Cyclic viscoelastoplasticity of polypropylene: effects of crystalline structure

Effect of crystalline structure on the mechanical response of polypropylene under cyclic deformation

“…Analysis of observations in cyclic tests with fixed maximum strains ε max demonstrates that the model of [24] is inapplicable for the description of stress-strain diagrams, because maximum plastic strain ε p max remains practically constant under cyclic loadings with mixed programs.…”

“…Treatment of W p as an internal variable distinguishes the present approach from that suggested in [24] , where plastic strain ε p was chosen for this role. Analysis of observations in cyclic tests with fixed maximum strains ε max demonstrates that the model of [24] is inapplicable for the description of stress-strain diagrams, because maximum plastic strain ε p max remains practically constant under cyclic loadings with mixed programs.…”

“…Firstly, coefficients γ and Σ are determined by matching observations in the relaxation tests with strain ε = 0.03 on mPP and iPP (figures 3 and 4 in [24] ). The best-fit adjustable parameters are reported in Table 1 of thermal treatment on the linear viscoelastic response of polyolefins [32,33] .…”

“…In particular, mPP reveals a novel relaxation mechanism (associated with rearrangement of chains in the mesomorphic phase [9] and rupture of small crystallites [23] ), and a unique viscoplastic behavior (attributed to deformation-induced transformation of α crystallites into the mesomorphic form [9,22] , and formation of an oriented mesomorphic phase, due to lamellae fragmentation [10] ). Investigation of the mechanical response under long-term creep [24] and ratcheting [25] shows that the time to failure under conditions of creep rupture and the number of cycles to failure under cyclic deformation, with a stress-controlled program of mPP, substantially exceeds those of iPP. These results are attributed to the presence of small crystalline domains in the amorphous phase of mPP, which reduce plastic flow in the matrix, acting as physical cross-links [6] .…”

“…The objective of this study, was to compare the viscoelastoplastic responses of iPP, non-annealed, and annealed mPP, under multi-cycle loading with a mixed deformation program (oscillations between fixed maximum strains ε max and the zero minimum stress) at room temperature [in our previous work [24] , the effect of annealing was not examined, and the mechanical behavior was analyzed under the stress-controlled program (ratcheting)]. Observations in a series of cyclic tensile tests with various maximum strains ε max in the range of small deformations, show that although the mechanical properties of iPP under tension are superior with respect to mPP (a higher strength and a less pronounced decay in stress with strain in the post-yield region of deformations), the residual strain in iPP (measured when stress vanishes under retraction) exceeds strongly that of mPP, which, in turn, is higher than the residual strain of annealed mPP.…”