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From elasticity to plasticity: the three stages of material deformation are so fascinating!
Understanding the properties and behavior of materials is crucial in engineering and materials science, and this is where stress-strain curves come in. These curves not only reveal how materials respond to different loads, but also help us better predict how they will perform in real-world applications.
The stress-strain curve shows key properties of the material, such as yield strength, ultimate tensile strength and Young's modulus.
The relationship between stress and strain can exist in many forms, so we usually divide these curves into several main stages. Let’s explore the three important stages of material deformation one by one: the linear elastic region, the strain hardening region, and the neck formation region.
Linear elastic region
The linear elastic region is the first stage in which a material deforms. In this stage, stress and strain are linearly related, that is, they obey Hooke's law. Here, the stress increases in direct proportion to the strain increase, and the slope is Young's modulus. This part represents a state of only elastic deformation, and its end marks the beginning of plastic deformation.
When the stress component reaches the yield strength, it means that the plastic deformation state begins.
Strain hardening zone
As the applied stress exceeds the yield point, the material enters a strain hardening region. At this stage, the stress reaches a maximum point, called the ultimate tensile strength. In the strain hardening region, stresses mostly remain elevated as the material stretches.
In some materials (e.g. steel), there is initially an almost flat region due to the formation and extension of Lüders bands.
During this process, as the plastic deformation increases, the number of dislocations inside the material will increase, suppressing the movement of further dislocations. In this case, higher shear stresses need to be applied to overcome the obstacle.
Neck formation zone
When the stress exceeds the ultimate tensile strength, it enters the neck formation region where the local cross-sectional area is significantly reduced. The deformation of the neck is non-uniform and further aggravated under stress concentration, eventually leading to the fracture of the material.
Even though the applied tensile force is decreasing, the actual stress in the material is still increasing because the reduction in local cross-sectional area is not taken into account.
After fracture of the material occurs, its percentage elongation and reduction in cross-sectional area can be calculated. These data are critical for engineering design and material selection.
Classification of materials
Based on the characteristics of the stress-strain curve, we can roughly divide materials into two categories: ductile materials and brittle materials. Ductile materials such as mild steel have good deformation characteristics at normal temperatures, while brittle materials such as glass usually do not exhibit obvious strain process and break directly.
Ductile materials are able to continue to deform after reaching their yield point, whereas brittle materials tend to break without significant deformation.
Toughness and Applications
Materials with excellent toughness can exhibit both strength and ductility, which makes toughness an important criterion in material design. Toughness is the area under the stress-strain curve, which can be thought of as the energy a material can withstand before breaking.
ConclusionIn summary, the three major stages of the stress-strain curve - the linear elastic region, the strain hardening region and the neck formation region - provide a deep understanding of material behavior. In materials science, these theories not only guide laboratory testing, but also affect the reliability and safety of engineering applications. Faced with the performance characteristics of different materials, we have to think: How do the characteristics of these materials affect our daily lives and the development of engineering technology?
