18.7
Plastic Behavior
A material's elastic behavior is characterized by the disappearance of stress once the load is removed, allowing the material to return to its origina…
Materials under stress due to tensile loading exhibit elastic and linear behavior until the yield point. Removing the load at the yield point, stress and strain decrease linearly. The strain does not return to zero, indicating plastic deformation.
Plastic deformation is influenced by the maximum stress value and the time elapsed before load removal.
Upon re-loading, the specimen follows the previous unloading curve until near the yield point, then aligns with the original stress-strain curve, implying a higher yield point than the initial strain-hardening.
However, the rupture point remains unchanged, suggesting reduced ductility.
If the initial loading is large enough to cause strain-hardening of the material at C', unloading occurs along line C'D'.
If a compressive load is applied, yield strength decreases to a maximum point H' via a curved path, yielding the material along H'J'.
While the compressive stress is less than the yield stress, the total change in stress between the maximum tensile and the compressive stress equals twice the yield stress.
View the full transcript and gain access to JoVE Core videos
Q1: What happens to strain when a load is removed before the yield point?
When a load is removed before the yield point, stress and strain decrease linearly, and the material returns to its original state. This elastic behavior demonstrates that no permanent deformation occurs. The material's elastic limit has not been exceeded, allowing complete recovery of its original dimensions.
Q2: How does plastic deformation differ from elastic deformation?
Elastic deformation is reversible—the material returns to its original shape when the load is removed. Plastic deformation is permanent; the strain does not return to zero after unloading. This permanent set occurs when stress exceeds the yield point, causing irreversible changes in the material's structure.
Q3: What is strain-hardening and how does it affect the yield point?
Strain-hardening occurs when a material is loaded beyond its initial yield point, increasing its proportional and elastic limits. Upon reloading, the specimen follows the previous unloading curve until near the yield point, then aligns with the original stress-strain curve. This indicates a higher yield point than initially observed, though ductility is reduced.
Q4: Why does the rupture point remain unchanged after strain-hardening?
The rupture point represents the material's ultimate tensile strength and remains constant despite strain-hardening. While strain-hardening increases the yield point and elastic limits, it reduces ductility—the material's ability to deform before breaking. This trade-off means the material fails at the same maximum stress level.
Q5: What is the Bauschinger effect and when does it occur?
The Bauschinger effect occurs when a second load is applied in the opposite direction to the first load. This reversal causes yield strength to decrease, and the stress-strain diagram becomes curved without a distinct yield point. The material's response to directional stress changes demonstrates its complex mechanical behavior under alternating loads.
Q6: How does the total stress change relate to yield stress under compressive loading?
When substantial initial loading causes strain-hardening, the total stress change between the highest tensile and maximum compressive stress equals twice the yield stress. While compressive stress remains less than the yield stress, this relationship illustrates the material's symmetric response to significant stress variations in opposite directions.
Q7: What factors influence the amount of plastic deformation in a material?
Plastic deformation is influenced by the maximum stress value applied and the time elapsed before load removal. Materials subjected to higher stresses experience greater permanent set. The duration the material remains under stress also affects plastic behavior, with longer loading periods potentially increasing permanent deformation.