5 3EI bEh3Eq

where L is the length of the beam, E is Young's modulus, I is the moment of inertia, b is the width of the beam, and h is the thickness of the beam. This equation is modified, for the case of thin and broad plate-like specimens (b • h), where plane stress is applied in the direction of the beam thickness and plane strain is applied in the direction of its width, to read:

where v is Poisson's ratio. Young's modulus is derived from the slope of S(P).

The simple beam theory described above can also be employed in the determination of the yield strength of the beam material. When the beam is bent under a downward load applied to its free end, the maximum tensile stress occurs at the fixed end. When this stress reaches the yield strength, the load-deflection curve ceases to be linear. The load that marks this deviation from linearity is denoted by Pv, so that the yield stress is (Ref 7):

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