Extracción de conocimiento

The second phase testing was designed to improve upon initial methodology presented in section 4.1. Primarily by addressing three considerations;

1. The time delay in section 4.2 between joint cure and joint testing of approximately 1 hour poorly represented the removal of fixtures and load application immediately after the cure cycle as in a production scenario. To address this the curing and test phase were combined and performed within the test machine, enabling an immediate tensile test following the curing cycle.

2. Single sided TS-CFRP induction heating yielded high variability in bondline temperature distribution in previous work, detailed in section 4.2.1. It was identified that this was a more significant problem with smaller geometry TS-CFRP coupons. To address this problem aluminium (AL)/TS-CFRP joints were used for subsequent coupon level development. The high thermal conductivity of the aluminium was

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aimed to reduce the temperature variation within the joint. The specimen width was also increased from 25 mm to 50 mm to assist in TS-CFRP heating efficiency. The aluminium and TS-CFRP substrates had a thickness of approximately 2.5 mm. TS-CFRP substrates were the same as that used in section 4.2 unless otherwise stated. Aluminium substrates were 5754 grade unless otherwise stated, initial development work used some 6000 series aluminium due to material availability.

3. The bondline temperature at the point of starting the tensile test, Ttest, was identified as highly influential to joint SLS strength in discussions with adhesive manufacturers. With joint cooling initially limited by natural cooling processes a forced air jet was added to accelerate the cooling profile, thus reducing total cycle time. The quench method used in the initial data collection was unsuitable compared to an air jet for use within a manufacturing environment. Further, the quench method was unsuitable for use in conjunction with the tensile test machine without removing the sample first, introducing undesirable delays into the test method.

An in-situ test method was developed to perform the investigation requirements based on the above consideration, the equipment set up is shown in in Figure 56. This utilised a two-sided induction coil which was developed through collaboration with the induction equipment supplier, EFD Induction. This coil was mounted horizontally between the grips of the tensile test machine. The same Instron universal test machine was used as detailed in section 4.2. A modified drill press stand was used enabling the coil to be slid into position around the SLS coupon for the curing stage and retracted immediately prior to tensile test. The air cooling jet was developed for the investigation and used to cool the joint to Ttest, as detailed in the experimental results. The cooling jet is shown in Figure 57, with a typical cooling profile in Figure 58. Following a substrate isopropanol wipe, adhesive was applied to the AL and CFRP substrates prior to loading in the tensile test machine.

As in previous work tabs were inserted into the grips to compensate for the offset geometry of the SLS test. A centrally embedded K-type thermocouple of 0.1 mm wire diameter was inserted into the adhesive joint, connected to the PID feedback controller used to control the heating process. 0.3 mm glass beads were added to the adhesive prior to application, unless otherwise stated, to control bond thickness. The control thermocouple and feedback loop was not used in later component level testing, however provided a convenient way to control

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joint heating profiles for coupon level development. Joint overlap was measured using a Vernier calliper and adjusted to 12.5 mm prior to curing using the crosshead adjustment on the tensile test machine. Adhesive spew was removed from the joint prior to cure with a target spew shape as shown in Figure 20. An inconsistent spew shape was identified as a cause of joint variability within literature. The induction coil was placed around the sample, with non-conductive tabs placed either side of the specimen inside the grips. A spring clamp was then added to help control bondline thickness during cure. This applied some consolidation pressure ensuring the joint was closed until the glass beads were contacted at 0.3 mm bond gap, unless otherwise stated. The experimental set up can be seen in Figure 56.

Figure 56 Second phase testing methodology rapid cured adhesive joints

The cure cycle followed a ramp at constant heating rate to an isothermal dwell temperature, where an isothermal dwell period took place. The heating profile parameters are specified within experimental results for each investigation. Following the specified dwell, the coil was removed and cooling air jet applied. Upon the joint reaching the desired Ttest the SLS tensile test would be immediately started at 13 mm/minute until joint failure. A minimum of three repeat coupons were produced for each set of test parameters. Adhesive choices were the same as those discussed within section 4.1.3 . A silane primer, 3M P592, was identified as required to produce a cohesive failure mode with Henkel PU 1510 upon untreated aluminium substrates. This was identified as an effective aluminium pre-treatment in preliminary trials, detailed further within portfolio submission six. This primer was applied following IPA wipe in all subsequent bonding of PU 1510 adhesive to aluminium. At partial cure 3M™ SA9816 was identified as not producing a cohesive failure upon aluminium substrates, although PT3

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coating was recommended by 3M as the most suitable aluminium surface treatment (97). PT3 coated aluminium, grade 5754, was sourced and used with the 3M™ SA9816, 7666/522 and Lohmann adhesives in later work presented in this report, unless otherwise stated.

Figure 57 Cooling airflow jet image with annotations

The representative joint temperature distribution of the improved CFRP/AL experimental methodology is seen in Figure 59 with thermocouples located at 5, 25 (centre) and 45 mm through the 50 mm wide SLS joint overlap, inside of the adhesive joint. The in-joint temperature distribution with the double-sided coil and multi-material substrates is a considerable improvement upon the previous single sided coil with TS-CFRP substrates which can be seen for reference in Figure 52.

0 40 80 120 160 200 240 280 320 360 0 20 40 60 80 100 120 140 160 180 200 220

AL/AL Air cooling jet CFRP/AL No air cooling jet CFRP/Al Air cooling jet

Bond line t em pe ra tur e / o c Time / s

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With the multi-material joint and double sided coil the temperature differential within the joint was approximately +/- 5°c, compared to +/- 38°c in some cases with the TS-CFRP / TS- CFRP single sided arrangement. This enabled more accurate identification of achievable handling strength following heating cycles, with less variability. Optimisation of induction heating coils to provide a more uniform temperature distribution with CFRP/CFRP substrates remained an area for future work.

0 40 80 120 160 200 240 280 320 20 40 60 80 100 120 140 160 180 200 220 Te m pe rat ure / o c Time / s Position 1 5 mm Position 2 25 mm Position 3 45 mm

Figure 59 Temperature distribution AL/TS-CFRP coupon, 50 mm wide, 12.5 mm overlap with adhesive, 0.3 mm bond thickness