadpredicted by the analytical model. In this experiment, a premature failure oc-curred when the bulkhead induced a side displacement and made the universaijoint of the crosshead to pivot excessively. The loading was not purely com-pressive and a moment was created in the _joint. Local defects such as distortionof the weave or slight asymmetry could be the cause of the lateral displacementof the bulkhead. The failure mechanism started with skin wrinkling at the edgeof the bonding tape. The huckled wave form propagated on each side of the bond-ing tape edge resulting in the debonding of the panel skin from the corc of 1 alength of 130 mm towards the support and up to the first face oft!](? 1,,,11

&dquo; . The

joint itself presented a dchonding of the lillet and the base of th, 1 ,<iim &dquo;:,ld lrom

the panel face. When the buckled waveform propagated along thc ¡ :5’ ’11I11atc.

the increasing twiting movement of the bulkhead made the specimen ~~I~eic on the

Plate 3. FIller tillet Jomt faIlure (raaius u mm).



655

Plate 4. Filler fillet joint failure (radius 25 mm).

support,,, resuhmg in corc shear failure. The result co the lailurc mechanism 11,

shown in Plate 4.

3.6 Filler Fillet- Radius 40 nun

The tiller tillet radius 40 mm failed at a toad lowcr hv 5.2 (j; than the load pre-dicted by the analytical model. As m the previous case, the bulkhead displaced;ideways and the Ba)idity of the result was affected b~ this feature. Simrlar to allthe previous cases, the mechanism of failure was initiated at the edge of the bond-ing tape by local buckling of the panel face. The buckled wavc form propagatedon each side of this point tor a length of 1~0 mm towards the support, and up tothe edge of the bonding tape on the other arcle of the bulkhead. The core waspulled away with the core bond remaining bonded to the panel face. At each edgeof the tapered bonding tape. a larger amount of core was pulled away leaving acrev ice. The buckled wave form of large amplitude propagated a, a shock wav eand combined with the twisting of the bulkhead., the core was fractured in twolocations, near the edge ot the bonding tape, and at the start of the tiller fillet

(both opposite to the side where the failure was Initiated). Plate 5 &dquo;i shows the tau-ure mechanism tor this case.

4. THE DES!GB ’I’001.

4.1 Analytical 1B lodel

The analytical Solution outhned rn Section 2 (and described fullB in Reference



656

Plate 5. Filler fillet joint fallure (radius 40 mm).

[10]) has given satisfactory failure load prediction when compared with the ex-periment as shown in Tables 2 and 3. The model has been obtained using simplebeam theory. The hull bulkhead assembly has been modelled as a beam withvariable cross section and engineering properties. However, the analytical solu-tion could be further improved by reducing the number of approximations whichhave been made, together with keeping it simple so that it can be used in practicalengineering situations.The load on the bulkhead was idealised as a point load acting at mid-span of

the hull panel. The load is in reality transferred by the bulkhead faces, the coreand the bonding tape. Each of these carry a load proportional to the cross sec-tional area and the engineering properties. The effective loading on the panel istherefore lower and could be assumed to be represented by two point loads at thelocation of the faces and a uniformly distributed load over the panel area coveredby the core of the bulkhead.The validation of the model has been carried out in two stages. The first task

involved the building up of finite element (F.E.) models [13] and comparing theF.E. results with experimental values. Figure 6 illustrates the F.E. deflection

plots for joints A, B and Cl. From Figure 7 it will be noticed that F.E. predicteddeflections tally quite well with experimentally recorded values. The marginaldifferences could be accounted for by the fact that the actual material propertiesare different from the ideal values assumed in the F.E. and theoretical models.The next validation stage concerned comparing the F.E. model with the analyti-cal model. Figure 8 shows a comparison of the slope along the span in both cases



657



658



659



660



661

There is adequate correlation between the two. It can therefore be concluded thatthe beam theory-based analytical model is sufficiently accurate for preliminarydesign considerations.

4.2 Weight EfficiencyIn the design of racing yachts and powerboats, structural engineers are con-

stantly looking for ways of improving the transfer of the loads so that scantlingscan be reduced; thus weight and performance of the craft can be improved.Each hull bulkhead assembly has been weighed in its finished state and the

weight of the joint itself has been deduced. The results are presented in Table 4for a metre length of joint. Efficiency has been derived by computing the weightto strength and the weight to deflection ratios. Performance indices for both ratioshave been fitted to the data.From the performance index, it can be deduced that the filler fillet radius 10

mm presents the best characteristics followed by the foam pad joint. However,this investigation is limited to the transfer of a compressive load. If a side loadis applied to the bulkhead, the moment induced in the joint may inverse theresults and type A may perform better.

4.3 Production EfficiencyIn the construction of one-off craft, most of the fabrication time is spent in

building the plug and attaching the stiffeners (bulkheads, frames etc.) to the hullshell. Considerable fabrication time is involved, especially in connecting stiffeners such as girders at their intersection. Therefore, the efficiency of the jointconfiguration also relies on a simple fabrication process to save time and labour.From the fabrication time analysis (Table 5), it can be seen that the filler fillet

is the fastest process to fabricate a joint. Smaller radius (25 < mm) will shorten

Table 4. Weight efficiency.

N1 = Performance index: weight/failure load ranking.N2 = Performance index: weight/max deflection ranking.



662



663



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the time considerably while a larger radius may increase the fairing and sandingtime.The foam pad is more difficult to manufacture than the triangular foam insert

simply because the pad needs to be clamped in order to cut the other edge at 45°.

4.4 Material Cost Implications

The cost of the materials to fabricate the joint is also an important feature inthe efficiency study. The results are summarised in Table 6. The sundry items in-clude sand paper, brushes, mixing pots and gloves. The cost estimate is for a onemetre length of joint and wastage factors are neglected for simplification.From the material cost viewpoint, it can be seen that the filler joint radius

10 mm is the cheapest followed by the foam pad. The filler fillet above 25 mmradius becomes excessively expensive due to the large use of epoxy resin.

4.5 Overall Joint Efficiencies

The performance indexes N1 to N4 have been summarised in Table 7 to deter-mine the best overall joint configuration from a structural and economic point ofview.The filler fillet of radius 10 mm has achieved the best overall performance fol-

lowed by the foam pad joint. It can be seen that the triangular foam insert and thefiller fillet of large radius are to be avoided. Typical graphs of filler fillet radiusversus failure load and cost are shown in Figures 9 and 10 for design purposes.It can be seen that the optimum size of the fillet for strength and cost is in the re-gion of 10 to 25 mm radius. As a rule of thumb, optimum filler fillet radius shouldbe chosen as approximately the thickness of the sandwich panel, to correlate withempirical formulae used in wood epoxy construction.

If the weight criteria are not so important, i.e. for a monocoque structure withsmall number of bulkhead and girders, the foam pad will perform better from astrength point of view but should only be used if skilled workers and large build-ing budget are available.

5. CONCLUSIONS AND RECOMMENDATIONS

This paper has attempted to develop an analytical procedure for the design of

Table 7. Overall joint efficiencies.

N5 = ranking of performance index for total cost.



665



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composite tee joints for small boats. The procedure has been validated by a seriesof experiments on models of typical tee joints found in small, high performancecraft. The procedure has then been extended as a design tool incorporating dif-ferent performance/appraisal criteria.

Several features arising from this investigation have indicated that additionalconsiderations may need to be made. Firstly, it would have been helpful if the ex-perimentation had been conducted in a more extensive manner. It would be of in-terest to increase the number of variables, particularly the strength of the hullpanel and bulkhead, so that the joint itself would fail.

Secondly, a more thorough indication of the material response can be obtainedthrough the use of finite element analysis. Such detailed investigation is usefuland desirable when time and financial constraints allow.Because such hull bulkhead joints are very important in transferring the load

efficiently throughout the structure, it is of interest to pursue such investigationsin order to help understand the behaviour of joints in the transfer of out-of-planeloads.

6. ACKNOWLEDGEMENTS

This work was partially supported by Structural Polymer Systems Ltd. Theauthors gratefully acknowledge the help of Messrs. Ness, Belgrano and Cripps.

7. REFERENCES

1. Smith, C. S. "Bending, Buckling and Vibration of Orthotropic Plate-Beam Structures," Journalof Ship Research, 12(4):249-268 (1968).

2. Dixon, R. H., B. W. Ramsey and P. J. Usher. "Design and Build of the GRP Hull of HMSWilton," Proceedings of Symposium on GRP Ship Construction, Royal Institution of Naval

Architects, London, pp. 1-32 (1972).3. Gibbs and Cox. Marine Design Manual for Fibreglass Reinforced Plastics, McGraw Hill (1960).4. American Bureau of Shipping. Guide for Building and Classing Offshore Racing Yachts (1986).5. Allen, R. G. and R. R. Jones. "A Simplified Method for Determining Structural Design-Limit

Pressures on High Performance Marine Vehicles," Proceedings of American Institute of Aero-nautics and Astronautics/Society of Naval Architects and Marine Engineers Conference on Ad-vanced Marine Vehicles, Paper 78-754 (April 1978).

6. Allen, H. G. Analysis and Design of Structural Sandwich Panels, Pergamon Press (1968).7. Agarwal, B. D. and L. J. Broutman. Analysis and Performance of Fibre Composites, Wiley Inter-

science (1980).8. Butler, R. "The Structural Application of Advanced Fibre Reinforced Composites," Westland

Aerospace Course Notes, Unpublished (undated).9. Plantema, F. J. "Sandwich Construction—Bending and Buckling of Sandwich Beams, Plates and

Shells," Wiley Interscience (1966).10. Violette, F. ’An Investigation into the Behaviour of Structural Composite Hull Bulkhead Joints,"

Department of Ship Science Report, University of Southampton (1988).11. Cope, R. D. and R. B. Pipes. Design of Wingspan Joint—Fibrous Composites in Structural

Design, Plenum Press (1980).12. Gillespie, J. W. and R. B. Pipes. "Behaviour of Integral Composite Joint-Finite Element and Ex-

perimental Evaluation," Journal of Composite Materials, 12:408-421 (1978).13. Swanson Analysis Systems Inc.: User’s Manual-ANSYS (1986).

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A Study of Structural Composite Tee Joints in Small Boats