Can tensile strength be the same as yield strength? Can Flexural Yield Strength be related to the yield strength of a material in any way?

  • $\begingroup$ What question do you have after reading the various definitions of those terms? Why do you think that they can or can not be the same? $\endgroup$
    – hazzey
    Jan 11 '17 at 19:09

Strength can mean different things in different contexts and technical definitions can vary significantly from what is commonly understood by the word 'strength'.

For structures where actual forces are more relevant you might talk about rated loads, safe working loads or design loads in conjunction with factors of safety but this usually needs to be qualified by specific details of how the load is applied and is much less generally applicable than stress.

In terms of yield it is generally more precise to use the term yield stress. This is a numerical value which applies to a particular material. For example Ultimate Tensile Stress is a common indicator of the resistance of a material to failure. For many ductile materials eg steel UTS is somewhat higher than yield stress, usually by some concistant factor for a given material. For example metric bolt grades are defined by UTS and yield stress as a factor of UTS eg for an 8.8 bolt UTS is 800 MPa and yield stress is 80% of that.

Also crucial is how failure is defined in a specific context and there are quite a few possible modes of failure.

  • Yield : the point at which plastic deformation begins to occur. The material remains deformed after the load is removed but may still be capable of supporting a significant load.
  • Fracture : the material 'breaks' and is no longer able to support any meaningful load.
  • Excess strain : the material deflects elastically to the point where it is no longer able to carry out its intended function. This does not necessarily imply permanent damage or deformation to the material.

Note also that we tend to use different terms for material properties and structural properties, ie rigidity for a material vs stiffness for a structure.

When discussing flexing it is important to maintain a clear distinction between stiffness/rigidity and strength. Where stiffness is the degree to which a material deforms under a certain load expressed as a ratio (strain) and strength is the load it can resist per unit area without failing.

Equally the stresses experienced by a part are as much a function of its geometry as its material properties.

For homogeneous materials (including most engineeriong metal alloys) yield stress is a pretty generally applicable figure, what is more important is the model used to determine stresses as even a simple applied load can result in complex internal induced stresses.

However for certain other materials such as many natural materials, textiles and composites the situation becomes rather more complex as both strength and stiffness can be directional.

Also nominal values for stress are generally based on static loads and apparent material properties can change significantly with strain rate or loads applied over long periods of time. Some materials may also be subject to fatigue failure under cyclic loads well below the static yield stress.

  • $\begingroup$ The material in question is a plastic, EVOH, and I have found a data sheet for it that gives the tensile strength at yield. I suspect that this is the same as just saying that this is the yield strength (or yield stress) given the context that yield is the point that plastic deformation begins as you have said. Am I correct in this conclusion? $\endgroup$ Jan 11 '17 at 21:52
  • $\begingroup$ Yes tensile strength at yield it the usual definition of yield stress. Should be quoted in MPa or a dimensional equivalent unit. $\endgroup$ Jan 11 '17 at 22:13

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