ABSTRACT Notched specimens of polyethylene (PE) have been subjected to constant and cyclic loadin... more ABSTRACT Notched specimens of polyethylene (PE) have been subjected to constant and cyclic loading at 80°C, and their microdeformation behaviour investigated by OM, SEM and TEM. Under constant loading, a transition from full ligament yielding to slow crack growth (SCG) was observed as the stress intensity factor, K, decreased, reflected by a macroscopic ductile-brittle transition with decreasing applied load. SCG was characterized by formation of a wedgeshaped crack tip deformation zone, whose internal structure became progressively finer as K decreased further. The behaviour of relatively SCG resistant third and second generation grades of PE in the low K limit was inferred from TEM of specimens subjected to accelerated testing in IgepalTM to be breakdown of diffuse zones of interlamellar voiding rather than development of a mature fibrillar structure. This latter failure mode gave smooth facture surfaces, similar to those observed under conditions, which have been linked to a transition from discontinuous (stick-slip) crack growth to continuous crack growth with decreasing peak K. TEM again indicated the smooth fracture surfaces obtained under low level cyclic loading conditions to be associated with breakdown of regions of interlamellar voiding, suggesting the micromechanisms of failure to be similar in both types of accelerated test. On the other hand, at higher K, crack advance under dynamic loading led to more extensive fibrillar retraction than for static loading.
International Journal of Solids and Structures, 2001
An instrumented falling weight impact test is thoroughly investigated in two cases, that of a reb... more An instrumented falling weight impact test is thoroughly investigated in two cases, that of a rebound and that of a rupture in the sample. First, the force signal is analyzed with four methods: the continuous wavelet transform (with dierent wavelets: derivatives of a Gaussian and Morlet's), the Gabor transform, the Wigner±Ville transform and the classical Fourier analysis. It is shown how a proper use of the time±frequency methods allows to precisely detect the discontinuities in the signal, the moment of rupture, if any, and the frequencies that are excited at the impact. In a second part, modal and stress analyses are carried out, both by an analytical method and by ®nite element computations. The results con®rm those obtained in the ®rst part and agree with experimental ®ndings. Ó
Circumferentially notched specimens of a ®rst generation and a third generation pipe-grade of hig... more Circumferentially notched specimens of a ®rst generation and a third generation pipe-grade of high density polyethylene with similar weight average molar masses have been subjected to constant tensile loads at 808C. A transition from full ligament yielding to failure by stable sub-critical crack growth was observed as the applied load was decreased. The specimen lifetimes in this latter regime were dependent on the initial stress intensity factor, K i , and failure was associated with slow crack propagation preceded by formation of a wedge-shaped cavitational deformation zone at the notch tip. The ®bril diameters in the deformation zones decreased with stress intensity factor near the transition, the limiting behaviour of a relatively slow crack growth resistant third generation grade at the lowest K i being inferred from testing in Igepale to be the breakdown of diffuse zones of interlamellar voiding. This regime was not directly accessible to testing in air within the allotted experimental times. However, comparison with the results of accelerated testing in cyclic fatigue has indicated stable interlamellar voiding in the third generation grade not to necessitate the presence of Igepal. Moreover, in both grades, very similar modes of deformation were observed in air and in Igepal at relatively high K i . Igepal was therefore inferred not to lead to qualitative changes in the range of mechanisms that are characteristic of slow crack growth in polyethylene. q
Journal of the Mechanics and Physics of Solids, 2002
A multi-scale constitutive model for the small deformations of semi-crystalline polymers such as ... more A multi-scale constitutive model for the small deformations of semi-crystalline polymers such as high density Polyethylene is presented. Each macroscopic material point is supposed to be the center of a representative volume element which is an aggregate of randomly oriented composite inclusions. Each inclusion consists of a stack of parallel crystalline lamellae with their adjacent amorphous layers.
ABSTRACT Notched specimens of polyethylene (PE) have been subjected to constant and cyclic loadin... more ABSTRACT Notched specimens of polyethylene (PE) have been subjected to constant and cyclic loading at 80°C, and their microdeformation behaviour investigated by OM, SEM and TEM. Under constant loading, a transition from full ligament yielding to slow crack growth (SCG) was observed as the stress intensity factor, K, decreased, reflected by a macroscopic ductile-brittle transition with decreasing applied load. SCG was characterized by formation of a wedgeshaped crack tip deformation zone, whose internal structure became progressively finer as K decreased further. The behaviour of relatively SCG resistant third and second generation grades of PE in the low K limit was inferred from TEM of specimens subjected to accelerated testing in IgepalTM to be breakdown of diffuse zones of interlamellar voiding rather than development of a mature fibrillar structure. This latter failure mode gave smooth facture surfaces, similar to those observed under conditions, which have been linked to a transition from discontinuous (stick-slip) crack growth to continuous crack growth with decreasing peak K. TEM again indicated the smooth fracture surfaces obtained under low level cyclic loading conditions to be associated with breakdown of regions of interlamellar voiding, suggesting the micromechanisms of failure to be similar in both types of accelerated test. On the other hand, at higher K, crack advance under dynamic loading led to more extensive fibrillar retraction than for static loading.
International Journal of Solids and Structures, 2001
An instrumented falling weight impact test is thoroughly investigated in two cases, that of a reb... more An instrumented falling weight impact test is thoroughly investigated in two cases, that of a rebound and that of a rupture in the sample. First, the force signal is analyzed with four methods: the continuous wavelet transform (with dierent wavelets: derivatives of a Gaussian and Morlet's), the Gabor transform, the Wigner±Ville transform and the classical Fourier analysis. It is shown how a proper use of the time±frequency methods allows to precisely detect the discontinuities in the signal, the moment of rupture, if any, and the frequencies that are excited at the impact. In a second part, modal and stress analyses are carried out, both by an analytical method and by ®nite element computations. The results con®rm those obtained in the ®rst part and agree with experimental ®ndings. Ó
Circumferentially notched specimens of a ®rst generation and a third generation pipe-grade of hig... more Circumferentially notched specimens of a ®rst generation and a third generation pipe-grade of high density polyethylene with similar weight average molar masses have been subjected to constant tensile loads at 808C. A transition from full ligament yielding to failure by stable sub-critical crack growth was observed as the applied load was decreased. The specimen lifetimes in this latter regime were dependent on the initial stress intensity factor, K i , and failure was associated with slow crack propagation preceded by formation of a wedge-shaped cavitational deformation zone at the notch tip. The ®bril diameters in the deformation zones decreased with stress intensity factor near the transition, the limiting behaviour of a relatively slow crack growth resistant third generation grade at the lowest K i being inferred from testing in Igepale to be the breakdown of diffuse zones of interlamellar voiding. This regime was not directly accessible to testing in air within the allotted experimental times. However, comparison with the results of accelerated testing in cyclic fatigue has indicated stable interlamellar voiding in the third generation grade not to necessitate the presence of Igepal. Moreover, in both grades, very similar modes of deformation were observed in air and in Igepal at relatively high K i . Igepal was therefore inferred not to lead to qualitative changes in the range of mechanisms that are characteristic of slow crack growth in polyethylene. q
Journal of the Mechanics and Physics of Solids, 2002
A multi-scale constitutive model for the small deformations of semi-crystalline polymers such as ... more A multi-scale constitutive model for the small deformations of semi-crystalline polymers such as high density Polyethylene is presented. Each macroscopic material point is supposed to be the center of a representative volume element which is an aggregate of randomly oriented composite inclusions. Each inclusion consists of a stack of parallel crystalline lamellae with their adjacent amorphous layers.
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