torque loss that adds to the total loss. From tests with single laminates it was found that interlaminate  friction is not a significant cause of the hysteresis. (The single laminate has approximately the same  hysteresis as multiple laminates). As of now, it is not known how much of the hysteresis is internal to the  material from stress related yielding of the spring stock, and how much is due to bearing friction.  One broad objective is to better understand why the physics (applied mechanics) behind the theoretical  formula for torque (Equation 4) does not account for the downward slope of the torque vs. deflection  curves. This will assist in design optimization of the relationships between the variables affecting torque  and torque loss. It may be that optimum design would be achieved at lower stress levels, by reduced  laminate thickness and increased number of laminates. It is desirable to minimize this decreasing torque  slope, because the maximum torque is usually needed at the end of deployment to actuate end-of-travel  latches or wind-up cable bundles.  Conclusions and Recommendations  This poster paper has discussed how general practice for the design and manufacturing of constant  torque springs using empirical design data provides springs that may be stressed beyond the yield  strength. Typically, highly stressed springs are subject to stress relaxation and related torque loss under  prolonged load and elevated temperature conditions.  It appears that torque loss due to thermal cycling, for the springs cited herein, had achieved a state of  stability, in that the torque output of the unwound spring actuated hinges was not continuing to decrease  with each successive cycle. Additional cycles are expected to confirm this. It also remains to be  determined whether maintaining stresses above the yield strength over periods of prolonged storage in  aerospace environments can be done without excessive loss of torque from stress relaxation and without  loss of immunity to delayed cracking.  The final outcome from this work will likely be that Type 301 extra hard corrosion resistant steel will  continue to be an optimum material for one-time operation in space of constant torque spring actuated  hinges. Particularly for the extra hard condition, which is not controlled by ASTM or military specifications,  the end user and spring manufacturer should be mutually cognizant of spring material requirements, such  as yield and ultimate strength and ductility, and stress relieving and pre-stressing procedures.  Acknowledgments  This work has been conducted interactively with the Lockheed Martin Corporation, Sunnyvale CA. The  Lockheed Martin portion of the investigation is under the cognizance of Stuart Loewenthal. The hinge  drawing (Figure l), test data (Figure 3), and sample springs (Figure 4), were provided by Lance Lininger,  courtesy of Lockheed Martin. Usage of the single figure from Reference 2 has been verbally approved by  telephone discussion with the publisher, Purdue University.  References  1. “The Theory and Design of Long-Deflection Constant-Force Springs”, F. A. Votta, Jr., ASME Paper  No.51-F-11, June 11,1951.  2. Desiun Handbook, Enaineerinu Guide to Sprinu Desian, Associated Spring, Barnes Group Inc., 1987  Edition.  3. Aerospace Structural Metals Handbook, 39th Edition, Vol. 2; Code 1301, Original Code1 301 Author -  W. D. Kloop, March 1988; Published and Updated by Purdue-University.  4. Metallic Materials and Elements for Aerospace Vehicle Structures, MIL-HDBKdF, DOD, Nov. 1,  1990.  “Stress Corrosion of Steels for Aircraft and Missiles”, E. H. Phelps, and A. W. Loginow, Feb. 3, 1960, 1 6th  Annual Conference of Corrosion Engineers, March 14-1 8, 1960.  168