REAL-TIME FIBER OPTIC DOSIMETRY FOR E-BEAM CURING OF COMPOSITES ABSTRACT

Transcription

1 REAL-TIME FIBER OPTIC DOSIMETRY FOR E-BEAM CURING OF COMPOSITES Andrea E. Hoyt Haight and Ronald E. Allred Adherent Technologies, Inc. Development Laboratories Cochiti SE Albuquerque, NM ABSTRACT Recent developments with the technology of electron beam curing of composites has led to the identification of many issues that must be addressed before full implementation of this promising processing method can occur. Verification that a processed part has received the dose required for full cure is one such issue. Adherent Technologies, Inc. has developed a real-time fiber optic dosimetry system specifically to monitor dose and dose rate during the cure process. This system is currently being employed by the electron beam processing facility at the University of Dayton Research Institute (UDRI) as a research tool in the advancement of electron beam processing of composites. KEY WORDS: E-Beam Curing, Quality Control, Dosimetry, Real-Time Measurement

2 INTRODUCTION The manufacture of large composite structures by current methods (organic resin matrix, fiber composite lay-up techniques followed by a thermal cure of the matrix resin) is slow and expensive and can lead to internal stresses in the structure that result in microcracks and voids. A large fraction of these stresses is generated by the curing process due both to resin shrinkage during cure and to thermal expansion mismatch between the composite constituent phases at the high temperature necessary for the matrix resin cure. Nonthermal or radiation curing processes can reduce or eliminate thermal curing residual stresses and appear to offer significant manufacturing cost savings advantages as well [1,2,3]. Electron beams (e-beams) have been used by Aerospatiale [4] to successfully cure composite rocket motorcases up to 7 in. (17.5 cm) thick. In e-beam processed composites, absorbed dose may determine such important properties as toughness or even interfacial adhesion. Resin cure by e- beams is initiated by radiation-produced reaction initiators or catalysts and the quantity or concentration of these initiating species can strongly affect the resin s final properties. Overcure or resin damage by excess deposited dose can cause embrittlement. Despite these difficulties, e- beam technology appears to be the best potential alternative to thermal cure in terms of manufacturing costs and resultant material properties, especially for large structures. If e-beams are to be used effectively in the production of composites for aerospace or civilian applications, real time dosimetry at the point of application is necessary to control the process and address the state of cure of the irradiated resin. Excess deposited dose may result in low degrees of polymerization because too many initiating species are present. Excess dose may also generate damage products that act as plasticizers or cause the formation of brittle regions. On the other hand, insufficient generation of initiating species will result in longer stabilization times and fiber or resin redistribution. In addition, unlike thermal cure where the entire structure is baked, a scanning e-beam can malfunction, missing part of a structure and leaving it uncured. There are other concerns in e-beam processing of composites as well. Complex structures present many opportunities for uneven dosage distribution in the part. In addition, thick parts can exhibit uneven dosing through the cross-section. Clearly, methods are needed to assess the exposure delivered in real time for adequate process control and quality assurance. Adherent Technologies' fiber optic dosimeters take advantage of a "Dual Cladding" fiber optic sensor configuration for scintillator-based detection of radiation (patent pending). Since nonconductive fiber optic leads are used for connection to data acquisition equipment, these dosimeters will allow real-time examination of dose that cannot be accomplished with any of the more conventional dosimetry techniques. Additionally, the small size of the fiber optic devices allows small processing areas to be monitored and the devices may be incorporated into composite structures without compromising their physical properties.

3 EXPERIMENTAL Sensor Fabrication Fused silica fibers of 0.2 millimeter in diameter with a removable cladding were obtained for sensor fabrication (3M Hard-Clad Fiber (FT-200-UHT) distributed by Thorlabs in Newark, New Jersey). This fiber features a mechanically strippable buffer and cladding. The fibers were cut to a length of approximately one meter, and a 2 3 cm section on one end of each test fiber was stripped of buffer and cladding. The unclad length was coated with a plastic scintillator formulation by applying a scintillator casting resin followed by photopolymerization. An overcoating of silicone was applied to the fibers after application of the active layer. The non-sensor end of the fiber was terminated using ST-type crimp-on connectors after which the entire length of the optical fiber sensor was encased in black heat shrink tubing. A reflective metal coating was applied to the tip of the sensor end, followed by a drop of black paint to seal out stray light. Reader System Design Real-time readout from the devices is accomplished through user interface electronics and software. The reader system consists of a photomultiplier tube (PMT) with integrated analog to digital conversion capability and RS232 output that allows direct interfacing to a PC-based data acquisition system. A Microsoft Excel-based data acquisition module was developed to collect and process real-time data acquired using this PMT system. Electron Beam Exposures Electron beam exposures were conducted at North Star Research Corporation in Albuquerque, New Mexico. The active section of the fiber optic dosimeters were placed into the beam path with fiber optic leads going outside the shielding area to the detector and data acquisition computer. In this manner, real-time data acquisition is possible.

4 RESULTS AND DISCUSSION Figure 1 shows the typical reflection configuration used for Adherent Technologies fiber optic dosimetry probes. The optics applying to the sensor probe are discussed below. Adherent Technologies Dual-Clad fiber optic radiation sensors (patent pending) take advantage of a simple method of manufacturing sensor elements from preformed fiber optic material. Removal of the factory buffer and cladding from the core fiber optic is followed by replacement with a sensor layer and a new outer layer that functions as both reflective cladding and buffer. Figure 2 illustrates the path of a light ray in a dual-coated optical light guide. Refractive index requirements for this operational mode are that n 2 > n 1 > n 3. In this discussion, the critical condition is assumed (n 2 > n 1 > n 3 ). A calculation of the critical angle for the mode and the variation of effective pathlength (sensitivity) with illumination angle follows. The critical angle for configuration at the interface between the sensor layer and the cladding is given by: sin d 2 = n 3 /n 2 (1) for the interface between the core and the sensor layer: giving, at the critical angle Figure 1. Reflective configuration for fiber optic dosimeter construction. In this application the High Index Sensor Material is a plastic scintillator formulation. n 1 sin d 1 = n 2 sin d 2 (2) sin d 1 = (n 2 n 3 )/(n 1 n 2 ) = n 3 /n 1 (3) Note that Equation 3 is the same result one would derive for the critical angle between the interior and the cladding in the absence of the sensor layer. The fiber optic continues to act in the normal, expected mode despite the fact that a radiation-sensing interior cladding has been added.

5 Cladding Scintilator layer Core c n3 n 2 n1 Figure 2. Optical path in dual coating lightguide marking the critical angle C and the reflective indexes n 2 > n 1 > n 3. Sensitivity Model for Emission from the Interlayer For this application, we have elected to use a sensor layer consisting of a scintillator material doped into a polymer host. For the case of emission within the inner layer, we have used two approximations to derive the sensitivity for light detection. The first case utilizes a planar guide to estimate the collection efficiency. In this model, the light that can propagate in the core is calculated from the critical angle for a core/cladding interface, the same critical angle derived above for the absorption case (Equation 4 below) sin c =n 3 /n 1. (4) Light with emission angles greater than this value will either be trapped in the inner layer and not propagate in the core, or will be lost by transmission through the cladding. The fraction that will propagate in the core is given by: F P =1.- (1.-(n 3 /n 1 ) 2 ) 0.5 (5) For typical refractive indices of core (1.41) and cladding (1.25), about 50% of the light emitted in the inner layer will propagate in the core. By careful shaping of the ends of the inner layer, it may be possible to force the light propagating in the inner layer alone to be captured by the core and increase the light yield. For thin inner layers, the planar approximation allows a good estimation of the fraction captured, it is an upper limit. In cylindrical geometry where the inner layer is comparable or thicker than the core, a different approximation must be used. In this case, the fraction captured varies from the outer region of the inner layer where it has the above value ( 50%), to the value at the interlayer-core interface. At the interlayer-core interface ( for a small diameter core) the fraction propagating in the core is: F P = 1-n 3 /n 1 (6) Here a typical value for capture is 10-11%. The sensitivity is then related to the position by: F P = 2ΠRL((F PO -F PI )(R-R I )/(R O -R I )+ F PI (R- RI )/(R O -R I ))/V (7) where F PO is the fraction captured at the outer surface of the inner layer, F PI is the fraction capture from emission at the inner layer/core interface, R O is the radius of the inner layer/cladding interface, R I is the core radius, R is the radius where the emission takes place, V is the volume of sensor (scintillator) in the length L of sensor.

6 To estimate the mean fraction of light captured, this expression must be integrated over all possible positions from which emissions can occur. For typical values of refractive indices, the cylindrical geometry should allow about 35-40% of the emitted light to be captured and propagate in the fiber optic core. This model does not allow light, which propagates in the inner layer to propagate in the core as well. With suitable shaping of the sensor ends, some of this light could be trapped into the core region. These analyses also assume that the core is operating far from its propagation wavelength cutoff (multimode operation). Clearly, a scintillation fiber optic sensor of dual clad structure harvests a substantial fraction of the available light from scintillation. Real-Time Dosimeter System As fabricated, the fiber optic sensors are single-ended probes capable of connection to the detection system via a standard ST fiber connection. Figure 3 shows a photograph of the sensor and the PMT detector housing. Typically, a relatively short probe segment is used and connected to double-ended fiber optic extensions that are then connected to the detector housing. This is to more easily allow for probe changes should one become broken or degrade as a result of long-term exposure to the electron beam. It should be noted that the probe lifetime in the electron beam has not yet been determined. The probe end is placed in the electron beam, the shutter to the PMT detector is opened and data acquisition is controlled by a Microsoft Excel module written especially for this application. PMT Connection for fiber optic probe ST Connector Figure 3. Photograph of PMT detector, detector housing, and fiber optic dosimeter with ST connections.

7 Total Dose (Kilogray) Total_Dose Re ading Time from start (seconds) Figure 4. Representative real-time data acquired using Adherent Technologies fiber optic dosimeter system. The probes must be calibrated; this is accomplished by irradiating and measuring dose via an alternate method. To date, our calibrations have been conducted using Far West radiachromic dosimetry film and a 50 kgy irradiation. A calibration factor is then calculated using the total counts accumulated during the irradiation and the total dose as measured using the film. The calibration factor for each probe is entered into the software and is automatically applied when the probe is selected during the first stages of the data acquisition process. Representative data is given in Figure 4. This data was collected using the 300 kev electron beam available at North Star Research in Albuquerque, NM. It should be noted that this beam system is not an optimum system: some issues in control of the beam parameters and therefore the penetration depth of the beam were encountered during data collection. The maximum penetration depth expected for this system is on the same order as the diameter of the sensor elements and any fluctuations in beam parameter can lead to inconsistencies in sensor response. The much higher energies used for electron beam processing of composites (3 10 MeV) will eliminate these inconsistencies of sensor response. 0 Reading (Counts/second) CONCLUSIONS Adherent Technologies has demonstrated fiber optic dosimeter system based on an optical fiber coating configuration that collects the light emitted from a scintillator segment on the sensor end. This fiber optic dosimeter system provides real-time output of scintillator response, which is related to the dose rate, and total dose (when properly calibrated). This technology offers a new quality control tool for the electron beam processing of composite structures and is expected to find particular advantage in the curing of thick and/or complex structures.

8 ACKNOWLEDGEMENTS The authors wish to thank North Star Research Corporation of Albuquerque, NM for their participation in the demonstration of these fiber optic sensors. We would also like to thank the Electron Beam Processing Facility at the University of Dayton Research Institute, and particularly Dr. Donald Klosterman, for funding the development of the prototype instrument discussed in this paper. REFERENCES 1. D. L. Goodman and D. L. Blrx, "Composite Curing With High Energy Electron Beams," Proc. 41st Intl. SAMPE Symposium, Society for the Advancement of Material and Process Engineering, Covina, CA, 1996, pp R. B. Vastava, et al., "E-Beam Processing of Composite Structures," Proc. 42nd Intl. SAMPE Symposium, Society for the Advancement of Material and Process Engineering, Covina, CA, 1997, pp F. Abrams and T.B. Tolle, "An Analysis of E-Beam Potential in Aerospace Composite Manufacturing," Proc. 42nd Intl. SAMPE Symposium, Society for the Advancement of Material and Process Engineering, Covina, CA, 1997, Pp (a) D. Beziers and J. Denost, "Composite Curing: A New Process", AIAA , American Institute of Aeronautics and Astronautics, Washington, DC (1989), (b) Daniel Beziers, "Electron Beam Curing of Composites", 35th Intl. SAMPE Symposium, p. 1220, (1990).

9 BIOGRAPHIES Dr. Andrea E. Hoyt received her Ph.D. in Polymer Science from the University of Connecticut Institute of Materials Science in She has a B.A. in Chemistry from the University of Colorado and an M.S. in Polymer Science from the University of Connecticut. Dr. Hoyt is the Manager of Polymer Projects at Adherent Technologies. In this position, she has led projects in the development of electron beam curing resin systems, coatings for chemical sensors, liquid crystalline thermoset adhesives, and new moisture resistant coupling agents for glass fiberreinforced composites. Her current research focuses on the development of UV- and electronbeam curable resin systems for a variety of space and aerospace applications and on the development specialty foam systems for self-deployable space structures. Before joining Adherent Technologies, she was engaged as a postdoctoral research associate in the Microsensors Research and Development Department at Sandia National Laboratories. She has also done research in liquid crystalline polymers and liquid crystalline thermosets at Los Alamos National Laboratory and in polyimide synthesis and characterization at the University of Connecticut. Dr. Hoyt has over 20 publications in technical journals and conference proceedings and holds 4 patents in the areas of liquid crystalline thermoset materials and chemical sensors. Dr. Ronald E. Allred earned his ScD in the Polymerics Panel of the Department of Materials Science and Engineering at MIT. He has a BS in chemistry and an MS in nuclear engineering materials, both from the University of New Mexico. Before establishing Adherent Technologies in 1990, Dr. Allred was Director of the Materials Development Department at PDA Engineering and a Member of the Technical Staff at Sandia National Laboratories. Dr. Allred's research has included polymer recycling, interfacial adhesion environmental effects on composite materials, heat damage in aircraft composite systems, materials for electronic packaging, chemical microsensors, and the development of specialty polymers, adhesives, coatings, fiber sizings, and foams. Over 30+ years in the materials industry, Dr. Allred has over 80 publications in technical journals and conference proceedings and has 20 patent disclosures and over 50 company reports in polymer and composites technology.