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5.3 Hooke's law

For elastic deformations the normal stress is directly proportional to strain. The ratio of axial stress/strain is a constant. This constant is known as Young's modulus or modulus of elasticity E. Young's modulus is directly proportional to the axial stress and inversely proportional to the axial strain. For three-dimensional stress state the formulas include more than one stress component. The expressions for isotropic elastic body include three elastic constants of material: modulus of elasticity E, Poisson's ratio m and shear modulus G. The elastic constants are characteristics of the material. They do not depend on the shape or size of the specimen. The shear modulus G can be expressed by the first two elastic constants. There are only two constants that are independent parameters for an isotropic material. Poisson's ratio m characterizes transverse deformation under axial tension or compression. It is a dimensionless parameter. For m=0.3, axial tension of 10% corresponds to a transverse compression of 3%. The expressions show another form of the Hooke's law. Normal stress sx depends on all three linear components of strain ex, ey and ez. Volume expansion e characterizes an increase of an element size, not a change of its shape. It depends on the linear strains only. The volume expansion e can be defined by the elastic constants of the material E, m and the sum of the normal stresses sx, sy and sy.

5.4 Plane problems

There are two special states of stress: plane stress for thin plates and plane strain for thick plates. For plane stress there are four non-zero strain components (ex , ey , ez , gxy ) and three non-zero stress components (sx , sy , txy ). The plane stress is typical for thin-walled structures such as in airplane wings. For plane strain there are three non-zero strain components (ex, ey, gxy) and four non-zero stress components (sx, sy, sz, txy). Plane strain is typical for thick plates. There are two equilibrium equations that must be satisfied for absence of mass forces. Elastic plates have stress fields that satisfy Hooke's law and the strain compatibility equations. The two stress components shown correspond to an embedded beam with a force applied to the right end. The equilibrium equations help solve the plane problems of the theory of elasticity. There are two inclined principle planes where there is no shear stress and normal stresses (s1 and s2) are at a maximum. Principle axes are inclined 45o to the direction of shear stress t for pure shear. Indication A is negative. There are zero indications for B and D. The figure shows typical stress patterns for sx, sy and txy. The normal stress sx is usually higher than other stress components for a long beam, where Length >> Height. There is concentration of shear stress in the area where force is applied. Shear stress is equal to zero at the surface and is at a maximum in a neighboring region. Normal stress sy at axis x can be positive outside the zone of compression. Here, the compressive stress is significantly higher than the tensile stress. Saint-Venant's principle. At a distance equal to width of the beam (2a), the maximum stress does not exceed the nominal stress by 4%. The stress pattern becomes practically uniform.

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