A rapid test for evaluating the degree of cure in CFRP composites

Abstract

A noncontact optothermal method is described for possible application to routine industrial evaluation of the degree of cure in polymeric composites. The surface of the part is heated by a laser beam or other radiative source while its temperature evolution is continuously monitored with an infrared detector. A strong exothermal peak is observed when the material is partially or totally uncured. Changes in the signal shape related to variations of the part geometry or environmental conditions are minimized by a differential approach comparing subsequent heat cycles on the same area. Results obtained with cured or uncured graphite-epoxy prepreg sheets are presented.Quality control of composite materials and structures is a very active research field [1,2]. Graphite-epoxy composites are particularly subject to strict inspection procedures because of their increasing utilization as primary structures in safetysensitive fields such as in the aircraft industry. The mechanical properties of carbonfiber-reinforced-plastics (CFRP) structures are much affected by the degree of cure of the resin matrix both before and after processing. Before processing, the slightly precured prepreg sheets which are shipped to the manufacturer in refrigerated cells are normally inspected to verify, that the required pre-cure level has not been exceeded [3]. After lay-up and autoclave curing, the degree of polymerization must be sufficiently high to assure the required mechanical performance.A number of approaches are possible to evaluate the degree of polymer cure, including spectroscopic, calorimetric, mechanical, electromagnetic or ultrasonic methods [1,4,5], the most widely used being the spectroscopic and the thermal techniques. Infrared spectroscopy can provide quantitative data concerning the amount of unreacted epoxy groups. For best results measurements must be made in transmission and, in the case of cured composites, this requires destruction (grinding) of the sample. The diffuse reflectance method has the advantage of being noncontact and nondestructive, but is of limited value for quantitative work [4].

Lumber moisture evaluation by a reflective cavity photothermal technique

“…-the front surface temperature reaches relatively high levels with low-moisture materials, arising material damage concerns in industrial applications ; one way to reduce the temperature excursion signal would be to insert a feedback loop switching off the heating power when a predetermined temperature level has been reached, and measuring the length of the heating period or the total source energy, as described in reference [35].…”

Photothermal radiometry monitoring of light curing in resins

“…The kinetics of light curing resins has been studied extensively in recent years due to its importance in a wide variety of applications including technology and dentistry [8,9] using different methods based on calorimetry, spectroscopy, mechanics, nuclear and electronic magnetic resonance, etc, in its different modalities [8][9][10], that have provided useful results in the understanding of the complex processes involved. Photothermal techniques have been applied to the monitoring of the increment of temperature during the process [11], inhomogeneous curing [12], evolution of thermal conductivity [13], adhesive curing [14], the effect of the hardener content on the thermal diffusivity [15], thermal characterization of the cured material [16] and the evolution of the photothermal signal during the curing of resins [17][18][19]. However, the values obtained for the thermal properties, during the curing process, based on photothermal techniques, can be overestimated, due to the fact that the measurement is affected by several factors, such as the change in sample thickness and the non-modulated heating of the sample due to curing, among others [17].…”