How to find stiffness using load versus defelection curve?

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Discussion Overview

The discussion revolves around determining the stiffness of a steel plate using its load versus deflection curve. Participants explore various methods and considerations for calculating stiffness, including the implications of nonlinearity in the load-deflection relationship.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants assert that stiffness is defined as load per unit deflection and inquire about its calculation from the load versus deflection curve.
  • One participant suggests translating the load-deflection curve into a stress-strain curve to find the E-modulus, contingent on knowing the geometry of the plate.
  • Another participant notes that while stiffness can be represented as the slope of the curve at any point, the nonlinearity of the curve complicates this, recommending the use of the initial slope for approximation.
  • It is proposed that the definition of stiffness may vary based on the intended application, particularly in cases of nonlinear load-deflection relationships.
  • A participant mentions the concept of average stiffness, suggesting it could be represented as the slope of a chord over a specific working deflection range, acknowledging that this may not be accurate at all points.

Areas of Agreement / Disagreement

Participants express varying views on how to define and calculate stiffness, particularly in the context of nonlinear behavior. There is no consensus on a single method or definition, indicating ongoing debate.

Contextual Notes

The discussion highlights the importance of context in defining stiffness, including the potential nonlinearity of the load-deflection curve and the relevance of specific applications in determining the appropriate method for calculation.

perfectz
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I know that stiffness is load per unit deflection. I have the load versus deflection curve for a steel plate. Is it possible for me to find the stiffness of the plate?
Thank you.
 
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You could translate it into a stress strain curve and the slope in the elastic zone is the E-modulus. If you have the geometry.
 
perfectz said:
I know that stiffness is load per unit deflection. I have the load versus deflection curve for a steel plate. Is it possible for me to find the stiffness of the plate?

By definition the stiffness would be the slope of the line at any given point. However, due to nonlinearities, your line is probably not linear. You could simply take the initial slope of line.
 
yes of course
 
THank you...
 
The stiffness value you want depends somewhat on your purpose for finding the stiffness, particularly if the load-deflection relation is nonlinear.

For many purposes, it is useful to define an average stiffness that represents the average slope over some working deflection range, whatever is appropriate to the situation appropriate to your problem. This would be the slope of a chord to the load-deflection curve, rather than the slope at a point. Obviously it will not be exactly correct at more than one point (the mean value point), but it will be somewhat close over the whole range.
 

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