Cure of adhesives is a process that converts the polymeric material from one state (e.g. liquid resin mixture, paste or flexible film) into a solid by means of chemical reactions, often activated by heat. As cure progresses, there is an increase in molecular weight accompanied by an increase in viscosity, until eventually the system “gels” (i.e. no longer able to flow) and finally “vitrifies” (Tg of the polymer rises above the cure temperature). During cure changes in physical state or chemical reaction occur and these changes can be monitored by a number of techniques including:
Optical fibre methods can be broadly split into 3 groups [4], based on the resin characteristics, which are monitored:
Optical techniques require probes or sensors to be embedded within or, at least, to remain in intimate contact with the adhesive during processing. It is possible however, for electrical sensors to be mounted on the outer surfaces of a component or to be integrated into a mould tool wall. The optical sensors can also be used for subsequent structural health and lifetime monitoring of a component.
Optical fibre with Bragg grating [4]
Fibre Bragg strain measurements can be used to determine gel point, end of cure, an estimate of shrinkage due to cure and cooling processes, Tg, coefficient of thermal expansion (CTE), and information relating to annealing/relaxation processes. The FBG technique uses optical fibres embedded in the resin to monitor the accumulation of strain within the material, due to constrained thermal deformation and chemical shrinkage during cure. Optical fibres, waveguides used to confine light, usually consist of silica glass cores surrounded by cladding of a lower refractive index. As a result, total internal reflection occurs at the boundary and light propagates along the fibre core. Additional polymer layers provide damage protection. The Bragg sensor is a segment (typically 3–15 mm long) of optical fibre with a longitudinal periodic modulation in the core refractive index, which acts as a narrowband reflection filter. The basic principle of operation is the measurement of changes in the wavelength of the reflected signal (i.e. centre or Bragg wavelength lB, when illuminated with a broadband light source). The Bragg wavelength is dependent on the effective refractive index of the core neff and the grating periodic spacing, L .
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Typical spectral response of a fibre Bragg grating [4]
Bragg grating cure sensors are not re-usable, as they remain embedded within the final product. Care needs be taken to ensure that the fibre optic sensor is not damaged due to handling or as a result of processing conditions. Fibre optic sensors are fragile, particularly at the resin exit point, thus requiring reinforcement in this region. Tight radii of curvature in fibre optics result in signal loss and are best avoided. The influence of temperature is 10 times more significant than the effect of strain and so temperature compensation is critical for accurate strain determination. Pressure also affects the Bragg wavelength, although the effect on the resin is likely to be far greater than that on the grating. FBG sensors generally only sample small volumes of material. Several gratings can be produced in-line on a single fibre and interrogated individually, in order to extend the measured region.
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XD4601 dielectric comparisons • confirms gel point and end of cure • dielectric of subsequent thermal cycle confirms strain relaxation and transition points • not cure related |
FBG strain measurement of cure for one-part epoxy adhesive [5]
Next: Dielectrometry
