Stress whitening in polyethylene

Cherry, B. W.; Hin, Teoh Swee

doi:10.1016/0032-3861(81)90372-4

Cited by 29 publications

(21 citation statements)

References 8 publications

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“…The whitening effect was previously hypothesized to be due to microdamage, 12 similar to stress whitening detected in polymers. 18 In a subsequent experiment on a bone sample from the previously healthy human donor, a whitened region was clearly correlated to microcracks seen in the same area using an SEM, proving the hypothesis right. It should be noted that the cracks shown in Fig.…”

Section: Discussionmentioning

confidence: 76%

High-speed photography of the development of microdamage in trabecular bone during compression

et al. 2006

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The mechanical properties of healthy and diseased bone tissue were extensively studied in mechanical tests. Most of this research was motivated by the immense costs of health care and social impacts due to osteoporosis in post-menopausal women and the aged. Osteoporosis results in bone loss and change of trabecular architecture, causing a decrease in bone strength. To address the problem of assessing local failure behavior of bone, we combined mechanical compression testing of trabecular bone samples with high-speed photography. In this exploratory study, we investigated healthy, osteoarthritic, and osteoporotic human vertebral trabecular bone compressed at high strain rates. Apparent strains were found to transfer into to a broad range of local strains. Strained trabeculae were seen to whiten with increasing strain. Comparison of whitened regions seen in high-speed photography sequences with scanning electron micrographs showed that the observed whitening was due to the formation of microcracks. From the results of a motion energy filter applied to the recorded movies, we saw that the whitened areas are, presumably, also areas of high deformation. In summary, high-speed photography allows the detection of microdamage in real time, leading toward a better understanding of the local processes involved in bone failure.

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Section: Discussionmentioning

confidence: 76%

High-speed photography of the development of microdamage in trabecular bone during compression

et al. 2006

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“…Negative volume variations were observed during creep tests on semi-crystalline polymers [4,13,26]. Cherry and Hin [13] measured volume variation of PE using three mechanical extensometers under creep at ambient temperature.…”

Section: Bibliographic Data On Compaction Amplitudementioning

confidence: 99%

“…Cherry and Hin [13] measured volume variation of PE using three mechanical extensometers under creep at ambient temperature. After the elastic volume expansion, these authors systematically recorded a compaction process that reached ε v = -0.017 for t = 25 000 s under σ 33 = 17.1 MPa.…”

Section: Bibliographic Data On Compaction Amplitudementioning

confidence: 99%

“…Pioneer systems were essentially based on fluid dilatometers [1,3] but their performances were rather limited because they only gave access to global volume changes and were very sensitive to temperature fluctuations. Another family of systems utilized multiaxially disposed mechanical extensometers but: -they were complicated to manipulate; -they caused unwanted indentation at the surface of soft polymers; -they were generally limited to low temperatures [4,[8][9][10]13]. Although the above techniques are still applied by some researchers, the computerized video techniques developed during the last decade brought a decisive contribution to the assessment of volume strain in polymers.…”

Section: Introductionmentioning

confidence: 99%

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Volume Variation Process of High-Density Polyethylene During Tensile and Creep Tests

Addiego¹,

Dahoun²,

G’Sell³

et al. 2006

Oil & Gas Science and Technology - Rev. IFP

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Résumé -Processus de la variation volumique du polyéthylène à haute densité au cours d'essais de traction et de fluage -Des éprouvettes de polyéthylène à haute densité ont été soumises à des essais de traction et des tests de fluage à l'aide d'un dispositif d'essais mécaniques à pilotage vidéo-métrique (VidéoTraction ® ). L'évolution du volume spécifique de ce polymère semi-cristallin est caracté-risée en fonction de la déformation vraie. Dans le stade élastique, on mesure une expansion hydrostatique puis, dans le stade plastique, on note une compétition entre un effet de compaction et un phénomène de dilatation. La compaction, sans doute surestimée dans les expériences, est toutefois un mécanisme important dû à l'orientation des chaînes amorphes au cours de l'étirage. Ce phénomène est attesté par des mesures de diffraction des rayons X qui montrent une réduction de la distance moyenne entre les chaînes amorphes. Le processus de dilatation s'explique par la diminution du taux de cristallinité et par la formation, la croissance et la coalescence de craquelures au sein et entre les sphérolites. L'observation d'échan-tillons déformés par microscopie électronique révèle ces défauts. La compétition compaction/dilatation est dictée par la mobilité de la phase amorphe influencée elle-même par la température et le temps. Abstract -Volume Variation Process of High-Density Polyethylene During Tensile and Creep Tests -Samples of high-density polyethylene have been subjected to tensile tests and creep experiments by means of a video-controlled testing system (VidéoTraction © ). The evolution of specific volume in this semi-crystalline polymer is determined versus true strain. In the elastic stage, we measure a hydrostatic expansion and then, in the plastic stage, we observe a competition between a compaction effect and a dilatation phenomenon. Although compaction is probably overestimated in the present testing technique, it represents a pertinent mechanism that is ascribed to the orientation of the amorphous chains during stretching. This phenomenon is characterized by X-ray diffraction measurements that show a reduction of average distance between amorphous chains. Dilatation process is explained by the diminution of crystallinity and by the formation, growth and coalescence of crazes inside and between spherulites.Electron microscopy reveals these defects. The competition between compaction and dilatation, controlled by the mobility of the amorphous phase, depends on temperature and time.

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“…Hence, the craze-like features in semicrystalline polymers have been addressed by different terminologies in the literature: transverse markings or strain bands, 59 sub-microcracks, 60 Craze-like features have also been observed during the deformation of semicrystalline 'hard' fibres 59,64-66 and highly oriented films. 67,68 It is well known that semicrystalline polymers stress whiten around the yield region when plastically deformed in tension [69][70][71][72][73] phenomenon has been attributed to the formation of microvoids in the interlamellar amorphous regions. In the case of die drawing, it has been shown that the voids are initiated at the die entry owing to high shear stresses 74 as suggested for amorphous polymers 57 and their growth is then aided by the tensile stresses 58 that are predominant at the die exit.…”

Section: Visual Examination and Semmentioning

confidence: 99%

Damage in large scale solid state deformation of thermoplastic polymers at elevated temperatures and varying strain rates

Mohanraj¹,

Barton²,

Ward³

2008

Plastics, Rubber and Composites

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A key factor which limits the production speed of the polymer die drawing process is the premature fracture of the material on exit from the die. In this paper, the growth of damage in the material during the die drawing process has been studied using a combination of thermoplastic finite element analysis and structural characterisation by means of scanning electron microscopy and small angle X-ray scattering for the specific case of die drawing of polyoxymethylene. It is demonstrated that special profiled dies offer a more beneficial strain rate distribution than the conventional conical dies and allow higher production speeds to be obtained. Voids grow in the material as a result of the tensile stresses pertaining near the die exit and then, crazes appear from within the material at a critical stress level leading ultimately to final fracture. The results suggest that although the crazes initiate at a critical stress, the extent of crazing at the maximum draw ratio obtained (y13) is independent of the type of die and hence the stress level. Fracture of the drawn product occurs at different stresses for different die profiles but always at the maximum draw ratio of 13, suggesting that this relates to the limiting extensibility of a molecular network.

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Stress whitening in polyethylene

Cited by 29 publications

References 8 publications

High-speed photography of the development of microdamage in trabecular bone during compression

High-speed photography of the development of microdamage in trabecular bone during compression

Volume Variation Process of High-Density Polyethylene During Tensile and Creep Tests

Damage in large scale solid state deformation of thermoplastic polymers at elevated temperatures and varying strain rates

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