Yield Stress

Yield stress is a material property that characterizes the point at which a material undergoes a transition from elastic deformation to plastic deformation. In simpler terms, it’s the amount of stress that must be applied to a material before it starts to deform permanently. This concept is particularly important in fields such as materials science, engineering, and rheology  which is the study of how materials deform and flow.

It is typically measured in units of pressure, such as megapascals (MPa) or pounds per square inch (psi). 

When a material is subjected to stress, it initially deforms elastically, meaning it returns to its original shape once the stress is removed. However, when the applied stress exceeds the yield stress, the material starts to deform plastically, meaning it undergoes permanent deformation even after the stress is removed. This transition from elastic to plastic deformation is known as yielding.

Yield stress is commonly represented graphically in stress-strain curves, where stress is plotted against strain. In such curves, the yield stress corresponds to the point where the curve starts to deviate from a straight line, indicating the onset of plastic deformation.

Understanding yield stress is crucial in various engineering applications, such as designing structures and materials that can withstand specific loads without undergoing excessive deformation or failure. Different materials exhibit different yield stress values, and these values can be influenced by factors such as temperature, strain rate, and material microstructure.

In some materials, the yield stress can vary depending on the duration of the applied stress. This phenomenon is known as time-dependent or viscoplastic behavior, and materials exhibiting this behavior are said to have a yield stress that is sensitive to time.

Methods of Measurement

Measuring yield stress accurately is crucial in various industries, including materials science, engineering, and rheology. Several methods are commonly employed to measure yield stress, each with its advantages and limitations.

The most common methods are listed her:

Stress-Strain Testing

In this method, a material specimen is subjected to gradually increasing stress while measuring the corresponding strain. The stress-strain curve generated from the test provides valuable information about the material’s mechanical properties, including yield stress. Yield stress is typically determined as the point where the stress-strain curve deviates from linearity, indicating the onset of plastic deformation. However, this method requires sophisticated equipment, and interpretation of the results can be complex, especially for materials with non-linear stress-strain behavior.

Rheological Testing

Rheology deals with the flow and deformation behavior of materials. Rheological tests, such as shear stress versus shear rate or shear stress versus shear strain measurements, can be performed using rheometers. Yield stress in rheological measurements is often determined by identifying the point where the material begins to flow under applied stress. Common rheological techniques used to measure yield stress include oscillatory shear tests, stress relaxation tests, and creep tests. These tests can provide valuable insights into the viscoelastic properties of materials and their response to applied stress over time.

Compression Testing

Compression tests involve applying a compressive force to a material specimen and measuring its deformation response. For materials with yield stress, the onset of plastic deformation can be observed as a sudden increase in strain or a change in the stress-strain curve’s slope. Compression testing is commonly used for bulk materials and is particularly useful for assessing the yield behavior of materials under compressive loading conditions.

Indentation Testing

In indentation testing, a hard indenter is pressed into the surface of a material, and the resulting indentation depth or impression size is measured. By analyzing the indentation response at various loads, researchers can determine the yield stress of the material. However, indentation testing is more commonly used for assessing hardness rather than yield stress, and interpretation of results can be influenced by factors such as specimen geometry and material properties.

Pressure-Volume Measurement

In some applications, such as in geotechnical engineering or soil mechanics, yield stress can be determined indirectly by measuring the pressure-volume relationship of a material under confined conditions. By subjecting the material to increasing pressure while monitoring volume changes, researchers can identify the point at which the material undergoes significant deformation, indicating the onset of yielding.

Each method has its strengths and limitations, and the choice of method depends on factors such as the type of material being tested, the desired level of accuracy, and the available equipment and expertise. Multiple methods may be employed in combination to obtain a comprehensive understanding of a material’s yield stress behavior.

Just to note there is a difference with yield value. Yield value, on the other hand, is a term commonly used in rheology, which is the study of how materials flow and deform. In rheology, yield value represents the minimum stress or force required to initiate flow in a material. It is similar to yield stress in that it indicates a transition from a solid-like behavior to a more fluid-like behavior, but it specifically applies to materials that exhibit a yield stress in their flow behavior.

Overall, yield stress plays a significant role in determining the mechanical behavior and performance of materials in various applications, making it an essential parameter to consider in engineering and material science analyses.