Tensile strength at yield is not always reported. 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.
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.
For some polymers, orientation during tensile testing significantly alters the materialÆs mechanical behavior as the test proceeds. In metals, I believe this is referred to as ôwork hardeningö. 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.
Other materials may exhibit "strain softening"; that is, localized heating may result from the shear generated during the necking process. This phenomenon would cause the apparent tensile strength to decrease after yielding.
Also, keep in mind that while most ductile polymers ôyieldö, they may not exhibit the classical ôyield pointö (the point at which there is a zero slope). Many ductile polymers exhibit ôpseudo-yieldö, that is, they deviate significantly from linear behavior but their stress continues to rise with increased strain.