Through-transmission mode
(inset—received signal)
Pulse-echo mode
(inset—received signal + signals from mould walls)
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Through-transmission mode (inset—received signal) |
Pulse-echo mode (inset—received signal + signals from mould walls) |
Time-of Flight [6]
Ultrasonic techniques (e.g. time-of-flight) may be used to establish relationships between changes in the characteristics of propagating ultrasound and the real-time mechanical properties of an adhesive during cure. The velocity of a sound wave is directly related to the modulus (stiffness) and density of the material, through the relationship:
El = rVl2
El is the longitudinal elastic modulus
r is the density
Vl is the longitudinal wave velocity.
Changes in viscosity during cure coincide with changes in the modulus, first a decrease followed by an increase, which will similarly change the velocity of sound in the material—a decrease followed by an increase in the velocity. By using probes that emit and detect ultrasound these changes in modulus can be detected, which then provide information on the viscosity and hence state-of-cure of the adhesive. It is also possible to determine information about the visco-elastic properties of the adhesive by monitoring the attenuation of the ultrasound signal. The attenuation of the ultrasound is at a maximum when losses due to viscous effects in the material are at a maximum, and this coincides with the glass transition temperature (Tg).
Adhesive materials are usually in the form of a thin layer, which makes the thickness dimension small, so that very accurate time measurement, of the order of nano seconds, is required to detect the change in velocity, which might only change by a factor of two or three. A second problem is that adhesive materials sometimes shrink during cure, which changes the dimensions, so that the time change is now due to both a modulus change and a distance change. The method, however, is non-intrusive (i.e. probes can be incorporated into mould walls). The time-of-flight can reflect the state of cure directly where thickness and density variations are negligible.
Through-transmission (T-T): For this mode of operation, two transducers are required, aligned collinearly. An electronic excitation unit produces a voltage spike, which triggers a pulse of ultrasound from the transmitting transducer. The pulse travels through the material and is received by the second transducer. Measurements are repeated at regular intervals during processing to produce a trace directly related to the material properties throughout cure. This procedure can be automated using software control and analysis. The availability of robust high temperature ultrasonic transducers and coupling media enables this method to be used for high temperature processes. The frequency, power and transducer diameter determine the maximum thickness/attenuation of material, which can be investigated.
Typical time of flight trace showing changes during elevated temperature cure [6]
I—heating stage, II—curing, III—over—curing
Pulse-echo: This mode of operation is based on the same principles of through-transmission, but only employing a single transducer. A pulse is sent through the material and at the boundary between the material and air or mould wall the pulse is partially reflected due to the acoustic impedance mismatch. The same transducer then captures the reflected signal and the transit time is determined. Pulse-echo has limited use for very thick or high attenuation material, as the pulse travels through the sample twice and experiences twice the signal loss. This method is ideal where only one-sided access is available. It is also more sensitive than through-transmission (twice the transit time therefore twice the difference in the measured time-of-flight) and has lower equipment costs.
Pulse-echo transit times with degree of conversion for two-part epoxy resin [6]
