Interferenzoptische Vermessung der Craze-Zone vor der Rißspitze in PMMA unterschiedlichen Molekulargewichtes

Weidmann, G. W.; Döll, W.

doi:10.1007/bf01517034

“…

In recent years there has been an increasing interest in investigating the openings of cracks in materials under load.In particular this interest has concentrated on the value of the critical displacement at the crack tips at fracture (variously designated as 6c, COS, COD, 2Vc) since these displacements, being essentially determined by the plastic deformations in the crack tip region, should provide a fracture criterion to characterize plastic fracture behaviour.In metals a direct measurement of the critical crack opening is seldom practicable and is, therefore, usually determined by an extrapolation of crack openings measured remote from the crack tip.In optically transparent materials such as glassy thermoplastics, on the other hand, it is possible to measure not only the crack opening very close to the crack tip but also the plastic zone (or craze zone) ahead of the crack tip using optical interference microscopy [1][2][3][4]. Essential requirements for the application of this method are that the crack opening and craze zone thickness should both be comparable in order of magnitude to the wavelength of l~ght and that, additionally, for the craze zone, the refractive index of the crazed material should differ significantly from that of the bulk material and the boundary between the two regions should be sharp.

In this work we would like to report results of measurements of the critical displacements of the elastic-plastic boundary directly at the crack tip (here designated as 2v c and marked by an arrow in Fig.

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mentioning

confidence: 99%

“…In optically transparent materials such as glassy thermoplastics, on the other hand, it is possible to measure not only the crack opening very close to the crack tip but also the plastic zone (or craze zone) ahead of the crack tip using optical interference microscopy [1][2][3][4]. Essential requirements for the application of this method are that the crack opening and craze zone thickness should both be comparable in order of magnitude to the wavelength of l~ght and that, additionally, for the craze zone, the refractive index of the crazed material should differ significantly from that of the bulk material and the boundary between the two regions should be sharp.…”

mentioning

confidence: 99%

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Some results of optical interference measurements of critical displacements at the crack tip

Weidmann

¹

,

Döll

²

1978

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In recent years there has been an increasing interest in investigating the openings of cracks in materials under load.In particular this interest has concentrated on the value of the critical displacement at the crack tips at fracture (variously designated as 6c, COS, COD, 2Vc) since these displacements, being essentially determined by the plastic deformations in the crack tip region, should provide a fracture criterion to characterize plastic fracture behaviour.In metals a direct measurement of the critical crack opening is seldom practicable and is, therefore, usually determined by an extrapolation of crack openings measured remote from the crack tip.In optically transparent materials such as glassy thermoplastics, on the other hand, it is possible to measure not only the crack opening very close to the crack tip but also the plastic zone (or craze zone) ahead of the crack tip using optical interference microscopy [1][2][3][4]. Essential requirements for the application of this method are that the crack opening and craze zone thickness should both be comparable in order of magnitude to the wavelength of l~ght and that, additionally, for the craze zone, the refractive index of the crazed material should differ significantly from that of the bulk material and the boundary between the two regions should be sharp.In this work we would like to report results of measurements of the critical displacements of the elastic-plastic boundary directly at the crack tip (here designated as 2v c and marked by an arrow in Fig. 4) and their variation with temperature in some glassy thermoplastics.These measurements were performed on miniature compact tension specimens in an experimental arrangement which permitted the simultaneous recording of the interference fringe pattern and the load acting on the specimen [4]. The materials were PMMA of two different molecular weights (M w ~ 2,200,000 and M w = 120,000) and PVC. Examples of the interference fringe patterns just before the onset of slow crack propagation at room temperature are shown in Fig. i. From the different numbers of fringes in the craze zones it is directly apparent that there are differences in the critical crack tip opening displacements.Based on a quantitative analysis of such fringe patterns (details of which have been given in earlier work [3,411 Fig. 2 shows the critical displacements at the crack tip 2v c in the three materials for temperatures between 0 deg C and 80 deg C. Three quite different patterns of behaviour are apparent: the 2v c values for PVC fall sharply with increasing temperature, those for the lower molecular weight PMMA remain practically constant whilst those for the h~gher molecular weight PMMA show a significant increase.*now at the Open University, Milton Keynes MK7 6ka, UK. Int Journ of Fracture 14 (1978) RI90In contrast to this, the corresponding results for the fracture toughness at the onset of slow crack propagation, Kio , decrease, albeit at different rates, with temperature for all three materials as is shown in Fig. 3. Since, in ge...

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“…For the optical interference investigations compact tension specimens, with characteristic dimensions 8 x 8 x 4 mm 3, have been used. Details on specimen preparation and on evaluation of the photographs and measurement have been reported [10,11].…”

Section: Methodsmentioning

confidence: 99%

Micromechanics of fatigue crack propagation in glassy thermoplastics

Schinker

¹

,

Könczöl

²

,

Döll

³

1984

Colloid & Polymer Sci

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By the aid of the optical interference method the size of the craze zone at the crack tip has been measured during fatigue crack propagation (FCP) in two glassy thermoplastics thus giving a basis to re-examine proposed models. In contrast to previous assumptions it has been found, that in PMMA of high molecular weight crack propagation occurs only during a short interval of the loading cycle when the fibrils are stretched most severely and it is not limited by crack tip blunting; between the dimensions of the craze zone and the crack advance per cycle which is also reflected by markings on the fracture surface no simple correlation has been found. In PVC first the craze grows continuously during many loading cycles up to its final size and then the crack propagates by a jump separating the craze zone only partly. Thus at all stress intensity levels investigated the length of the final craze zone has been found to be distinctly larger than the jump spacing on the fracture surface. By aid of SEM-photography it is shown that in PVC during FCP cracking occurs by separation of fibrils instead of void coalescence.

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“…Bei den mikrostrukturellen Parametern wurde insbesondere der Einflu6 der Struktur der Monomeren (1-5) und des Molekulargewichtes (6)(7)(8) aufgezeigt, w~ihrend bei makroskopischen Parametern die Einfliisse yon Temperatur (9,10) und Spannungsfaktor (6, 10) beriicksichtigt wurden. Betont sei, dat~ die Untersuchungen entweder an stehenden Rissen durchgefi~hrt wurden, um u. a. Aussagen zum Deformationsverhaken verstreckter Molekiilbiindel an Rit~spitzen zu erhalten (11), oder aber haupts~ichlich bei langsamer Bruchausbreitung im Bruchgeschwindigkeitsbereich von ca.…”

Section: Introductionunclassified