Internal Force, Stress and Strain of Metal Materials

1. What is metal internal force?

Internal force refers to the interaction force (additional internal force) between adjacent parts in an object caused by external force.

As shown in the figure, it is any object, which is composed of infinitely many particles. There is an interaction force between any two adjacent particles in the component, and the magnitude of the force is related to the relative position of the particles. When an object is subjected to an external force, the object deforms, the relative position of its internal particles changes, and the interaction force between them changes accordingly. We call the change of the force produced by the external force the additional internal force, or internal force for short.

2. What is metal stress?

When the metal material is deformed, a reaction force of equal size but opposite direction is generated inside to resist the external force, and the concentration of the distributed internal force at one point is called stress.

According to the causes of stress, there are:

(1) Thermal stress

Thermal stress is the stress generated when the object cannot completely freely expand and contract due to external constraints and mutual constraints between internal parts when the temperature changes. Also known as variable temperature stress. Different cooling rates and uneven plastic deformation are called thermal stress.

(2) Phase transformation stress

Phase transformation stress is due to the phase transformation of certain alloys during the cooling process after solidification, resulting in changes in volume and size, or the internal stress generated by the asynchronous transformation of different parts of the workpiece during the thermal process. Different parts at different times due to internal phase transitions.

(3) Shrinkage stress (also called mechanical resistance stress)

Shrinkage stress refers to the tensile stress caused by the resistance of the mold and core when the metal material shrinks. This stress is temporary and generally disappears automatically when the box is opened.

3. Displacement, deformation and strain of metal

(1)Displacement

The position change of a point in the object before and after deformation, the displacement in material mechanics has linear displacement and angular displacement. As shown in the figure below, a concentrated force is applied to the free end of the cantilever beam, and the beam bends and deforms. If we examine the displacement of a certain section, such as the displacement of the free end, it is obvious that the centroid of the section will have a downward displacement, resulting in a linear displacement, and at the same time, the normal direction of the section will also change, that is, the section will rotate, resulting in an angular displacement. displacement.

(2) Deformation

The change of size and shape of an object under the action of external force is called deformation.

(3)Strain

The degree of deformation at a certain point of a component is called strain, and the strain is for a certain "point".

a. Linear strain (measures the degree of change in the size of a point in an object).

b. Angular strain (measures the degree of change in the shape of a point in an object) is also called shear strain or shear strain.

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