Understanding Tension and Stress: The Relationship Explained

1. What is tension? What is microtension? What is the unit of deformation?

First, most of the safety monitoring industry measures the deformation of the structure being tested. Too much deformation can cause accidents.

For example, cracks in structures, subsidence and displacements between the structure and a fixed reference, are large deformations that can be seen with the naked eye, and can be measured in millimeters using gauges such as crack gauges, static levels and displacements. Manometers.

But how can the small deformation caused by compression within the structure being tested or by bending outside the beam-shaped object be represented?

The answer is tension.

Suppose the length of a structure with length L undergoes deformation under tension and its length changes to L', then its change in length ΔL = L' – L, and the strain ε is the ratio of the change in length ΔL to the length original L, the formula is as follows:

So what is the unit of deformation?

As can be seen from the formula, deformation is a ratio and is dimensionless, which means it is unitless.

So what is microtension?

Because ΔL is very small, generally in the micron range, the calculated strain value is very small, with many decimal places, making it inconvenient for display and visualization, so the scientific notation 10-6, called με microstrain, is introduced, which It can be understood as the microstrain unit is 10-6, and our strain gauge measurement range is ±1500 microstrains, positive indicating stretching and negative indicating compression.

2. What is stress? What is the relationship between tension and stress? How does strain calculate stress?

Creep is a small deformation within the structure being tested, so why does it deform? Because it is subject to external forces.

Taking a bridge pier as an example, if a fully loaded truck passes over the bridge, the pier will bear additional pressure and produce compression and compression deformation, while the pier will produce an internal force to counteract the external force and overcome the deformation.

This internal force is stress. Stress is defined as the force per unit area, which is actually pressure, with units of MPa.

So, what is the relationship between the variable strain and the change in withstand stress? See the calculation formula:

In the formula, σ represents tension, E is the modulus of elasticity of the material being tested, also known as Young's modulus, which is a physical quantity that describes the elasticity of the material.

It can be seen as the material's ability to resist deformation (rigidity), and from a micro perspective, it is the binding strength between atoms and molecules.

Two materials with the same strain (the same strain value), the material with greater resistance to strain (a higher modulus of elasticity) will withstand greater stress.

For example, tofu and iron block of the same size, if their height is compressed by 1mm, the former only needs to be pressed gently by hand, while the latter needs to be assisted by a tool.

The modulus of elasticity of common engineering materials can be found in tables, such as the modulus of elasticity of C30 concrete is 30000MPa (1N/ ^mm2 = 1MPa), and the modulus of elasticity of carbon steel is 206GPa.

The modulus of elasticity Ec of concrete under compression and tension must be adopted according to Table 4.1.5.

The shear deformation modulus Gc of concrete can be adopted at 40% of the corresponding elastic modulus value.

The Poisson's ratio Vc of concrete can be adopted at 0.2.

Table 4.15 Modulus of elasticity of concrete (×10 ⁴ N/mm ² ).

Observation:

1. When reliable test data is available, the elastic modulus can be determined based on actual measured data;

2. When a large amount of mineral additives is added to concrete, the modulus of elasticity can be determined based on actual data measured according to the specified age.

Table 1.1-13 Modulus of elasticity and Poisson's ratio of commonly used materials

After all, when internal stresses cannot be measured directly, stress can be calculated by measuring the deformation and multiplying it by the modulus of the material.