cure monitoring

8/7/2019 CURE MONITORING

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STUDENT ID: 4104669

3.0 ULTRASONIC EVALUATION AND MONITORING OF THE CURE STATE OF

EPOXY ADHESIVE

Adhesive bonding technologies have found various and wide applications in the

production and assemblage processes of many manufacturing industries.

The need for quality control during production and post-production quality assurance and

reliability of adhesive bonded joints gave rise to the need for monitoring and evaluation

of adhesive curing process. It has been noted that the curing process impacts on the

joints performance and defects such as delaminations, kissing-bonds, cracks, voids,

missing adhesion etc usually found in adhesive bonded joints can result from improper

curing process [11].

The curing or hardening of the epoxy adhesive is a chemical process during which the

polymeric epoxy mixture translates from a viscous liquid characterized by low molecular

weight into a highly cross-linked solid with the application of heat [12,13]. The curing

process involves gelation and vitrification stages. At gelation, the mixture cease to flow

and begins to form elastic cross-links. While at vitrification, a solid glass is formed and

chemical reactions cease as glass transition temperature goes above the cure

temperature of the epoxy[14,15]. The real-time tracking of the chemical reactions and

the accompanying changes in the physical state that take place during cure is referred to

as cure monitoring.

Effective curing of adhesive is a function of a number of parameters such as resin-

hardener mix ratio, cure temperature, time etc [12]. In order to achieve a properly

cured adhesive and hence a good adhesion strength, cure parameters are appropriately

set by monitoring and evaluating the progress of the hardening process in real time. In

order words, if the curing process is not monitored and it goes wrong, though the

adhesive may be bonded to its substrates, the required cohesive properties of the

adhesive joint would not be achieved i.e it is either too rubbery or too brittle. Thermoset

epoxy is usually cured for longer time than necessary in order to avoid undercure, butthis is not time efficient. In order to avoid undercure and also minimize time and cost, a

cure monitoring technique that can detect the time the cure reaction ends is very

essential.

Not only this, cure monitoring is being used in a feedback loop system to control and

efficiently optimize cure temperature and time. This is to say that if the curing process is

going too fast(which could lead to overcure), the temperature could be lowered to slow it

down and if the cure rate is too slow, the temperature could be raised to restore the

process to normalcy. Also, through cure monitoring, mix ratio variations i.e excess and

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insufficient hardener/curing agent have been noted to have significant effect on curing

as well as cure rate.

Various techniques have been employed to follow the process of epoxy curing, but the

Ultrasonic method has been favoured as it is simple, convenient, non-destructive andallows in-process monitoring of the hardening process. The ultrasonic techniques, during

curing, employ the changes in the behaviour of sound wave as it propagates through the

epoxy mixture, to follow and characterize the state of cure. Parameters such as

ultrasonic velocity, amplitude attenuation and acoustic impedance can be used to follow

curing process. Ultrasonic velocity increases as the epoxy translates from liquid to solid.

Consequently, the arrival time of echo from the epoxy decreases as cure progress. Also,

this increase in sound velocity coupled with the assumption that the density(ρ) of the

epoxy is constant during cure, results in a higher acoustic impedance(Z=ρV) as also

noted by Challis et al [3].

The ultrasonic pulse-echo mode, shown in figure 3.0, can be simply employed for these

velocity, acoustic impedance and attenuation measurements using the ultrasonic signal

waveforms.

Fig. 3.0: Pulse-echo mode (a) and typical ultrasonic signal waveform (b)

The sound velocity(V) in the epoxy layer can be calculated from the thickness of the

sample and a measure of the time of flight difference between the reflected pulses from

the sample boundaries as follows;

V= 2D/∆t (3.0)

where D is the thickness of the epoxy sample and ∆t is the time of flight difference

between echoes from upper and lower epoxy interfaces/boundaries.

The attenuation measurement is based on the changes in the echo amplitude from the

epoxy interface. The echo amplitude is dependent on the interface refelection

coefficient(r), which in turn relates with the acoustic impedance(Z) by equation (1.0) in

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1 Al

2 Epoxy

3 Al

(a) (b)



STUDENT ID: 4104669

part 1, where medium 1 is the Al substrate and medium 2 is the epoxy mixture. Hence,

as both velocity and acoustic impedance increase as cure progresses; reflection

coefficient changes, which in turn leads to changes in echo amplitude(from which

attenuation can be measured). Figure 3.1 presents typical changes in sound velocity

and attenuation during cure. It is also noteworthy that acoustic impedance only gives

information about the cure state at the epoxy interface.

Fig. 3.1: Typical changes in Sound velocity(a) and attenuation during adhesive cure(b)

Many researchers have investigated the use of ultrasonic techniques to monitor and

evaluate adhesive cure state. Maeva et al. [12] recently used the pulse-echo reflection

mode to analyse the acoustic parameters and how they relate with the degree of cure

and cohesive strength of the epoxy adhesive. They used the same transducer to transmit

pulse and receive echo reflections from the interfaces of an epoxy-metal substrate

layered sample as shown in figure 3.2 taken from Maeva et al. [12].

Fig 3.2: Showing origin of echoes(a) and typical A-scan(b):1-echo from buffer-rod/epoxy

interface; 2-echo from epoxy/metal interface. (copied from Maeva et al. [12]).

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Cure time(mins)

S

oundveloc

ity(m/s)

Cure time(mins)

Attenuation(Np/m)

(a)

0(b)



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Using the echo waveforms, the longitudinal sound velocity and the attenuation of sound

wave in samples of known thicknesses were measured during the cure at different and

increased cure temperatures.

Their results show that longitudinal sound velocity increases during the curing processand the velocity curve’s (shown in Fig 3.1a) slope rises as the cure temperature

increases [12]. This means that, at temperatures higher than specified cure

temperature, gelation occurs earlier, which leads to overcuring or the degrading of the

polymer as it becomes more brittle. Also, sound attenuation peaks at the onset of the

curing process and diminishes afterwards.

In the same vein, Faiz et al . [16] employed the pulse-echo reflection method to follow

the curing process of an epoxy resin and then did a comparison with the results achieved

from using a pulse transmission mode. In their experiment, the vessel containing the

resin and hardener mix and the transducer were immersed in a water tank as shown in

figure 3.3, taken from Faiz et al . [16].

Fig. 3.3: Pulse-echo experimental set-up from Faiz et al . [16]

Phase velocity, attenuation and acoustic impedance measurements, extracted in the

time domain, were used to monitor the curing of different mix of epoxy(resin &

hardener), in order achieve the best percentage of hardener needed for proper curing.

Their results show that phase velocity increase during the curing process, while

attenuation peaks as curing progresses, but diminishes as the epoxy sets(solidifies).

Also, epoxy normal setting time reduces(i.e earlier gelation time) as the percentage of

the hardener increases, which represents overcure of the epoxy. Slow cure rate and

probably undercure result, when the hardener percentage is below the manufacturer’s

recommended value. Moreso, they showed that acoustic impedance is a measure of

epoxy cure state at the vessel-epoxy interface and that the curing process is

nonhomogenuous throughout the sample as curing actually starts from the centre

towards the vessel walls.

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Their transmission mode, which incorporates a reference epoxy sample, gave results

that are in corformity with the ones from the reflection mode.

On the other hand, the use of ultrasonic guided waves in wires to monitor cure reaction

was investigated by Vogt et al . [17]. Figure 3.4, taken from Vogt et al . [17], shows amagnetostrictive device used to generate longitudinal and torsional waves at low

frequencies, which were then guided into the epoxy sample by a steel wire. For high

frequencies, a piezoelectric transducer was used to excite a longitudinal mode as an

alternative and the waveguide in this case is steel bar.

Fig.3.4: Cure Monitoring using a magnetostrictive device to guide waves in steel wire

(copied from Vogt et al .[17])

They presented two methods using a wire waveguide that is partially inserted in the

epoxy adhesive. Attenuation measurements, due to bulk wave leakages from the

waveguide into the surrounding epoxy, form the basis of their first method. The second

method measures the wave reflection that occurs at the entry point of the waveguide

into the epoxy as shown in figure 2.5, taken from Vogt et al . [17]. Both methods employ

the tangible changes in attenuation and reflection during cure.

Fig 3.5: Schematic representation of entry reflection (copied from Vogt et al . [17])

Using a software tool called DISPERSE, they modelled the attenuation of sound wave in

the inserted steel wire waveguide as the epoxy translate from liquid(precure state) to

solid (postcure state). The prediction shows that only bulk longitudinal waves leak away

in an ideal liquid epoxy, while both longitudinal and shear waves radiate in solid epoxy.

Also, the entry reflection was modelled, using the finite-element and modal prediction,

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based on the idea that the shear velocity increases as the epoxy cures; and as the

impedance mismatch between the free portion of the waveguide in air and the inserted

portion increases, the entry reflection becomes large. The variation in density of the

epoxy resin was assumed to be negligible during the curing process and the reflection

coefficient was treated to be independent of the longitudinal bulk wave.

Their experimental results, using measurement of attenuation and reflection, when

waveguide is in free air as a reference, show wave’s amplitude reduction(attenuation) on

insertion(due to leaking bulk waves) and as the epoxy solidifies, the shear wave velocity

rises and hence, attenuation rises monotonically till curing ends. Also, the entry

reflection rises as cure progresses as a consequence of the rising shear velocity in the

epoxy.

Furthermore, Lionetto et al. [18] investigated the practicality of using a customised, air-

coupled ultrasonic device to monitor the cure process of epoxy samples at room

temperature. Their experimental set-up involves the appropriate positioning (in terms of

incident angle and stand-off distance from the sample) of the two non-contact

transducers in a pitch-catch mode in order to transmit and receives compression waves

from the sample throughout the curing process as shown in figure 3.6, taken from

Lionetto et al. [18].

Fig.3.6: Wave propagation paths in the sample using air-coupled transducers to monitor

cure ( copied from Lionetto et al. [18])

The hardening process was followed by measuring the changes in the transit time of the

propagating waves from the transducer, through air and the sample, then back to the

receiver. The compression wave velocity is then calculated using the transit time (ttotal ),

the sample thickness(s), the refraction angle(β) and the wave path number(N) in the

sample as follows;

V=(N x d)/ttotal (3.1)

d=s/cos β and ttotal=ta1+ts+ta2.

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STUDENT ID: 4104669

where ts is the transit time of sound wave travel through the sample and ta1 & ta2 are the

transit times of sound wave travel in air from the transmitter to the sample and from the

sample to the receiver respectively.

Their results show that the transit time and the velocity are constant at the onset of thecure reaction, but as it progresses, transit time reduces while the velocity increases. The

increase in sound velocity results in an increase in the refraction angle(β), according to

Snell’s law, and thus an increase in the ultrasound path-length in the sample.

Consequently, during cure, the exit point of the ultrasound from the sample moves

further to the right as shown in figure 3.7 taken from Lionetto et al. [18]. Tangible

changes also were noted in the intensity and shape of the echo waveforms.

Fig.3.7: Change of exit point of ultrasound from the sample during cure (copied from

Lionetto et al. [18])

3.1 PRACTICALITIES

The Lionetto et al. [18] technique proposes the elimination of contact and coupling

effects etc, but the complexity that is inherent in analysing ultrasound propagation in air

and in optimizing the transducers to excite and receive compression waves throughout

the curing process requires skill and great effort and hence poses a limitation on the

application of this method.

The ultrasonic guided waves technique is fascinating, in that it allows the curing process

to be followed all through, even from the onset, by measuring the shear properties of

the waves [17]. However, its application requires good system design that suits the test

sample at hand. Also, the problem of multimode wave propagation in the waveguide can

complicate the measurements. Apart from this, the waveguide is stuck permanently

inside the adhesive. This is unsuitable for some applications as there is the possibility of

formation of voids around the waveguide, thereby weakening the joint strength. This

could also lead to an increase in the overall weight of the structure.

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STUDENT ID: 4104669

The pulse-echo reflection technique is a simple and effective method for monitoring and

evaluating the cure state of epoxy adhesive. Although this techique could be affected by

errors arisng from couplants, surface coatings, coupling pressure and surface roughness

[3], with appropriate signal processing, the errors can be minimized. It is also good to

note that the water tank immersion, used by Faiz et al . [16], can cause water ingress

into sample, which is a problem in practical applications.

The pulse-echo reflection method will be suitable for the test component in that it is

versatile enough to monitor all cure states. Moreso, an automated device for testing

adhesive joints is available commercially.

3.2 BONDMASTER 1000E+

The Bondmaster 1000e+ shown in figure 3.8 is a robust, multipurpose instrument

produced by Olympus NDT [19]. This device incorporates three test modes: Resonance,

Mechanical Impedance analysis and Pitch-catch mode.

Fig. 3.8: BondMaster 1000e+ (Copied from www.olympusNDT.com[19])

The pitch-catch mode shown in figure 3.9 is relevant to this application. It has three

inspection methods namely pitch-catch swept, pitch-catch impulse and pitch-catch RF

methods. This mode uses ultrasonic technique (propagation of high frequency sound

wave) to excite surface waves into the test sample. Another probe, set at a distance

from the transmitting probe, receives the energy propagated into the material. Changes

in phase and amplitude of the transmitted and received signal are detected and a phase-

amplitude display is used to inspect adhesive bond and locate defects.

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STUDENT ID: 4104669

Fig. 3.9: Pitch-catch Test Mode (Copied from www.OlympusNDT.com[19])

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