Language
Country/Area
No result found
The curious dance of stress and strain: How do materials deform under pressure?
In engineering and materials science, stress-strain curves are key to understanding material behavior. This curve shows the relationship between stress and strain, which is obtained by gradually applying a load to a sample of test material and measuring its deformation. These curves not only help engineers predict the performance of a material, but can also reveal many important material properties, such as Young's modulus, yield strength and ultimate tensile strength.
Stress-strain curves can reveal the properties of materials at different stages of deformation, making them an important tool that cannot be ignored in the engineering community.
Definition of stress and strain curve
In general, a stress-strain curve represents the relationship between stress and strain in any form of deformation. These relationships may be normal, shear, or a mixture of the two, and may be uniaxial, biaxial, or multiaxial, and may even vary over time. The deformation can be in the form of compression, tension, torsion, rotation, etc.
Future discussions will focus primarily on the relationship between axial normal stress and axial normal strain, which is obtained from tensile tests. In many practical situations, different materials will show different stress-strain curves that reflect the unique behavior of the materials.
Different stages of the stress-strain curve
The stress-strain curve of many materials can be divided into several different stages, each of which displays different behavior. Taking low carbon steel as an example, its stress-strain curve at room temperature shows the following main stages:
Linear elastic region
The first stage is the linear elastic region. In this region, stress is proportional to strain, that is, it follows Hawke's law, and the slope of this region is Young's modulus. Here, the material undergoes only elastic deformation until it reaches the point where plastic deformation begins, and the stress at this point is called the yield strength.
Strain hardening region
The second stage is the strain hardening region. In this region, stresses gradually increase as they exceed the yield point until the so-called ultimate tensile strength is reached. This region is characterized by a stress increase that occurs primarily as the material stretches. Since the material is subject to work hardening at this stage, increasingly greater stresses need to be applied to overcome the internal resistance.
During the strain hardening process, plastic deformation increases the dislocation density inside the material, which will have a significant impact on the subsequent deformation behavior.
Neck area
The third stage is the necking region. When the stress exceeds the ultimate tensile strength, the local cross-section of the material will be significantly reduced, forming a neck. At this point, the deformation is uneven and the pressure is concentrated at the reduced position, leading to faster necking development and eventually fracture. Although the tensile force decreases at this time, work hardening continues and the actual stress continues to rise.
The end of the necking region represents the fracture of the material, and the elongation and cross-sectional reduction after fracture can be calculated for the benefit of the engineering community in designing materials and manufacturing processes.
Classification of materials
Based on the common characteristics shown by the stress-strain curve, we can roughly divide materials into two categories: ductile materials and brittle materials.
Ductile Materials
Ductile materials, such as structural steel and most other metals, exhibit the property of yielding at normal temperatures. The stress-strain curves of such materials usually contain a well-defined yield point and show a range of deformation behavior during the plastic deformation stage. The toughness of a ductile material is often related to the area under its stress-strain curve, which is an indicator of the energy the material absorbs before fracture.
Brittle Materials
Brittle materials, such as cast iron, glass, and some stones, exhibit very different behavior from ductile materials. These materials often have no well-defined yield point, and when fracture occurs, the rate of deformation remains almost unchanged. Its stress-strain curve is usually linear, and no significant plastic deformation occurs during the deformation process.
A characteristic of brittle materials is that they tend to reform into their original shape upon fracture, in contrast to the necking fracture of ductile materials.
Understanding how a material behaves under different pressures is undoubtedly crucial to the design and selection of suitable materials. In applied engineering, we need to conduct in-depth research on the properties of various materials and how they perform in different situations. Have you ever thought about what other potential factors should be considered besides strength when choosing materials?
