Fiber-Optic and Ultrasonic Measurements for In-Situ Cure Monitoring of Graphite/Epoxy Composites

Abstract

To produce high-quality composites with high density and low void content, the knowledge of cure process is very important, and the sensors capable of monitoring the cure process are therefore desirable. Since the term of "fully processed" should be a reflection of the ultimate material application, the objectives of the present work are to monitor how material properties have been developed during the cure and justify when "end-of-cure" has been achieved by measuring chemical and mechanical properties of the curing composites. As a reference, differential scanning calorimetry is used to evaluate the degree of cure while fiber-optic sensors are used to measure the evolution of process-induced strains, and ultrasonic sensors are used to monitor the development of viscoelastic properties of the curing composites. Moreover, an ultrasonic cure monitoring system has been developed, by using conventional broadband ultrasonic sensors together with clad buffer rods. The major merits of this system can be summarized as (i) workable inside an autoclave at high temperature (for example, up to 2500C if by water-cooling) and internal pressure (for example, 100 psi); (ii) high signal to spurious ultrasonic noise ratio, and high signal strength; (iii) suitable for longitudinal or shear wave measurement in reflection/transmission mode.

Process Monitoring of Fiber-Reinforced Polymer Composites

“…87 Chen et al 88 demonstrated the feasibility of using an optical-fiber-based ultrasound sensor that operated in an autoclave. 87 Chen et al 88 demonstrated the feasibility of using an optical-fiber-based ultrasound sensor that operated in an autoclave.…”

Automated In-Process Cure Monitoring of Composite Laminates Using a Guided Wave-Based System With High-Temperature Piezoelectric Transducers

“…For thermoset resins, ultrasonic velocity has been used to infer the degree of cure because of its direct relation with the modulus of the resin [5][6][7]. Bulk wave contact ultrasound, usually conducted in pulse-echo mode in the low megahertz frequency range, can monitor the completion of resin cure based on when the time delay plateaus/ultrasonic velocity becomes constant value in graphite/epoxy composites [8] and epoxy resins [9]. Other ultrasonic phenomena have also been used for monitoring degree of cure including, attenuation (i.e., amplitude of signal) [7,8,[10][11][12][13], instantaneous phase, and the "mean value of each frequency curve weighted by the maximum corresponding spectral amplitude" [14].…”

Instrumented sphere method for measuring thermal pressure in fluids and isotropic stresses and reaction kinetics in thermosetting resins

“…In situ monitoring of strain development during cure can be done by fiber optic sensors embedded in composite materials. 6,7 However, the value of residual stresses cannot be measured directly and have to be computed based on a model which includes the viscoelastic properties of the material, which may not be known. 6 In measuring or calculating the thermally induced isotropic stresses in any material, the parameter of interest is the thermal pressure coefficient 3,5,8,9 ␥ = ͑‫ץ‬P / ‫ץ‬T͒ V = ␣K, i.e., P = ␥⌬T for a temperature change in the constrained system, where ␣ and K are the coefficient of thermal expansion and bulk modulus of the material, respectively.…”