Yield Point Determination Using Cyclic Loading in Polymer

Defining the yielding point of semicrystalline polymers is a matter to be established in the literature. ASTM D638-14 and ISO 527-1 standards define the yielding point as the point where there is an increase in strain without an increase in stress, which coincides with the beginning of necking of the test samples. The literature has been reevaluating this matter, taking into account the methods used and their respective damage generation in the material. Polymer materials are used in the oil and gas industry, for example, in risers. The understanding of the transition between the elastic and plastic regions is necessary, as well as the understanding of the damage done in both regions. This study is about the effects of cyclic loadings with triangular and sinusoidal loading, with different strain levels and their effect on the mechanical behavior of a fluorinated polymer(Halar ECTFE). The cyclic loading tests were strain-controlled and done with frequencies around 0.1Hz, equivalent to a strain rate of 0.2 and 0.4%/s, and strains up to 2%, with the effects on the transition from the elastic and on the stress relaxation being observed. The results show that up to strains of 0.5% the material has elastic behavior, irrespective of the loading. When the strains are greater than 0.75%, the material shows relaxation on all loadings cycles. Between 0.5% and 0.75%, the triangular loading led to cyclic hardening, while the sinusoidal lead to stress relaxation. The stress relaxation is then related to the damage accumulation on the structure of the material, while the hardening to the chain orientation.