Elogesh:
Let’s take your points one at a time.
1. For some plastics (usually without additives), only tensile strength is specified and compressive strength is not specified. Can, I consider tensile strength is equivalent to compressive strength?
Compressive strength is not an often-used property; therefore it is not usually measured nor reported as a typical property for a material. Do not consider it to be the same as the tensile strength. If you need that specific property for the material, have it measured by a competent laboratory.
2. For some plastics tensile strength at break or ultimate tensile strength is specified without referring to yield strength. Whether these plastics has brittle fracture...
Tensile strength is defined as the highest tensile stress achieved --- sometimes it is the tensile yield strength, for other materials it is the tensile strength at break. If tensile yield strength is not reported, DO NOT ASSUME that the material exhibits brittle fracture (i.e., rupture prior to yield). The material may just exhibit a higher stress at break than at yield.
3. For some plastics ultimate tensile strength values mentioned are usually less compared to yield strength. Can someone explain how this is possible? In metals tensile strength is higher than yield strength.
I am not sure if it’s always the case with metals that the tensile yield strength is higher than the strength at break. However, for some polymers, orientation during tensile testing significantly alters the material’s mechanical behavior as the test proceeds. In metals, I believe this is referred to as “work hardening”. In any event, as the material elongates the polymer molecules orient, in some cases enhancing crystallization, and the polymer becomes stronger and more rigid. The tensile stress will begin to rise with further extension and may exceed the yield stress.
Also, keep in mind that while most ductile polymers “yield”, they may not exhibit the classical “yield point” (the point at which there is a zero slope). Many ductile polymers exhibit “pseudo-yield”, that is they deviate significantly from linear behavior but their stress continues to rise with increased strain.
4. For some plastics, elongation at yield is specified as 3%. Does this means that strain greater than 0.03 will result in permanent set? For the same material elongation at break is specified as 1.9-150%. The lower limit of elongation at break less than corresponding value for break!!!
Actually, permanent set can occur well before the yield strain. Viscoelastic flow occurs as soon as the elastic limit is exceeded, i.e., anywhere after the linear portion of the stress-strain curve. Most of that viscoelastic flow that occurs prior to yield is generally recoverable with time --- but not all of it. That which is not recoverable is permanent set.
The second part of this question I don’t understand. Rather than guess at what you might mean, I’d rather not answer it without further clarification.
5. How flexural modulus is different from tensile modulus and compressive modulus?
Ideally, they should all be the same. The fact of the matter is that we can’t measure the properties of materials in ideal states. Because of the way we have to test materials, not everything can be held constant --- things change --- and these changes, many of which occur during the test, alter the outcome of the test.
A case in point, flexural modulus is generally measured in a 3-point bend test. A rectangular specimen is supported horizontally by two steel pins and the plastic bar is loaded at the midpoint of the two supports.
One limitation with flex test is that it is not a “pure” stressed state. The stress is calculated as the maximum “fiber” stress that occurs directly under the load on the underside surface of the bar --- this is the only point at which that maximum fiber stress exists. Actually, the stress distribution through the bar varies from tensile on the underside surface of the specimen to compressive stress on the topside surface. The compressive stress tends to inhibit the deflection of the specimen, artificially raising the apparent modulus of the material.
Also, as the specimen deflects, the bar must move along the supports to accommodate the deflection. If the calculated fiber strain on the underside surface exceeds about 5%, a significant portion of the load is consumed as the driving force to “push” the specimen through the supports --- rather than merely bend the specimen.
In addition, at some strain level, probably around 5%, the actual strain on the bar at the point of loading stops increasing --- no further curvature occurs in the central area of the specimen --- but the specimen continues bend toward the ends of the bar as it slides through the supports. Inasmuch as the apparent fiber strain is calculated based on the amount of deflection from the original horizontal position, the method begins to yield erroneous data.
The latter two points most likely would not affect the reported modulus, but should be considered in any other data reported for the test.
Rich Geoffroy
Polymer Services Group
POLYSERV@aol.com
