Yield value and yield stress are both physical quantities used to describe the deformation characteristics of substances under the action of force, but they have different meanings and application scenarios.
Yield value refers to the minimum shear force at which a substance begins to undergo reversible deformation after being subjected to a certain shear force. Simply put, yield value is a parameter describing the rheological properties of a substance, which is used to characterize the resistance and deformation ability of a substance during the shearing process. In fluid mechanics, yield value is usually used to describe the rheological properties of non-Newtonian fluids, such as slurries, colloids and pastes. When these substances are subjected to shear force, their viscosity gradually decreases with the increase of shear rate, so it is necessary to introduce the parameter of yield value to describe their special rheological behavior.
Yield stress refers to the minimum stress at which a material begins to deform plastically after being subjected to a certain tensile or compressive force. Simply put, yield stress is a parameter that describes the plastic deformation characteristics of materials and is used to characterize the strength and toughness of materials. In material mechanics, yield stress is usually used to describe the mechanical properties of metals, plastics and other materials, such as elastic modulus, yield point, ultimate strength, etc. Yield stress is the critical point at which a material begins to deform plastically after being subjected to a certain force. After this critical point, the material will undergo permanent deformation.
In a word, yield value and yield stress are two physical quantities that describe the deformation characteristics of substances under the action of force, but their application scenarios and physical meanings are different. Yield value is mainly used to describe the rheological properties of non-Newtonian fluids, while yield stress is used to describe the strength and plastic deformation properties of materials.
In material mechanics, tensile test or compression test is usually used to measure the yield stress of a material. Tensile test is to place the material in a sample fixture and stretch it under the tensile force of the upper and lower clamps. The stress-strain curve of the material during the tensile process is measured, and the yield stress of the material is determined by analyzing the slope or inflection point of the curve. Compression test is to place the material in the sample fixture and compress it under the pressure of the upper and lower clamps. Measure the stress-strain curve of the material during the compression process, and determine the yield stress of the material by analyzing the slope or inflection point of the curve.
In fluid mechanics, a Rotational Viscometer or shear stress meter is usually used to measure the yield value of non-Newtonian fluids. A Rotational Viscometer is an instrument that measures the viscosity of a sample by rotating a disk or cone inside the viscometer to subject the sample to shear force. A shear stress meter is an instrument that measures the shear strain of a sample by applying a constant shear stress. When measuring the yield value of a non-Newtonian fluid, the rotational speed or shear stress is usually gradually increased until the sample begins to undergo reversible deformation. At this time, the rotational speed or shear stress is the yield value of the sample.
The deformation behavior of different materials or fluids under the action of force may be complex and diverse, so for some materials or fluids, the yield value and yield stress cannot fully describe their characteristics. In addition, different test methods and test conditions may also affect the measurement results, so it is necessary to choose the test method carefully in practical applications, and carry out data analytics and interpretation in combination with specific experimental conditions and actual situations.