EFFECT OF SURFACE CONTAMINATION ON COMPOSITE BOND INTEGRITY AND DURABILITY

Vishal Musaramthota, Tomas Pribanic and Dwayne McDaniel

Florida International University, Applied Research Center Miami, FL, U.S.A.

Xiangyang Zhou

University of Miami, Department of Mechanical and Aerospace Engineering Coral Gables, FL, U.S.A.

ABSTRACT

Previous research has shown that surface contamination plays a key role in the initial bond strength of adhesively composite joints (ABCJ’s). Recent advances in experimental methods offers improvements in capturing changes in bond strength which are significant in predicting the performance of ABCJs. This study presents an experimental method to evaluate the effects of contamination on the durability of ABCJ’s. Initially, bonded double cantilever beam (DCB) specimens were manufactured with laminates that were contaminated prior to bonding (secondary cure). Surface characterization of the laminates was conducted prior to bonding using FTIR and water contact angle measurements techniques. After manufacturing, specimens were exposed to an elevated humidity/temperature environment or fatigued via cyclic loading to accelerate the aging process of the bond. DCB testing of the specimens provided the fracture toughness and mode of failure for each group of conditioned specimens. Results were compared with baseline tests in which no contamination was introduced but a similar aging process was utilized. Additionally, the moisture absorption of bonded and non-bonded specimens in unstressed conditions was used to obtain the equilibrium saturation point over a period of time.

1. INTRODUCTION

Composite material systems are used in a number of commercial industries including automotive, aerospace, marine and civil. Their popularity is primarily due to their high strength to weight ratio [1-3]. The use of composite materials has broadened over time due to their other benefits including material toughness, damage tolerance, and fatigue endurance. Despite the advantages in material properties, issues related to the joining of composite systems still need to be addressed. The optimal joining of composites is accomplished using adhesive bonding which does not induce typical stress risers observed with riveting or other classical techniques [5-7]. Adhesive bonding provides uniformly distributed load transfer has improved environmental resistance over the classical joining methods [8, 9]. The primary goal for adhesively bonded composite joints (ABCJ’s) is to sustain static and cyclic loads for extended periods of time

without deleterious effects on the load bearing capacity of the structure [10]. A fundamental understanding on the performance of bonded joints using the initial bond strength has been demonstrated and documented as well [10-13]. Multiple factors have been identified that impact joint performance leading to a reduction in the bond strength. Some of these factors include operating temperatures [14, 15, 16], cleanliness of bonding surface [17, 18], relative humidity [19] and service loads [20]. Surface contamination originating from the peel ply fabric has also been shown to cause a reduction in the bond strength [21]. Although there has been significant effort placed on understanding how these factors effect bond strength, less focus from industry and academia has been place on understanding the factors that affect the long term durability of a bond. Identifying the key processing parameters, material characteristics, and the effects of surface contamination on durability performance is critical in establishing quality control procedures to produce bonds that will be effective for their design life. In support of these needs, the objective of this study is to evaluate the durability of ABCJ’s utilizing various contamination procedures and evaluating their bond strengths after conditioning with environmental aging and mechanical fatiguing.

2. BACKGROUND

Although there are a few experimental studies that have been conducted to evaluate bond strength durability, there is little agreement of the definition of durability and the processes needed for evaluation. Silva and Ochsner note that “Durability is a general term that is related to the residual strength of the joint when subjected to water or temperature. The loading can be static but also dynamic. This subject is probably the major challenge that the adhesion community faces today” [23]. The following sections provide information of recent efforts at FIU as well as other efforts in terms of evaluating the effects of contamination on durability.

2.1. FIU Research Efforts

Previous research efforts at FIU included developing a test methodology to evaluate the durability of ABCJ’s [22]. In the process of developing this procedure, various sets of peel ply fabrics were utilized in manufacturing composite specimens to evaluate the effect of peel ply on the strength of the bonds. Additional surface preparation methods that included sanding with various types of sand paper were also investigated [24]. Surface characterization on the composite laminates was conducted on various coupon sets prior to bonding to determine what correlations could be made between the bond strength and the resulting characterization. Techniques used for the characterization included atomic force microscopy, electrochemical sensors [25], Fourier transform infrared spectroscopy and contact angle measurements [26]. To evaluate the durability of bonded joints, a fatigue fixture was developed that loads multiple specimens in a 3 point bending arrangement, generating a uniform shear stress at the bondline (Figure 1). Specimens used for this study are DCB specimens geometrically similar to those found in ASTM D790. The design configuration and specifications have been reported in

previous papers [26, 27]. Contaminated and non-contaminated specimens will be fatigued in the fixture and resulting fracture toughness values will be evaluated to establish the effect of contamination on fatigue.

Figure 1. Schematic of three point loading (left) and rendering of the fatigue fixture (right).

Understanding the surface chemistry of substrates prior to bonding and how it affects bond strength is of significant importance. Various analytical methods have been utilized in our previous studies to determine if correlations between the parameters evaluated could be correlated with bond strength. These methods include FTIR, wettability measurements, atomic force microscopy (AFM) and a solid state electrochemical sensor [22]. In this paper, emphasis will be on the surface characterization of contaminated specimens using FTIR and contact angle measurements. To assess the durability of bonded DCB specimens, strain energy release rates (SERR) were determined for specimens as per ASTM 5528. Specimens were grouped into the following categories:

(a) baseline specimens – no conditioning (b) environmentally conditioning - 95% RH, 50°C (c) mechanical fatiguing (2.6 million cycles at a one double amplitude displacement) (d) combined environmental exposure and mechanical fatiguing

The GIC values obtained for each set is shown in Figure 2. Note that the results are for the non-contaminated specimens. Details of the procedures and analysis can be found in [24].

Figure 2. Average fracture toughness values for the non-contaminated specimens.

2.2. Other Contamination Studies

A number of published studies have been conducted on the effect of contamination on the initial bond strength of ABCJ’s but little effort has focused on the long term durability of the bond. Some of these efforts [18, 21, 28-30] have focused on understanding the effect of contaminants that stem from peel ply residues on the bond performance. Hart Smith et al. noted that a silicon residue is left on the laminate surfaces after a nylon peel ply is removed [29]. Phariss et al. [31] used XPS measurement to show that a siloxane coated peel ply left contamination residue on a composite surface after ply removal. Additionally, Van Voast et al. [30] used a method in which the peel ply is soaked in solutions of contamination prior to the use of the ply. They observed the effects of contamination on bond performance after the peel ply was removed and the contamination was transferred to the composite surface. These efforts have provided insight to the present work in utilizing several contamination procedures in order to assess the durability of bonded joints.  

3. EXPERIMENTAL METHODS

3.1 Methodology

A workflow of the proposed methodology utilized to access durability of the bonded joints is shown in Figure 3. The specimens are categorized into pristine (non-contaminated) and contaminated sets. Results of the pristine samples were reported in [22]. This study will emphasize the methods of contamination and durability assessment on contaminated specimens. Cured laminates are first contaminated and analyzed prior to bonding. Bonded panels are then cut to size and conditioned under fatigue loading, environmental aging and combined fatigue loading and environmental aging. A separate set of specimens with no conditioning (baseline) is

used to provide the initial strength of the joints. The objective of the presented approach is to aid the in the assessment of the long-term durability of bonded joints by replicating conditions similar to those experienced by bonded assemblies in service. Conclusions drawn from the experimental results (fracture toughness, mode of failure) and their relationship with the type of conditioning combined with contamination procedures will contribute to a better understanding long term durability of ABCJ’s.

Figure 3. Durability assessment procedure for adhesively bonded composite joints.

3.2. Specimen Preparation

Specimens were fabricated from 12 in x 12 in composite panels. Bonding and cutting of the unidirectional lay-up DCB test coupons is performed according to ASTM D5528. The material system employed included:

• Prepreg - T800 Unidirectional carbon fiber (P2362W-19U-304) from Toray • Adhesive - AF 555M film adhesive from 3M • Peel ply - 60001 polyester ply from Precision Fabric

The details of fabrication process and bonding can be found in [22]. The contaminated bonded specimens were categorized as described above in section 2.1.

3.3. Specimen Conditioning

A Thermotron 2800 environmental chamber, set to 50°C and 95% relative humidity (RH), was utilized to environmentally age the contaminated specimens. The time of exposure in the environmental chamber was 2 months. Fatigue conditioning of the specimens was performed using a double amplitude cyclic loading three point bending mode. Cyclic load parameters were kept under the failure limit for the desired number of cycles. Specimens were loaded with a

double amplitude of 1 inch at a frequency of 1 Hz for a total of 2.6 M cycles. The deflection parameters and its specifications were documented in [27].

3.4. Contamination Procedures

The aim of developing a procedure to contaminate the laminate surfaces prior to bonding is to create weaker links between the adherend and adhesive and determine the effect of contamination on bond durability in ABCJ’s. Several approaches were explored at FIU to create measurable and repeatable bonding conditions. This was done by systematically controlling the amount of contamination created or deposited on the laminate surface. Two contaminants were use in this study: 100% pure Silicone spray from CBS Aerosol & Paint, Inc and Freekote 700-NC mold release agent. After investigating several contamination procedures, two approaches were selected. The following section provide descriptions for each.

Mesh Approach

For this approach, a perforated stainless steel mesh (0.305 m x 0.305 m) with evenly spaced holes having a diameter of 3.2 mm, a thickness of 0.91 mm, and a stagger of 22.2 mm was utilized. The mesh was vertically placed at a 90° angle and the contaminant was sprayed in the perpendicular direction towards the mesh as shown in the Figure 4. Care was taken to create a uniform layer of contaminant on one side of the perforated mesh. The side of the mesh containing the contaminant is brought in contact with the laminate surface leaving and imprint of the mesh. It should be noted that a gravimetric analysis was conducted on the laminates before and after contamination in order to record the amount of contaminant deposited.

Figure 4. Contamination procedure- mesh approach.

Stamp Approach

For the stamp approach, a rubber stamp with a dotted pattern was wetted with a sponge saturated with the contaminant. The stamp was then brought in contact with the laminate surface as shown in Figure 5. The imprint generated using this procedure provides higher control of the contaminated areas than those areas generated with the mesh approach. As mentioned above, gravimetric analysis was conducted on laminates before and after contamination in order to monitor the weight gains.

Figure 5. Contamination procedure- stamp approach.

In both approaches, the spatially ordered patterns can be varied (altering the diameter of the holes on the mesh or varying the dotted pattern on the stamp). Once the approach is optimized, two contaminants were evaluated to observe their effect on bond strength. Three aspects regarding the contamination procedure were investigated:

• Application of the contaminant (mesh and stamp approach). • Contaminant material (Aerosol and Freekote). • Side of application of contaminant to laminate (contaminating one laminate vs.

contaminating both laminates prior to bonding)

The objective of varying the side of application of contaminates onto the laminate was to capture changes in DCB test results stemming from an increase in the contaminant concentration and to differentiate the failure analysis between contaminated and non-contaminated surfaces.

3.5. Surface Characterization

Analysis of the surface chemistry and surface characteristics of laminates prior to bonding can be conducted using a broad range of surface characterization techniques (i.e. AFM, ECS, contact angle, and FTIR). Results from the analysis can be used to correlate surface properties with bond strength. In this investigation, a comparative assessment of non-contaminated and contaminated specimens was conducted using wettability measurements and FTIR. Additionally, optical microscopy was used to investigate failure surfaces and determine bondline thicknesses.

Contact Angle Measurements

Contact angles were obtained using the sessile drop method with a KYOWA Contact angle meter (model no: DM-CE1) equipped with a dispensing needle. Three liquids were used for measuring the contact angles: DI water, ethylene glycol and diiodomethane. A 2 µl droplet (sessile) was generated by rotating the needle and approaching the substrate perpendicular to the needle direction with a gentle feed rate of a few micrometers per minute. All the tests were carried out in ambient air at room temperature. Ten different drops for each liquid at 10 different locations on the substrate were analyzed. With the aid of an image analysis system, the contact angle formed between the laminate and the droplet was measured and surface free energy parameters of were obtained. FTIR

Infrared spectra were collected using the JASCO bench-top infrared spectrometer using the ATR protocol. The ATR measures the change in the internally reflected infrared beam when the beam comes in contact with the sample. The changes in the energy due to absorption of the incident beam are detected which provides information about available functional groups. Care was taken to ensure full and intimate contact between the composite surface and the ATR germanium crystal with the application of moderate pressure over the surface. The FTIR spectrum of the material can be compared for “best matches” with libraries of spectra that have been cataloged for known materials. The residual peel ply was removed from the composite surface and measured for its IR spectroscopy. The spectra were collected in 600 cm-1 to 1800 cm-1 region with 2 cm-1 resolution, 128 scans with scanning speed of 2 mm/sec. Optical Microscopy

Bondline thickness consistency is critical in evaluating bond performance. Bondline thickness measurements were taken at three locations - 1, 3 and 5 cm along the specimen length after the pre-crack of the DCB coupons prior to testing. The optical microscope utilized was a petrographic Olympus BH2 and SZ9 scope with a camera Q-Imaging Micropublisher 3.3 RTV. In addition to the optical microscopy measurements prior to testing, the failed surfaces from DCB specimens were analyzed for mode of failure and a line profile was generated.

3.6. Gravimetric Analysis

Assessing the weight changes of the laminate due to the addition of contamination is of key importance to quantify the effect of contamination in bond performance. A Metler Toledo AB304-S scale was utilized to measure the increase in the mass of the specimens after contamination. Additionally, two sets of specimens (one bonded and the other laminate only) are being monitored to evaluate the net water ingress % (absorption) into the specimen. (bonded and non-bonded).

3.7. Experimental Testing

DCB testing was conducted on an MTS 858 Table Top system. Control of the jaws was defined using displacement control at a constant rate of 2.5 to 5 mm/min. Changes in the delamination length (crack length) were measured on one side of test specimen with the aid of a travelling microscope. The delamination onset was visually recorded using gradations on the side of the specimen starting from the pre-crack for every 1 mm for the first 5 mm and continuing the vertical gradation for every 5 mm from thereon (ASTM D5528). The results of all the DCB tests were obtained as per ASTM D 5528.

4. RESULTS

4.1. Gravimetric Analysis

Gravimetric analysis was conducted to determine the net mass increase of contaminants using the two approaches mentioned previously. Laminate panels were contaminated and the percent increase in mass is shown in Table 1 below.

Table 1. Laminate Mass Increase with Si Aerosol

Before (gms) After (gms) B % Increase

Aerosol Si spray (mesh)

Laminate 1 123.59 123.63 0.032

Laminate 2 122.52 122.53 0.008

Aerosol Si spray (stamp)

Laminate 1 123.04 123.06 0.022

Laminate 2 124.18 124.21 0.024

The distribution of contamination was found to be non-uniform after numerous trials via the mesh approach as shown in Figure 6a. Significant variations in the percent increase in mass were also observed. The stamp approach showed promising results with a very low deviation in the percent increase and a uniform distribution as shown in Figure 6b. This method was selected for our contamination procedure. Two materials used for contamination were also considered for this study: Freekote 700-NC and a Silicone aerosol spray. Table 2 show the percent increase in

mass for both contaminants using the stamped approach. No significant change in mass increase was observed for each of the contaminants.

a)

b)

Figure 6. Distribution pattern of Aerosol contaminant in a) mesh and b) stamp.

Table 2. Laminate Mass Increase Using Stamp Approach

Before (gms) After (gms) B % Increase

Aerosol Si spray (stamp)

Laminate 1 123.04 123.06 0.022

Laminate 2 124.18 124.21 0.024

Freekote 700NC (stamp)

Laminate 1 121.35 121.37 0.016

Laminate 2 125.46 125.49 0.023

The effect of contaminating both sides and a single side were also investigated in this study. Table 3 shows the percent increase in mass when contaminating a single side and both sides of the panel. Little change for the percent increase was observed.

Table 3. Laminate Mass Increase for Single and Dual Sided Contamination

Before (gms) After (gms) B % Increase

Freekote 700NC (Dual side)

Laminate 1 121.35 121.37 0.016

Laminate 2 125.46 125.49 0.023

Freekote 700NC (Single side)

Laminate 1 122.41 122.44 0.024

Laminate 2 NA NA NA

*Laminate 2 is designated as not applicable (NA) as only one side is contaminated Table 4 depicts the percent mass increase for panels that will be used to manufacture the contaminated baseline, contaminated environmentally exposed and contaminated fatigue specimens. No significant changes were observed with the percent increase in mass

demonstrating that the contamination procedure is providing consistent amount of contaminants on the laminate surfaces. .

Table 4. Laminate Mass Increase for Contaminated Panels Used in Durability Assessment

Stamp approach - Single side Before (gms) After (gms) B Gain %

Freekote 700NC

Baseline 122.41 122.44 0.024 Environmentally

exposed 118.45 118.48 0.025

Fatigue loaded 120.8 120.82 0.016 Combined fatigue and

environmentally exposed

TBD TBD TBD

*TBD- to be determined

4.2. Surface Characterization

Contact Angle Measurements

Contact angles were measured on all specimens including baseline (non-contaminated), mesh contaminated and stamp contaminated with aerosol and stamp contaminated with Freekote. Lower contact angles were observed for the baseline specimens indicating better wettability for this group. Additionally, the aerosol mesh contaminated specimens had slightly higher angle measurements than the aerosol and Freekote stamp contaminated specimens using water and ethylene glycol measurements. It should be noted that with the stamp contaminated specimens, it was visually obvious that the mesh approach covered more surface area than the stamp approach.

Figure 7. Contact angle measurements for baseline, aerosol_mesh, aerosol stamp and freekote stamp.

Surface free energies (SFEs) were also determined for the specimens and are shown in Table 5. The SFE of the baseline specimens and the aerosol mesh contaminated specimens did not vary significantly; however, there was a large deviation associated with the aerosol mesh values. The aerosol stamped specimens did exhibit a large reduction in SFE as compared to the Freekote stamped specimens. In general, a surface with low surface energy has poor adhesion i.e. large contact angles. In general, the data correlates well with the contact angles measured for the specimens. However, the aerosol mesh specimens had a larger mean value and a greater deviation which can be expected based on the non-uniformity of the contamination.

Table 5. Surface Free Energy Components

Sample Surface Free Energy, mJ/m2

Toray_Baseline 51.0 ± 4.5

Toray_Aerosol mesh contaminated 51.3 ± 28.1

Toray_Aerosol stamp contaminated 36.2 ± 10.9

Toray_Freekote stamp contaminated 46.2 ± 3.9

Fourier Transformed Infrared Spectroscopy

FTIR results from T800 baseline specimens contaminated with the aerosol spray using the mesh and stamp method is shown in Figure 8a. Silicone based peaks were identified at 1260 cm-1 [32, 33], 1018 and 800 cm-1 [32]. It can easily be seen from Figure 8a, that a minimal silicone peak is detected in baseline specimens whereas significant peaks of silicone are detected on specimens contaminated with the stamp and mesh approach. At 1140 cm-1 the stretching of carbon and oxygen bonds (C-O stretch) was identified and is known to be a derivative of polyester which may be attributed to the peel ply residue [34]. Peak intensities at 1458, 1506 and 1592 cm-1 are characteristic stretching vibrations of carbon and hydrogen bonds (C-H groups) [27]. The CH3 group was identified in all the three specimens which can be attributed to the prepreg [35]. Figure 8b shows the same results in Figure 8a, except results from the Freekote stamp contaminated specimens are included. No silicone peaks were identified in Freekote contaminated specimen; however, an ester peak and a CH3 peak can be seen which likely stems from the peel ply and prepreg, respectively.

(a)

(b)

Figure 8a) FTIR Spectra for Toray as tooled specimen (black), Aerosol stamp contaminated specimen (red) and Aerosol mesh contaminated (cyan). 8b) FTIR Spectra for Toray as tooled (black), Aerosol stamp contaminated

specimen (red) and Freekote stamp contaminated (blue).

Optical Microscopy

Optical microscopy was utilized to evaluate the bondline thickness variation on all the contaminated specimens after bonding (Table 6). A consistent bondline for each contaminant was obtained with values ranging from 9.17 ~ 9.65 mils in aerosol contaminated specimens and 12.57 ~ 12.99 mils in Freekote contaminated specimens. A total of 4 specimen for each set where evaluated.

Table 6. Average Bondline Thickness for Contaminated Specimens

Specimens (n=4) Bondline (mils)

Toray/Aerosol (mesh) contaminated 9.17

Toray/Aerosol (stamp) contaminated 9.65

Toray/Freekote (stamp) contaminated-dual side 12.57

Toray/Freekote (stamp) contaminated-single side 12.99

Additionally, bondline thickness measurements for specimens that were part of the durability assessment is provided in Table 7. The bondline thickness between all the sets varied between 9.29 mils to 17.47 mils.

Table 7. Average Bondline Thickness for Specimens in Durability Assessment

Specimens (n=4) Bondline (mils)

Non Contaminated Contaminated

Baseline 9.29 12.99

Environmentally Exposed 15.72 13.86

Fatigue loaded 17.47 13.50

Combined fatigue + Environmental exposure 11.96 TBD

4.3. DCB Test Results

DCB specimens were sectioned from the bonded non-contaminated and contaminated laminates and strain energy release rates associated with crack growth were calculated according to ASTM D5528. In determining the choice of contaminants for the study, DCB tests were conducted with the aerosol and Freekote contaminants with contamination on both sides of the plies. The GIC values for the baseline, and contaminated sets are show in Figure 9.

Figure 9. GIC values for baseline (non-contaminated), aerosol (mesh and stamp) and Freekote (stamp).

The baseline bonded specimens (non-contaminated) had an average G1C value of 4.22 in-lb/in2. The aerosol mesh and stamp specimens had values similar to the baseline set but the Freekote stamp set provided a reduced average of 3.61 in-lb/in2. For the subsequent tests, the Freekote stamp method was utilized. We also investigated the use of contamination on one side of the bonded laminate and both sides of the bonded laminate. Results provided in Figure 10 show the single side contaminated sets (red dots on one side) had a slightly larger reduction in bond strength than when contaminated on both sides. Thus, single side contamination was adopted for subsequent testing.

Figure 10. GIC values for baseline, dual side and single side contamination specimens with Freekote (stamped).

For the durability assessment, specimen sets were categorized into the following sub-groups: (a) baseline specimens (b) environmentally conditioning specimens (c) fatigued specimens in ambient air (d) environmental exposed and fatigued specimens

Contamination specimens were conducted using Freekote on a single side of the bonded specimen with the stamp approach. The GIC values for the non-contaminated and contaminated sets are shown in Figure 11. The baseline bonded specimen (non-conditioned) had an average GIC value of 4.22 in-lb/in2. Pristine specimens that were environmentally exposed had an average value of 4.24 in-lb/in2 while the fatigued specimens in ambient air exhibited a slight increase (4.90 in-lb/in2). Specimens that environmentally exposed and fatigued simultaneously exhibited larger GIC values with a large deviation from the average value. For the contaminated sets, the baseline specimens (non-conditioned) had a lower GIC value of 3.04 in-lb/in2. Specimens that were environmentally exposed had an average GIC value of 4.0, slightly higher than the baseline contaminated set. Specimens fatigued in ambient air exhibited an average value of 5.04 in-lb/in2. It should be noted that at the time this paper was submitted, the combined fatigue and environmentally exposed contaminated specimens was not complete. To understand the increase in GIC values from the baseline set, we are currently investing aspects of the DCB tests including the relationship between compliance and changes in material properties.

Figure 11. Fracture toughness values for non-contaminated and contaminated specimens.

Mode of Failure

Example failure modes for the specimens used to define/evaluate the contamination procedure are provided in Figure 12a and 12b. The specimen sets in Figure 12a include: (i) baseline – non contaminated, (ii) aerosol mesh contaminated, (iii) aerosol stamp contaminated and (iv) Freekote stamp contaminated. All specimens showed a mixed mode failure between cohesion and interlaminar. No adhesion failures were observed. Baseline and aerosol mesh contaminated specimens show more cohesion failure, while the aerosol stamp method was dominated by interlaminer failures. The Freekote stamped specimens showed a pure mixed mode failure with adhesion failures patterned within cohesive failures. Figure 12b shows a comparison of the modes of failure for our analysis using single and dual sided specimens contaminated with Freekote using the stamp approach. Baseline specimens, again, exhibited a pure cohesive failure with minor interlaminar failure at the edges. Dual sided contamination specimens (Figure 12b (ii)), again, showed a mixed mode failure with distributed adhesive and cohesive failures similar to Figure 12a (iv). Figure 12b (iii) showed a mixed mode failure on the contamination side and a pure cohesive failure on the non-contaminated side.

a)

b) Figure 12. a) Failure modes of baseline (non-contaminated) (i), aerosol mesh contaminated (ii), aerosol stamp

contaminated (iii) Freekote stamp contaminated (iv) and b) baseline (non-contaminated) (i), dual side contaminated (ii) and single side contaminated (iii) specimens.

Samples of the failure modes for the durability assessment using non-contaminated (a) and single side contaminated (b) specimens are provided in Figure 13. Here, we include results for (i) baseline, (ii) environmental exposed, (iii) fatigued and (iv) combined fatigue and environmental exposure specimens. It is clear that the environmental exposed specimens had an increase in the interlaminar failure at the edges due to water ingression. The contaminated sets, as mentioned previously, had mixed mode failures with both adhesion and cohesion failures.

(i)

(ii)

(iii)

(iv)

(i)

(ii)

(iii)

(a) (b)

Figure 14. Failure modes for durability assessment (a) non-contaminated and (b) contaminated each with following (i) baseline, (ii) environmental exposure, (iii) fatigued, and (4) combined fatigue and environmental exposure.

Line Profile Analysis

Optical microscopy with line profiling capabilities was utilized to investigate the fracture surfaces after DCB testing. Examples of the non-contaminated and contaminated fracture surfaces are shown in Figure 14a and b, respectively. A pure cohesion failure is observed in the non-contaminated specimens while a visible woven pattern of adhesion and cohesion can be seen on the contaminated surfaces. Equally spaced circular regions of adhesion failure can easily be observed with surrounding regions corresponding to cohesive failure. A line is drawn on this fracture surface to observe the depth/topography of the fracture as shown in Figure 15a. Figure 15b shows the resulting line profile where the peak regions correspond to the adhesive and the valleys correspond to the substrate (no adhesion). Figure 15c shows a spectral image mapping of the line where regions of white correspond to the adhesive and regions for black correspond to the non-bonded region (adherend).

Figure 14. Failure surface of non-contaminated and contaminated specimens after testing.

Figure 15. Line profile analysis including – line/region for analysis, the line profile and the spectral mapping of the line.

4.4. Environmental Exposure Analysis

For this study, water uptake of both bonded and non-bonded specimens (4 each) in 95% RH has been monitored. Epoxy polymers are known to absorb moisture when subjected/exposed to humid environments. This process occurs through diffusion and the moisture uptake percentage as a function of time is monitored to determine the saturation levels. Figure 16 shows the moisture uptake curve increasing asymptotically as a function of time. The percentage uptake was fairly rapid for the first few weeks and subsequently slowed in the following weeks. Monitoring of the uptake is still in process and to date, has not reached the saturation limit.

Figure 16. Moisture percent uptake for bonded composite laminates and laminates only.

5. CONCLUSIONS

Promising contamination procedures were developed to create “less-than-ideal” conditions on composite laminates prior to bonding. Two different procedures were presented: one utilzing a stainless steel mesh and a second using a rubber stamp. The rubber stamp approach proved to provide more consistent contaminant patterns on the laminate. Two contaminants were evaluated (an aerosol silicone spray and a mold release agent - Freekote 700NC) to study their effect on bond durability. Specimens contaminated with Freekote 700NC experienced a larger reduction (25%) of the fracture toughness than the group contaminated with silicone spray.

Gravimetric analysis was conducted on contaminated specimens using both the mesh and stamp approach. Mass was recorded for both contaminant sets in addition to the use of single and dual sided contamination specimens. For all cases, the stamp approach delivered 15% more contaminant by weight than the mesh approach. For the panels manufactured having both

laminates contaminated, the weight increase was o approximately 38% larger compared to the single sided contaminated panels. Surface characterization was conducted on contaminated specimens using FTIR and contact angle measurements. Contact angle measurements for the pristine specimens were lower than measurements for the contaminated specimens - indicating lower wettability. Additionally, Silicone contaminated specimens showed the highest hydrophobicity. Bond strength results demonstrated a good correlation between the pristine and contaminated specimens. FTIR results showed a clear peak for Silicone (1250 cm-1) for the specimens contaminated with Silicone spray. The results of the Freekote – 700NC demonstrated a peak in the 1500 cm-1 range. Due to its proprietary chemical formulation, it was not possible to relate the peak to a chemical group.

Accelerated aging of the DCB specimens for contaminated and non-contaminated sets was conducted via 1) environmental conditioning and 2) mechanical fatiguing. Contaminated sets were manufactured with Freekote as the contaminant on a single laminate side using the stamp approach. For contaminated sets, reductions in GIC values were obtained for the baseline and environmental specimens. The contaminated fatigued specimen set had approximately the same values obtained for the non-contaminated fatigued specimen set. Generally, it was found that the conditioning of the specimens increased the GIC values. To understand this increase, we are currently investing aspects of the DCB tests including the relationship between compliance and changes in material properties.

Mode of failure analysis of the baseline contaminated specimens showed a mix mode adhesion/cohesion failure with adhesion failure occurring at the areas where contamination was introduced. The contaminated environmentally aged specimens showed and additional interlaminar failure at the edges of the specimens potentially caused by weakening of the laminate due to water ingression. Line profile analysis of pristine and contaminated specimens showed a clear change in the topography of the fracture surfaces due to the lack of adhesive in the contaminated areas. Future efforts will include conditioning contaminated specimens with environmental aging and fatigue loading simultaneously. DCB results of these specimens will then be compared with the fracture toughness values and mode of failure from previously tests. Finally, water uptake of specimens in the environmental chamber will continued to be monitored to determine the time require for the material system to reach its moisture intake saturation limit.

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