Technology definitions

Chinese name:

stress

English name:

Stress

definition:

The degree of internal force on the cross-section of a stressed object is the internal force per unit area.

Subject:

Hydraulic Science and Technology (Grade 1); Engineering Mechanics, Engineering Structure, Building Materials (Grade 2); Engineering Mechanics (Water Conservancy) (Grade 3)

This content was published by the National Science and Technology Terms Examination and Approval Committee

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stress

When the material cannot be displaced under external forces, its geometry and dimensions will change. This deformation is called Strain. When the material is deformed, an equal and opposite reaction force is generated inside to resist the external force. The degree of distribution of internal forces at a point is called stress, and the product of stress and micro area is the micro-internal force. When an object or an object is deformed due to an external cause (such as stress, humidity change, etc.), an internal force interacting between each part of the object is generated to resist the effect of the external cause, and the object is intended to be restored from the position after the deformation to the position before deformation. s position.

table of Contents

Definition of stress

Classification of stress

Line strain

Cut strain

A little bit of strain

Measuring tools

Edit the definition of stress in this paragraph

Stress is defined as "additional internal force per unit area." The formula is marked as

Where σ is the stress; ΔFj is the force in the j direction; ΔAi is the force area in the i direction.

Because the area and force are both vectors, if the force area is the same as the direction of force, it is called normal stress, as shown in Fig. 1 σx and σy; if the force area and the direction of force are mutually orthogonal, the shear stress is called shear stress. ), as shown in Figure 1 τxy and τyx.

"Inner stress [1]" refers to the different types of materials that make up a single structure, resulting in different types of deformation due to material differences, followed by various stresses.

When the material cannot be displaced under external forces, its geometry and dimensions will change. This deformation is called Strain. When the material is deformed, an equal and opposite reaction force is generated inside to resist the external force. The degree of distribution of internal forces at a point is called stress, and the product of stress and micro area is the

Positive stress and shear stress

Internal force. When an object or an object is deformed due to an external cause (such as stress, humidity change, etc.), an internal force interacting between each part of the object is generated to resist the effect of the external cause, and the object is intended to be restored from the position after the deformation to the position before deformation. s position. The internal force per unit area at a point in the cross-section surveyed is called stress. According to the relationship between stress and strain, stress can be divided into normal stress σ and shear stress τ. The direction of normal stress is parallel to the direction of strain, and the direction of shear stress is perpendicular to strain. According to the different forms of load, the stress can be divided into tensile compression stress, bending stress and torsional stress.

Edit this paragraph stress classification

Positive stress and shear stress

The perpendicular to the same section is called normal stress or normal stress, and the one that is tangential to the section is called shear stress or shear stress. Stress increases with the increase of external forces. For a certain material, the increase in stress is limited. Above this limit, the material must be destroyed. For a material, this limit that stress can reach is called the ultimate stress of that material. The ultimate stress value is determined by the mechanical test of the material. The limit stress determined is appropriately reduced to specify the maximum stress that the material can work safely. This is the allowable stress. If the material is to be used safely, the stress within it should be lower than its ultimate stress, otherwise the material will be destroyed in use.

When some materials work, their external forces do not change with time. At this time, the internal stress is constant, which is called static stress; and there are some materials whose external forces are periodically changed with time. The internal stress also periodically changes with time, called alternating stress. The failure of materials under the action of alternating stress is called fatigue failure. In general, damage can occur when the material undergoes an alternating stress that is far less than its strength limit under static load. In addition, the material will cause a local increase in stress due to changes in the cross-sectional dimensions. This phenomenon is called stress concentration. For a brittle material with uniform microstructure, stress concentration will greatly reduce the strength of the component, which should be especially noted in the design of the component.

When objects are deformed by force, the degree of deformation at various points in the body is generally not the same. The mechanical quantity used to describe the degree of deformation at one point is the strain at this point. For this purpose, a unit body can be found at this point, and the changes in size and shape of the unit body before and after the deformation can be compared.

Unit: Pa, Psi

Edit this paragraph line strain

When the element body taken in Cartesian coordinates is a regular hexahedron, the ratio of the length of the three mutually perpendicular edges before and after the deformation to the original length is defined as the line strain, which is denoted by ε. The line strains in the x, y, and z directions are εx, εy, and εz, respectively. The line strain is positive with elongation and shortened with negative.

Edit this paragraph cut strain

The two perpendicular edges of the unit body, after the deformation of the right-angle change, defined as the angular strain or shear strain, denoted by γ. One point is the shear strain in the x-y direction and the z-x direction in the y-z direction, which is divided into γxy, γyz, and γzx. The shear strain is reduced to positive at right angles, negative to negative.

Edit the strain status of this paragraph

When the strain components εx, εy, εz, γxy, γyz, γzx of a point are known, the line strain in any direction at this point and the amount of right angle change between any two line segments passing through the point can be transformed according to the coordinate of the strain component. Find the formula. The strain state at this point is also determined.

The strain tensor composed of strain components εx, εy, εz, γxy, γyx, γyzγzy, γzx, γxz at one strain state

In the formula, the shear strain in the right tensor is expressed by εxy, εxz,---, which is applicable to the notation of the conjugated label with tensor.

The shear strain in the left tensor is expressed by γxy, γxz, and ---, and is the engineering habit representation.

The two concepts are the same and their sizes are doubled. The strain tensor is also a second-order symmetry, in which the shear strain component εxy=εyx

Edit this paragraph measurement tool

A strain gauge or strain gauge is an instrument that measures the internal stress of an object. Generally, the signals of the strain gauges are collected and converted into electrical signals for analysis and measurement.

In this method, the strain gauges are attached to the object to be measured so that they expand and contract along with the strain of the object to be measured, so that the inner metal foil material is elongated or shortened along with the strain. Many metals change in resistance as they mechanically elongate or shorten. Strain gages use this principle and measure change by measuring the change in resistance. The strain gages of general strain gages use copper-chromium alloys, and their rate of change of resistance is a constant, which should become a positive proportional relationship.

With a Wheatstone bridge, the proportional relationship of this resistor can be converted to voltage. Different instruments can then convert this change in voltage into measurable data.

For the stress gauge or strain gauge, the key indicators are: test accuracy, sampling speed, number of channels that the test can support, dynamic range, and supported strain gage types. Moreover, the software that the stress meter is equipped with is also very important. It needs real-time display, real-time analysis, real-time recording, and other functions. The high-end software also has various signal processing capabilities.

In addition, some instruments are designed using the principles of spectroscopy and diaphragms.