Sensors, as referenced in this report, refer to devices both intrusive and extrusive that detect or measure physical and/or chemical quantities, and convert measurements from one signal domain into another. Within the realm of manufacturing (or processing) science, the role of sensors is to measure process variables so that they can be controlled in real time or precisely determined for off- line process modification to attain microstructural or chemical compositional goals, and thereby attain performance specification. Most sensors used in the composites area are not developed primarily for this application, and are modifications of those used in the metals and polymer processing area. Sensors for use in composites manufacturing can be classified into four major areas:
Electrical measurements include those of capacitance and conductance especially for cure monitoring, and can be directly related to molecular structure and mobility of chains. Several commercial systems, including those from Miomet and Kranbuehl, are already on the market using parallel plate geometry (Mopsik 1984), as well as comb electrode (Sheppard 1981, Kranbuehl 1987), which depend on fringing fields. The outstanding problems today relate to (1) the presence of conductive fibers such as carbon that short out the system, and (2) the selection of the proper measurement frequency to facilitate process monitoring with rapidly curing systems.
Wave propagation techniques are the only mechanical-type tests with the potential for on-line processing. They are, however, widely used at present for post- processing inspection of composite parts. Voids, porosity, fiber volume fraction, viscosity, moduli, glass transition temperature (Tg), degree of cure, etc., can be empirically correlated to wave characteristics. All these tests depend on the generation of an oscillatory disturbance and determination of the propagation characteristics of the wave. Measurement techniques include the use of ultrasonics, interface waves, and wave guide methods. Ultrasonics is the best developed for use in composites as shown in Table 6.6, which relates categories to parameters.
Optical and spectroscopic techniques can directly sense the state of the reactants and hence are of most use in cure monitoring. No commercial systems are currently available, but there exists the potential for the technical barriers to be removed in the near future. Measurement techniques include the use of vibrational spectroscopy, UV visible spectroscopy, and optical fibers. Table 6.7 shows specific techniques within each of these measurement categories.
Depth of Use of Ultrasonic Techniques
Status of Specific Measurement Techniques
Table 6.8 compares the three basic types of sensing techniques and ranks them in terms of currently available technology. A brief review of non-destructive inspection (NDI) techniques and applicability is given in Table 6.9.
Ranking of Sensing Techniques
Quality control as related to the Japanese environment and manufacturing science is achieved through the integration of:
Although U.S. companies make routine use of non-destructive evaluation (NDE) systems, they seem to be refined to a significant degree in Japan. NDE is considered critical to the success of a manufacturing process by all companies and universities in Japan, and even small laboratories have some level of NDE apparatus. Techniques such as ultrasonic scanning, thermo-mechanical stress analysis, acoustic emission analysis, and X-ray CT scans are used widely, and codes have been developed in-house for image enhancement. In-house development is preferred (such as at the National Aerospace Laboratories) over commercially developed packages, so as to enable attention to detail and greater attention to image enhancement and speed. Some examples of NDE-related equipment and capabilities present at the National Aerospace Laboratories (NAL) are given in Figures 6.13, 6.14 and 6.15.
Review of Existing NDE/I Techniques (1)
Review of Existing NDI/E Techniques (2)
Figure 6.13. Ultrasonic Scanning Tank
Figure 6.14. Multi-Channel Acoustic Emission Analyzer
Figure 6.15. X-Ray CT Scanner
Further details on the NDE tools depicted in Figures Figures 6.13, 6.14 and 6.15 are given in the trip report for NAL (Appendix C).
Figure 6.16 depicts an overall scheme developed by Professors Kageyama and Kimpara at the University of Tokyo, where again the emphasis is on NDE/I methods.