Experimental Details

When joining large components as in the case of longitudinal fuselage joints, it may not be feasible to control curing temperature evenly and as such, an adhesive capable of both curing at room temperature and elevated temperature is desirable. During FSW of FS weld-bonded joints, the uncured adhesive will be subjected to an elevated temperature which may locally accelerate the curing process. To assess the curing process of the chosen adhesive differential scanning calorimetry (DSC) was used. DSC analysis was performed on a Netzsch® DSC 200 F3 equipment on specimens with a mass of ≈50 mg, at a constant heating rate of 20 K/min from 21ºC to 320ºC in an atmosphere of constant flow of 20 mL/min of N2. Figure 1 shows a representative curve of a DSC analysis.
Figure 1 Representative curve of DSC analysis of the uncured epoxy adhesive
Even though the adhesive may cure at room temperature, DSC analysis showed that the majority of the curing process occurs at elevated temperatures, with peek curing at ≈120ºC. An endothermic event is also observed at about 200ºC in all samples tested, which is believed to be evaporation of water, after exceeding the sealing limit of the sample container (water vapor pressure at 200ºC is ≈15 Atm).
From the DSC analysis it may be inferred that full curing does not occur at room temperature, leading the adhesive to have different mechanical behavior with different curing conditions. To assess the tensile mechanical properties of the adhesive, bulk tensile were performed at 1 mm/min crosshead speed in an Instron testing machine. The bulk tensile specimens were made with 4 different curing conditions, room temperature for 7 days, 120ºC for 1 hour as indicated in the adhesive data sheet, 165ºC and 200ºC for 30 minutes. After curing specimens were milled according to ASTM D638 standard. In Figure 2, the resulting stress vs strain curves for the 4 curing conditions are presented.
Figure 2 Representative Araldite 420 stress vs. strain curves with curing temperature
An increase in ultimate strength was observed with increasing curing temperature, accompanied by a reduction in elongation at break. This behavior may be due to increased cross-linking of the polymeric chains with increasing cure temperature. The change is more significantly from room temperature to 120ºC cure condition than from 120ºC to further higher temperatures. This is consistent with the DSC analysis results were most of the curing was shown to happen around 120ºC.
As during the welding process, the adhesive will be subjected to high temperatures, having an adhesive with a high degradation temperature is important. Thermogravimetric analysis (TGA) of the uncured epoxy adhesive was made on a Netzsch® Tg209 F3 Tarsus at 20 K/min from 21ºC to 600ºC. Figure 3 presents resulting TG curves where the onset of degradation was found to be at \(357_{-3}^{+2}\)ºC. Temperatures in the adhesive during FS weld-bonding were reported to be between 200ºC and 250ºC, which leads to conclude that no significant degradation will occur during the welding procedure.6
Figure 3 TGA of uncured epoxy adhesive
To determine shear strength and shear modulus, thick adherend shear test (TAST) and Poisson ratio measurement were made according to ASTM D5656 – 10 and ASTM E132 – 17 respectively. Two curing conditions were assessed, room temperature for 1 week and 120ºC for 1 hour. The resulting (\(\tau_{u}\)) and shear modulus are presented in Table 1.
Given the complex loading case of single lap joints, with combination of peel and shear load on the adhesive, fracture toughness of the adhesive in mode I and II was assessed through double cantilever beam (DCB) specimens and end notch flexure (ENF) specimens. In both tests, specimens had the same dimensions, being composed of two steel beams with 320 x 25 x 12.7 mm bonded by a layer of 0.2 mm thick adhesive, differing only on the loading method. Specimens used for fracture toughness assessment were cured at room temperature for 7 days and as an approximation it was assumed that fracture toughness remained unchanged with curing condition. However, it may be expected that some fracture toughness is lost with increasing curing temperature and as such the real values may be lower.7
DCB specimens were loaded at 1 mm/min crosshead speed and the resulting load vs. displacement curves were used to plot the corresponding R-curves using the compliance based beam method (CBBM).8 A resulting representative R-curve is shown in Figure 4. The critical fracture toughness in mode I measured was 3\(\pm\)0.37 N/mm. This value is relatively high compared with other structural adhesives9,10 and continuous fiber reinforced composites11, showing that the chosen epoxy has high fracture resistance.
Figure 4 Representative adhesive R-curve for mode I
A digital twin of the experimental procedure was created to confirm measured experimental data using Abaqus. Cohesive zone modeling (CZM) was used to model the adhesive failure. The load vs. displacement curves were in good agreement as shown in Figure 5.
Figure 5 a) von Mises stress in 3D DCB Abaqus model at 5 mm displacement and b) load displacement curve comparison between numeric and experimental
For mode II, ENF testing was performed at 0.2 mm/min crosshead speed. Similarly to the DCB tests in mode I, the ENF tests were also analyzed through CBBM method, but in this case for mode II loading.12 A representative R-curve obtained in the ENF tests is presented in Figure 6. The critical fracture toughness measured in mode II was 11.6\(\pm\)0.3 N/mm. However, the maximum bending load was relatively high, which may have induced local plasticization, and as such the measured mode II fracture toughness may be artificially high. A parametric study was then used to find an adequate adhesive fracture toughness in mode II, by keeping constant all other material parameters and comparing numeric and experimental loads vs displacement curves.
Figure 6 Representative adhesive R-curve for mode II
The Abaqus model showed that indeed 11.6 N/mm was an overestimation of the critical fracture toughness in mode II and by an iterative process that 9 N/mm resulted in better agreement with the experimental load vs. displacement curves as shown in Figure 7. This value is still relatively high fracture strength when compared with adhesives reported in the literature.13,14 There was a small difference in terms of stiffness between numerical model and experimental data which was consistent in all numeric runs and is probably due to the experimental loading configuration. As the ENF test is performed in 3-point bending, the machine is operated in compression and the displacement values measured include all the slack within the system, while in the numerical model no such limitations exist.
Figure 7 a) Shear stress in 3D ENF Abaqus model at the onset of damage, b) displacement curve comparison between numeric and experimental.
The adhesive mechanical properties are summarized in Table 1, for two curing conditions.
Table 1 - Araldite 420 mechanical properties
The alloy used in this study was the AA6082-T6. The chemical composition is show in Table 2 and the mechanical properties in Table 3.
Table 2 Chemical composition of AA6082-T6(% mass)15
Table 3 AA6082-T6 mechanical properties15
All welding procedures were performed on a dedicated FSW ESAB® LEGIO 3UL numerical control machine. In FS weld-bonding, the welding procedure was done with the adhesive in a non-cured stated and right after adhesive lay-up and joint closing. Calibrated metal strips with 0.2 mm thickness were strategically positioned in-between the shim plates to assure a more uniform adhesive thickness.
Prior to bonding surfaces to be bonded were degreased and sanded. In the case of adhesive bonded joints phosphoric acid anodization (PAA) according to ASTM D3933 - 98(2017) standard was used, while FS weld-bonded joints were subjected to chemical treatment with 3M® AC-130, which is a sol-gel anodization replacement normally intended for aeronautical repair 16.
The FSW tool used had 5 mm diameter cylindrical threaded pin with 3 mm length and 16 mm diameter grooved shoulder. The FSW process parameters used are listed in Table 4. These were chosen based on literature review and past experience. Various levels of downward force were tested to assess its effect on joint performance and maximize joint strength.
Table 4 FSW process parameters
All joint configurations were tensile tested with three specimens each. Tensile testing was done in an Instron® 5566 machine at 1 mm/min crosshead speed. Joint efficiency was calculated dividing maximum axial load by the substrate cross-section outside of the overlap as in previous works.3,17 Figure 8, compares the joint efficiency of the joint configurations tested.
Figure 8 Joint efficiency of FSW and FS weld-bonded joints with differing downward force
When comparing FS weld-bonded joints to FSW it was possible to observe an improvement of 20-30 % in most cases. It was possible to observe that for FSW joints the in downward force results in an increase of joint strength. This may be related with higher thermal input which leads to further softening of the workpiece. The further softening of the workpiece may result in better mixing and as such diminishing the hook defect size, as presented in.18 For FS weld-bonded joints the trend is not as clear as in FSW joints, as it increases from 400 to 450 kgf but diminishes from then on. The reasoning for this decrease may be due to high downward force leading to excessive adhesive thinning or possibly the higher temperature may lead to degradation of the surface to bond and/or the adhesive. FS weld-bonded joints also showed higher dispersion in terms of joint strength than FSW which may be due to variations in surface treatment, as the joint strength is very sensitive to the bonding strength. Figure 9, compares the highest strength FSW and FS weld-bonded joints with adhesive bonded joints.
Figure 9 Stress vs. displacement of highest strength FSW and FS weld-bonded joints along the adhesive bonded.
In Figure 10, the microhardness profile and joint cross-section of the best performing FS weld-bonded joint is presented. It is possible to observe that the hook defect formed by the upward flow of material generated in the advancing side is present. The size and shape of this defect is critical to overlap FSW joint strength.18Along with the hook defect it is also observable a cold lap defect on the retreating side of the weld, which is a result of the initial upward flow under shearing effect of the pin followed by a downward flow in order to fill the space at the bottom of the pin. However, these defects become less critical in FS weld-bonded joints when compared to FSW as the adhesive increases the effective joined overlap and reduces loading the weld edges.
Figure 10 a) Microhardness profile and b) joint cross-section of FS weld-bonded joint with 450 kgf
A loss of hardness is presented in the microhardness profile, which is due to the loss of T6 condition during welding. Temperatures increase towards the top of the joint due to contact between workpiece and tool and as such, a wider cross-section of the joint has lower hardness at the top.
An assessment of the fatigue strength of FSW, FS weld-bonded and adhesive bonded joints was then made at a load ratio of R=0.1 in an Instron 8874 machine. For this study both FSW and FS weld-bonded joints were made using 450 kgf. Figure 11 shows the resulting S-N curves including 50% and 5% probability of failure calculated using ProFatigue software.19 The S-N curve regarding the AA6082-T6 base material reported in 20 was also included.
Figure 11 a) p-S-N curves of the three joint type and b) failure modes
FSW joints showed lower fatigue strength than adhesive and FS weld-bonded joints as it would be expected given the lower quasi-static strength. The FS weld-bonded showed similar fatigue strength to adhesive bonded joints. Adhesive bonded joints still had the highest fatigue strength of all joints tested, which is to be expected given the more continuous stress distribution in these joints. At 106cycles adhesive bonded joints have a 50% probability of failure at 56.4 MPa, while FS weld-bonding and FSW at the same number of cycles the 50% probability of failure is achieved at 45.1 MPa (79.9% of adhesive bonded joints) and 23.5 MPa (41.6% of adhesive bonded joints). The failure modes were consistent with the quasi-static ones, with the adhesive failing in adhesive/cohesive way, the FSW failing through the hook defect and the FS weld-bonded ones failing through the adhesive layer followed by cracking through the hook.