Yield stress

Definition

Yield stress is the minimum amount of stress required to cause a material, such as a polymer, to deform plastically. This concept is crucial in understanding how polymers behave under applied forces and relates to their flow characteristics, where below this stress, a polymer will not flow or deform significantly, while above this threshold, it will start to yield and exhibit permanent deformation.

5 Must Know Facts For Your Next Test

1. Yield stress is an important parameter in determining the processing and handling characteristics of polymers, affecting their flow behavior during manufacturing.
2. Polymers with higher yield stress require more force to initiate flow and are often utilized in applications that demand enhanced strength and durability.
3. The measurement of yield stress can vary based on factors such as temperature, molecular weight, and the presence of additives in the polymer.
4. Understanding yield stress is essential for predicting how polymers will perform under real-world conditions, including impacts and stresses during use.
5. In rheological studies, yield stress helps distinguish between materials that behave as solids and those that behave as liquids under various loading conditions.

Review Questions

• How does yield stress influence the processing of polymers during manufacturing?
  • Yield stress directly affects how polymers flow during processing operations like extrusion or molding. If the yield stress is too high, the polymer may not flow easily, leading to difficulties in shaping and forming it into desired products. Conversely, a lower yield stress allows for easier processing but may compromise the final product's strength and durability. Therefore, understanding yield stress is crucial for optimizing processing conditions and material selection.
• Discuss the relationship between yield stress and plastic deformation in polymers.
  • Yield stress marks the transition point at which a polymer begins to undergo plastic deformation. Below this stress level, the polymer behaves elastically, returning to its original shape once the load is removed. However, once the applied stress exceeds the yield point, the polymer will experience irreversible changes in shape, leading to plastic deformation. This relationship is vital for engineers when designing materials that need to withstand certain loads without permanent changes.
• Evaluate how temperature and molecular weight affect the yield stress of polymers and their implications for real-world applications.
  • Temperature and molecular weight are critical factors that influence the yield stress of polymers. Generally, as temperature increases, yield stress decreases because the polymer chains become more mobile, allowing for easier deformation. Higher molecular weight often leads to increased yield stress due to enhanced entanglement between chains. Understanding these relationships is crucial for applications like automotive parts or consumer goods where materials must perform reliably under varying temperature conditions while maintaining strength.

Related terms

viscosity: A measure of a fluid's resistance to flow, which plays a significant role in determining how a polymer behaves under stress.

plastic deformation: The permanent change in shape or size of a material when subjected to stress beyond its yield point.

rheology: The study of the flow and deformation of materials, particularly focusing on how they respond to applied forces and stresses.