Advanced cure monitor

Nondestructive Testing. Nondestructive testing (NDT) is far more economical than destructive test methods, and every assembly can be tested if desired. Several nondestructive test methods are used to check the appearance and quality of structures made with adhesives or sealants. The main methods are simple ones such as visual inspection, tap, proof, and more advanced physical monitoring such as ultrasonic or radiographic inspection. The most difficult defects to find are those related to improper curing and surface treatments. Therefore, great care and control must be exercised in surface preparation procedures and shop cleanliness. 

Figure 6.29. (a) A photograph of an autoclave custom-modified to enable the accommodation of optical and electrical sensor systems for cure monitoring of advanced reinforced composites via contact and non-contact IR spectroscopy and measurements of residual strain and temperature. 

Dielectric cure monitoring generally relies on measin-ement of the ionic conductivity (a in eq. 19). The conductivity during cure of epoxy-amine systems have been characterized to establish relationships between conductivity and viscosity (103,104), conductivity and Tg (104), and relationships to the conversion of epoxide (103). Recently, models were established to relate changes in the dipole component of the complex permittivity to the advancement of cure through the Tg-con version relationship, expanding the capabilities of dielectric sensing to monitor cure (102). 

In the past, impedance or dielectric studies have been examined as an experimental technique to monitor the flow properties, effects of composition, and the advancement of a reaction during cure [1]. Until a paper by Zukas et al [2], little emphasis had been placed on the frequency dependence except to note the shift in position and magnitude of impedance maxima and minima. Furthermore, most measurements on curing systems reported results in terms of... 

The magnitude of the ionic mobility c and the rotational mobility of the dipole t depends on the extent of the reaction and the physical state of the material (5). As such, c and t determined from the frequency dependence of (u), provide two molecular probes for monitoring the reaction advancement and the viscosity during cure. 

In situ frequency dependent electromagnetic-impedence measurements provide a sensitive, convenient, automated technique to monitor the changes in macroscopic cure processing properties and the advancement of the reaction in situ in the fabrication tool. This chapter discusses the instrumentation, theory, and several applications of the techniques, including isothermal cure, complex time—temperature cure, resin film infusion, thick laminates, and smart, automated control of the cure process. 

Overall an intelligent closed-loop system produces more consistent parts because the advancement to the next stage of cure is based on the resin achieving a certain molecular state rather than on time. The advancement of the viscosity and degree of cure is monitored. Final cure is defined by a universal degree of cure. This is a more consistent way to produce composite parts than simply subjecting them to the same time-temperature schedule when batch variations and differences in prefabrication handling are present. 

Instrument manufacturers offer a variety of devices, including monitoring systems for standard process or for spot curing technology and systems for process control. Online UV radiometers can measure and display the peak intensity and total UV energy used in the process. More-advanced systems map the entire process and evaluate the lamp focus, reflector focus and reflector efficiency within a specific... 

Bar-Dohen, K.H. Nguyen, and R. Botsco Eddy Currents Monitor Composites Cure. Advanced Materials Processes, 41 (April 1991). 

The beginning of the polymerization reaction can be followed by NMR spectroscopy. Thus the nadimide polymerization advancement is easily monitored by the disappearance of the ethylenic proton and carbon resonances. However these criteria should be handled with care. For instance, the ethylene proton disappearance was particularly misleading in the case of the 2,3,4,6-tetrahydroph-thalimide curing study [38]. An isomerization stage took place with the formation of a tetrasubstituted double bond. Moreover, an oxidation reduction pathway was also evidenced, which gave saturated and aromatic derivatives. 

Among the advanced techniques employed to follow the cure reaction, Fiber Optic Raman Spectroscopy has been an effective tool. By this technique, both the temperature build-up and the cure advancement of AroCy L-10 could simultaneously be followed. The local temperature of the sample, determined by Ra-man-Stokes and anti-Stokes scattering of a reference peak correlated well with the temperature measured using a thermocouple probe. The extent of cure could be monitored using either individual peaks associated with the reactant or product or by using the entire spectrum [104]. 

R. L. Levy (McDonnell Douglas Research Laboratories) (1) Yes. (2) Probably, but a series of probes with progressively smaller rotating groups might be needed for monitoring advanced stages of cure. 

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