Mechanical Shock Testing for LIGA Materials Characterization Vesta I. Bateman Alfredo M. Morales Principal Member of Technical Staff Sandia National Laboratories* P.O. Box 58, MS553 Albuquerque, NM 87185-553 Abstract LIGA is a German acronym for a multi-step process involving, Lithographie: synchrotron X-ray patterning of Polymethyl Methacrylate (PMMA) resists, Galvoformung: electroplating metal features in the developed resist, and Abformung: molding of replicates using an electroplated master. LIGA enables the production of electroformed metal parts with high aspect ratios. Electroforming is commonly done with nickel and nickel alloys. This paper will describe the design of shock and material test specimens, shock testing with a Hopkinson Bar Flyaway configuration, static and rate dependent material parameter tests, and modeling of design elements and material performance including shock environments. Introduction Sandia National Laboratories (SNL) has built a LIGA program to prototype and analyze metal, ceramic, and plastic microcomponents. The mechanical shock testing of the nickel LIGA microcomponents are the focus of this paper. Both the Department of Defense and the Department of Energy have microfuze, trajectory sensing, pressure sensing, safing and arming applications for LIGA components in mechanical shock environments with high accelerations of thousands of g s. This project was initiated because of insufficient data about strain rate effects at high acceleration during mechanical shock, alternate energy dissipation mechanisms, and size effects on material properties. A Hopkinson bar test configuration was developed for quantitative evaluation of material properties at high acceleration. Prior to nickel LIGA parts fabrication, several analyses using ABAQUS was performed to size the individual components and predict what information would be derived from the mechanical shock testing. Figure 1 Shear Rate Dependence (Beam/Ball Devices) Flyer (Free to Move) Nickel-Nickel Friction and Surface Deformation Elastic/Plastic Deformation of Comb Figure 1: LIGA Test Device to Study Material Properties. *Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under DE-AC4-94AL85.
shows the LIGA test device created to study the nickel material properties. The shear rate dependence is indicated by the frequency of oscillation and the acceleration level at which the beam/ball devices shear off on the outer edges. The nickel-nickel friction and surface deformation is indicated by flyer and comb interactions. Finally, the elastic/plastic deformation of the shuttle fingers indicates basic material properties at high acceleration levels. The initial nickel material properties for the modeling were obtained from LIGA tensile tests. By fabricating the tensile specimens in the same manner as the LIGA device in Figure 1, a higher degree of accuracy was obtained from these tests. The results as stress-strain curves are shown in Figure 2. The outer beams were modeled using a transient dynamic analysis in ABAQUS. The comb/flyer was analyzed with a kinetic energy analysis. The modeling predicted the results of the flyer/comb to impulsive mechanical shock loads and that the flyer would move in response to thousands of g s acceleration as shown in Figure 3. The actual packaged LIGA devices are shown in Figure 4. 2 Nickel Stress vs. Strain Curves 16 Stress (MPa) 12 8 4 Watts Sulfamate 2 4 6 8 1 Strain (%) Figure 2: Tensile Test Results for LIGA Nickel Material Initial Flyer Position Final Flyer Position Figure 3: Model Predictions of Flyer/Comb Response to Impulse Loading.
Test Configuration Figure 4: Packaged LIGA Nickel Device (Approximately 8 cm by 2 cm). A Hopkinson bar apparatus was constructed out of 7.62 cm diameter and 183 cm long Type 661 aluminum. An aluminum vacuum chuck that holds a flyaway device to the end of the Hopkinson bar was used. The flyaway device is also made of Type 661 aluminum, and the end that mates to the Hopkinson bar is round. The opposite end holds the LIGA device in an orientation so that six digital cameras can record the motion of the flyaway. The portion of the flyaway holding the LIGA part was machined down to a one-quarter inch thick rectangular cross-section in order to minimize the Poisson s effect or radial motion of the aluminum on the LIGA part and thus create a pure impulsive mechanical shock load in the direction of the comb motion. Two views of the Hopkinson bar test configuration bar are shown in Figures 5 and 6. The impulse compressive load is created in the Hopkinson bar by the impact of a projectile fired by a pressurized nitrogen gun at the opposite end of the Hopkinson bar from the flyaway device (not shown). The compressive wave travels down the Hopkinson bar and into the flyaway device. The compressive wave reverses and becomes a tensile wave at the end of the flyaway device. As the tensile wave returns to the bar/flyaway interface, it causes the flyaway device to leave the bar when the tensile wave magnitude is high enough to break the vacuum. A felt lined steel tube catches the flyaway device after it leaves the vacuum chuck and moves out of the camera field of view. The impulsive mechanical shock was measured by a commercial Laser Doppler Vibrometer (LDV) focused on the end of the flyaway device, and a typical acceleration time history is shown in Figure 7. The inertia of the flyer causes it to remain in place while the beams with the combs moved past the flyer with the rigid body velocity of the flyaway device. Figure 5: LIGA Flyer/Comb Device As Viewed by Digital Cameras.
Figure 6: Low and High Resolution Digital Cameras Focused On the LIGA Part. Input Acceleration Curve for 5, g Test Acceleration (kg) 6 5 4 3 2 1 1 2 3 4 5 6 Time (µs) Test Results Figure 7: Typical Hopkinson Bar Acceleration Time History as Measured by a Commercial Laser Doppler Vibrometer Initial test results showed that the model was over predicting the response of the flyer as shown in Figure 8. The motion of the flyer was predicted in rachets (or single comb fingers) as a function of the speed of the flyaway device for various materials, nickel alloys and other material candidates. Also, damping mechanisms were observed that were not predicted by the model. The model predicted a highly oscillatory, lightly damped response not observed in the beam/ball devices. Data for second largest nickel Watts device tested at 21 m/s final velocity are shown in Figures 9 and 1. A comparison of the impulse mechanical shock pulse used in the analysis of the LIGA test device and the measured impulsive shock pulse showed that the pulse for the modeling had an unrealistically short duration, and this is reflected in the comparison in Figure 9. The model prediction with the short duration pulse oscillates with no apparent damping, but the model prediction with the measured pulse does show damping although not as much as the experimental measurement. Figure 1 compares the response of three beam/ball devices on one LIGA part; the fourth beam/ball device, with the largest ball, sheared off. These data show that three oscillating beam/ball devices have similar and repeatable response and were used to update the nickel material properties in the model.
1 Ni Watts Ni Sulfamate PMMA Polycarbonate NiWattsData NiSulfamateData PMMAData PCData 1 Ratchets 1 1.1 5 1 15 2 25 3 Fly Away Speed (m/s) Figure 8: Comparison of Predicted Motion of Flyer as Compared to Flyaway Device Speed. Finger C Y-Displacement (microns) 1 5-5 -1-15 -2 Experiment Modeled with ideal sharp pulse Modeled with experimentally determined acceleration profile 2 4 6 8 Time (microseconds) Figure 9: Comparison of Experimentally Measured and Modeling Results with Both Idealized and Measured Acceleration Time History for LIGA Beam/Ball Devices.
Finger C Y-Displacement (microns) 1 5-5 -1-15 -2 Lower right finger Upper left finger Lower left finger 2 4 6 8 Time (microseconds) Figure 1: Comparison of Response from Three Beam/Ball Devices on a Nickel Watts LIGA Part. Uncertainty Analysis The uncertainty in the LDV measurement results is attributed to: the uncertainty due to the data acquisition system and the uncertainty in the laser Doppler vibrometer. The sensor and data acquisition uncertainty is monitored on a continual basis in the SNL Mechanical Shock Laboratory as required by the SNL Specification 99583 [1]. These requirements include the performance of both the hardware (sensors, amplifiers, digitizers etc.) and the IMPAX software that controls the data acquisition system through a computer [2, 3, 4]. The 99583 specification allows an accuracy of +1% for amplitude, +5% for duration, and +8% for rise and fall time for any measured pulse greater than 5 µs in duration. The current data acquisition system and software have an accuracy for all three of these requirements of +.5%, and documentation of these results is maintained in the Mechanical Shock Laboratory. In this test series, the LDV was used for measurements with the target (flyaway device) moving towards the LDV, and in this mode, the LDV is used, this LDV has a 2-3% uncertainty over 9% of its range for all specified frequencies and velocities [5]. The uncertainty decreases for decreasing velocity scales. The LDV provides a reference velocity measurement for velocities up to 1 m/s and for frequencies up to 1.5 MHz. The uncertainty in the displacement measurements of the various LIGA parts made with the digital cameras is.4mm. All uncertainties have a 95% confidence level. Conclusions A LIGA mechanical shock test device was developed with a simulation based on quasi-static properties and fabricated as nickel devices. A Hopkinson Bar with a unique flyaway designed to eliminate the Poisson s effect to the LIGA part was designed and implemented. High speed video and analysis was used to record the LIGA performance. It was learned that an understanding of damping mechanisms is necessary for accurate prediction of device shock response
Acknowledgements All digital camera photography and analysis was performed by Mr. Mark Nissen and Dr. Bruce Hansche of SNL. References 1. Ulibarri, Davie, and Kuehnle, "Mechanical Shock Test Instrumentation," Sandia National Laboratories Specification 99583-4, Internal Sandia National Laboratories Publication, pp. 1-19, 1992. 2. Bateman, V. I., Data Acquisition System Hardware Fidelity Check Procedures, Internal Sandia National Laboratories Publication, pp. 1-21, February 14, 1997. 3. Bateman, V. I., "Software Management Plan for Software Supporting Production Lot Acceptance Testing," Internal Sandia National Laboratories Publication, pp. 1-5, January 11, 1996. 4. Bateman, V. I., "Software Quality Requirements for Area I Mechanical Shock Laboratory," Issue A, Internal Sandia National Laboratories Publication, pp. 1-1, January 11, 1996. 5. V. I. Bateman, B. D. Hansche, and O. M. Solomon, "Use of a Laser Doppler Vibrometer for High Frequency Accelerometer Characterizations," Proceedings of the 66th Shock and Vibration Symposium, Vol I, Biloxi, MS, November 1995.