In materials and metallurgy, a stress-strain curve in a material provides the mathematical relationship between strain and stress. It's obtained by slowly applying load to an instrument, taking readings of the deformation and calculating the stress. This process is called load cell application.
The key to interpreting a stress-strain diagram is to know what the curve means and what it depicts. A stress-strain curve can have many different shapes, but it always follows a decreasing slope or U-shape. The most widely used shapes are the L-shaped curve (also called the isotonic curve) and the curved line. It's very important that you learn and understand these two important shapes.
The stress-strain curve shows the relationship between the applied force and the deformed point. This can be thought of as the yield curve or strain yield curve of the particular material being tested. The yield curve is a measure of the energy change due to porosity, where the energy change is graphed as a function of time and the distance from the breaking point. The stress-strain curve shows the variation in this yield curve as a function of applied force. The stress at the break point is typically much greater than the yield curve, indicating that the material has exceeded its elastic limit. If it has not, then the material is considered to have a limited elastic limit.
Learning how to interpret a stress-strain curve is an important part of understanding a mechanical design. There are several different types of diagrams that are commonly used in metallurgical design and analysis. You will need to learn how to interpret a mechanical design and analyze the stresses and strain curve. Understanding how to interpret a stress-strain diagram is critical to understand a testing design and the stresses that are encountered during manufacturing. There are two basic types of diagrams that are commonly used in the mechanical design.
The first type of diagram is called the tensile stress strain graph. This shows the applied load on the material being tested in compressive loads (like welding) or in compressive tension (like pulling a spring). It shows the stress as a function of time and the distance to the nearest failure point. The tensile stress graph is usually more reliable than a strain Gage because of the proximity of failure points.
The second type of diagram is called the dynamic stress strain curve. This shows the applied load, applied bending loads, and the breaking stresses and strains at the breaking point. It is normally graphed in the x-y plane with the x axis pointing along the x-axis.
As you study how to interpret a stress-strain curve, you will come across various other graphical examples. The parabolic curve shows the bending stresses as a parabola, and the elliptical graph depicts the bending stresses as a circle. Finally, there is the proportional limit diagram, which depicts the applied load divided by the critical stress level. You need to be careful with these different examples because they all illustrate different aspects of a complex mechanical problem.
Understanding the stress-strain curve is the first step toward understanding the operation of a mechanical design. The purpose of the curve is to show the mechanical properties of an object as it experiences changes over time. Understanding what happens as an object changes through time is essential to designing better machines for future use. You can get a more detailed explanation of the curves in a future article, but in the mean time, make sure you understand the basic diagrams so you can practice drawing them on your own. When you learn how to draw a stress-strain curve in your own drawings, you will have the knowledge to draw similar diagrams in future articles and test them on your machine.