18.16
Déformation de cisaillement
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Q1: What is shearing strain and how does it differ from normal strain?
Shearing strain is the angular change in a cubic element when subjected to shearing stress, transforming the cube into an oblique parallelepiped without affecting normal strains. Unlike normal strain, which involves length changes along axes, shearing strain causes angular distortion. When shearing stress is applied alone, the cube morphs into a rhomboid shape, demonstrating pure angular deformation independent of dimensional changes.
Q2: How is shearing strain measured and what does a positive value indicate?
Shearing strain is measured as the angular change between the axes of a cubic element under shearing stress. The strain is considered positive when the angle between the axes shrinks, indicating compression of the angle. This measurement quantifies the degree of angular distortion the material experiences, providing a numerical value for the deformation caused by shearing forces.
Q3: What is the shear modulus and how does it relate to other elastic constants?
The shear modulus, also called modulus of rigidity (G), is the constant in Hooke's law for shearing stress that relates shearing stress to shearing strain. It is expressed in the same units as shearing stress and is less than half but more than one-third of the modulus of elasticity. The relation between poisson s ratio modulus of elasticity and modulus of rigidity determines material behavior under combined loading conditions.
Q4: How does Hooke's law apply to shearing stress and strain?
Hooke's law for shearing stress states that shearing stress is proportional to shearing strain within the elastic region. The shearing stress-strain diagram shows an initial straight line indicating this linear relationship, with the shear modulus (G) as the slope. This proportionality holds until the material reaches its elastic limit, allowing prediction of strain from applied shearing stress.
Q5: What is the generalized Hooke's law and how is it derived?
The generalized Hooke's law extends Hooke's law to general stress conditions by combining shearing strain relations with the basic law using the principle of superposition. This generalized form involves three constants: two determined experimentally and one obtained computationally. It enables prediction of material deformations from various stress combinations, accounting for interactions between normal and shearing stresses.
Q6: How are the constants in shearing strain relationships determined?
The constants in shearing strain relationships and generalized Hooke's law are determined experimentally through testing. Shearing stress-strain diagrams are created by applying known shearing stresses and measuring resulting strains. Once these experimental constants are established, they can be used to predict material deformations under various stress combinations without additional testing.
Q7: What shape does a cube take when subjected to pure shearing stress?
When a cubic element is subjected to pure shearing stress alone, it transforms into a rhomboid shape. This geometric transformation clearly demonstrates the effect of shearing strain, as the cube's right angles become oblique angles while maintaining its volume. The rhomboid shape visually represents the angular distortion characteristic of shearing deformation.