WHY DOES A SPRING BREAK
With this article, we introduce you to our technical portal and we do so by talking about the main issue involved in springs, in other words spring breakage.
Technically, the breakage of a spring occurs when there is an interruption in the continuity of the physical structure of the wire: in short, the wire breaks in two or more parts.
Why does the spring break?
The theory of materials tells us that a material breaks when the breaking load is exceeded in any point of its section. Working backwards, as in the 5Why problem solving technique, we go down one level and discover what the breaking load is: it is a physical quantity typical of each material, which has the dimensions of pressure (it is measured in Mpa , mega Pascal). In fact, the force applied is divided by the area of the section of the material.
We meet this quantity by studying one of the first physical laws formulated at the beginning of the scientific revolution of the seventeenth century, Hooke’s law.
Given σ the stress and ε the elongation, Hooke’s law expresses the direct proportionality bond between σ and ε through the constant E, which represents Young’s modulus.
Hence the relation is σ = E x ε.
This proportionality bond is valid only in the elastic part of the material, that is, when the stress suppression brings the material back to its original size (ε = 0). In the graph this area is the straight one.
Successively the material enters the elastoplastic zone, exceeds the yield strength (Rp0.2, by convention the value obtained by intersecting the graph with a straight line parallel to that of the elastic zone, displaced by 0.2% of elongation), reaches the maximum load (Rm) and shortly after it breaks.
Conventionally, we use Rm as the breaking load, even if the breaking occurs at a lower load, this happens to steel for springs.
So, to go back to our story, if the system of forces applied to the spring leads the material to exceed the value of Rm, there is first a permanent deformation (yield) and then the breaking.
But is this what happens in reality?
The answer is no, springs usually break before reaching the load Rm of the material they are made of. Indeed, they break even when far from reaching the yield strength.
We are going into the field of resistance to fatigue. To understand what are the dynamics that determine the fatigue strength of a spring and what are also the factors that influence it, it is necessary to familiarize with the following concepts to which we will dedicate the next articles:
- Differences between static and dynamic stress
- Wöhler and Goodman diagram
- Materials and their structure (surface defects)
- Shot peening
- Usage environment (aggressive environments and corrosion)
- Interference and risk factors
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