discuss the point propotional limit point and permanent set by stress strain curve
Answers
Answer:
The proportional limit is the maximum stress that a dental material sustains without any deviation, or the magnitude of elastic stress above which plastic deformation occurs. Below the proportional limit, no permanent deformation occurs; and when the stress is removed, the structure returns to its original dimension. Proportional limit (PL) can be easily deciphered in Fig. 17.3:
In preceding figure, as the stress increases, the strain also increases. From O to A, a linear relation is established between stress and strain. As the stress doubles, the strain also doubles. After point A, no linear proportional relation is maintained between stress and strain. Here, the value “A” is identified as the proportional limit. So, the proportional limit is defined as the highest stress at which the stress-strain curve is a straight line. Below the proportional limit, there is no permanent deformation in a structure, that is, the object returns to its original position after the removal of applied force. Thus, the material is elastic in nature below the proportional limit, and the curve before the proportional limit is called the “elastic region,” and above the proportional limit, is called the “plastic limit.” The connectors of plastic dentures should have a high proportional limit. Materials such as cobalt/chromium (alloy) have high proportional limits; thus, they should be used to fabricate connectors, because they can withstand high stresses without permanent deformation.
Hooke’s Law and Stress-strain Curve
By now, we know that the stress and strain take different forms in different situations. In this article, we will understand the relationship between stress and strain by looking at the Hooke’s law and the stress-strain curve.
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Solution to Question
The following figure shows some examples.
Hooke's Law
Hooke’s Law
Hooke’s Law states that for small deformities, the stress and strain are proportional to each other. Thus,
Stress ∝ Strain
Or, Stress = k × Strain … where k is the constant of proportionality and is the Modulus of Elasticity. It is important to note that Hooke’s Law is valid for most materials.
Stress-Strain Curve
To determine the relation between the stress and strain for a given material, let’s conduct an experiment. Take a test cylinder or wire and stretch it by an applied force. Record the fraction change in length (strain) and the applied force needed to cause the strain. Increase the applied force gradually, in steps, and record the readings.
Now, plot a graph between the stress (which is equal in magnitude to the applied force per unit area) and the strain produced. The graph for a typical metal looks as follows:
Hooke's Law
The stress-strain curves can vary with the material in question. With the help of such curves, we can understand how the material deforms with increasing loads.