Effects of hygrothermal aging on the fatigue behavior of two toughened epoxy adhesives

Engineering Fracture Mechanics 79 (2012) 61–77

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Effects of hygrothermal aging on the fatigue behavior of two toughened epoxy adhesives N.V. Datla a, A. Ameli a, S. Azari a, M. Papini b, J.K. Spelt a,⇑ a b

Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario, Canada M5S 3G8 Department of Mechanical and Industrial Engineering, Ryerson University, 350 Victoria Street, Toronto, Ontario, Canada M5B 2K3

a r t i c l e

i n f o

Article history: Received 20 April 2011 Received in revised form 26 September 2011 Accepted 6 October 2011

Keywords: Fatigue threshold Crack growth rates Aging Glass transition temperature Toughened epoxy Open-faced specimen

a b s t r a c t Aged open-faced asymmetric double cantilever beam (ADCB) specimens made with two different rubber-toughened epoxy adhesives were subject to cyclic loading under mixedmode conditions. The contrasting results illustrated the effects of environmental degradation on the matrix and toughener. The fatigue threshold strain energy release rate, Gth, and the crack growth rates of adhesive 1 degraded in two stages: Gth initially decreased with aging time until it reached a constant minimum value at long aging times. Similarly, fatigue crack growth rates initially increased with aging time until reaching a limiting upper value. However, Gth reached the minimum value sooner than did the crack growth rate. In contrast, the Gth of adhesive 2 decreased significantly with aging while the crack growth rates remained unchanged even after prolonged aging. These differences in fatigue threshold and crack growth rate behavior were attributed to changes in the size of the plastic zone at the crack tip as the applied loads changed. The differences in the degradation behavior of both adhesives were explained using gravimetric and dynamic mechanical thermal analysis (DMTA). The degradation of the toughening mechanism of adhesive 1 was related to retained, bound water disrupting the chemical bonds at the rubber/matrix interface. Having no retained water, adhesive 2 was unaffected by this degradation. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Water can degrade adhesive joints by damaging the adhesive–adherend interfacial region or the adhesive itself; therefore, degradation in an adhesive joint exposed to varying amounts of water is closely related to the water absorption and desorption behavior of the adhesive [1–3]. Water absorption in rubber-toughened epoxy adhesives usually exhibits anomalous (non-Fickian) behavior [4–6], whereas moisture desorption on drying follows Fick’s law [7]. Some adhesives retain water after drying [8–10], because of the relatively strong bonds formed between water molecules and the epoxy [9]. Water molecules in the bulk epoxy are either in a free or bound state [2]. Free water molecules plasticize and soften the adhesive, decreasing its glass transition temperature [11]; however, these effects are reversible upon drying. Bound water molecules, on the other hand, introduce irreversible damage to the adhesive by hydrolysis and chain scission [12]. Structural adhesive joints such as those used in automotive applications can be subjected to hygrothermal aging under conditions of elevated temperature in humid or wet environments. Such conditions often lead to the degradation of adhesive joint strength. Moreover, the fatigue strength of adhesive joints is recognized as being lower than that measured under quasi-static loading. Therefore, an understanding of how long-term hygrothermal degradation affects adhesive joint fatigue behavior is of importance in the design of many bonded structures. Closed joints are usually used in degradation studies, ⇑ Corresponding author. Tel.: +1 416 978 5435; fax: +1 416 978 7753. E-mail address: [email protected] (J.K. Spelt). 0013-7944/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfracmech.2011.10.002

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Nomenclature a crack length C moisture concentration before drying C1 saturated moisture concentration C11, C21 moisture concentration of the first and second stages retained water concentration Cr D diffusion coefficient D1, D2 diffusion coefficients of the first and second stages Dd diffusion coefficient of the desorption stage degradation coefficient Ddeg ERT room temperature storage modulus EI exposure index EIa, EId exposure index during absorption and desorption total exposure index EIT G strain energy release rate GI, GII mode I and mode II strain energy release rates Gmax maximum strain energy release rate threshold strain energy release rate Gth Gth,1 threshold strain energy release rate at stabilization Gth,fresh threshold strain energy release rate of fresh joint h thickness of primary adhesives layer M1 saturated water concentration M11, M21 fractional mass uptake in the first and second stages Mr minimum fractional retained water after drying fractional mass uptake Mt N fatigue cycle number t time t0 drying time transition time between the two stages td Tg glass transition temperature x distance from exposed surface dmin, dmax minimum and maximum displacement W phase angle U Heaviside step function ADCB asymmetric double cantilever beam DMTA dynamic mechanical thermal analysis FESEM field emission scanning electron microscope RH relative humidity SDF sequential dual Fickian

although they take a long time to degrade due to the length of the diffusion paths, and the degradation is non-uniform across the joint area, being greatest at the exposed edges. This non-uniform degradation makes it difficult to associate a loss of joint strength with a particular level of degradation. These limitations can be overcome using open-faced specimens in which the adhesive is applied to only one adherend, subject to environmental aging, and then bonded to a second adherend to make the final fracture specimen [13–18]. This reduces the water diffusion path to the thickness of the adhesive layer over the entire joint area, thus producing a relatively uniform state of moisture concentration and degradation in a relatively short period of time. A previous study [18] correlated the loss of fatigue strength to a particular level of degradation, but was limited to the effects of aging temperature only. The present work, used open-faced specimens to evaluate the effects of both the temperature and the relative humidity (RH) of the aging environment. It has been proposed that the degradation of the  fracture toughness of an adhesive joint aged at a given temperature is a R function of the exposure index EI ¼ Cðx; tÞdt , defined as the time integral of the water concentration, C, in the joint [14,17]. This concept is motivated by the desire to combine the effects of time and water concentration in a single parameter which could be used to quantify the severity of the aging condition. In order for this concept to be practically useful, the degradation should be uniquely related to EI such that a given amount of degradation corresponds to a particular EI regardless of the exposure pathway; i.e. long exposure to a low RH environment should be equivalent to shorter exposure to a high RH environment. This hypothesis has been confirmed for the degradation of fracture toughness [14,17] at relatively high EI values, but its validity has not been examined for the degradation of the fatigue threshold and crack growth rates. In the present work, aged open-faced ADCB specimens made with two different rubber-toughened epoxy adhesives were subject to cyclic loading under mixed-mode conditions. The degradation of the fatigue thresholds and crack growth rates

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were quite different, and illustrated the effects of environmental degradation of the matrix and toughener as a function of aging time, temperature, and RH on both the Gth and crack growth rates. A degradation model analogous to Fick’s law was proposed to characterize the decrease in fatigue threshold with degradation that was seen for adhesive 1. The EI hypothesis was also evaluated for fatigue threshold and crack growth rates. Differences in the water absorption properties and glass transition temperatures of the two adhesives were used to explain the differences in the degradation behavior. 2. Experimental 2.1. Open-faced specimen preparation Open-faced ADCB specimens were prepared by casting a 0.4 mm thick ‘‘primary’’ layer of adhesive on a 12.7 mm thick P2etch treated AA6061 bar using a smooth backing plate coated with polytetrafluroethylene release agent (Fig. 1). Adhesive 1 and adhesive 2 were used as the primary adhesives for system 1 and system 2, respectively. Both adhesives were single-part, heat-cured rubber-toughened epoxies designed for structural automotive applications. The desired bondline thickness was achieved by placing 0.4 mm diameter piano wires in the primary layer. The assembly was clamped using large binder clips (25.4 mm wide by 50.8 mm long, ACCO, Booneville, MS, USA) that were centered directly above the spacing wires. The assembly was then cured using the cure profile (180 °C for at least 30 min for both adhesives) recommended by the adhesive manufacturer. The backing plate was removed after curing and the open-faced specimens were exposed to various environments for a range of times as shown in Table 1. Since the present experiments focused on the effects of irreversible degradation, the aged specimens were dried in a vacuum oven containing anhydrous calcium sulfate at 40 °C for approximately 7 days. This drying procedure removed the absorbed moisture (unbound water molecules) thereby eliminating the reversible effects such as plasticization by water molecules. After drying, the complete ADCB specimen (Fig. 2) was made by bonding the primary adhesive layer of the open-faced specimen to a 25.4 mm thick P2-etched aluminum AA6061-T6 bar using a 0.25 mm thick ‘‘secondary’’ layer of adhesive. Adhesive 2 was used as the secondary layer for both adhesive systems. The desired bondline thickness of the secondary layer was achieved by placing 0.65 mm diameter (combined thickness of primary and secondary layer) piano wires between both adherends in locations without adhesive. To improve bonding between the primary and secondary adhesive layers, the degraded primary layer was sanded lightly with a 100 grit sand paper, wiped with acetone and then dried prior to the application of the secondary adhesive. The assembly was given a secondary cure following the cure profile (180 °C for at least 30 min) recommended by the adhesive manufacturer. After the secondary cure, the excess adhesive on the sides of the specimen was removed using a belt sander with a 120 grit sand paper and water as coolant, followed by hand sanding with a 600 grit sand paper. 2.2. Aging and test conditions As mentioned above, the open-faced specimens were exposed to environments maintained at constant humidity and temperature (Table 1). A constant RH was achieved by placing the specimens in air-tight plastic containers above a saturated salt solution. The containers were then placed in temperature controlled ovens for aging. These procedures were identical to those followed in [18] which presented fatigue durability results for adhesive 1 aged only at 95% RH. 2.3. Fatigue testing procedures and environment Fatigue tests were performed with a servo-hydraulic load frame under displacement control using a sinusoidal waveform with a frequency of 20 Hz. A constant displacement ratio (i.e. ratio of minimum to maximum displacement, dmax/dmin) of 0.1 was used. The testing began with the application of the highest strain energy release rate, G, which then decreased as the crack grew under constant displacement until the threshold crack growth rate of 106 mm/cycle was reached at the threshold strain energy release rate, Gth. During fatigue testing, the specimens were enclosed in a chamber to ensure a room temperature, dry air (RD) condition (21 ± 2 °C and <10% RH). It was assumed that crack tip heating during cycling was negligible based on the work of Hwang et al. [19] who showed that the temperature of bulk rubber-toughened epoxies increased by less than 1 °C for loading frequencies of 1–100 Hz. Moreover, this temperature rise would be much smaller with aluminum adhesive joints due to the high thermal conductivity of the adherends.

Fig. 1. Open-faced specimen used for aging. The arrows indicate the direction of moisture diffusion into the primary adhesive layer. The adherend is the thinner one in the ADCB (Fig. 2) and was therefore subject to greater bending strain during fracture testing.

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Table 1 SDF diffusion model parameters (Eqs. (1) and (2)) of the adhesives for the various humid environments studied (data from [22]). Each data point is the average of three repetitions, where SD indicates the standard deviation. Environment

Absorption

Desorption

D1 ± SD (1014 m2/s)

D2 ± SD (1014 m2/s)

M11 ± SD (%)

M1 ± SD (%)

1=2 td

RH RH RH RH

113 ± 11 134 ± 17 271 ± 24 314 ± 25

0.0 3.8 ± 0.7 4.3 ± 0.8 8.6 ± 0.9

1.65 ± 0.04 3.36 ± 0.09 1.38 ± 0.03 3.73 ± 0.11

1.65 ± 0.04 4.78 ± 0.15 1.62 ± 0.04 6.98 ± 0.18

Adhesive 2 60 °C–95% RH

248 ± 29

8.1 ± 1.5

3.16 ± 0.09

4.78 ± 0.12

Adhesive 1 40 °C–43% 40 °C–95% 60 °C–43% 60 °C–95%

Dd ± SD (1014 m2/s)

Mr (%)

1 536 924 329

242 ± 19 214 ± 22 186 ± 18 172 ± 22

0.86 ± 0.04 1.40 ± 0.09 0.98 ± 0.03 1.76 ± 0.11

219

143 ± 13

0.16 ± 0.03

(s

1/2

)

Fig. 2. Configuration of open-faced ADCB specimen after being closed (dimensions in mm, not to scale). The thicknesses of the primary and secondary adhesive layers were 0.4 and 0.25 mm, respectively. The location of the clip gauge is also shown. The upper adherend is the open-faced adherend shown in Fig. 1.

A phase angle W = 18° was achieved when equal loads were applied to both adherends of the ADCB [20]. The phase angle, pffiffiffiffiffiffiffiffiffiffiffiffiffi GII =GI , where GI and GII are the mode I and mode II strain energy release rates, respectively.

W, is defined as w ¼ atan

The crack length was measured using both optical and specimen compliance methods. Optical measurements were performed using a CCD camera mounted on a motorized linear stage. A telescopic lens attached to the camera allowed a field of view of 2 mm. To obtain clear photographs of the location of the crack tip, the load cycling was stopped and held at the mean load for 15 s every 9000 cycles. The specimen compliance was obtained from the relationship between the crack opening and the applied force during the unloading portion of the loading cycle. A clip gauge (model 3541, Epsilon Technology Corp., Jackson, WY, USA) recorded the opening displacement at the loading pins (Fig. 2). For each specimen, a polynomial relationship between the optically observed crack length and the specimen compliance was established according to ASTM E647 [21]. Using this relationship, the crack length was inferred from the continuous clip gauge compliance data, and used in all calculations of crack growth rate and G. A beam-on-elastic-foundation model for unequal adherends was used to calculate G and W from the measured force and crack length [20]. 2.4. Adhesive rubber tougheners A field emission scanning electron microscope (FESEM) was used to examine the rubber toughener morphology of both adhesives. Bulk adhesive wafers were freeze-fractured in liquid nitrogen to obtain a planar surface. These fractured surfaces were carbon coated and then examined under the microscope. The micrographs shown in Fig. 3 indicate that the size of rubber particles were approximately 1 lm and 0.2 lm for adhesives 1 and 2, respectively. 2.5. DMTA A dynamic mechanical thermal analyser (DMTA Q800, TA Instruments, New Castle, DE, USA) was used to measure the dynamic mechanical properties of bulk samples of the two adhesives in the fresh, wet, and dried states, as a function of temperature. Cast adhesive wafers (10 mm  20 mm  0.8 mm thickness) of both adhesives were tested under a tensile strain of 0.1% using a frequency of 1 Hz at a temperature ramp of 10 °C/min between room temperature (25 °C) and 190 °C. The Tg was taken as the temperature at which the loss modulus was maximum. The wet samples were tested immediately after removal from the environment chambers, and the dry samples were dried using the procedure explained above for the open-faced specimens. The DMTA temperature scan was also repeated a second time, immediately after the first scan, for a few dry and wet samples of adhesive 1.

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Fig. 3. FESEM micrograph that shows the rubber particles dispersed in the epoxy matrix for (a) adhesive 1 and (b) adhesive 2. Approximate size of rubber particles is 1 lm and 0.2 lm for adhesives 1 and 2, respectively.

3. Results and discussion 3.1. Gravimetric analysis The water absorption of both adhesives was previously found to be non-Fickian, with a pseudo-equilibrium state being reached at intermediate exposure times before ultimately reaching a final saturation state; however, water desorption showed simple Fickian behavior [22]. The absorption and desorption behaviors for both adhesives were modeled using a sequential dual Fickian (SDF) model [22] and a simple Fickian model, respectively, as illustrated in Fig. 4. The fractional mass uptake, Mt, at any time t, for the SDF model during absorption is given by [22]:

Mt ¼

1

1 8 X

p2

 1

n¼0

1 ð2n þ 1Þ2

1 8 X

p2

n¼0

exp

1 ð2n þ 1Þ2

D1 ð2n þ 1Þ2 p2 t

exp

!!

2

4h

 M11 þ /ðt  t d Þ

D2 ð2n þ 1Þ2 p2 ðt  td Þ 2

4h

!!  M 21

ð1Þ

where M11 and M21 correspond to the fractional mass uptake in the first and second stages, respectively, and M11 + M21 = M1. D1 and D2 are the diffusion coefficients of the first and second moisture uptake stages, respectively. td is the transition time between the two stages, and U(t  td) is the Heaviside step function which is equal to zero for negative values of t  td and equal to one otherwise. During drying, the Fickian model gives Mt as:

Mt ¼ Mr þ

1 8 X

p2

n¼0

1 ð2n þ 1Þ2

exp

Dd ð2n þ 1Þ2 p2 t 2

4h

!!

 ðM 1  Mr Þ

ð2Þ

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Fig. 4. Illustration of the sequential dual Fickian (SDF) model for water absorption and the simple Fickian model for water desorption.

where Mr is the minimum fractional retained water after drying and Dd is the diffusion coefficient of the desorption mechanism. Table 1 lists the absorption and desorption parameters for the different exposure conditions for both adhesives as found in [22]. These parameters were obtained by fitting the model to experimental data obtained from gravimetric measurements as described in [22]. These parameters were used to calculate the water concentrations at given degradation times in the aged open-faced ADCB joints. It is seen that D1 for both adhesives was largely independent of RH, but depended on temperature (Table 1). Furthermore, M1 for both adhesives was a function of both temperature and RH. Significant differences in water desorption behavior were observed between adhesive 1 and adhesive 2. A fraction of the absorbed water in adhesive 1 was retained even after prolonged drying, whereas the absorbed water in adhesive 2 was almost completely eliminated by drying (Table 1). This amount of retained moisture, Mr, for adhesive 1 was proportional to the saturated water concentration, M1, so that Mr increased as the temperature and RH of the environment increased. Ameli et al. [22] used XPS to confirm that water molecules were present in adhesive 1 after prolonged drying at 40 °C. 3.2. DMTA Table 2 summarizes the conditioning history of fresh, wet, and dry samples of both adhesives with the corresponding fractional water uptake (Mt), glass transition temperature (Tg), and room temperature storage modulus (ERT) obtained using DMTA. The water absorption behavior in the chosen conditioning environments was dual-Fickian (0 < td < 1, see Table 1), except for the wet A1 sample where the water absorption was simple Fickian (td = 1 at 40 °C–43% RH environment, see Table 1). The Tg of the wet samples of both adhesives was smaller than the Tg of the fresh samples (Table 2), decreasing approximately 9° and 7 °C, respectively for adhesives 1 and 2, for each 1% increase in water concentration. These values agree well

Table 2 Conditioning environments and the corresponding fractional water uptake (Mt), glass transition temperature (Tg), and storage modulus at room temperature (ERT) of fresh, wet, and dry samples of both adhesives. Percentage change in Tg and ERT values from the fresh sample values of the corresponding adhesive were also included. Sample

Conditioning

Mt (%)

Tg (°C)

% Change in Tg

ERT (N/mm2)

% Change in ERT

Adhesive 1 Fresh, A1 Wet, A1 Wet, B1 Wet, C1 Dry, A1 Dry, D1 Dry, C2

Cured and dried Saturated at 40 °C–43% RH 5 days aging at 60 °C–95% RH 7 days aging at 60 °C–95% RH Saturated at 40 °C–43% RH and dried Saturated at 60 °C–43% RH and dried Saturated at 60 °C–95% RH and dried

0 1.68 4.54 4.81 0.86 0.98 1.76

121 117 75 81 124 122 121

3 38 33 +2 +1 0

1402 1628 1126 1097 1575 1543 1711

+16 20 22 +11 +10 +22

Adhesive 2 Fresh, A2 Wet, A2 Dry, A2

Cured and dried 7 days aging at 60 °C–95% RH 7 days aging at 60 °C–95% RH and dried

0 4.32 0.16

75 42 78

44 +4

995 770 1078

20 +7

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67

with the 8 °C drop in Tg per 1% water concentration reported with a DGEBA epoxy resin [23]. As seen in Table 2, this decrease in Tg was reversible for both adhesives, and drying restored Tg to the value of the fresh samples of both adhesives, irrespective of the amount of retained water after drying. Noting that the reversible decrease in Tg with absorbed water was due to plasticization from free water molecules [24], it can be concluded that the retained water in adhesive 1 was not in a free state and was bound strongly to the adhesive constituents since it did not affect Tg. This is consistent with the conclusions reached in [17] for these same adhesives. To further test this conclusion, a second DMTA temperature scan was performed immediately after the first, for few wet and dry samples of adhesive 1. As observed in [9,12], the high temperature of the DMTA scan decreased the water concentrations in the samples. Gravimetric measurements before and after the first scan showed that the retained water in the dry samples decreased from 1.68% to 1.28%, and that, for the wet samples, there was a larger decrease from 4.68% to 1.71%. This loss of water affected the Tg differently for the wet and dry samples. The Tg of the dry samples remained almost constant (112 °C and 113 °C for the first and second scans, respectively) indicating that retained water in dry samples had no plasticizing effect. In contrast, the Tg of the wet samples significantly increased (75 °C in first scan to 111 °C in the second scan) indicating that the water present in the wet samples had a plasticizing effect. These results support the argument that the retained water after drying is in the bound state. The data for the room temperature storage modulus, ERT, was used to understand the effects of water on the modulus of the adhesive. Table 2 shows that the ERT of the adhesive 1 samples that had been wet and then dried increased as the amount of retained water after drying, Mr, increased. As discussed above, since the retained water in adhesive 1 after drying was in a bound state, it can be concluded that elastic modulus increased as the amount of bound water in the adhesive increased. This increase in modulus with bound water can be explained as an increased resistance to molecular mobility within the adhesive constituents as a result of the formation of strong bonds with water molecules. Table 2 also shows that ERT of the wet samples of both adhesives decreased below the ERT of the fresh samples, except for the wet A1 sample, which was aged at a relatively low temperature and RH (40 °C–43% RH) and therefore contained little water. This inconsistency in the change in ERT of the wet samples compared with that in the dry samples of adhesive 1 is expected, because water in wet samples exists in both the free and bound states, and each affects the elastic modulus differently; i.e. water in the bound state was shown above to increase the elastic modulus, while water in the free state is known to plasticize and decrease the elastic modulus [25,26]. Therefore, the ratio of bound to free water in the adhesive determines whether ERT increases or decreases with water absorption; i.e. ERT increases when the ratio of bound to free water is high where the increase in stiffness due to bound water dominates the decrease in stiffness due to free water, and ERT decreases when the ratio is low. Since ERT increased above the value of the fresh sample only for the wet A1 sample, the ratio of bound to free water is the highest in this sample compared to other samples. Furthermore, since water absorption followed a simple-Fickian relationship only for the wet A1 sample as a consequence of the relatively low temperature and RH of the aging environment (40 °C–43% RH), and was dual-Fickian for all other samples, it can be concluded that the ratio of bound to free water is highest for simple-Fickian absorption and that the ratio was relatively low for dual-Fickian absorption when the aging environment was at a higher temperature and RH. It is noted that absorbed water decreased the Tg of the wet adhesive 2 sample (A2) well below the temperature of the conditioning environment (60 °C); however, the Tg of the wet adhesive 1 samples (A1, B1 and C1) remained well above the temperatures of the respective conditioning environments (40 °C and 60 °C). The effects of aging these adhesives below or above their Tg is discussed below. 3.3. Fatigue behavior of joints with adhesive 1 3.3.1. Effects of aging environment on the fatigue threshold Fig. 5 shows the variation of Gth with aging time under the two humidity levels at 40 °C and 60 °C, and the average Gth of an unaged (freshly bonded) closed specimen of adhesive 1. Two stages of degradation were observed in all aging environments. In the first stage, the fatigue threshold decreased relatively quickly with aging time, but then Gth remained approximately constant at a low value (Gth,1) with further aging. A similar two-stage degradation of the fatigue threshold was observed for joints with pretreated AA5754 adherends bonded with adhesive 1 that were aged and tested under similar environments [18]. These similarities were expected, because in both studies the crack path remained cohesive in the adhesive layer; i.e. since the crack path was in the adhesive layer, the degradation in threshold behavior was sensitive to the adhesive and was insensitive to the pretreatment. It was hypothesized that the variation in threshold with time, Gth(t), could be modeled, using a simple Fickian-type relation since it appeared to depend mostly on the amount of water absorbed, or equivalently, on the amount of retained water. Therefore, following the form of Eq. (2)

" Gth ðtÞ ¼ Gth;fresh  1 

1 8 X

p2

n¼0

1 ð2n þ 1Þ2

# expðDdeg tÞ ðGth;fresh  Gth;1 Þ

ð3Þ

where Gth,fresh and Gth,1 are the fatigue thresholds of the fresh joint and the joint after prolonged aging, respectively. Ddeg is defined as a degradation coefficient reflecting the rate of degradation. Treating Ddeg as an adjustable coefficient and minimizing the least-squares error gave the solid curves in Fig. 5. In general, the agreement between the fitted curves and the data was good, especially at 60 °C for both humidities and at 95% RH for both temperatures. Under the driest conditions

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(a) 200

40ºC

3,2

175

3,2

150

Gth (J/m2)

Fresh 40ºC 43% RH 40ºC 95% RH

2,2

125

2,1

4,2

100

2,1

2,2

75

2,2

50

2,1

25 0 0

5

10

15

20

t1/2 (Day1/2)

(b) 200

60ºC

Fresh 60ºC 43% RH 60ºC 95% RH

2,2

175

2,2

3,2

Gth (J/m2)

150

2,1

2,1

125

2,1

100 75

3,2

50

2,1

2,1

25 0 0

5

10

15

20

t1/2 (Day1/2) Fig. 5. Fatigue threshold as a function of square root of aging time for adhesive 1 specimens aged at 95% and 43% RH at temperatures of (a) 40 °C and (b) 60 °C. Trend lines are the least-square fits of Eq. (3) to the measured data. Numbers next to each data point indicate the number of thresholds reached and the number of specimens tested, respectively; these two numbers are different in cases where more than one threshold was reached using a single specimen. Error bars represent the range of the measurements.

(40 °C–43% RH) the fit was good at short and long times, but tended to overpredict the decrease in Gth, with aging time at intermediate times. This supports the contention that the degradation of Gth, under hot–wet aging tends to develop in proportion to the amount of absorbed water, or retained water in the dry adhesive since they were related as discussed above (Table 1). Fig. 6 shows that Gth,1, the stable value after relatively long exposure times, was affected more by the RH than by the temperature of the aging environment; i.e. at each RH, increasing the aging temperature from 40 °C to 60 °C had a statistically insignificant effect on Gth,1, whereas at each temperature, increasing RH from 43% to 95% decreased Gth,1 significantly (t-test, 95% confidence level). This can be attributed to the much larger change in the amount of water in the adhesive due to changes in RH than due to changes in temperature; i.e. Table 1 shows that M1 increased by 190% at 40 °C and 330% at 60 °C as the RH increased from 43% to 95%. In comparison, M1 increased by only 46% at 95% RH when the temperature increased from 40 °C to 60 °C. 250

Gth, J/m2

200 150 100

Adhesive 1 172 122

116 77

68

50 0

Fig. 6. Gth,1 values in different aging environments and the Gth of fresh specimens. Average threshold values are shown above the columns and the error bars represent ±1 standard deviation.

N.V. Datla et al. / Engineering Fracture Mechanics 79 (2012) 61–77

200

95% RH

175

40ºC 95% RH

150

Gth (J/m2)

69

60ºC 95% RH

125 100 75 50 25 0 0

5

10

15

20

t1/2 (Day1/2) Fig. 7. Least-squares fits of Eq. (3) for the data of Fig. 5 at 95% RH.

Fig. 7 compares the variation in threshold with aging time for specimens aged at 95% RH and temperatures of 40 °C and 60 °C. Increasing the aging temperature increased the rate at which the joints degraded and decreased the time to the onset of the steady-state stage; i.e. the aging time required for Gth to decrease to 80% of the ultimate degradation was approximately 60 days at 60 °C and 87 days at 40 °C. 3.3.2. Effects of aging environment on the crack growth rate behavior of adhesive 1 Fig. 8 shows representative adhesive 1 crack growth rates with aging time for specimens aged at 60 °C–95% RH. It can be seen that the scatter is slightly larger in the degraded specimens than in the fresh specimens. Fig. 9 shows the variation of the crack growth rate curves with aging time for adhesive 1 in the various aging environments. The lines in the figure are the best-fit lines to the Paris regions of the crack growth rate curves. As with threshold degradation, the crack growth rates degraded in two stages, with rapidly increasing crack growth in the first few weeks of aging followed by a stabilization where the crack growth rates remained largely unchanged with further aging. This is illustrated in Fig. 10 for specimens aged at 60 °C–95% RH. Gmax at a fixed crack growth rate was obtained from Fig. 9d by finding the intersection point between the crack growth rate curve and a horizontal line corresponding to the crack growth rate of interest. It can be seen that in the first stage Gmax decreased relatively quickly with aging time until 90 days of aging, but then in the second stage Gmax remained approximately constant at a relatively low value. As expected, Fig. 11 shows that the crack growth rate curves for the different aging environments became more widely separated as the aging time increased. Furthermore, as was observed in threshold degradation, at longer aging times the crack growth rates were affected more by RH than by the temperature of the aging environment. In other words, for the longer aging times at each RH, increasing the aging temperature from 40 °C to 60 °C had an insignificant effect on the crack growth rate (Fig. 11e and f). The first stage of degradation ended sooner for the threshold than for the crack growth rate; for example, for specimens aged at 60 °C and 95% RH the threshold stabilized after 48 days (Fig. 5b) and after 90 days for the crack growth rate (Fig. 9d).

60ºC 95% RH

Log (da/dN), mm/cycles

-1

Fresh

-2

7 days -3

20 days 48 days

-4

90 days -5

149 days -6

231 days

-7 1.6

2

2.4

2.8

3.2

Log (Gmax), J/m2 Fig. 8. Measured fatigue crack growth rate curves for adhesive 1 specimens aged at 60 °C–95% RH. Aging times in days are given in the legend.

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Log (da/dN ), mm/cycle

(a)

-1

40ºC -43% RH

-2 -3 -4 -5 -6 1.6

2

2.4

Log (Gmax), J/m

Log (da/dN ), mm/cycle

(b)

2.8

3.2

2.8

3.2

-2 -3 -4 -5

2

2.4

Log (Gmax), J/m

Log (da/dN ), mm/cycle

3.2

40ºC -95% RH

-1

-6 1.6

(c)

2.8 2

2

60ºC -43% RH

-1 -2 -3 -4 -5 -6 1.6

2

2.4

Log (Gmax), J/m2

Log (da/dN ), mm/cycle

(d) -1

60ºC - 95% RH

-2 -3 -4 -5 -6 1.6

2

2.4

2.8

3.2

Log (Gmax), J/m2 Fig. 9. Variation of crack growth rate curves with aging time for adhesive 1 specimens aged at: (a) 40 °C–43% RH, (b) 40 °C–95% RH, (c) 60 °C–43% RH, and (d) 60 °C–95% RH. Each line is the least-squares fit to the linear Paris region of the crack growth curves. Aging times in days are given in the legend.

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1600

60ºC -95% RH

Gmax, J/m2

1200

10

2

mm/cycle

10

3

mm/cycle

10

mm/cycle

800

400

0 0

50

100

150

200

250

t, Days Fig. 10. Gmax at a fixed crack growth rate vs. aging time for adhesive 1 aged at 60 °C–95% RH. Gmax at a fixed crack growth rate was obtained from Fig. 8, by finding the intersection point between the crack growth rate curve and a horizontal line corresponding to the crack growth rate of interest.

These transition points between the first and second stages of threshold degradation were defined as the minimum aging times required to degrade Gth to 80% of the ultimate degradation measured at stabilization. Similar differences between the degradation of the fatigue threshold and the crack growth rates were observed in earlier work with joints made with pretreated AA5754 adherends bonded with adhesive 1 [18]. It is hypothesized that this can be explained by differences in the fatigue failure mechanisms near the threshold and at higher crack growth rates, which are related to differences in the plastic zone size at the crack tip. Because of the uncertainty of predicting the size of the crack tip damage zone which contains cavitation voids and dispersed micro-cracks ahead of the continuous macro-crack, Datla et al. [27] assumed that the size of the damage zone would be proportional to the plastic zone, and that a relative comparison of damage zone sizes under different loads and conditions could be made in this manner. Datla et al. [27] used finite element modeling of the ADCB specimen (W = 18°) used in this study to show that the thickness of the plastic zone (dimension normal to the crack plane) of adhesive 1 near the threshold at an applied Gmax = 150 J/m2 was around 80 lm. This grew to approximately 120 lm at Gmax = 200 J/m2 at the start of the linear Paris region of the crack growth rate curve. In comparison, Fig. 3a indicates that the rubber particle size in adhesive 1 was approximately 1 lm. Therefore, fatigue at the threshold will involve a smaller volume of rubber particles and the behavior of the epoxy matrix will be relatively more important than at higher crack growth rates. Since the degradation of Gth stabilizes sooner than the crack growth rate, this hypothesis implies that the degradation of the cross-linked epoxy matrix stabilizes before the degradation of the toughening associated with the rubber particles. Similarly, using the same adhesive system, Ameli et al. [17] concluded that the fracture toughness of the epoxy matrix degrades, but only at longer aging times. Their conclusion was based on the observation that initiation fracture toughness, a reflection of the toughness of the matrix, decreased for very long aging times; for example, for specimens aged at 60 °C and 95% RH, the initiation toughness values decreased only after 360 days of aging, while the fatigue threshold decreased from the start of the exposure. This difference in the aging times required to observe degradation suggests that fatigue is more sensitive to degradation than is fracture. This is similar to the concept proposed by Azimi et al. [28] who found that the fatigue threshold of a rubber-toughened epoxy adhesive was the same as that of an untoughened epoxy, suggesting that rubber toughening mechanisms were absent at these small crack growth rates close to the threshold. They attributed this to the size of the plastic zone ahead of the crack tip being smaller than the rubber particles, thereby minimizing their influence in crack propagation. 3.3.3. Effects of aging environment on the crack path of adhesive 1 Fig. 12 shows that the crack paths in both the unaged and aged joints were cohesive at all crack growth rates. Furthermore, the thickness of the residual adhesive on the more highly-strained open-faced adherend decreased monotonically with decreasing crack growth rate (decreasing Gmax) in all specimens. A similar trend has been explained in terms of the decreasing size of the plastic zone at the tip of the crack as the applied G decreased [20]. This relation was illustrated above for Gmax = 150 J/m2 and 200 J/m2. Assuming that the average crack path tends toward the center of the plastic zone, the residual adhesive thickness will decrease as the applied G decreases and the crack slows. Fig. 12 also shows that the aging time did not affect the thickness of the residual adhesive. 3.3.4. Exposure index (EI) behavior of adhesive 1 Based on the SDF model for water absorption, the analytical expression for EIa as a function of absorption time t in an open-faced specimen; i.e. the time integral of the water concentration within the adhesive at a given location x from the exposed surface is given by [17]:

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Fresh

(a)

Fresh

-1

(d) -1

7 days

40ºC 43% RH

Log (da/dN ), mm/cycle

Log (da/dN ), mm/cycle

40ºC 43% RH -2 40ºC 95% RH -3

60ºC 43% RH 60ºC 95% RH

-4

-5

-6

90 days

40ºC 95% RH -2 60ºC 43% RH -3

60ºC 95% RH

-4

-5

-6 1.6

2

2.4

2.8

3.2

1.6

2

Log (Gmax), J/m2

2.4

2.8

3.2

Log (Gmax), J/m2 Fresh

-1

(e) -1

21 days

Fresh 40ºC 43% RH

-2

Log (da/dN ), mm/cycle

Log (da/dN ), mm/cycle

(b)

40ºC 95% RH -3

60ºC 43% RH 60ºC 95% RH

-4

-5

-6

150 days

40ºC 43% RH 40ºC 95% RH

-2

60ºC 43% RH -3

60ºC 95% RH

-4

-5

-6 1.6

2

2.4

2.8

3.2

1.6

2

2

2.4

2.8

3.2

2

Log (Gmax), J/m

Log (Gmax), J/m

Log (da/dN ), mm/cycle

-1

45 days

Fresh 40ºC 43% RH

-2

40ºC 95% RH -3

60ºC 43% RH 60ºC 95% RH

-4

-5

-6

(f)

-1

Log (da/dN ), mm/cycle

Fresh

(c)

-2

240 days

40ºC 43% RH 40ºC 95% RH 60ºC 43% RH

-3

60ºC 95% RH -4

-5

-6 1.6

2

2.4

Log (Gmax), J/m2

2.8

3.2

1.6

2

2.4

2.8

3.2

Log (Gmax), J/m2

Fig. 11. Effect of aging environment on the crack growth rate curves for adhesive 1 specimens aged for: (a) 7, (b) 21, (c) 45, (d) 90, (e) 150, and (f) 240 days. Each line is the least-squares fit to the linear Paris region of the crack growth curves.

" ! #  ) 2 1 16h X ð1Þn D1 ð2n þ 1Þ2 p2 t ð2n þ 1Þpx  1 cos EIa ¼ t þ 3 exp C 11 2 2h p D1 0 ð2n þ 1Þ3 4h " ! # (  ) 2 1 16h X ð1Þn D2 ð2n þ 1Þ2 p2 t ð2n þ 1Þpx  1 cos þ uðt  t d Þ ðt  t d Þ þ 3 exp C 21 2 2h p D2 0 ð2n þ 1Þ3 4h (

ð4Þ

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Fig. 12. Fracture surfaces on the more highly-strained adherend for adhesive 1 specimens that were: (a) unaged, (b) aged for 21 days at 40 °C–95% RH, and (c) aged for 150 days at 40 °C–95% RH. In each case, the fatigue region is to the left of the arrow showing where Gth occurred. After reaching Gth, specimens were fractured, except for (c) where the fatigue process was repeated.

where C11 and C21 are the water saturated concentrations of the first and second diffusion mechanisms such that C11 + C21 = C1, where C1 is the total saturation concentration. Assuming a uniform distribution of water concentrations at saturation, C11 = M11 and C21 = M21. h is the thickness of the primary layer of adhesive. Since the absorbed water can

(a) 200

40ºC 43% RH

175

40ºC 95% RH

G th , J/m2

150

40ºC 95% RH

125 100 75 50

y = 200.56x-0.236 R2 = 0.9219

25 0 0

50

100

150

EIT ,106 g/g.s

(b)

60ºC 43% RH 150

60ºC 95% RH

Gth , J/m2

125

60ºC 95% RH 100 75 50

y = 176.55x-0.222 R2 = 0.8104

25 0 0

50

100

150

200

EIT ,106 g/g.s Fig. 13. Fatigue threshold vs. EIT for adhesive 1 specimens aged at 43% and 95% RH and temperatures of (a) 40 °C and (b) 60 °C. Trend lines show the best-fit power law curves to the data at 95% RH.

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also degrade the adhesive during the drying period (7 days in vacuum oven), which sometimes is a significant portion of the exposure time, the environmental exposure during desorption process, EId was also considered [17]:

" ! #  ) 2 1 16h X ð1Þn Dd ð2n þ 1Þ2 p2 t0 ð2n þ 1Þpx  1 cos ðC  C r Þ exp 2 2h p3 D1 0 ð2n þ 1Þ3 4h

( EId ¼ C r t0 

ð5Þ

where Cr is the retained water concentration, C is the water concentration before drying, and t0 is the drying time. Assuming a uniform distribution of water concentration after drying, Cr = Mr. The total exposure index, EIT from start of exposure to hot–wet environment to the end of the drying period is thus

EIT ¼ EIa þ EId

ð6Þ

Fig. 13 shows the variation in the average Gth with the total exposure index, EIT, for joints aged using two different aging paths: 43% and 95% RH at temperature of 40 °C and 60 °C. In these cases, EIT, was evaluated at x = h; i.e. at the interface of the primary adhesive and the adherend, because this approximated the location of the crack path (for long t, the calculation is insensitive to the choice of x). For both temperatures, at any particular EIT value the decrease in Gth was greater for specimens aged at 95% RH than at 43% RH. For example, at an EIT of 25  106 g/g s at 60 °C the Gth decreased by 57% after exposure at 95% RH for 48 days (from 172 ± 20 to 74 J/m2) and by 32% for specimens aged at 43% RH for 154 days (from 172 ± 20 to 117 ± 7 J/m2). Furthermore, Fig. 14 shows significant differences in the crack growth rate curves for similar EIT values reached using two different aging paths. For both temperatures, at any particular applied G the crack growth rates were higher for specimens aged at 95% RH than at 43% RH, even though the EIT values were quite similar. This indicates that degradation in the fatigue threshold and crack growth rates depended on the path of aging, thereby invalidating the EI hypothesis for both the fatigue threshold and crack growth rates, at least for relatively small values of EIT < 36  106 g/g s. A similar dependence on the path of the aging at lower EIT (less than 25  106 g/g s) was observed for fracture toughness using the same adhesive system (1) as in present study [17]. However, at higher EIT values (above 25  106 g/g s) aging path independence was observed. Since the maximum EIT reached at the lower humidity level in the present tests was 36  106 g/g s, which is relatively small, it is possible that aging path independence may exist at higher EIT values.

Log (da/dN ), mm/cycle

(a) -1

T = 40ºC

-2 -3 -4 -5 -6 1.6

2

2.4

43% RH (13.3 10

g/g.s)

95% RH (15.1 10

g/g.s)

43% RH (35.6 10

g/g.s)

95% RH (34.6 10

g/g.s)

2.8

3.2

Log (Gmax), J/m2

(b) -1 Log (da/dN ), mm/cycle

T = 60ºC -2 -3 -4

43% RH (8.4 95% RH (10.3 43% RH (21.5 95% RH (26.9

-5 -6 1.6

2

2.4

2.8

10 g/g.s) 10 g/g.s) 10 g/g.s) 10 g/g.s) 3.2

Log (Gmax), J/m2 Fig. 14. Differences in the crack growth rates for adhesive 1 with similar EIT values that were aged at different RH at aging temperatures of (a) 40 °C and (b) 60 °C. EIT values of specimens are given in the legend. Each line is the least-squares fit to the linear Paris region of the crack growth curves.

N.V. Datla et al. / Engineering Fracture Mechanics 79 (2012) 61–77

75

3.4. Fatigue behavior of joints with adhesive 2 Fig. 15 shows the variation of Gth with aging time for specimens aged at 60 °C–95% RH, and the average Gth of unaged closed specimens. It can be seen that the threshold decreased significantly by 36% (from 129 ± 15 to 82 ± 2 J/m2) after 60 days of aging, and by 43% (from 129 ± 15 to 73 ± 21 J/m2) after 180 days of aging. However, Fig. 16 shows insignificant changes in crack growth rates with aging at relatively high crack growth rates, corresponding to G greater than approximately 102.4 = 250 J/m2. In other words, the fatigue performance at loads above Gth was undegraded even after 180 days of open-faced aging. This is in marked contrast to the behavior of adhesive 1 which showed significant degradation of the threshold and the crack growth rate from the onset of aging. As with adhesive 1, Fig. 17 shows that the crack path in both unaged and aged joints with adhesive 2 was cohesive at all crack growth rates. It was also observed that at high crack growth rates the crack path was inconsistent for aged specimens; i.e. the crack path was in the primary layer for some specimens (Fig. 17b) and in the secondary layer for the remaining specimens (Fig. 17c). Furthermore, since the changes in the crack growth rate curves were insignificant (see Fig. 16), this inconsistency in crack path was probably caused by the approximately equal toughness of the aged (primary) and unaged (secondary) layers. Fig. 17b and 17c also show that the crack path remained in the primary adhesive near the interface of the more highly-strained adherend as the crack growth rate approached the threshold. 3.4.1. Degradation mechanisms of adhesive 2 As discussed above, Figs. 15 and 16 show that aging of adhesive 2 caused Gth to decrease significantly with aging time, but that crack growth rates were mostly unaffected by aging. Following the reasoning used in Section 3.3.2 to explain the differences between the threshold and crack propagation behaviors in adhesive 1, it is possible that the rubber toughening mechanisms that were present at relatively high crack growth rates above threshold were absent as the crack growth rates reached threshold. A marked difference was observed between both adhesives in their degradation behavior at crack growth rates above threshold: while adhesive 1 degraded with aging time adhesive 2 did not degrade, even after prolonged aging. This difference 150

Gth, J/m2

100

50

0 0

30

60

90

120

150

180

Aging time, days Fig. 15. Fatigue threshold vs. aging time for adhesive 2 specimens aged at 60 °C and 95% RH. Numbers next to each data point indicate the number of thresholds reached and the number of specimens tested, respectively. Error bars represent the range of the measurements.

Log (da/dN ), mm/cycle

-3

-4

-5

Fresh 60 days 180 days

-6

-7 1.6

2

2.4

2.8

3.2

Log (Gmax), J/m2 Fig. 16. Measured fatigue crack growth rate curves for adhesive 2 specimens aged at 60 °C–95% RH. Aging time is given in the legend.

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Fig. 17. Fracture surfaces of adhesive 2 specimens on the more highly-strained adherend for an unaged closed joint (A) and for two 180 days aged specimens at 60 °C–95% RH with crack path at higher crack growth rates in the primary layer (specimen B) and in the secondary layer (specimen C). Crack growth was from left to right corresponding to an applied G that decreased from left to right toward the threshold. Specimens were fractured after reaching Gth.

in degradation combined with the key observation that retained water in the form of bound water was present only in adhesive 1 and not in adhesive 2, suggests that the bound water in adhesive 1 may have been responsible for degradation at relatively high crack growth rates. Since rubber toughening mechanisms are strongly affected by the adhesion between the rubber particles and the matrix [29–31], a possible explanation for the degradation in adhesive 1 was that the retained bound water disrupted chemical bonds between the rubber particles and the matrix by being bound at the rubber-matrix interface. This hypothesis was supported by an earlier observation by Ameli et al. [13] that showed that the degraded fracture toughness of adhesive 1 decreased to a low value which was approximately equal to the fracture toughness of an untoughened epoxy. The negligible retained water in adhesive 2 suggests that the chemical bonds at the rubber-matrix interface could not have been disrupted in this way, thereby leaving the toughening mechanisms and consequently the crack growth rates unaffected in adhesive 2. This is consistent with the work in Ref. [13] showing that the quasi-static fracture toughness of adhesive 2 remained unchanged when aged under these same conditions. It is interesting to note that aging was done at temperatures below the wet Tg for adhesive 1 and above the wet Tg for adhesive 2 (see Table 2). Further work is required to determine if aging at temperatures above Tg, when the adhesive was in the rubbery state, had an effect on degradation that was independent of the effect attributable to the absence of retained water in adhesive 2. 4. Conclusions Two rubber-toughened epoxy adhesives exhibited very different degradation behaviors when aged in hot-wet environments. The reasons for these differences were investigated by comparing the water absorption/desorption behavior and the results of dynamic mechanical thermal analysis (DMTA). In adhesive 1 a significant amount of absorbed water was retained in the adhesive even after prolonged drying, whereas in adhesive 2 the amount of retained water was negligible. The DMTA results showed that retained water in adhesive 1 was bound to the adhesive constituents and was not in a free state. Aged open-faced ADCB specimens made with these adhesives were subject to cyclic loading under mixed-mode conditions. The contrasting results illustrated the effects of environmental degradation of the matrix and the rubber-toughening particles. The fatigue threshold strain energy release rate, Gth, of adhesive 1 initially decreased with aging time until it reached a constant minimum value for long times. Similarly, fatigue crack growth rates initially increased with aging time until reaching a limiting upper value. However, Gth reached the minimum value sooner than did the crack growth rate. In contrast, Gth of adhesive 2 decreased significantly with aging time while the crack growth rates remained unchanged, even after prolonged aging. These differences in fatigue threshold and crack growth rate behavior were explained by the changes in the size of the plastic zone at the crack tip as the applied loads changed. At relatively high crack growth rates, rubber-toughening mechanisms were active because of the relatively large plastic zone. These mechanisms were much less effective when the crack tip plastic zones became smaller with the decreasing applied loads as the crack growth rates approached the threshold. The differences in the effects of degradation between the two adhesives at relatively high crack growth rates is believed to be primarily related to the amount of retained, bound water. The experimental observations were consistent with the action of bound water disrupted the rubber/matrix interface in adhesive 1, thereby degrading the toughening mechanisms. Having no retained, bound water, adhesive 2 was unaffected by this degradation.

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The hypothesis that degradation can be correlated with the time integral of the water concentration in the adhesive layer (the ‘‘exposure index’’, EI) was evaluated using different combinations of water concentration and exposure time that gave the same EI. For the range of EI values that were investigated, it was found that such path independence did not exist. This limits the applicability of the EI approach, at least for the relatively small EI values that were studied. Acknowledgments The work was supported by General Motors Canada Ltd. and the Natural Sciences and Engineering Research Council of Canada. References [1] Wahab MA, Ashcroft IA, Crocombe AD, Shaw SJ. Diffusion of moisture in adhesively bonded joints. J Adhes 2001;77:43–80. [2] LaPlante G, Ouriadov AV, Lee-Sullivan P, Balcom BJ. Anomalous moisture diffusion in an epoxy adhesive detected by magnetic resonance imaging. J Appl Polym Sci 2008;109:1350–9. [3] Fernandez-Garcia M, Chiang MYM. 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