Adhesive Cure
Introduction
Firstly, it is important to explain why we should want to monitor the ultrasonic properties of a curing adhesive.
We can look at the measurement from two perspectives: firstly at the physical properties of adhesive and how and why they change during cure and secondly from the point of some quality or process control. Previously many workers have predominantly used contacting compression wave transducers to monitor the change in longitudinal waves in a curing polymer and have made some interesting measurements. However it is the shear wave measurement that contains the most important information on the mechanical properties of the curing adhesive. While using contact shear wave transducers is a viable approach to measure these changes in the lab, it will never be a viable technique on real samples as the traducer needs bonding to the adherent.
The shear wave EMAT offers several advantages over contact shear wave generators and has been used to perform ultrasonic measurements on a range of 2 part epoxy resins during cure.
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The EMATs are broadband (a wide frequency range is used in a single measurement), and being non-contact they can also operate at elevated cure temperature.
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The ultrasonic waves that the EMATs generate here are normal incidence SH waves which provide the most sensitive probe of shear wave velocity or shear modulus at the frequency of the shear wave.
For some applications it is not possible to access both sides of the bonded sample and in these cases a single send-receive EMAT can be used.
Figure 1
The above waveform animation shows how the shear wave develops with time through a curing 2 part epoxy resin (Araldite 2013 - Vantico). The resin was cured between two aluminium plates over a period of 24 hours. Notice how the amplitude and arrival time change during cure. The experimental set-up is described further on down the page. The bottom axis is time in microseconds and the left axis is amplitude in arbitrary units.
The adhesives that we have monitored during cure are 2 part epoxy adhesives that are available in a two component cartridge form, that have been mixed by using of an application nozzle to deliver the epoxy onto an adherent face prior to bonding. The rig we have used consists of two temperature controlled heated aluminium adherents each 12mm thick. Thick adherents were used to give better temperature stability during measurement. A schematic diagram of the rig is shown below.
Figure 2
The above schematic diagram shows the set-up used for adhesive cure monitoring using EMATs. The EMATs are held exactly opposite each other using a supporting frame not shown in the above diagram.
The adhesive samples are cured between two temperature controlled 10mm thick aluminium plates. The plates were chosen for their low acoustic birefringence [1] which was negligible and not measurable for several plate transits. The plates have been surface ground to provide a very flat uniform surface. Relatively thick plates were used in order to provide good temperature stability due to the large thermal mass of aluminium. Each plate is heated by four identical temperature controlled high power resistors in good thermal contact with the plates.
In our recent experiments we started to study the changes in frequency dependent attenuation and velocity of the shear wave during cure as a function of temperature. The adhesive is a viscoelastic material and thus attenuation and velocity are frequency dependent. The simplest approach in measuring the acoustic wave changes during cure is a peak amplitude measurement, taking no account of the change in frequency content of the wave (see figure 3). The curves have been normalised for the final cured state amplitude. Also plotted on the graph are the points at which the adhesive reaches lap shear strengths (LSS) greater than 1N/mm2 and 10N/mm2, obtained from fitting the data manufacturers' data to an exponential function. The point at which the LSS exceeds 1N/mm2 occurs shortly after the shear wave is detectable above the background level, and interestingly the point at which the LSS exceeds 10N/mm2 is roughly 0.76 of the maximum (fully cured) shear wave amplitude in all the curves. Note that the higher temperature leads to an increased cure rate and consequently the adhesive is able to support a shear wave at much earlier cure times.
Figure 3
The above diagram shows the development and change in shear wave amplitude during cure through the adhesive (Araldite 2013). The cure temperature decreases from right to left from around 55oC to 30oC in roughly equal steps.
Cartridges of epoxy resins are used in the manufacture of our adhesive bonds because they are a convenient and reliable way of ensuring correct mixing and composition of the adhesive. Cartridges are not usually used in the initial construction of aerospace components (a 'pre-preg' sheet is normally used) but cartridges are commonly used in small area repairs. In the automotive industry adhesive bonding is ideally carried out on a production line basis where obviously relatively small cartridges are not used. However, it may be the case that some of the implications from the work presented here are relevant to the way in which the adhesives may be stored or contained. The cartridges with mixing nozzles are also a convenient source of material for scientific research and are ideal for the experimentalist as they should be representative of typical 'real' adhesive properties.
The epoxy adhesive cartridges used initially appeared to have been an ideal source of material for scientific investigation and were chosen as they are a quality product that has been used by other experimental workers. It was found that in this case (Araldite 2013 from a 50ml cartridge) there was a significant variation in shear wave velocity (and consequently shear modulus), dependant on the order of extrusion from the tube; i.e. the adhesive first extruded from the tube has a shear modulus 35% higher than the shear modulus of the adhesive extruded from the tube when it is half empty. These measured differences in shear modulus (velocity) may not be significant in the critical plastic failure mechanism but do show that the cartridges cannot be used for a systematic study of adhesive cure behaviour. We are currently looking into what causes this difference in shear modulus. Figure 4 below is a series of 'spot' measurements of adhesive shear wave velocity as the tube is progressively used up.
Figure 4
The above diagram shows the change in shear wave velocity in terms of the 'order that the adhesive has been extruded from the tube'. For each measurement the temperature of cure was held constant at around 40oC.
Our next step is to find out why we get this variation in adhesive properties within one cartridge. For this particular adhesive the effect is highly repeatable. We are looking at other adhesives now to see if they exhibit similar behaviour when extruded from a cartridge.
References for further reading
Also see the EMAT section for relevant references.
[1] Lindrose A M, Ultrasonic wave moduli changes in a curing epoxy resin, Exp. Mech, vol.18, 1978, pp227-239
[2] Matsukawa M and Nagai I, Ultrasonic characterisation of polymerising epoxy resin with imbalanced stoichiometry, J. Soc. Am., 1996, pp2110-2105
[3] He F, Rokhlin S I and Adler L, Application of Sh and Lamb wave EMATs for evaluating adhesive joints, Rev. of Prog. in QNDE, ed. Thompson DCand Chimenti DE, 1987, pp911-918
[4] Freemantle R J and Challis R E, Combined compression and shear wave ultrasonic measurments on curing adhesive, Meas. Sci. Tech., (, 1998, pp1291-1302
[5] Dixon S, Edwards C E and Palmer S B, Experiment to monitor adhesive cure using electromagnetic acoustic transducers, Insight, vol.37, 12, 1996, pp969-973