durability of adhesively bonded single lap joints between ...£o... · durability of adhesively...

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Durability of adhesively bonded single lap joints between epoxy and polyurethane and GFRP

J.A.O.G.V. De Carvalho

___________________________________________________________________ Abstract The current MsC dissertation was performed in collaboration with Instituto Superior Técnico (IST) and Laboratório Nacional de Engenharia Civil (LNEC) with the main purpose of evaluating the durability of adhesively bonded connections between Glass Fibre Reinforced Polymers (GFRP) adherends. Two types of adhesives were used (resin based epoxy and polyurethane) to perform a single lap joint between GFRP pultruded profiles made of polyester resin with rectangular section. More detailed, this durability study consisted in the quantification of the degradation of the physical, chemical and mechanical properties of both types of joints. Thus small specimens were elaborated and were subjected to several degradation environments, namely water and salt water immersion at different temperatures. The tensile properties, rupture mechanism and performance analysis of the single lap joints were addressed after the established ageing times. Several mechanical and visual alterations were verified through the attained results in some of the ageing environments. It was noted that aqueous solutions and high temperatures significantly degrade the bond strength. Sodium Chloride does not seem to affect in a noticeable way the single lap joints. Furthermore, an inverse relation between the tensile strength and stiffness of the studied joints was noted. The majority of failure modes occurred by fibre delamination of GFRP adherends. However, a large percentage of the stated delamination was at a superficial level. It was not possible to note a trend of the adhesive force deterioration by the degradation agents, through the failure mode analysis, Keyword: Durability, GFRP, Adherends, Polyurethane, Epoxy, Ageing environments. _________________________________________________________________________________ 1. Introduction Nowadays, the high maintenance and reparation costs of structures built with traditional materials and the well-known durability problems of steel and reinforced concrete have been promoting the search for viable alternatives in the construction industry. Therefore there is an increasing interest in new structural materials, lighter, more resistant and with less required maintenance while keeping adequate performance [1]. Fibre reinforced polymers (FRP) application in the construction industry has been increasing significantly in the last decades. Within these materials, glass fibre reinforced polymers (GFRP) have the higher application rate. They present several advantages that include high specific strength and stiffness, low self-weight, high durability even in harsh environments, electromagnetic transparency, and low maintenance requirements. However, there are still some drawbacks to their widespread usage like high production costs, the lack of specifications and guidelines, significant deformability, susceptibility to instability phenomena and bond technology difficulties [2]. Consistent knowledge of bonding technology and behaviour between GFRP components is a necessary requirement mostly due to their dimensional limitations, restricted by handling operations and transportation. This is a critical

factor for the evolution of GFRP profiles in civil engineering. Bolted connections between GFRP profiles are the most used, mimicking constructive details from the metallic construction. However, there are significant differences between both materials. In addition, some studies have been showing promising results with bonded connections. The adaptability to GFRP intrinsic characteristics favour those types of connections. In other fields of FRP application, like aerospace industry, bonded connections present more advantages at mechanical behaviour. The widespread application of bonded connection technology in applications for civil engineering is today at an early stage, not being too much frequent. This fact is related with the uncertainty of its long term behaviour and durability and the limited experimental research available [1]. Vallée et al. [3] tried to establish the influence between the bond area in the global resistance, as well as finding the optimal thickness of the applied adhesive. The authors noted that the bond stiffness is generally higher with increasing bond length and subsequently with its area. The higher bond strength resulted for 1 mm thickness. Tsai and Morton [4], Lang and Mallick [5] and Beligarde et al. [6] studied the effects of different finishing ends of the bonded connection, having concluded that the triangular end on the extremity of the bonded

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connection proved to be the most adequate. This triangular type has the ability to absorb a significant portion of the tangential stress, narrowing its localized peak value at the bond extremity. Magalhães et al. [7] determined the stress distributions along the bond length. The results were consistent with the same triangular shaped endings stated before. In addition, the authors concluded that with higher bond stiffness values were noted increasing stress concentrations as well as lower bond strength. The current dissertation aims to continue two studies on the degradation of adhesively bonded connections between GFRP adherends, when exposed to ageing environments simulating different types of common environments in the civil engineering industry. The durability gap is a key aspect on the performance on any component in civil engineering. Only guaranteeing a compatibility between the appropriate life cycle and its design application can provide a more widespread usage of these materials, both at economic and performance aspects. 2. Experimental work In order to amplify the samples degradation, they were subjected to the following artificial accelerated ageing environments, mimicking different civil engineering applications: (i) immersion in demineralized water at 20 °C, and 40 °C (I20 and I40, respectively); (ii) immersion in salt water at the same temperatures (IS20 and IS40, respectively); (iii) continuous condensation at 40 °C (CC); (iv) thermal cycles (CT); and (v) natural ageing (EN). Table 1 presents the different ageing environments considered as well as their respective duration. The materials used in the experimental investigation were GFRP profiles, obtained through pultrusion process and provided by ALTO Perfis Pultrudidos, Lda. These profiles are made of a polyester based matrix reinforced with E-glass fibres in the longitudinal direction and superficial layers containing randomly oriented fibres. For the GFRP adherends two types of resins were used: an epoxy adhesive (Sikadur®-330) consisting of an impregnating resin, thixotropic, solvent free and presenting an epoxidic nature, and a polyurethane bi-component structural adhesive (SikaForce®-7888 L10 – VP).

Table 1 – Ageing environments.

Environment (Designation) Conditions

Duration

epoxy polyurethane

I20 Temperature: 20 (±2) °C

2, 4, 6 and 9 months 9 months

I40

Temperature: 40 (±2) °C

composition: 35 g/l NaCl


IS20 Temperature:: 20 (±2) °C


IS40


composition: 35 g/l NaCl

2, 4, 6 e 9 months

9 emonths

CC


relative humidity: 100

%


CT

6 hours at -5 ºC alternated with 6 hours

at 40 °C

150 cycles -

EN Variable 2, 4, 6 and 9 months 9 months

The single lap bonded specimens were obtained from GFRP specimens, cut from the provided GFRP profiles with approximate dimensions of 180 mm x 33 m x 5 mm. These specimens were later prepared through an abrasion process in order to improve their adherence. It is important to note that all these specimens were placed in a chamber at 80 °C for 48 hours in order to consolidate the curing state of the material. The single lap bonded specimens were then obtained after considerate preparation to ensure approximately 1 mm of the bond thickness. The single lap bond specimen configuration is represented in Figure 1, with a 60 mm bond length.

Figure 1 – Single lap bonded specimen

configuration.

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In what concerns the characterization methods, all of the experimental procedures were according with ISO 291 standard [8] that indicated the atmospheric conditions. The initial material characterization was already performed in two previous investigations by Marinho [9] and Barros [10]. Material characterization after ageing comprised the tensile properties of the single lap bonded GFRP specimens, in order to adequately characterize their tensile strength and stiffness as well as their rupture modes. This test (Figure 2) was performed according to ASTM D1002-10 and ASTM D5868-01 standards [11,12]. The first standard is adequate to tensile tests in simple bond connections between metallic profiles. The second is applicable to FRP materials and complements the considered testing procedure. 5 sample specimens were tested until rupture at a constant rate of 2 mm/min, for each considered environment and duration period of ageing.

Figure 2 – Single lap bonded specimen tensile

test. Flexure characterization of the adhesive specimens was also done. This test aimed to evaluate the flexural strength (σfu,) and modulus (Ef), as well as the deformation associated to rupture (εfu). This test was based on the ISO 178 [13] standard that aims to characterize the flexure properties of plastic materials. Thus, 80 mm x 10 mm specimens with 4 mm thickness were made, considering a 64 mm span. Both the described testing procedures were performed using an Instron 4803 universal testing machine. Dynamical-mechanical analysis (DMA) was also performed concerning the adhesive specimens, utilizing a Q800 DMA tester from TA Instruments. This test allows the viscoelastic characterization of the considered

material in flexure environment, and was performed according to ISO 6721 standard [14,15]. A three point bending, dual cantilever system was utilized with a 50 mm span. 3 specimens with 60 mm x 15 mm and 5 mm thickness were used. The temperature varied between 25 °C to 150 °C for the epoxy adhesive and -50 °C to 150 °C, with 2 °C/min heating rate. DMA testing allows the characterization of the glass transition temperature (Tg). Mass variation of the specimens immersed throughout the different ageing environments was addressed as well, in order to evaluate the different materials water absorption. This testing procedure was adopted for I20, I40, IS20, IS40 and CC environments. 3. Results 3.1. Initial characterization Initial characterization of the GFRP profiles had as its main goal to determine the constituent materials, glass fibre content, density, glass transition temperature and its mechanical properties (flexural, tensile and interlaminar shear). The results are listed in Table 2 according to Marinho [9] and Barros [10]. Attained results in the initial characterization somewhat differ from those provided by the manufacturer, especially on the inorganic content, flexural strength and flexural modulus. However the manufacturer values are indicative. DMA characterization allows obtaining three glass transition temperature values. These values differ from the experimental curve that provides them: early decay from E’ curve, tan δ curve peak and E’’ curve peak. Only the first two curves were considered as they are more relevant for structural application. The Tg value obtained from the early decay of the storage modulus curve shows the temperature in which the material starts to soften due to the viscoelastic nature of the constituent materials. This value is considered relevant for structural applications as it affects the elastic behaviour presented by these materials. Tan δ curve value allows evaluating the end of the material vitreous transition region. Above this temperature the value the material does not have elastic characteristics suitable to structural applications. The loss modulus curve (E’’) has interest in the consideration of damping properties of these materials.

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Table 2 – Initial characterization of GFRP profiles.

Property (Test) Standards Results

Constituent materials (FTIR)

ASTM E 1252

Unsatured polyester, E-glasss fibres,

calcium carbonate

Content percentage (TGA)

EN ISO 11358

28 % unsatured polyester resin,

65 % E-glass fibre, 7 % calcium carbonate

Density ISO 1183 ρ = 1,97 (g/cm3)

Glass transition temperature

(DMA) ISO 6721

Tg(E´) = 103,7 °C Tg(tanδ) = 137,7 °C

Tensile properties ISO 527

σtu = 470,0 MPa εtu = 1,5 %

Et = 35,0 GPa

Flexural properties ISO 14125

σfu = 873,3 MPa εfu = 0,035 %

Ef = 33,7 GPa

Interlaminar shear strength

EN ISO 14130 τu ≥ 14,1 MPa

In the same way of the GFRP profiles characterization, both adhesives were subjected to the same initial testing. Table 3 and Table 4 present the adhesive initial characterization results.

Table 3 – Initial characterization of epoxy adhesive.



ASTM E 1252

epoxidic nature adhesive, silicates

and calcium carbonate


EN ISO 11358

67 % epoxy resin, 11% calcium carbonate,

22 % silicates



(DMA) ISO 6721

Tg(E´) = 59,5 ºC Tg(tanδ) = 78,2 ºC

Tensile properties ISO 527 σtu = 33,8 MPa εtu = 1,0 %

Et = 4,2 GPa


σfu = 71,8 MPa εfu = 1,6 %

Ef = 3,6 GPa

Table 4 – Initial characterization of polyurethane adhesive.



ASTM E 1252

polyurethane resin, silicates, calcium

carbonate


EN ISO 11358

64 % polyurethane resin, 10 % calcium

carbonate, 26 % silicates



(DMA) ISO 6721

Tg(E´) = 9,0 ºC Tg(tanδ) = 49,9 ºC

Tensile properties ISO 527

σtu = 15,8 MPa εtu = 27,6 % Et = 1,1 GPa


σfu = 30,4 MPa εfu = 8,8 %

Ef = 1,2 GPa

3.2. Characterization after-ageing 3.2.1. Water absorption Water absorption was taken in consideration by means of weighting control specimens at regular intervals. Figure 3 and Figure 4 present the experimental curves obtained for the water absorption of both adhesives: epoxy and polyurethane, respectively. The analysis of the obtained results shows that the early monitored stages present the higher absorption rates throughout all of the considered ageing environments. Both figures show an upward trend to a constant value after the quick initial absorption rate, where the material presents a saturated value. The epoxy based adhesive specimens immersed in IS20 presented the lower mass variations throughout the exposure time (1,44 %). On the other hand the higher absorption values registered for the I20 environment were of 2,09 %. In what concerns the 40 °C immersions, IS40 presented 2,20 % as for the I40 and CC environments, both followed an identical trend up to 2000 hours. After this period, IS40 showed higher absorption rates than CC presenting their highest values of 2,80 % and 2,52 %. For the polyurethane adhesive results, IS20 and IS40 ageing environments presented the lowest absorption values: 1,93 % and 1,87 %, respectively. While I20 environment showed higher results, in the magnitude of 3,77 %, the highest absorption rates were noted for the I40 and CC environments with 6,61 % and 7,93 % of the highest mass change registered, respectively.

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Figure 3 – Epoxy adhesive water absorption

curves.

Figure 4 – Polyurethane adhesive water

absorption curves. Polyurethane adhesive presented higher absorption rates in comparison with epoxy adhesive with a higher variation in the environment at higher temperatures. The higher mass changes registered at the higher temperature environments for both adhesives is associated with the increase of diffusion processes and also with the increase in the permeability coefficient. This fact is confirmed with the higher initial slope of the experimental absorption curves at higher exposure temperatures. Salt water environments presented lower water absorption rates than demineralized water environments. This fact is due to the material pores being preferentially filled with sodium chlorine particles leaving less void content to be filled with water molecules. Karbhari et al. [16] reported similar conclusions. It is also noted that the experimental mass curves show different trends when comparing demineralized water and continuous condensation at the same temperature. According to Petrie [17] water absorption of polymeric materials is generally quicker in high humidity environments (near 100 %) when compared to immersion environments. However the obtained results show the opposite. These results can be explained with the higher direct contact with liquid water, thus increasing its impregnation

properties. Another possible justification for this fact is related to the various nature of polymeric materials, which are highly customizable and present significant differences regarding their production type and manufacturer. These can affect material properties such as porosity. In Figure 5 and Figure 6 are represented the experimental curves obtained for the mass changes throughout exposure time for different ageing environments of both epoxy and polyurethane GFRP single lap bonded specimens, respectively.

Figure 5 – Epoxy adhesively bonded joints water

absorption curves.

Figure 6 – Polyurethane adhesively bonded

joints water absorption curves. Through Figure 5 shows that there are some resemblance between the trends of the absorption experimental curves of the GFRP bonded slip specimens and the adhesive specimens, although with different absolute values. In a general way all sorption curves showed approximate Fickian behaviour, showing a significant absorption rate in the first exposed hours. The rate gradually decreases until a saturation point, almost constant. The IS40 environment showed the lowest mass change saturation points (0,56 %), whereas the IS20 environment showed similar change (0,59 %). The specimens aged in the I20 environment presented a 0,67 % higher mass change and the I40 and CC environments

0,0 0,5 1,0 1,5 2,0 2,5 3,0

0 2000 4000 6000 8000

Mas

s ch

ange

s (%

)

Time (hours) I20 I40 IS20 IS40 CC

-1,0 1,0 3,0 5,0 7,0 9,0

0 2000 4000 6000 8000

Mas

s ch

ange

s (%

)


0,0 0,2 0,4 0,6 0,8 1,0

0 2000 4000 6000 8000 Mas

s ch

ange

s (%

) Time (hours)

I20 I40 IS20 IS40 CC

0,0 0,2 0,4 0,6 0,8 1,0

0 2000 4000 6000 8000 Mas

s ch

ange

s (%

)


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presented the higher absorption values: 0,92 % and 0,93 % respectively. Figure 6 presents similar experimental curves and mass changes when comparing epoxy and polyurethane GFRP singe lap joint specimens which does not occur in the adhesive specimens. This fact is mainly due to the relative proportion between the different analysed materials. In this case the GFRP laminate quantity is the majority of the studied specimen. It is also noted that the GFRP single lap specimen experimental absorption curves are very similar and consistent with Marinho [9] and Barros [10] studies. This additional data validate the attained results. IS40 environment showed the lower mass change when compared to other environments, presenting a maximum value of 0,46 %. IS20 and I20 environments showed 0,51% and 0,69 % mass change peak values. The highest peak value registered were for CC and I40 environments: 0,84 % and 0,97% respectively. The same conclusions about the effects of the salt water, temperature and humidity are valid for the bonded specimens, relatively to what was observed in the adhesive specimens. 3.2.2. Adhesive properties 3.2.2.1. Dynamical Mechanical Analysis It is possible to obtain two values for the glass transition temperature, according to the experimental curve E' and the second experimental curve tan δ. Fgures 7 and 8 bar charts represent the Tg values obtained, according to the experimental curves E´ and tan δ, for the epoxy adhesive specimen samples, respectively. The curves obtained in the epoxy adhesive DMA test do not present similar forms for all samples analyzed, being this discussed later in this subchapter.

Figure 7 – Epoxy adhesive Tg values

corresponding to the beginning of the decay in the storage modules (E´) experimental curve.

Figure 8 – Epoxy adhesive Tg values

corresponding to the peak value of the tan δ experimental curve.

In relation to the demineralized water immersion environments, one can observe that the changes in the experimental curves are indicative of the presence of material plasticization phenomenon due to the contact with water during the immersion phase. During the 9 months of aging, the glass transition temperature value decay shows a monotonous and relatively linear tendency up to 6 months, showing an inversion between 6 and 9 months. Comparing the experimental curves with the reference for water immersion at 40 °C, one observes that the results show a more pronounced plasticization when compared to the immersion at 20 °C, as expected, since temperature favors water transport by diffusion via the adhesive. During 9 months of ageing, the decay of the value of glass vitreous transition temperature shows a relatively flat and linear trend. In regard to the seawater immersion environments, it is possible to observe changes in experimental curves similar to those observed in water at 20 °C, suggesting material plasticization due to contact with saline solution during immersion. During the 9 months of ageing, the vitreous transition temperature value shows a decreasing tendency, although the variation is not monotonous. The results obtained with the epoxy adhesive DMA experimental curves after immersion on seawater at 40 °C is similar to the ones attained with water immersion at the same temperature, and again suggests material plasticization due to contact with saline solution. During the first 6 months of ageing, the vitreous transition temperature value decreases, in spite of the variation not being monotonous and of having an inversion between 6 and 9 months of ageing. The presence of salt in the soaking solution causes less decrease in both the temperature at the beginning of the decay curve of the storage modulus as well as the temperature value of the tan δ curve over time. With regards to the continuous condensation environment,

0 10 20 30 40 50 60 70

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Tg (E

’) (°

C)

Initial value

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Tg (t

an δ

) (°C

)

Initial value

7

changes in experimental curves of aged material in the condensation chamber continuously at 40 ° C are similar to those observed in water immersion at the same temperature. As previously mentored, these results suggest the plasticization of the resulting material due to contact with a saturated atmosphere at 40 °C. During the 9 months of aging the vitreous transition temperature value points to a decreasing tendency, in spite of the variation not being uniform. It was verified that in all studied environments at 40 °C (i.e. immersion in water at 40 °C, seawater immersion at 40 °C and continuous condensation at 40 °C) a maintenance of Tg value in the beginning of aging suggests the existence of a post-cure mechanism associated to the material staying at a higher temperature than room temperature. As a matter of fact, the inversion of the trend in Tg variation for longer immersion periods indicates that the mechanism for post-cure is overriding the antagonistic mechanism of material plasticization by hygrometric effect. This process is verified although the specimen samples cure was undertaken in a 50 °C incubator, revealing itself insufficient. The plasticization phenomenon can be identified by the tan δ curve enlargement, which occurs for the temperature interval in which material phase transition takes place. The immersion in water at 20 °C causes a monotonous Tg decrease until 6 months of age, showing an inversion at 9 months. The retention evolution of seawater solution immersion in the first 6 months is smaller compared to water immersion observed at the same temperature. However, in the wider period, similar retentions are observed. This reduction can be explained by the plasticization phenomenon that the resin suffers in contact with water. In immersions in water and saline solutions at 40 ° C, the Tg value taken from the beginning of the decay curve of the storage modulus is always close to the value obtained with the unaged material. This behavior is also observed in the aging carried out by the continuous condensation at the same temperature. In the first 4 months of immersion in seawater solution the value after immersion is higher than the one initially observed. The explanation for this behavior is, as mentioned previously, the post-cure effect of the epoxy resin when exposed to high temperatures. It is noted, however, that during the exposure time, there is a decrease in the Tg value due to plasticization phenomena, as observed in immersions at 20 ° C. There is a competition between two phenomena, which present an antagonic influence over the Tg value: the

plasticization that diminishes Tg and the post-cure that elevates it. This can explain that at a higher immersion temperature us observed a less pronounced reduction of the Tg value. With respect to polyurethane adhesive, and unlike the one performed for the epoxy adhesive, the results are only analyzed up to six months of aging, obtained by Barros [10]. For economic reasons, the author has analyzed a specimen sample only for each study collection. Figures 9 and 10 represent the Tg values obtained in the DMA test, showing the experimental curves E´ and tan δ, respectively with bar charts. Contrary to what happened for the specimen samples of epoxy adhesive, the curves attained in the DMA test to the polyurethane adhesive present similar forms for all specimen samples analyzed, not suggesting plasticization phenomena. Taking into account the fact that only one sample of each sample had been tested, the statistical validity of the observed trends is somewhat limited, since it lacks information related to the variability associated with the results. As for the demineralized water immersion environments, it is possible to verify that the material has suffered a post-cure effect during aging.

Figure 9 – Polyurethane adhesive Tg values

corresponding to the beginning of the decay in the storage modulus (E´) experimental curve.

Figure 10 – Polyurethane adhesive Tg values corresponding to the peak value of the tan δ

experimental curve.

0 5

10 15 20 25

1464

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28

4368

14

64

2928

43

68

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Tg (E

’) (°

C)

Initial value

45

50

55

60

1464

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2928

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Tg (t

an δ

) (°C

)

Initial value

8

Increments in the Tg value taken from the tan δ curve peak are observed after 1464 and 4368 hours, (for the 2928 hours period this variation was null) for the immersion environment at 20 °C. For this same environment, an increase in the Tg value taken from the beginning of E´ decay can also be obtained, although more pronounced, for the three aging periods. Comparing these results with the reference for water immersion at 40 °C, are can observe that the post-cure effect is more pronounced at a higher immersion temperature. For the immersion environment at 40 °C, there is a null variation in the Tg value taken at the tan δ curve peak after 1464 hours of water immersion at 40 °C, a 4% increase and a 1% reduction for the 2928 and 4368 hours respectively. Regarding the Tg results taken from the beginning of the E´ decay, there is a decrease after 1464 hours and increments of 60% and 56% after 2928 and 4368 hours, respectively. The increase in the value of Tg with increasing immersion times is compatible with the presence of a post-curing process associated with the permanence of the material at a temperature above room temperature. This process happens even after a specimen sample cure has been carried out in a 50 °C incubator. In the immersion environments in saline solution, there is an increase in Tg taken from the tan δ curves' peak, at the end of three periods of immersion in the solution at a temperature of 20 °C. On the other hand, there are highly significant increments in the Tg value taken in the beginning of the storage module curve decay, after the same immersion periods in saline water at 20 °C. Regarding the saline solution at 40 °C one equally verifies increments in the Tg value taken in the tan δ curve peak, and similarly to what has been observed, the most significant increments for Tg value taken in the beginning of the storage module curve for the three environments under study. The increment in the Tg value during the immersion period, and similarly to what had been observed for demineralized water immersion environments, is compatible with the presence of a post-cure mechanism associated to the material’s permanence at a higher temperature when compared to room temperature. The presence of salt in the soaking solution seems to cause an increase in the temperature at which the decay curve of the storage module initiates. In relation to the condensation in continuum environment, there are increases of 6% and 4% and a decrease in the Tg value taken from the tan δ curve peak after 1464, 2928 and 4368 hours, respectively. When analyzing the Tg results from the

beginning of the storage modulus curve decay, there is a 108%, 132% and 60% increase after 1464, 2928 and 4368 hours of continuum condensation, respectively. It is worth noting that the variations observed especially in the beginning of the storage modulus curve decay are more exuberant than the ones observed in water immersion at the same temperature. Only with the exception of 40 °C water immersion after two months, all the remaining aging conditions and exposure durations led to higher Tg values (again taken in the beginning of the storage modulus curve decay) than with unaged material. When compared with immersion in demineralized water it can be seen that immersion in saline solution causes a larger increase of Tg. Nevertheless there is, in general, a non-monotonous Tg variation over time, and a tendency to increase between 2 and 4 months, followed by a decrease from 4 to 6 months of exposure. These results show that the temperature at which the material starts loosing rigidity is substantially superior after aging. The most significant variation is the highest one observed after 4 months exposure in a continuous condensation chamber, in which the temperature at which the rigidity starts to decrease is 21 °C (instead of 9 °C), value attained for the initial characterization of polyurethane adhesive. The Tg value variations taken at the tan δ experimental peak curve, although less pronounced, do not contradict the analysis carried out. Lastly, by observing the experimental curves evolution, the dynamic mechanical analysis excludes the degradation due to physical processes in the polyurethane adhesive, namely resulting from mechanisms of material plasticization. 3.2.2.2. Flexural test Figures 11 and 12 depict bar charts representing the flexural strength and modulus, respectively in the epoxy adhesive specimen samples flexure tests.

Figure 11 – Average values and standard

deviation obtained for the flexural strength of the epoxy adhesive.

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Flex

ural

str

engt

h (M

Pa)

Initial value

9


deviation obtained for the flexural modulus of the epoxy adhesive.

Analysing the results, it appears that in most cases, the evolution of strength is similar to the evolution of the flexural modulus. All aging environments led to a reduction in both the flexural strength and elasticity modulus. Generally, it was in the last period of aging (9 months) that was verified the highest maximum tension reduction, which varied between 4% and 18%. Additionally, in the elasticity modulus, was only in the last aging period that the most significant variation was registered, ranging from 12% to 19%. For both environments – demineralized water immersion and seawater solution immersion – the maximum tension values are generally higher for the environments at 40 °C. Regarding the elasticity modulus, contrary to what happened for the maximum tension values, the highest values are attained for 20 °C temperatures. In spite of the observed differences, in the case of epoxy adhesive specimen samples it seems that there is not a direct relation between temperature and their degradation. In addition, the difference between the highest flexural strength values for adhesives exposed to immersion environments at different temperatures is very reduced and does not follow any tendency over time. These results suggest that there is no clear degradation of the epoxy adhesives at high temperatures (40 °C), when compared with lower temperatures (20 °C). Another reason towards this resides in the post-cure process observed in the DMA test, as mentioned previously, the most significant degradation at highest temperatures. DMA tests detected evidence of plasticization in the present case. This way, one can assume that the deterioration effects detected on the flexion test are mainly the result of plasticization and possibly hydrolysis mechanisms. Taking into account the results obtained for the immersion environments, from both demineralized water and seawater solution, one can find that for the

same temperatures the deterioration is more noticeable in environments without sodium chloride. As previously mentioned and proven by the epoxy adhesive water absorption test, when these are immersed in seawater solution, the material’s pores are preferentially filled with NaCl particles, reducing the permeability coefficient. This way, the amount of water that has access to the adhesive specimen interior is less for seawater immersion environments and, subsequently, the prevalence of hydrolysis mechanisms in the polymeric material will be equally small. Flexural properties results did not present significant variations when comparing demineralized water immersion and continuous condensation environments at the same temperature. Water absorption rates compared between these two environments may justify these results. Figure 13 and Figure 14 present the flexural strength and modulus bar chart as a function of time for polyurethane adhesive specimens.


deviation obtained for the flexural strength of the polyurethane adhesive.


deviation obtained for the flexural modulus of the polyurethane adhesive.

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Flex

ural

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(GPa

)

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Pa)

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ural

mod

ulus

(GPa

)

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The results obtained present similar trends in flexural strength and modulus. As also noted in the epoxy adhesive specimens a flexural strength and modulus reduction was observed in all ageing environments. However, this reduction was significantly higher in the polyurethane adhesive. The last ageing batch presented the higher reductions in flexural strength, varying from 26 % to 55 % considering the different exposure environments. The same remark can be made regarding the elasticity modulus, showing degradation between 21 % and 55 % for the last ageing periods and different environments. The polyurethane adhesive results show higher flexural strength and modulus in the 40 °C environments in both demineralized and salt water immersions. In addition, the I40 and CC environments presented increased values in the flexural modulus: 25 % and 18 % increases at 1464 hours, respectively. The higher values registered for the 40 °C environments soften throughout exposure time, minimizing its difference. This fact suggests the evidence of post-cure phenomena more pronounced in the higher temperature environments. DMA tests showed little evidence of plasticization mechanisms in the studied case. Thus the deterioration effects may be due essentially to hydrolysis. DMA results for 9 months exposure was not possible to accomplish in the polyurethane adhesive specimens. However, the flexural properties suggest that the post-cure phenomena have been reduced to some extent at 9 months of

exposure time, that somewhat was concealing material degradation. It is also noted that in the polyurethane adhesive results present higher levels of deterioration in sodium chlorine free environments. In addition, flexural strength and modulus of the polyurethane adhesive were similar in value and trends at the I40 and CC environments, which is consistent with the epoxy adhesive. 3.2.3. Bonded joint tensile properties 3.2.3.1. Failure load Figure 15 and Figure 16 present ultimate strength values for the different considered ageing environments as a function of exposure time for the GFRP single lap bonded epoxy and polyurethane joints, respectively. Figure 15 show for decreasing values in ultimate strength all ageing environments for the first exposure time (1464 hours) showing 10 % – 26 % variations. Natural ageing showed the highest variation while CC environment showed the lowest. It is important to note that for the majority of the ageing environments, the first exposure time results presented high values of standard deviation, whereas these considered reductions have limited statistical significance. Besides natural ageing, all of the other ageing environments showed long-term bond degradation. The ultimate strength reductions varied between 5 % and 22 %.

Figure 15 – Average values and standard deviation obtained for the failure load of the epoxy adhesively

bonded joint.

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Figure 16 – Average values and standard deviation obtained for the failure load of the polyurethane

adhesively bonded joint. The lowest strength values were registered for IS20 environment while the highest was noticed in the I40 environment. Concerning natural ageing, the last exposure period showed a slight increase in the ultimate strength when compared to their initial values (approximately 4 %). The ultimate strength of these bonded specimens did not show a constant or uniform trend from the first to second exposure times: natural ageing and 20 °C immersions presented slight increases in ultimate strength. From the second to the third exposure time the degradation trend showed ultimate strength reductions in all considered environments, with the exception of natural ageing. However in the last period the ultimate strength trend is again somewhat inconclusive. Only I40 environments presented ultimate strength reductions. These observations suggest the evolution of the ultimate strength deterioration may tend to a constant value at the I20, IS20 and CC environments. The thermal cycles environment does present slight reductions in the ultimate strength. However a significant trend of deterioration cannot be suggested. Concerning natural ageing, there is no clear trend of degradation showed by the specimens. In fact, the ultimate strength presented increased values throughout exposure time. Figure 16 evidences that all of the considered environments, with natural ageing being an exception, revealed increasing values in the ultimate bond strength when comparing the first exposure period with the initial material characterization. These values ranged between 3 % and 35 % being the lowest value

registered in IS40 environment and the highest in the I20 environment. Natural ageing specimens showed a 5 % ultimate strength loss for the first exposure time. However in I20, I40, IS20 and EN environments, where the ultimate strength variations were more pronounced, the standard deviations were also higher. According to this fact, the strength increase may have a lower expression. The results show that the post-cure phenomena are also present in the polyurethane bonded specimens, in accordance to the DMA results. Besides IS20, all the other ageing environments showed evidence of long term deterioration of the ultimate strength. The higher reduction in the majority of the studied environments occurred between the second and third exposure times. This period presents an ultimate strength reduction between 5 % (IS20) and 22 % (I40 and CC). The general reduction trend is noticeable from the first to the second exposure times and from the second to the third, except for natural ageing at the latter period. After 4368 hours it is possible to observe ultimate strength gains. It is of interest to note that afterwards the ultimate strength values show slight variations. This observation suggests the same referred trend to a constant deterioration value, already noted in the GFRP single lap bonded epoxy specimens. 3.2.3.2. Stiffness Figure 17 and Figure 18 present the GFRP single lap bonded specimens stiffness (K) for

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epoxy and polyurethane adhesive as a function of exposure time and different ageing environments, respectively. Figure 17 shows a general downward trend for the bond stiffness, usually verified for different exposure times of the studied environments when compared to the initial material characterization. The tensile elasticity modulus of the GFRP laminate is essential to the bonded connection stiffness, since the based displacement that

provided stiffness results is calculated in function of the claw span of the testing machine. As such, the general stiffness reduction may be related with the tensile modulus decay of the GFRP laminates when exposed to similar environments, as the results in Silva [18] investigation suggest. Through the analysis of Figure 18 it is possible to note that the same general downward trend is noticeable, resembling the epoxy bonded specimens.

Figure 17 – Average values and standard deviation (erros bars) obtained for the stiffness of the epoxy

adhesively bonded joints.

Figure 18 – Average values and standard deviation (erros bars) obtained for the stiffness of the

polyurethane adhesively bonded joints.

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Figure 17 and Figure 18 show that, in a general way, the stiffness trend throughout exposure time for the majority of the studied environments has an opposite evolution when compared to the ultimate rupture strength. These results seem to agreement with those of Magalhães et al. [7], who suggest that material stiffness significantly affects the tangential stress peaks in the bond plane, that are critical to its rupture. It is expected, to some extent, that the ultimate bond strength diminishes while stiffness presents higher values. It is also of interest to note that the epoxy adhesive stiffness presented a slight decrease as a function of the exposure time. According to Silva et al. [19] the adhesive stiffness greatly influences the bond strength, and as such the related stress peaks. However, it is not possible to establish a clear relation between the observed stiffness variations with the epoxy bonded specimens and the epoxy adhesive specimens. On the other hand, the same cannot be said about the polyurethane. In fact there are some similarities between both GFRP bonded and adhesive polyurethane specimens. This fact evidences the adhesive stiffness as a significant factor of the general bond stiffness. These similarities are best observed in the latter exposure period, where the stiffness loss in the polyurethane GFRP bonded specimens is highly noticeable, and a considerable loss in the polyurethane adhesive specimens is registered as well. The significant deterioration trends of the flexural modulus of polyester GFRP laminates studied by Costa [20] for similar ageing environments resembles the obtained results of the single lap bonded specimens stiffness. This evidence suggests flexural modulus influence of the GFRP laminate. This observation may be explained by the stress peaks apparent dependence on the superficial layers of the laminate. The results show in Figure 16 suggest that after 9 months exposure is higher ultimate bond strength when compared with the initial value in the I20 and IS20 environments. The specimens bond stiffness in these environments after 9 months presented the higher losses in stiffness. Considering these observations and comparing these results with the ones obtained in the polyurethane adhesive characterization an improvement appears to be evidenced in the bonded interface for the first 6 months of exposure, with increasing stiffness from both the adhesive and the bonded specimens. After 6 months a high reduction in adhesive and bonded specimen

stiffness is noted. This fact suggests a compensation effect for the ultimate strength loss. These results suggest that water may help the chemical adhesion in the first exposure periods. However it is of importance to address that all polyurethane specimens up to 6 months were made in a different experimental campaign, which can somewhat have affected results, even with the same preparation methods and guidelines. 3.2.3.3. Failure modes The single lap bonded GFRP specimens failure modes were analysed after the tensile tests. It was not possible to distinguish adhesive rupture and cohesive rupture with the optical means available. Thus three failure modes were addressed: adhesive rupture (Figure 19), cohesive rupture through superficial fibres delamination (Figure 20) and cohesive rupture through deep fibre delamination (Figure 21). Figures 22 and 23 present the individual percentage values for each failure modes, as a function of total bond area, for each exposure environment for both epoxy and polyurethane GFRP single lap bonded specimens, respectively. In Figure 22 it is possible to verify that the global bond rupture was mainly by the GFRP fibre delamination (66 %) for the majority of the exposure times. In 71 % of those cases it was noted a superficial delamination. It is of interest to note that the failure mode of all specimens was never due to solely deep fibre delamination. Rupture always started within a preferential point near the bonded plane. The preferential start points are in accordance with the critical rupture points stated by Magalhães et al. [7]. The large majority of the verified failure modes are also related to the propagation mode presented by Keller and Vallée [21] along the bond plane. Adhesive failure modes (34 %) suggests the effectiveness of the adhesion process. However, in a large number of cases the weakest element was still the bond interface. In addition, the majority of specimens that presented adhesive rupture were epoxy bonded specimens. These specimens had ultimate rupture strength below the average values, and had the presence of liquid cyanoacrylate glue, used in the specimen preparation. These gaps may have caused critical rupture points leading to an early rupture and compromising the bond effectiveness.

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The results presented in Figure 23 suggest the same conclusions regarding failure modes percentage. Globally considering all of the exposure environments, 83 % of the failure modes were due to GFRP fibre delamination and in 71 % of those cases were at a superficial level. The same observation about the inexistence of a solely deep fibre delamination failure mode may also be addressed. In addition, the vast majority of the observed failure modes coincides with the propagation

mode referred about the epoxy adhesive bonded specimens. Adhesive rupture (17 %) also evidences the high effectiveness of the adhesion process, although the interface was not the weakest link in the polyurethane bonded specimens. Comparing the adhesive ruptures between the immersion environments it is noted a higher occurrence in the demineralized water environments. However, the comparison between results of the same environment at the same temperatures were inconclusive.

Figure 19 – Adhesive failure.

Figure 20 – Light-fibre-tear failure.

Figure 21 – Fibre-tear failure.

Figure 22 – Failure modes and respective percentages observed for each environment and exposure

duration of the epoxy adhesively bonded joints.

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Figure 23 – Failure modes and respective percentages observed for each environment and exposure

duration of the polyurethane adhesive bonded joints. The opposite results are evidenced when comparing the delamination ruptures that occurred in immersion environments at the same temperature. These results can be explained by the higher degradation verified by the connection in the demineralized water environments, in accordance to the current study. Temperature does not seem to affect significantly the failure modes registered. In light of the attained results, and according to [4-6], the finishing fillet of the connection extremity has a high impact on the failure beginning. Some cases presented irregularities in the extremity fillet. In these cases the failure mode had preferential critic points. On the other hand specimens with excessive adhesive in the fillets did not affect the bonding effectiveness. In fact, in these cases, no impact was noticed in the failure beginning. The adhesive rupture modes were higher in the un-aged polyurethane specimens. This observation was not expected due to the supposed progressive degradation of the adhesion stress, when exposed to the ageing environments. Barros [10] suggested that the adhesion process and the polyurethane resin full curing were not complete when the initial characterization tests were conducted, although they occurred after the curing time recommended by the manufacturer. 3.2.3.4. Visual Analysis The visual observations performed on the connection specimen samples bended with epoxy and polyurethane adhesive enabled the

detection of certain defects such as bubbles in the adhesive and irregularities in the fillet’s extremity. In spite of all caution in the bonding process to eliminate the maximum number of void content from the adhesive interior layer, bubbles were detected both in the lateral face and in the connection’s extremity zone. In addition, a study was also carried out to analyze the thickness variation of the epoxy and polyurethane adhesive’s connection over the ageing period. Nevertheless, the results of the latter did not enable to verify any clear tendency of variation - there was no increase or decrease in thickness for the aging environments studied. In the immersion environments case, it would be expected a thickness increase of the connection adhesive due to water absorption, however, in all these environments there are periods that show negative variations. 4. Conclusions The durability of GFRP single lap bonded joints with both polyurethane and epoxy adhesives were studied through chemical, physical and mechanical analyses. The main conclusion taken from the study presented here are: (1) The water absorption tests carried out in

the materials under study (polyurethane adhesive, epoxy adhesive and both polyurethane and epoxy GFRP bonded specimens) suggested that the temperature does have an influence on

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the permeability coefficients and diffusion coefficient of polymeric materials.

(2) For higher temperatures the absorption coefficients were also higher (this was highlighted by the absorption curve initial slope).

(3) The presence of salt induced the reduction of permeability and diffusion coefficients, as well as the quantity of water that is absorbed at the material’s saturation point.

(4) Osmosis mechanisms were detected in the contact area between the two adhesives and GFRP material.

(5) The finishing fillet irregularities seem to have a direct influence in the connection´s behavior, since they limit the beginning of their rupture.

(6) There is an increase of plasticization phenomena with the epoxy adhesive, with the increase of aging time, and even more significantly with the increase of temperature.

(7) A post-cure process of the epoxy and polyurethane adhesives was detected, consequence of its exposure to higher temperatures than room temperature.

(8) Salt presence in water solutions has led to an increase of the temperature where the vitreous transition region of the adhesive starts.

(9) It was not possible to detect the presence of physical degradation mechanisms in the polyurethane adhesive, namely plasticization, resulting from exposure to aqueous solutions.

(10) The resistance of the adhesive specimens is related with their accelerated aging.

(11) The presence of sodium chloride in a seawater solution slows down the adhesives deterioration process.

(12) It was not possible to verify a direct and clear relation between the temperature and the degradation of the adhesive specimen samples.

(13) There seems to be an inverse relation between the stiffness and the resistance of the connection.

(14) The aqueous solutions and temperature led to a deterioration of both types of bonded connections.

(15) The presence of sodium chloride did not influence significantly the bonded specimens durability.

(16) The adhesion process for the polyurethane adhesive was benefited by the bonded connection’s exposure to aqueous solutions in the first periods of ageing.

(17) There is a good compatibility between the epoxy adhesive, the polyurethane adhesive and the GFRP material, where the adhesion process was extremely effective.

Acknowledgements The author would like to thank all the support given by the Laboratório Nacional de Engenharia Civil. The author also acknowledges Sika Portugal, Produtos Construção e Indústria, S.A. for providing the adhesives and ALTO Perfis Pultrudidos, Lda. for the pultruded profiles. References [1] J.R. Correia, “Polymer Matrix Composites”, “Ciência e Engenharia de Materiais de Construção” (Editoras: F. Margarido e M.C. Gonçalves), IST Press, Lisboa, 2012 (In Portuguese). [2] J.R. Correia, F.A. Branco, J.G. Ferreira, “Mechanical behavior of pultruded GFRP profiles and their connections”, Mecânica Experimental, APAET, 12, 2006, 59-70 (In Portuguese). [3] T. Vallée, J.R. Correia, T. Keller, “Optimum thickness of joints made of GFRP pultruded adherends and polyurethane adhesive”, Composite Structures, 92, 2010, 2102-2108. [4] M.Y. Tsai, J. Morton, “The effect of spew fillet on adhesive stress distributions in laminated composite single-lap joints”, Composites Structures, 32, 1995, 123-131. [5] T.P. Lang, P.K. Mallick, “Effect of spew geometry on stresses in single lap adhesive joints”, International Journal of Adhesion and Adhesives, 18, 1998, 167-177. [6] G. Belingardi, L. Goglio, A. Tarditi, “Investigating the effect of spew and chamfer size on the stresses in metal/plastics adhesive joints”, International Journal of Adhesion and Adhesives, 22, 2002, 273-282. [7] A.G. Magalhães, M.F.S.F de Moura, J.P.M. Gonçalves, “Evaluation of stress concentration effects in single-lap joints of laminate composite materials”, International Journal of Adhesion and Adhesives, 25, 2005, 313-319. [8] ISO 291, “Plastics – Standard atmospheres for conditioning and testing”, International Organization for Standardization, Genève, 2008. [9] A.C. Magalhães Marinho, “Durability of adhesively bonded single lap joints GFRP profiles and epoxy used in rehabilitation”, MsC Dissertation in Civil Engineering, Instituto Superior Técnico, Universidade Técnica de Lisboa, Lisboa, 2012 (In Portuguese).

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[10] P.F.C. Correia de Barros, “Durability of adhesively bonded single lap joints between polyurethane and GFRP”, MsC Dissertation in Civil Engineering, Instituto Superior Técnico, Universidade Técnica de Lisboa, Lisboa, 2012 (In Portuguese). [11] ASTM D1002-10, “Standard Test Method for Apparent Shear Strength of Single-Lap-Joint Adhesively Bonded Metal Specimens by Tension Loading (Metal-to-Metal)”, American Society for Testing and Materials, West Conshohocken, PA, 2004. [12] ASTM D5868-01, “Standard Test Method for Lap Shear Adhesion for Fiber Reinforced Plastic (FRP) Bonding”, American Society for Testing and Materials, West Conshohocken, PA, 2008. [13] ISO 178, “Plastics – Determination of flexural properties”, International Standard Organization, Genève, 2001. [14] ISO 6721-1, “Plastics – Determination of dynamic mechanical properties – Part 1: General principles”, International Organization for Standardization, Genève, 1994. [15] ISO 6721-5, “Plastics – Determination of dynamic mechanical properties – Part 5: Flexural vibration – Non-resonance method”, International Organization for Standardization, Genève, 1996. [16] V.M. Karbhari, J. Rivera, J. Zhang, “Low-temperature hygrothermal degradation of ambient cured E-glass/Vinylester composites”, Journal of Applied Polymer Science, 86, 9, 2002, 2255-2260. [17] E.M. Petrie, “Handbook of Adhesives and Sealants”, McGraw-Hill, NY, 2000. [18] B.M. Abreu da Silva, “Durability of GFRP pultruded profiles made of polyester resin used in rehabilitation”, MsC Dissertation in Civil Engineering, Instituto Superior Técnico, Universidade Técnica de Lisboa, Lisboa, 2012 (In Portuguese). [19] L.M. da Silva, A.G. Magalhães, M.S.F. de Moura, “Structural Adhesive Joints”, Publindústria, Ed 1, 2007 (In Portuguese). [20] R.L. Costa, “Durability of GFRP pultruded profiles made of polyester resin”, MsC Dissertation in Civil Engineering, Instituto Superior Técnico, Universidade Técnica de Lisboa, Lisboa, 2009 (In Portuguese). [21] T. Keller, T. Vallée, “Adhesively bonded lap joints from pultruded GFRP profiles. Part I: Stress-strain analysis and failure modes”, Composites: Part B, 36, 2005, 331-340.

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