Assessing Composite Aerospace Structures using Distributed Dielectric Cure Monitoring

Topics Covered

Introduction
D-DEA with LT-439 Dielectric Channel
DEA in Manufacturing
Conclusion

Introduction

In research, quality control and manufacturing applications, distributed dielectric cure monitoring (D-DEA) represents a novel concept for applying dielectric analysis (DEA). D-DEA does not use long extension cables and a costly base instrument, but rather uses low-cost instruments that are linked along an RS-485 communications line (Figure 1).

Figure 1. Distributed DEA compared to conventional DEA (Composite wing in autoclave)

With the help of D-DEA, DEA can be used to track multiple locations in ultra-large autoclaves, such as those utilized for composite aerospace structures. Moreover, D-DEA also eliminates the mesh of extension cables that are utilized to track components demanding multiple sensors, such as components of spacecraft and aircraft.

In comparison to traditional DEA instruments, D-DEA is less expensive, does not use lengthy sensor cables which degrade signals to base instrument but instead uses short sensor cables which preserve signals to D-DEA units, allows simultaneous operation of up to 256 DEA channels, and enables DEA monitoring and process control of extremely big structures. Furthermore, it also combines DEA into a standard RS-485 process control network and extends the distance of DEA channels to 1890m, which is the RS 485 distance limit.

The same sensors are used in D-DEA for measurements in research, manufacturing applications, and quality control. DEA correlates with laboratory tests such as dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC), thereby making DEA to serve as the “go between” to bring data from the research lab to the work area, and then from the work area to the product quality manager.

D-DEA with LT-439 Dielectric Channel

The LT-439 dielectric channel (Figure 2) is a compact and affordable instrument used for thermoset cure monitoring in research and development, quality control/quality assurance, and manufacturing. The LT 439 dielectric channel enables D-DEA in a variety of processing environments when it is coupled with either reusable or disposable dielectric/conductivity sensors (Figure 3).

Figure 2. LT-439 Dielectric Channel for Distributed DEA

Figure 3. Disposable (left) and reusable (right) dielectric sensors

The measurements obtained from the LT-439 can be transmitted to a control network via its standard RS-232 serial port or optional RS-485 serial port. Each LT 439 is designed to support one thermocouple and one dielectric sensor. It is possible for about 256 individual dielectric channels to be connected to a single RS-485 line. This versatile feature eliminates the need for numerous cabling and allows instantaneous monitoring of cure state via multiple channels at any point in a large part. The standard LT-439 dielectric channel has four excitation frequencies - 10Hz, 100Hz, 1kHz and 10kHz. An optional extended frequency expands the range from 1Hz to 10kHz with supplementary frequencies within each decade.

DEA in Manufacturing

Every step relating to the manufacture of highly critical parts, such as composite spacecraft or aircraft components, is well documented for the purpose of future analysis in case of a failure as well as for keeping record to prove that each part meets standard specifications. Temperature is often measured by several manufacturers to deduce the progress of cure, which is a very indirect and incorrect method. However, the DEA is a direct method for indicating cure state. DEA can measure the frequency independent electrical resistivity called ion viscosity.

Dielectric cure monitoring is very useful for the purpose of documentation as no other method is capable of real-time monitoring of cure state during production. During the manufacture of composite materials, curing of parts is normally performed with a fixed schedule for temperature and time. DEA is quite frequently used to verify the consistency of parts. Figure 4 illustrates nominal cure profiles of a carbon fiber reinforced prepreg (CFRP).

Figure 4. Nominal cures of carbon fiber reinforced prepreg

Productivity for aerospace components, such as high value composite fuselages or wings, can be instantly enhanced using dielectric cure monitoring. These components are frequently more than 30ft long, and are fabricated in an autoclave. The part’s thickness and the exotherm during processing differ with location. As a result, the cure rate also differs based on location. Figure 5 illustrates the temperature dependency of the time to a defined end of cure for CFRP.

Figure 5. Variation of CFRP cure time with temperature

If a part is removed too early during a cure, there is a risk of imperfect cure and poor strength. Similarly, delaying the removal of a part causes wastage of energy, throughput reduction, and a hike in cost. Therefore, it is important for manufacturers to use their skill, compressive analysis, and guesswork to decide the most favourable processing time.

With the aid of dielectric sensors installed in at key locations, it can be established whether the desired level of curing has been achieved along the along the whole part. Based on this data, parts can be removed from the autoclave. By studying the various cure profiles, each panel can be evaluated against a nominal group of curves. The results can be recorded for statistical quality control (SQC), and any deviations beyond the defined limits signal a drift in the process.

Conclusion

The D-DEA with the LT-439 dielectric channel is an innovative technique of applying dielectric cure monitoring for composite aerospace parts as well as other large composite structures. D-DEA avoids the need for lengthy extension cables, which can degrade the sensor signal, by fitting economical, individual DEA instruments at desired points. A process control network is capable of communicating with up to 256 LT- 439 dielectric channel units, thereby enabling cure monitoring of numerous points at various distances, even as far as 1890m away from the base station.

As a simple electrical measurement, dielectric cure monitoring applies the same sensors and techniques in research, manufacturing applications, and quality control. Dielectric analysis data are in good agreement with the results of laboratory analyses, such as DSC or DMA. Thus, DEA can serve as the “go between” to transmit data from the lab to the work area, and from the work area to the product quality manager.

This information has been sourced, reviewed and adapted from materials provided by Lambient Technologies.

For more information on this source, please visit Lambient Technologies.

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