Elasticity means that strain is generated little by little after applying a load to a spring or material. However, the strain disappears when the load is released when the load is not too great. This is called elasticity.
Plasticity refers to the fact that when a load is applied to a spring or material and released, it does not return to its original state and retains strain (permanent strain).
Stress is a force generated inside a material that resists deformation or destruction when a load is applied to the object. The resistance generated inside the material is called internal force, and the internal force per unit area is called stress. Compression coil springs and extension coil springs are subjected to torsional stress, and torsion coil springs are subjected to bending stress. Normal stress is represented by the Greek letter σ (sigma), and shear stress is represented by τ (tau).
Strain is the deformation of an object when a load is applied to it. At this time, the ratio of deformation to the shape before deformation is called strain. In addition, strain when a compressive load or tensile load acts is called longitudinal strain, which is represented by the Greek letter ε (epsilon), and shear strain is represented by γ (gamma).
Fig. 1 Simplified diagram of elasticity and plasticity
Does a metal bar (mild steel) stretch?
A coiled spring can be seen to expand and contract. However, does a metal bar expand or contract when a tensile load is applied? It is conceivable that when a metal rod is pulled, if a larger load is applied to it, it will separate into two or more parts at some point. To say the conclusion first, it depends on the type of metal, but all of them are accompanied by elongation to some extent. After that, when a further load is applied, the center area is stretched and the cross-sectional area is reduced while breaking at a certain point.
Fig. 2 Simplified diagram of metal rod fracture At this time, if the stress acting per unit area of the metal rod is σ and the amount of change in the length of the metal rod is strain ε, the stress and strain are proportional, and this proportionality constant E is elastic called a coefficient. This relationship also expresses Hooke's law. The formula is as follows. σ=Eε [N/mm 2 ] or [HPa]
Since this relationship holds, the elastic modulus corresponds to the spring constant. Therefore, the higher the number, the less likely the material will deform. Also, the ratio to the strain when stress is applied in the axial direction of the material is called the longitudinal elastic modulus or Young's modulus.
Also, when a metal rod is pulled until it breaks, the relationship between load and elongation is shown in Fig. 3, and the stress-strain diagram in Fig. 4 shows this in terms of the relationship between stress and strain.
Fig. 3 Relationship diagram of load and elongation
