International Journal of Adhesion & Adhesives 45 (2013) 150–157

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International Journal of Adhesion & Adhesives

0143-74http://d

n CorrE-m

journal homepage: www.elsevier.com/locate/ijadhadh

Design and durability of titanium adhesive jointsfor marine applications

B. Golaz, V. Michaud n, S. Lavanchy, J.-A.E. MånsonLaboratory of Polymer and Composite Technology (LTC), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland

a r t i c l e i n f o

Article history:

Accepted 4 April 2013

Comparative measurements of strength and Griffith's critical strain energy release rate GIc were carriedout on adhesively bonded joints with different surface treatments of titanium, before and along 13 weeks

Available online 30 April 2013

Keywords:Marine structuresEpoxy adhesivesBonded jointsTitanium surface treatmentsWet agingDimensioning

96/$ - see front matter & 2013 Elsevier Ltd. Ax.doi.org/10.1016/j.ijadhadh.2013.04.003

esponding author. Tel.: +41 21 693 49 23; faxail address: veronique.michaud@epfl.ch (V. M

a b s t r a c t

of accelerated aging in salted or deionized water at 50 1C. Thermo-mechanical measurements werecarried out on the bulk epoxy adhesive, with the same aging conditions. A combined surface treatment ofsanding, degreasing and chemical etching showed the best durability, whereas a treatment using anadditional sulphuric anodic oxidation showed the best adhesion before aging. Aging decreased thestrength and the critical strain energy release rate of bonded joints by 30–70%. Joint design with a finiteelement calculation using a cohesive failure law at the interface, accounting for surface treatment, agingeffects and safety factors, can thus be performed, from a limited set of experimental values.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Marine structures of all types increasingly use composite materi-als, in particular sailboats. Composite materials properties are wellrecognized in the marine industry, in particular specific density,mechanical performance, ease of implementation, adaptability tocomplex geometries, absence of corrosion, reduced maintenancecosts, fire resistance and insulation to heat and electricity. Theseadvantages are even more important for racing boats with the use ofcarbon fibre reinforced composites and hybrid composite-titaniumassemblies to benefit from the low density, high strength andexcellent corrosion resistance of titaniumwhere required. Traditionalassembly techniques like screws, rivets or bolts are inadequate forsuch composite parts as they generate stress concentrations and addweight. Shipbuilders therefore seek to replace these methods byadhesive bonding that offers the desired lightweight and stressspreading properties while reducing galvanic corrosion problemsand manufacturing costs. Bonding still suffers from an arduousprocess control and a low reproducibility. Sources of variability aremultiple: choice of adhesive, storage and processing conditions, bondthickness, geometry, presence of defects that are difficult to detect,and finally the lifetime behaviour in marine environment (tempera-ture, salt water, UV rays). One must also consider the importantdifference that exists between laboratory and shipyard productionconditions, in particular for bonding large composite structures thatare subject to heterogeneous surface conditions and anisotropicstrains. Titanium-composite bonding thus still remains an issue

ll rights reserved.

: +41 21 693 58 80.ichaud).

nowadays [1–3] with the necessity to better understand specificitiesof titanium surface preparation to determine the best strategy forbonding. Moreover, current design criteria do not often include theaging of composites in the presence of seawater. These doubts aboutdurability and variability of bonded joints and the lack of safetyfactors for design explain a certain lack of trust from shipbuilders.Solutions to improve properties and durability of bonded jointsrequire efficient titanium surface treatments to increase the adhesionof epoxy and its durability. Bonding as an assembly technique shouldbe considered from the design level and adapted to the mechanicaland environmental lifetime of the structure while achieving anoptimal weight reduction.

Metallic surfaces are usually covered with various impurities likedust, greases, adsorbed water and fragile oxide layers but regardlessof the substrate material, a good adhesion requires a clean and stablesurface [1–3]. Wetting by the adhesive requires a high surface energy,bonding requires physicochemical interactions, and mechanicalanchoring of the adhesive requires a given surface roughness. Allthis could be achieved by surface treatments which mostly consist ofchemical and mechanical techniques: cleaning with solvents ordetergents, abrasion, chemical acidic etching or anodizing in anelectrochemical cell to generate thick and strong oxide layers [1–3].The natural surface oxides of titanium alloys are thinner than those ofaluminium but much more stable, also providing micro-roughness.Recommended titanium etching solutions are based on a concen-trated nitric acid combined with hydrofluoric acid. Recommendedtitanium electrochemical anodizing solutions are usually chromic orsulphuric [1–4].

In epoxy-metal bonding, the metallic surface ions interact withthe amines during the process, creating an interphase withproperties different from those of bulk epoxy [5–8] in particular

Table 1Labels and steps of the different surface treatments.

«D» «S+D» «D+E» «S+D+E» «S+D+Ind. E» «B+D+Ind.A»

Degreased Sandblasted Degreased Sandblasted Sandblasted Beadblasted

Degreased Etched Degreased Degreasedhot

Degreased

Etched Etched(Industrial)

Etched

Anodized(Industrial)Cleaned/MEK

B. Golaz et al. / International Journal of Adhesion & Adhesives 45 (2013) 150–157 151

for strength and durability [9]. The thickness of the interphase wasfound to be about 600 mm for chemically etched titanium with adiglycidyl ether of 1,4-butanediol (DGEB)/Isophoronediamine(IPDA) epoxy [6,7]. Studies showed a significant influence of theloading speed, the temperature and the joint thickness on theultimate strength, the thinner the bond, the higher the joint shearstrength, with an optimum of less than 1 mm for epoxies [10–14].

The aging mechanisms of epoxy in marine environments aredominated by water absorption, which is mostly reversible. Otherinvolved mechanisms are the irreversible loss of additives in thesurrounding environment by diffusion, the polymer chains damageby UV radiation and oxidation, and the mechanical damage of cyclicrecrystallization of salt ions within the material. The seawater agingof a glass fibre reinforced epoxy for 18 months at 50 1C was reportedto result in no modulus loss but in an ultimate strength loss of 40%with a partial recovery after drying [15]; seawater was shown to beslightly less detrimental than deionized water. Adhesively bondedsteel/epoxy joints showed a significant drop in strength afterimmersion in seawater [16]. Metallic–epoxy interphase layers werealso found to be sensitive to water absorption [9]. The metal-oxide/adhesive interface of bonded joints is, however, recognized to be themost critical location during wet aging. Absorbed water moleculescondense into the interfacial porosity and dissolve the substrate'soxide layers, creating an osmotic pressure that will grow blisters.Interfacial water also disrupts the Van der Waals chemical bonds. Butin certain conditions, water might foster oxides layer growth into theinterfacial porosity and improve adhesion [17]. Aging mechanismsare accelerated by thermal activation according to the Arrhenius law,which is generally used to limit the duration of aging tests bypresenting an excellent symmetry with real world aging curves [15].

Design tools for adhesively bonded composite joints for marineapplications have been investigated previously [10,18] but pointedout the difficulty of failure modelling and the need for morereliable input data. In recent years, Cohesive-Zone Modelling(CZM) has been successfully applied to model fracture in adhesivejoints and composite repairs in various modes [19–25]. Numericalsimulations using a CZM appeared to be appropriate for a designpurpose by providing quantitative predictions for both thestrengths and failure mechanisms of adhesively bonded compositejoints presenting mixed-mode fracture [26].

The objectives of this study were therefore to reach performanceand durability for marine composite-titanium bonded joints bytesting different surface treatments, and to propose a finite elementmethod suited to the joint design. This was achieved by comparativemeasurements of fracture toughness GIc and ultimate strength onbonded joints with different surface treatments of titanium, beforeand after aging in salt water and deionized water. The focus of studywas the epoxy–titanium bond, which is less controlled than thethoroughly studied epoxy-composite bond. The surface treatmentswere chosen based on the literature review [1,6,7] and on the actualpractice of shipbuilders. The adhesive's bulk properties during agingwere characterized as well. A finite element model based on CZMand using the experimental data was then proposed to predict thefailure of complex structures. This model was developed for designpurposes with the aim of requiring few experimental data as inputand providing a conservative solution as output.

2. Experimental

2.1. Materials

The metallic substrates used were Ti–6Al–4V titanium grade5 alloy (Bibus Metals AG, Switzerland). This alloy has a density of4.43 g/cm3, a Young's modulus of 114 GPa and a yield strength of828 MPa (according to the manufacturer). The polymer adhesive

was the Araldite 420 A/B from Huntsman Advanced MaterialsGmbH, Switzerland. The weight ratio of the epoxy monomer Awith the amine hardener B was A¼100/B¼40. Once reticulated,this adhesive has a density of 1.15 g/cm3, a Young's modulus of1850 MPa and a tensile strength of 37 MPa (according to themanufacturer and from Ref. [23]). The Araldite 420 componentsA and B were mixed and vacuumed during 10 min to get thebubbles out. The curing cycle was a 12 h isotherm at roomtemperature followed by a 10 h isotherm at 80 1C, according toshipbuilding practice.

2.2. Surface treatments

Before any polymer application, the titanium substrates sur-faces were treated as presented in Table 1. The treatment “D” wasdegreasing, “S+D” and “D+E” are single surface treatments as theyconsist of one operation (sanding or etching) while “S+D+E”, “S+D+Ind. E” and “B+D+Ind. A” were combined treatments as theyconsist of two or more operations, including degreasing andetching or anodizing.

The degreasing step was achieved in an ultrasonic cleaning bathwith acetone during 5 min. Abrasion was carried out by sandblastingor bead blasting. The chemical etching treatment consisted inimmersing the substrate in a 7.5 wt% NH4HF2 solution during5 min followed by a rinse in an ultrasonic cleaner with distilledwater during 5 min. All these steps were performed at roomtemperature, except for “S+D+Ind. E” which used a proprietarycleaning and etching surface preparation by Steiger SA (Switzerland).The anodizing performed for “B+D+Ind. A” was an anodic sulphuricoxidation carried out by PMA Bonnans SA (France) 2 weeks beforethe bonding operation, this surface was therefore wiped clean withmethylethylcetone (MEK) just before bonding. In all other cases, theepoxy was applied directly after the treatment.

2.3. Methods

2.3.1. Bulk adhesive properties measurementThe polymer samples for bulk property and water intake

measurements were 60�10�2 mm3 samples cast between twoglass plates with release agent (Frekote 770-NC, Henkel, Germany)and cut to shape with a diamond saw. The thermo-mechanical andviscoelastic properties of the bulk adhesive polymer during agingwere monitored by dynamic mechanical analysis (DMA Q800, TAInstruments, USA) using a dual cantilever setup with a distance of35 mm between contacts. A sinusoidal strain of 0.1% was appliedat 1 Hz to the material and the resultant stress was measured witha temperature ramp of 5 1C/min between ambient and 110 1C, toreduce potential drying effects of the sample during test. The glasstransition temperature Tg was determined at peak tan∂ and peakE”. DMA and swelling measurements were repeated on a mini-mum of 3 samples. Water intake measurements were repeated on

B. Golaz et al. / International Journal of Adhesion & Adhesives 45 (2013) 150–157152

a minimum of 4 samples. Swelling and water intake measure-ments were carried out using a calliper (70.01 mm) and aweighing scale (70.001 g) on 41 g samples at room temperatureto avoid any thermal dilatation effect.

2.3.2. Surface observationsScanning electron microscopy (SEM) images and energy-

dispersive X-ray spectroscopy (EDX) of the metallic substrateswere acquired on a high-resolution scanning electron microscope(Phillips XL30 FEG, Eindhoven, The Netherlands).

2.3.3. Critical strain energy release rate measurementThe epoxy/titanium adhesion was measured by a three-point

bending test based on the standard ISO 14679:1997. The principleis to load a metallic-substrate/polymer-stiffener bonded sample ata constant speed until failure of the bonding interface. Themaximum force Fmax and displacement dmax are characteristics ofthe adhesion if the crack initiates at the metal–polymer interface.This test was designed to determine the critical strain energyrelease rate Gc [J/m2] in mode I or mode II depending on thedistance between supports. This results in intrinsic adhesionvalues with the advantage of being representative of interfacialphenomena and not related to the bond volume, making thismethod particularly suited for studying the influence of surfacetreatments on adhesion and durability. According to the standard,the distance between supports is set to 33 mm to load theinterface in mode II. However, the most critical loading for anadhesive is in mode I (cleavage), so the distance between supportswas modified to 90 mm, so that the bonding interface was in purebending mode [5,7,8]. The standard displacement speed of0.5 mm/min was changed to 1.5 mm/min to maintain an equiva-lent deflection.

The critical strain energy release rate was calculated using a bi-layer mechanical model with the materials being homogeneousand isotropic. The calculation was based on the beam theoryfollowing the developments of Bouchet et al. [7]. Plastic deforma-tion must be avoided but non-linear elastic force–displacementcurve may occur because of the large displacements and rotationsinduced by this geometry. Fig. 1 shows the geometry of the test,which can be considered as a non-homogeneous beam with avariable section and supports on both ends with a force F appliedin its centre.

The total strain ε from a point in the system is defined only byHooke's mechanical strains as follows (adhesive strains beingneglected):

εðyÞ ¼ y−y0R1

¼ sEðyÞ ð1Þ

If the bonding failure occurs almost instantaneously, theexternal work is negligible and the elastic energy Welastic releasedby area unit A of crack extension can be written as:

G¼ −∂Welastic

∂Að2Þ

The critical strain energy release rate GIc at crack onset (a-0)with the assumption of pure mode I in the interface is calculated at

Fig. 1. Schematic representation of the three-point bending test specimen, w

constant displacement δ since the bonding failure is almostinstantaneous:

GIc ¼−1br

∂∂a

Z L′−l

l

ZSðxÞ

12Eðx; yÞ y−y0ðxÞ

R1ðxÞ

� �2dS

!dx

" #" #δ

ð3Þ

As a-0 it can be assumed that the beam theory is valid andfracture mechanics apply. Fmax is then the critical value for crackpropagation. Full details are in Refs. [5,7].

The three-point bending samples for adhesion measurements(ISO14679:1997) consisted of a 3.6�5�25 mm3 polymer blocksmoulded in a silicon mould held against a 100�10�0.6 mm3

titanium substrate. A total of 18 samples could be processed in onecycle. The tests were carried out at room temperature at constantdeformation rate (1.5 mm/min) using a universal tensile andcompression machine (UTS TestSysteme, Ulm, Germany) equippedwith a 100 N load cell. Three-point bending measurements wererepeated on a minimum of 6 samples.

2.3.4. Bonded joints strength measurementsA modified Arcan setup was used to determine the ultimate

strength envelope by stressing Ti/Epoxy/Ti bonded joints in thenormal and tangential directions by varying the angle of thesample clamps in the tensile machine. The modified Arcan setupused followed the developments proposed by Cognard et al.[10,11,18,27,28] over the classic Arcan configuration to allowcompression/shear testing. The geometry of the specimens fol-lowed the proposed optimizations [18,27] to improve the uni-formity of the stress field into the bonding plane using beaks alongthe joint edges to reduce the stress concentration, with a beakangle of 451 and a connexion radius of 0.8 mm. An almost uniformstress field was therefore assumed in the plane of bonding near tothe failure due to the stress spreading properties of viscoplasticadhesives [11,18,27], so that the measured ultimate strength wasrepresentative of the intrinsic strength of such joints regardless oftheir geometry. The bonded metal–metal samples consisted of twoidentical titanium pieces, with a 70�10 mm2 rectangular bondingarea, assembled by a 0.4 mm thick adhesive layer. These tests werecarried out at room temperature at constant deformation rate(0.5 mm/min) using a universal tensile and compression machine(UTS TestSysteme, Ulm, Germany) equipped with a 100 kN loadcell. Loading directions tested were 01 tension, 451 tension–shearand 901 shear. Arcan measurements were repeated on a minimumof 3 samples.

2.3.5. Aging proceduresIn addition to DMA, three-point bending and Arcan tests on

newly bonded samples, tests were also performed on artificiallyaged samples to determine the durability in salted or deionizedwater. The accelerated hydrothermal aging procedure consisted inthe immersion of the samples in salt water (35 g/l NaCl) at 50 1C,with aging durations of 1, 3, 7 and 13 weeks. For comparison, thisprocedure was repeated with distilled water at 50 1C on somesamples. The aged samples were not dried before testing.

ith dimensions for modified ISO-14679:1997 geometry in parenthesis.

Gc

Gc-aged

Shearseparation

Normalseparation

Traction(nominal stress)

Mixedmode

σu

τuτu-aged

σu-aged

Puremode I

Puremode II

σ = nominal stress in the normal direction

τ = nominal stress in the shear direction

σu(-aged) = ultimate strength in the normal direction (after aging)

τu(-aged) = ultimate strength in the shear direction (after aging)

Gc(-aged) = critical strain energy release rate (after aging)

separation δ = relative displacement

Damage initiation

Full damage

Fig. 2. Schematic «traction-separation» representation of the constitutive behaviour of the adhesive (in normal and shear directions), showing the damage initiation and thedamage evolution as well as the degradation of the cohesive properties after aging and the resulting mixed-mode response.

Fig. 3. Geometry and mesh of the FEM models in Abaqus. For the three-point bending tests, the sample cut on the two planes of symmetry (left). For the Arcan tests with 01angle (right).

B. Golaz et al. / International Journal of Adhesion & Adhesives 45 (2013) 150–157 153

2.3.6. Numerical method for joint designDesigning bonded joints requires a tool to predict local stresses

and failure in complex assemblies. This was achieved with finiteelement calculations using cohesive interface elements to simulatemixed-mode damage onset and growth. The model was developedwithin the code Abaqus 6.6 and its implicit solver. The aim of thismodel was to gain the capability of predicting the durability ofcomplex geometry marine bonded joints by using experimentaldata from the two simple tests presented before.

A traction-separation law based on fracture mechanics wasused to describe the joint's mechanical response and failure, asshown in Fig. 2. This geometry-independent energetic approachhas been widely reviewed by Campilho and de Moura[19,20,23,29]. The model was set in mixed-mode and based on alinear constitutive relationship between stresses and relativedisplacements. It required the knowledge of local strengths andof the critical strain energy release rates Gc. Damage onset waspredicted by a quadratic stress criterion. The in-plane shearstrength properties of the bonded joint were supposed to beisotropic and the input tension and shear strengths (smax and τmax)were set to the Arcan measured values. The damage evolution wasset linear with maximum (non multiplicative) degradation. Gc wasassumed to be mode independent, using the 3-point bending GIc

measured values (i.e GIc¼GIIc¼GIIIc). This independent modeapproach was conservative for a design purpose as GIIc and GIIIc

values are known to be higher than GIc [23].For the three-point bending simulation, the sample was cut on

its two planes of symmetry and discretized into three structuredmesh regions, for the metallic substrate, a single 0.35 mm thickcohesive elements layer and the bulk polymer stiffener. Thisquarter of sample was meshed with ∼9500 linear hexahedralelements (incompatible mode) with an edge size set to 0.35 mm,as shown in Fig. 3 (left). The element type used in the interfacelayer was the cohesive hexahedral 3 dimensional 8 nodes element.For the Arcan simulation, a single layer of 70�10�0.4 mm3 with∼2500 cohesive elements was tied between two 70�10 mm2 rigidshells where load was applied by displacement boundary condi-tions, as shown in Fig. 3 (right). The elastic properties of thecohesive elements were assumed identical to those of the bulkpolymer stiffener.

3. Results and discussion

3.1. Adhesive polymer aging

Swelling and water intake measurements during aging showeda linear increase (as function of square-root time) during the first3 days with the water intake value reaching 4% and the lineardilatation 1%, as shown in Fig. 4. This corresponded to the watersaturation period. Afterwards, a stable period was reached withthe water intake stabilizing to 4.5% and the linear dilatation to1.1%, which were typical values for epoxies [16].

The Fick's diffusion coefficient D during the water saturationperiod was calculated from the slope of the ratio of mass tosaturated mass (value after 60 days) by the square-root timeas shown in Fig. 4. At 50 1C, Dsalt water¼5.2�10−13 m2 s−1 andDdistilled water¼4.2�10−13 m2 s−1, which were again typical valuesfor epoxies [16]. Considering these values, the adhesive saturationlevel after 1 week or longer is equivalent in the resin block of theArcan, 3-point bending and bulk samples.

Fig. 5 presents DMA measurements at 30 1C showing a diminu-tion of the storage modulus of 40% and an increase of the lossmodulus by 50% during the first week of aging. After this diminution,the storage modulus value was stable and the loss modulusdecreased progressively by 25% during the following 12 weeks ofaging. The glass transition temperature Tg was around 60–80 1Cbefore aging, around 35–60 1C at the wet state after 1 week of aging,and around 45–80 1C at the wet state between 3 and 13 weeks ofaging (DMA heating might cause partial drying of wet samples). Thismechanical properties behaviour showed the plasticizing effect ofwater in the adhesive polymer as well as a post-cure effect at thelater stage. Besides the effect of water intake, the acceleratedhydrothermal aging during 13 weeks had no significant impact onthe bulk properties of the adhesive except the thermal post-cure. Nodifferences were observed between aging in salt or deionized water.

3.2. Substrates surface observations

SEM pictures of the treated surfaces present great differences ofroughness and specific area, modifying the mechanical anchoringand the area available for chemical bonds, Fig. 6. The degreased

0 300 600 900 1200 1500 1800 2100 24000%

1%

2%

3%

4%

5%

0 200 400 600 800 1000 12000.0%

0.2%

0.4%

0.6%

0.8%

1.0%

1.2%

0 100 200 300 400 5000%

10%20%30%40%50%60%70%80%90%

100%

Wat

er u

ptak

e M

t

Square root of aging time[s1/2]

Dila

tatio

n

Square root of aging time in salt water at 50°C[s1/2]

Diffusion coefficient: D [m2.s-1]

in salt water at 50°C 5.17.10-13

in distilled water at 50°C 4.18.10-13

Square root of aging time[s1/2]

Mt/M

Fig. 4. Water intake of the adhesive samples during aging (top left). Dilatation of the adhesive samples during aging (top right). Determination of Fick's diffusion coefficientD the adhesive samples during aging (bottom).

Fig. 5. Storage and loss modulus at 30 1C and Tg measurements using DMA after different aging conditions.

B. Golaz et al. / International Journal of Adhesion & Adhesives 45 (2013) 150–157154

surface was clearly the smoothest while both the sandblasted andthe etched ones were rugged. Combined sandblasting and etchingfurther increased roughness and dissolved the remaining sand-blasted particles in fluonitric acid. The dark spots present withtreatments “S+D”, partially removed in “S+D+E” and absent in “S+D+Ind. E” were found to be silicon at EDX analysis. The etchingtreatment produced 1–5 mm indentations and some regions with avery small angular structure (o1 mm); this effect was prominentin treatment “S+D+Ind. E”. The anodizing treatment “B+D+Ind. A”had the deepest roughness with 5–10 mm indentations but withoutthis small angular structure. Optical observations pointed that “S+D+Ind. E” was darker than “S+D+E”. Their different processingconditions led to a thinner titanium oxide layer (white) on “S+D+Ind. E”, indicating a more efficient etching. The oxygen layer after

the combined etching treatments for “S+D+E” and “S+D+Ind. E”was too thin to be quantified by EDX analysis; their chemicalcomposition was identical considering the standard deviation. Forthe anodized surface treatment “B+D+Ind. A” the atomic percen-tage of oxygen was measured to be 30% at EDX analysis, revealinga much thicker oxide layer. Atomic percentages of aluminium andvanadium were also lower for “B+D+Ind. A”, indicating that theoxide layer was mostly made of titanium oxides.

3.3. Mode 1 fracture energy of bonded joints

Fig. 7 shows a typical result of a three-point bending test.The failure of the stiffener-substrate interface was alwaysadhesive and occurred catastrophically, leading to a maximum

«D» «S+D» «D+E»

«S+D+E» «S+D+Ind. E» «B+D+Ind. A»

Fig. 6. Titanium substrates with the different surface treatments (secondary electrons SEM images).

0 1 2 3 4 5 6 7 8 902468

10121416182022

6.75 12. 1 17. 0 20. 2 22. 2 23. 5

Forc

e [N

]

Displacement [mm]

SampleS+D+ETitanium substrate

D S+D D+E S+D+ES+D+Ind. E

B+D+Ind.A

0

5

10

15

20

25

30

F max

[N]

Surface treatments

Fig. 7. Typical force–displacement curves of three-point bending tests for a samplewith a polymer stiffener comparatively to a simple titanium strip, the peak forcerecorded for the composite sample corresponding to the crack onset (top). Force atbond failure in the three-point bending tests for the different surface treatmentswith samples not aged (bottom).

0 1 2 3 4 5 6 7 8 9 10 11 12 130%

20%

40%

60%

80%

100%

496 578 641

S+D+E S+D+Ind.E B+D+Ind.A0

100200300400500600700800

Surface treatments

G1C

G1C

/G1C

not

age

d

Aging time in salt water at 50°C [weeks]

S+D+E batch1 S+D+Ind. ES+D+E batch2 B+D+Ind. A

Fig. 8. Mode 1 fracture energy in the three-point bending tests for the three mostcapable surface treatments with samples not aged (top). Mode 1 fracture energyafter aging relative to its value before aging in the three-point bending tests for thethree most capable surface treatments (bottom).

B. Golaz et al. / International Journal of Adhesion & Adhesives 45 (2013) 150–157 155

on force–displacement curves. Test repetitions revealed a highstandard deviation, around 30% on Fmax, which is a commonproblem with bonding adhesion measurements. The stiffenerthickness was measured for each sample and found to be respon-sible of 7% of the standard deviation. It was thus taken intoaccount in the GIc calculation of Eq. (3).

The results of the average force at bond failure shown in Fig. 7demonstrate the large influence of the different surface treatmentson adhesion. The superiority of the three combined surfacetreatments “S+D+E”, “S+D+Ind. E” and “B+D+Ind. A” over thesingle ones “D”, “S+D” and “D+E” is demonstrated. A quick agingof samples with treatments “D”, “S+D” and “D+E” resulted in a toosignificant loss of adhesion after only 24 h to consider them formarine applications. For that reason, further aging and strengthinvestigations focused on treatments “S+D+E”, “S+D+Ind. E” and “B+D+Ind. A”. They presented the best adhesion for epoxy–titanium

bonding, keeping in mind their realistic implementation for high-tech marine applications.

Fig. 8 shows GIc values calculated from Fmax measurements forthe combined surface treatments before aging. The best adhesionbefore aging was reached by the combined treatment of sanding,chemical etching and sulphuric anodic oxidation “B+D+Ind. A”.The difference between the two combined etching treatments “S+D+E” and “S+D+Ind. E” was attributed to the optimized industrialcleaning and etching parameters used for “S+D+Ind. E” which alsoreduced the standard deviation. Fig. 8 also presents the evolutionof GIc values during aging. These GIc values will then be used inFEM for joint design.

Different sample production batches gave different results, espe-cially for the laboratory treatment “S+D+E”. All treatments, andespecially “S+D+E” were quite sensitive to environmental variationsand the duration between surface treatment and bonding. Theetched surfaces should be bonded quickly because air exposuresupports the growth of weak oxides layers and contamination.

0

5

10

15

20

25

30

35

400.0

22.5

45.0

67.5

90.0

Surface treatmentsS+D+ES+D+Ind. E FEM / S+D+EFEM / S+D+Ind. E

Angle [°]

Ulti

mat

e st

reng

th[M

Pa]

Fig. 9. The modified Arcan fixture with 01 angle (left). Ultimate strength in function of the angle in Arcan tests without aging versus FEM calculations (right).

24.0 19.923.6 17.527.6 20.426.6 19.8

Not aged Aged 1 week in salt water0

10

20

30

40

Ulti

mat

e te

nsile

stre

ngth

[MPa

]

Surface treatmentsS+D+EFEM / S+D+ES+D+Ind. EFEM /S+D+Ind. E

Fig. 10. Ultimate tensile strength in Arcan tests with and without aging versus FEMcalculations.

0 1 2 3 4 5 6 7 8 90

2

4

6

8

10

12

14

16

18

20

22

Forc

e [N

]

Displacement [mm]

Sample S+D+ETitanium substrateFEM sample S+D+EFEM titanium substrate

Fig. 11. Typical measured force–displacement curves of three-point bending testsversus FE calculations for a sample with a polymer stiffener comparatively to asimple titanium strip. The peak force recorded for the composite sample corre-sponds to the fracture of the epoxy–titanium bond. The enclosed picture shows thegradient of damage of the cohesive elements at the simulated crack onset.

B. Golaz et al. / International Journal of Adhesion & Adhesives 45 (2013) 150–157156

This effect was less significant with a surface treatment creatingstable oxides layers as with the sulphuric anodic oxidation of“B+D+Ind. A”.

Overall, bonded joints showed a fall of their adhesion proper-ties during aging. During the first 3 weeks the GIc of “S+D+Ind. E”dropped by 10% while “B+D+Ind. A” dropped by 40% and the twobatches of “S+D+E” by 10% and 60%. The combined etching surfacetreatment “S+D+Ind. E” was the most durable with a GIc loss of 30%after 13 weeks while a batch of “S+D+E” lost 70%. The combinedoxidation surface treatment “B+D+Ind. A” which offered the bestadhesion properties before aging appeared to be less durable than“S+D+Ind. E”. Again, no significant differences were observedbetween aging in salt or deionized water.

3.4. Ultimate strength of bonded joints

Fig. 9 presents the modified Arcan test setup and the ultimatestrengths of bonded joints depending on the loading directionwith the combined etching surface treatments “S+D+E” or “S+D+Ind. E” and a measured bond thickness of 0.4370.03 mm.

Failure of the bonded joint interface always occurred catastro-phically and led to a maximum on the force–displacement curves.The three loading angles 01, 451 and 901 enabled the determina-tion of a partial ultimate strength envelope, showing that the

ultimate strength in tension (∼25 MPa) was higher than theultimate strength in shear (∼15 MPa). In agreement to that, thefailure mode changed from adhesive under 451 and 901 shearloadings to cohesive under tension loading. The results wereconsistent with the results of Cognard et al. [10,18,27]. Theultimate strength in tension–shear was quite elevated leading toan elliptic shaped strength envelope. This could be partly due to anunrepresentative batch of samples. Nevertheless this ellipticalshape fits perfectly the Ishai–Dolev–Gali model [30] for the failureenvelope of bonded epoxy adhesive. Samples having reached thewater content equilibrium of 4% after 1 week of aging were testedin tension loading and showed a strength decrease of 15–25%,staying inside standard deviation as shown in Fig. 10.

3.5. Bonded joints design

The numerical model was validated for design by simulatingthe 3-point bending and the Arcan tests, using the cohesiveelements as described earlier, with parameters: smax and τmax

taken from Fig. 9 for both treatments S+D+E and S+D+IndE and Gc

taken from Fig. 8 for the same treatments, either before aging, orafter 1 week aging in salt water. Simulation results enabled thevisualization of local stresses and strains, the failure onset and itsextension in real time during loading. Fig. 11 presents the results of

B. Golaz et al. / International Journal of Adhesion & Adhesives 45 (2013) 150–157 157

3-point bending simulations, which corresponded to the mea-sured force–displacement curves and crack onset. Results of Arcansimulations shown in Figs. 9 and 10 fitted very well the ultimatestrengths in tension and shear. The simulated Arcan ultimatestrengths at 451 were lower than the measured values (that werein a quite unrepresentative batch, as already discussed) and seemfor that reason more realistic. This conservative simulated resultwas a consequence of the quadratic mixed-mode damage initia-tion criterion used in the model, which was based on ultimatestrengths in pure tension and pure shear only. Simulated resultsfor both tests were close to the middle of the standard deviationon measured values before aging and after 1 week of aging forboth etching treatments “S+D+E” and “S+D+Ind. E”.

This good fit between simulated and measured values for thetwo different test geometries demonstrated the validity of themodel, considering that the adhesive saturation level after 1 weekor more is equivalent in the Arcan and 3-point bending samples.This model was then successfully used for the design of bondedjoints with complex geometries by selecting the experimentalvalues of GIc, smax and τmax according to the surface treatment andaging. Safety factors can be introduced on the maximum simulatedload. This model relied on simplifications (mode-independentto reduce the need of experimental data input) but providedadequate conservative data for design.

4. Conclusion

A combined surface treatment of sanding, degreasing and chemicaletching resulted in the best durability of the titanium/epoxy joint, withan adhesion loss of 30% after 13 weeks of accelerated aging. Anadditional sulphuric anodic oxidation resulted in the best adhesionbefore aging with GIc4600 J/m2 but with an adhesion loss of 40% afteronly 3 weeks. Combined etching treatments were thus proven to be asuitable preparation for titanium bonding in marine applications, andrecommended over simple degreasing, etching or sanding treatments.Arcan test results for the combined etching treatments showed thatthe ultimate strength in tension (�25MPa with cohesive failure) washigher than the ultimate strength in shear (�15MPa with adhesivefailure) with a strength decrease of 15–25% after 1 week of aging. Apartial ultimate strength envelope was determined between puretension and pure shear. The overall dispersion of results and thedifference between sample batches pointed out the high environ-mental and processing sensibility of bonding operations, as well as theinfluence of the failure mode for the different Arcan loading angles.Thermo-mechanical measurements carried out on the bulk epoxyadhesive Araldite 420 with the same aging conditions were consistentwith the elevated initial adhesion loss on 3-point bending samples.The thermo-mechanical properties of the adhesive were found to bestable after water saturation. No significant difference between agingin salt or deionized water appeared in any of the experimentations.

A numerical method for dimensioning was finally proposed bymeans of a finite element implementation with a cohesive zonemodel to simulate the interface in the commercial code ABAQUS.This method was validated by simulation of two representativemechanical tests corroborating the experimental results. This finiteelement model allowed the design of bonded joints with complexgeometries by selecting the desired experimental values of GIc, smax

and τmax according to the surface treatment and the aging conditions.

Acknowledgements

The authors gratefully acknowledge the financial support of theHydroptère Suisse SA, Team Alinghi SA and the Swiss InnovationPromotion Agency CTI. The use of unpublished data by V. Martin and J.

Plojoux is also gratefully acknowledged. The authors wish to expresstheir gratitude to Dr. J. Cugnoni, Dr. P. Volgers, Dr. J. Bouchet and D.Schmäh, who contributed to the work described in this article.

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