In-process monitoring using acoustic emission on cure cycle of Al/Al bonded joints
Adherends: 2024 T3 aluminium alloy (anodised and covered with primer)
Adhesive: modified epoxy
Cure cycle (heating 49°÷ 117°C, 90 min. at 117°C and cooling 117 ÷ 66°C)
Pressure: 0.25 MPa
Cycles with variations in time, temperature and pressure were performed.
Furthermore modifications of joint surface were introduced, by producing unbonded zones (Teflon layers) and applying mould release (dibuthyl ether) on some part of the surface to be bonded.
Acoustic emission outputs:
- The three phases of the cycle are easily discernible in AE diagrams (histories with time)
- Long duration events are preferably shown during temperature holding by ideal procedure joints.
- The polymerisation (long duration events) seems to stop after about 60 min.
Further considerations were that:
- The most exploitable results can be obtained by considering the cooling-down phase alone and comparing AE activity during this phase and during the whole cure cycle
- A reduction of the adhered surface of 50 and even 75 % did not produce a relevant decrease of joint quality
- Some low amplitude signals with frequencies around sensor peak were isolated as events due to adhesive shrinkage subsequent to adhesion
- C-Scan showed the effect of non-planar application of pressure as a relevant difference on ultrasounds transmission
- An investigation with ESPI (Electronic Speckle Pattern Interferometry) provided the idea, then confirmed by shear stress tests, that half-surface and whole surface bonded joints were of comparable quality.
- Digital scanning calorimetry (DSC) gave information about the level of polymerisation reached in joints produced with out-of-standard cycles, for as regards time and temperature.
Politecnico di Milano
Università di Napoli "Federico II"
Università di Roma – La Sapienza
ALENIA - Pomigliano