Force-Deflection Curve
Select Force-Deflection Curve in the Extensional Force-Deflection Variation drop-down list of the Spring Definition dialog box to define a variable stiffness spring. You can define the extensional properties of a spring in terms of a force-deflection curve. A force-deflection function specifies the force required to produce a deflection of the spring. A positive value of force denotes tension while a negative value denotes compression. A positive value of deflection denotes extension while a negative value denotes contraction.
When you select Force-Deflection Curve, the Spring Definition dialog box displays the 
button and the Force Factor box in the Extensional Force-Deflection Variation area. Type a nonnegative real number value for force factor.
button and the Force Factor box in the Extensional Force-Deflection Variation area. Type a nonnegative real number value for force factor.When you click , the Functions dialog box opens. You can create a symbolic or a table function for the force-deflection curve. The independent variable of the function is deflection which has units of length. The dependent variable is a quantity without units. Logarithmic interpolation is not available for table functions of force-deflection curves, since they must pass through (0,0).
, the Functions dialog box opens. You can create a symbolic or a table function for the force-deflection curve. The independent variable of the function is deflection which has units of length. The dependent variable is a quantity without units. Logarithmic interpolation is not available for table functions of force-deflection curves, since they must pass through (0,0).When you click the Mirror for negative deflections: f(deflection) = -f(-deflection) check box on the Function Definition dialog box , the specified function formula calculates the force in the spring for positive deflections. For negative deflections, the symmetric force function is used. If you do not select this check box, the formula calculates positive (extension) and negative (compression) deflections. This check box is selected by default.
Graphing the function also performs a stability check to ensure that the defined force-deflection curve is nonnegative and has positive slopes in a specified range.
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