No abstract

In this study, we present a method to detect cure-induced strain in an epoxy resin (EP) with a Fiber Bragg Grating (FBG) sensor. By embedding the optical fiber into the EP resin the characteristics during isothermal cure (gel point, vitrification) could be precisely detected due to changes in the fiber strain. In a follow up dynamic temperature scan the coefficient of thermal expansion and the glass transition temperature (T g ) of the fully cured EP were determined by the FBG sensor technique. All results obtained by the fiber optical method showed a very good agreement with those deduced by independent techniques, viz. rheological measurements, differential scanning calorimetry, and thermomechanical analysis.

ABSTRACT:The polymerization and crystallization of cyclic butylene terephthalate oligomers (CBT) were followed by fiber Bragg grating (FBG) and normal force measurements under isothermal conditions at T 5 170 and 1908C, respectively. It was found that the FBG and normal force sense only the crystallization-induced shrinkage. The course of the FBG signal and the normal force as a function of time suggested that crystallization of the polymerized CBT (pCBT) occurs in two steps. The primary crystallization-induced shrinkage is several hundreds by contrast to the secondary one showing several tens in ppm/min unit according to the FBG results. The two-step crystallization was confirmed by normal force measurements. The crystallinity and crystalline structure of the pCBTs were studied by differential scanning calorimetry and wideangle X-ray scattering. It was found that the crystallinity and the crystalline parameters slightly differ for the pCBTs formed at T 5 170 and 1908C, respectively. The pCBT produced at T 5 1908C had slightly higher crystallinity and more perfect crystals than the pCBT formed at T 5 1708C. The reliability of the FBG sensing was checked by thermomechanical analysis (TMA). A fair agreement was established between the thermal contraction and thermal expansion measured by FBG and TMA, respectively.

In this paper, the effects of different cure regimes on the strain development in an anhydride‐cured epoxy resin were investigated by fiber optical measurements. The course of the strain signal was detected by an embedded fiber Bragg grating sensor in the unconstrainedly curing epoxy. The build‐up of strain was detected for various cure regimes differing in the dwelling times of the first isothermal step, heating rates to the cure temperature, and final curing temperatures, respectively. Characteristic points (gelation, vitrification) of the cure regimes were identified by conversion‐ and Tg‐determinations via DSC and assigned to changes of the FBG signal. The fiber Bragg sensing technique allowed us to find those variables of the cure regimes which mostly affect the strain development and thus, the level of the residual strain. It was established that the dwelling time and heating rate to the cure temperature influence markedly the residual strain whereas the cure temperature affects this value to a lesser extent for the selected cure regimes. So, the above parameters should be selected properly for an optimum cure regime characterized by the build‐up of a minimum residual strain.magnified image

Abstract. The influence of adhesion to the mould wall on the released strain of a highly filled anhydride cured epoxy resin (EP), which was hardened in an aluminium mould under constrained and unconstrained condition, was investigated. The shrinkage-induced strain was measured by fibre optical sensing technique. Fibre Bragg Grating (FBG) sensors were embedded into the curing EP placed in a cylindrical mould cavity. The cure-induced strain signals were detected in both, vertical and horizontal directions, during isothermal curing at 75°C for 1000 minutes. A huge difference in the strain signal of both directions could be detected for the different adhesion conditions. Under non-adhering condition the horizontal and vertical strain-time traces were practically identical resulting in a compressive strain at the end of about 3200 ppm, which is a proof of free or isotropic shrinking. However, under constrained condition the horizontal shrinkage in the EP was prevented due to its adhesion to the mould wall. So, the curing material shrunk preferably in vertical direction. This resulted in much higher released compressive strain signals in vertical (10 430 ppm) than in horizontal (2230 ppm) direction. The constrained cured EP resins are under inner stresses. Qualitative information on the residual stress state in the molding was deduced by exploiting the birefringence of the EP.