Plastic deformation is central to the behavior and design of extension springs. It describes how a spring changes under stress or different conditions. Think about a spring in a garage door opener that repeatedly experiences stress; the design that accounts for plastic deformation can prevent early failure and extra replacement costs. Studying plastic deformation exposes the link between the force exerted and the spring's material properties. By grasping this connection, engineers can create better and safer springs.

Plastic Deformation vs Elastic Deformation

Deformation in extension springs can be categorized as either plastic or elastic. Understanding the distinction between these two types is necessary for effective spring design. Elastic deformation results in temporary changes to the shape of an extension spring. These changes reverse when the applied force is removed, leaving the spring's material properties unchanged. Put simply, the spring returns to its initial shape because of the balance between internal and external forces.

In contrast, plastic deformation leads to enduring shape changes that persist even after the removal of force. This unalterable transformation transpires when the applied force exceeds the yield strength of the material, causing permanent changes in the material structure. For extension springs, such deformation should be avoided, as it inhibits the spring's function. This is due to the spring being unable to restore its initial shape or size when it's stretched beyond its limit, limiting its ability to store energy. A practical example can be seen in a safety pin; if a safety pin is extended too far, it will not be able to close due to the effects of plastic deformation.

Whether elastic or plastic deformation takes place depends on the properties of the material and the quantity and duration of the applied force. Steel, which is a commonly used material for extension springs, remains elastic until it reaches its yield point. Beyond this, the steel undergoes plastic deformation. Therefore, engineers must carefully choose material and design parameters to ensure the desirable performance and useful life of an extension spring.

Why Plastic Deformation Occurs

Plastic deformation in extension springs occurs when the load applied surpasses the elastic limit of the spring, resulting in a change in its original form and a decrease in its energy storage capacity. In such cases, the function of the spring is compromised.

Plastic deformation is influenced not solely by the load, but also by various factors such as the material of the spring, temperature changes, different manufacturing techniques (including cold winding or heat treatment), and the speed at which force is applied. In an instance, extension springs made of stainless steel at room temperature have presented better resistance to plastic deformation compared to springs composed of other materials. Furthermore, particular manufacturing methods, like correct cooling procedures, can improve a spring's resistance against plastic deformation.

The tendency of a spring to deform is a variable characteristic. It can be affected by its surroundings and its operational conditions. This implies that a spring that withstands deformation in one set of conditions may not do so in other conditions. For this reason, these variables should be taken into account during the design phase. To reduce the probability of plastic deformation and to ensure reliable and secure function of extension springs, these factors should be considered by engineers in the design and selection processes.

Proportionality Limit vs Elastic Limit

The proportionality limit and the elastic limit are traits of extension springs that are relevant to plastic deformation. The proportionality limit denotes the peak stress a material can withstand while adhering to Hooke's law, which establishes a direct, predictable connection between stress and strain in springs. When a spring exceeds its proportionality limit, the stress-strain relationship becomes nonlinear.

For instance, consider an ordinary garage door extension spring. If the force on the spring remains within its proportionality limit, the spring stretches and contracts in accordance with the force in line with Hooke's law. However, if the load surpasses this limit, the correlation between the stress and strain diverges, leading the spring to operate in a nonlinear fashion, which could result in unexpected movements of the door.

The elastic limit, inversely, is the point at which a material starts experiencing unchangeable or 'plastic' deformation. When the stress exceeds this limit, the spring does not return to its original shape even when the force is withdrawn, leaving the deformation in the spring.

If we consider the garage door extension spring again, if the stresses pass the spring's elastic limit, the deformation will not mitigate even after the force is removed, potentially causing the door to operate incorrectly, which may necessitate a spring replacement.

The ability to distinguish between the proportionality limit and the elastic limit has practical significance. They function collectively in determining if a spring will undergo elastic or plastic deformation under varying stress levels. Therefore, knowledge of these limits aids in spring selection for engineering projects.


The principle of plastic deformation is vital in the process of designing and selecting extension springs. This consistent alteration within a spring, happening when forces exceed the elastic limit, signifies a reduction in the spring's functionality. Recognizing the boundaries of the proportionality limit and the elastic limit allows engineers to identify where this deformation originates and how to prevent such issues. This knowledge is crucial when creating extension springs to adequately withstand plastic deformation. Incorporating this understanding into your design process will contribute to better performing and longer lasting extension springs.