What Does Yield Stress Mean?
Yield stress refers to the minimum stress at which a material will deform without significant increase in load. In other words, the minimum stress required to make a material flow is the yield stress, and it is a measure of the strength of the material structure. The stress at which yield occurs is dependent on both the rate of deformation (strain rate) and, more significantly, the temperature at which the deformation occurs.
Some materials can better be described as soft solids than as fluids. Soft solids are excellent examples of materials that exhibit yield stress. This sudden yielding is characteristic of iron and annealed steels. In other metals, deformation begins gradually.
Yield stress fluids are commonly found in many applications, including foods (mayonnaise), cosmetics, hygiene (shaving creams and toothpaste) as well as those common to the drilling industry (mud and cement). It has long been observed that drilling fluids do not flow unless subjected to a certain load (stress); that is, they are yield stress materials.
Corrosionpedia Explains Yield Stress
Yield stress of a solid is essentially the point at which, when increasing the applied stress, the solid first shows liquid-like behavior of continual deformation. If this is the case, then we can say that conversely, when decreasing the applied stress, solid-like behavior is first seen—no continual deformation. There has been an ongoing debate in the literature on whether true yield stress fluids exist, and even whether the concept is useful. This is mainly due to the experimental difficulties in determining the yield stress.
For example, paint must flow under the brush, but remain fixed in a vertical film despite the force of gravity. Consumer products such as toothpaste, concrete, food products and even radioactive nuclear waste sludge exhibit yield stresses. The yield stress may serve to keep particulate fillers from settling, as in many consumer products and gelled propellants, or determine whether bubbles remain trapped in cement. For handling and using these materials, it is paramount to know the stress at which the material starts to flow. An understanding of plastic flow is vital in technology, particularly in the production of large numbers of objects in a variety of complex shapes.
The presence of a significant yield stress will impart various qualities to a fluid that may or may not be desirable. A yield stress will often inhibit flow under the relatively low stresses induced by gravity; giving sag and slump resistance to products such as adhesives, plaster, thick-film inks, molten chocolate, paint and fire-retardant coatings. With some products the presence of a yield stress is not so desirable, leading to, for example, dosing problems in gravity-feed systems or an excess of residue on the sides of inverted bottles.