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The bending stiffness is the resistance of a member against bending deflection/deformation. It is a function of the Young's modulus E {\displaystyle E} , the second moment of area I {\displaystyle I} of the beam cross-section about the axis of interest, length of the beam and beam boundary condition.
where is the flexural modulus (in Pa), is the second moment of area (in m 4), is the transverse displacement of the beam at x, and () is the bending moment at x. The flexural rigidity (stiffness) of the beam is therefore related to both E {\displaystyle E} , a material property, and I {\displaystyle I} , the physical geometry of the beam.
In mechanics, the flexural modulus or bending modulus [1] is an intensive property that is computed as the ratio of stress to strain in flexural deformation, or the tendency for a material to resist bending. It is determined from the slope of a stress-strain curve produced by a flexural test (such as the ASTM D790), and uses units of force per ...
Similarly, the torsional stiffness of a straight section is = where is the rigidity modulus of the material, is the torsion constant for the section. Note that the torsional stiffness has dimensions [force] * [length] / [angle], so that its SI units are N*m/rad.
The quantity is the extensional stiffness, is the coupled extensional-bending stiffness, and is the bending stiffness. For the situation where the beam has a uniform cross-section and no axial load, the governing equation for a large-rotation Euler–Bernoulli beam is
This results in a constant bending moment between the two supports. Consequently, a shear-free zone is created, where the specimen is subjected only to bending. This has the advantage that no additional shear force acts on the specimen, unlike in the 3-point bending test. [6] The bending modulus for a flat specimen is calculated as follows:
The persistence length can be also expressed using the bending stiffness , the Young's modulus E and knowing the section of the polymer chain. [2] [5] [6] [7] = where is the Boltzmann constant and T is the temperature. =
Approximate specific stiffness for various materials. No attempt is made to correct for materials whose stiffness varies with their density. Material Young's modulus Density (g/cm 3) Young's modulus per density; specific stiffness (10 6 m 2 s −2) Young's modulus per density squared (10 3 m 5 kg −1 s −2) Young's modulus per density cubed ...