Spring Steel loading by elastic deformation
Spring steels demonstrate durability, pliability, longevity and toughness, all of which relate to the spring property of reacting to loading by elastic deformation. Spring Steel materials should have the highest tensile strength and a high elastic limit. In addition to high strain properties in the elastic region, there must also be sufficient plasticity. This allows for the manufacture of cold formed applications which will not break when they experience unforeseen overloading.
Elasticity and Plasticity
Elasticity is the tendency of solid objects and materials to return to their original shape after the external forces (load) causing a deformation are removed. An object is elastic when it comes back to its original size and shape when the load is no longer present. Physical reasons for elastic behavior vary among materials and depend on the microscopic structure of the material.
Plastic behavior occurs when stress is larger than the elastic limit. In the plastic region, the object or material does not come back to its original size or shape when stress dissapates but acquires a permanent deformation. Plastic behavior ends at the breaking point.
Variance in expected Yield Strength for a metal is discussed very briefly here.
Stress Strain Diagram Insights
We can graph the relationship between stress and strain on a stress-strain diagram. Each material has its own characteristic strain-stress curve. .In the accompanying diagram we can see that in the region between O and A, the curve is linear. Hence, Hooke’s Law obeys in this region. In the region from A to B, the stress and strain are not proportional. However, if we remove the load, the body returns to its original dimension.
The point B in the curve is the Yield Point or the elastic limit and the corresponding stress is the Yield Strength (Sy) of the material. Once the load is increased further, the stress started exceeding the Yield Strength. This means that the strain increases rapidly even for a small change in the stress.
This is shown in the region from B to D in the curve. If the load is removed at, say a point C be-tween B and D, the body does not regain its original dimension. Hence, even when the stress is ze-ro, the strain is not zero and the deformation is called plastic deformation.
Further, the point D is the ultimate tensile strength (Su) of the material. Hence, if any additional strain is produced beyond this point, a fracture can occur (point E). Note that if
- The ultimate strength and fracture points are close to each other (points D and E), then the material is brittle.
- The ultimate strength and fracture points are far apart (points D and E), then the material is ductile.
Elastic Limit and Elastic Modulus
The two parameters that determine the elasticity of a material are its elastic modulus and its elastic limit. A high elastic modulus is typical for materials that are hard to deform. A low elastic modulus is typical for materials that are easily deformed under a load.
If the stress under a load becomes too high, then when the load is removed, the material no longer comes back to its original shape and size, but relaxes to a different shape and size: The material becomes permanently deformed. The elastic limit is the stress value beyond which the material no longer behaves elastically but becomes permanently deformed.
The performance of an elastic material depends on both its elastic limit and its elastic modulus. For example, all rubbers are characterized by a low elastic modulus and a high elastic limit; hence, it is easy to stretch them and the stretch is noticeably large. Among materials with identical elastic limits, the most elastic is the one with the lowest elastic modulus.
We refer to the Elastic Region as that part of the Stress-Strain Curve where as the load increases from zero, the resulting stress is in direct proportion to strain in the way given by (Figure), but only when stress does not exceed some limiting value.
The linearity limit (or the proportionality limit) is the largest stress value beyond which stress is no longer proportional to strain. Beyond the linearity limit, the relation between stress and strain is no longer linear. When stress becomes larger than the linearity limit but still within the elasticity limit, behavior is still elastic, but the relation between stress and strain becomes nonlinear.
For stresses beyond the elastic limit, a material exhibits plastic behavior. This means the material deforms irreversibly and does not return to its original shape and size, even when the load is removed. When stress is gradually increased beyond the elastic limit, the material undergoes plastic deformation.
Rubber-like materials show an increase in stress with the increasing strain, which means they become more difficult to stretch and, eventually, they reach a fracture point where they break. Ductile materials such as metals show a gradual decrease in stress with the increasing strain, which means they become easier to deform as stress-strain values approach the breaking point