Degradation of polycarbonate sheeting on outdoor exposure. Relationship between changes in molecular weight and tensile properties

To obtain better understanding of surface microcracking of polycarbonate (PC) glazing during outdoor exposure, and to determine the effect of weathering on its fracture resistance, changes in molecular weight and tensile properties during outdoor and various artificial weathering were studied. During outdoor weathering, the ultimate elongation and ultimate tensile stress decreased showing that PC became less ductile and more susceptible to fracture. The occurrence of surface microcracking of PC is attributed to a combination of a decrease in ultimate tensile stress and ductility of surface layer due to UV radiation, and alternating stresses due, primarily, to temperature changes.     

The effect of a residual stress field on fracture initiation in RPV steel

Whether flaws in structures containing residual (secondary) stresses will extend under particular operational (primary) loads depends on the extent to which the residual stress field affects: (a) the nature and distribution of initiators; (b) the combined (primary + secondary) stresses and strains experienced by potential initiators. This paper compares fractographic data from specimens loaded by only a primary stress with data from specimens also containing a tensile residual stress field. Three‐dimensional elastic–plastic finite element calculations are used to characterize the stress–strain conditions at the initiation sites at the onset of brittle fracture. The introduction of a residual stress changes the dominant stage in fracture nucleation from microcrack extension to particle cracking. This offsets some of the decrease in fracture toughness expected when the residual stress field increases specimen constraint.     

Photo-oxidation of polystyrene under load

The effect of a tensile stress on the rate of photo-oxidation of polystyrene was investigated. Molecular-weight measurements showed that tensile stress accelerates molecular scission in injection-moulded bars exposed to ultraviolet irradiation. Changes in the residual-stress distribution were observed during the exposure but they were not sufficient to reverse the sense of the residual stresses, and the residual stress near the surface remained compressive. Depth profiling indicated that the degradation process was oxygen-diffusion limited and an approximate analysis of the kinetics gave an activation energy for degradation of 4.2 kJm^–2, which is also consistent with a diffusion process.     

Vermessung von 3dimensionalen Eigenspannungszuständen an festgewalzten hochfesten Stäben

Measurement of three-dimensional residual stresses at rolled bars with high hardness Bars out of 56 Ni Cr Mo V 7, a diameter of 63 mm and a length of 100 mm were rolled with a force of 12 kN and 27 kN. The hardness of the bars was 2150 N/mm². In 8 different direction along the surface the residual stresses were determined with the help of x-ray diffraction. This was done several times after electrolytic reduction of the material at the measuring zone to get a stress profile in dependence of the depth. Afterwards in two dimensions along the surface and perpendicular to the surface the residual stresses inclusive the shear stresses were determined. High compressive residual stresses were along the surface, but therefore tensile residual stresses were perpendicular to the surface at a zone of around 1.5 mm depth. A comparison of the residual stresses with stresses induced by shot peening is done.     

嵌段共聚物在共混体系中的增容作用研究II.PS-b-PMMA对PVC/SBS共混体系相态结构的影响

合成了一系列分子量和单体组成比不同的聚苯乙烯 -聚甲基丙烯酸甲酯两嵌段共聚物 ( PS- b- PMMA)作为PVC/SBS共混体系的增容剂。系统研究了 PVC/SBS的组成比、增容剂的用量以及增容剂的分子结构等因素与共混体系相态结构之间的关系。结果表明 ,增容剂的不同结构对共混体系的相态结构影响很大 ,在所合成的 4种增容剂中 ,分子量相对较小 ,分子内聚甲基丙烯酸甲酯嵌段的分子量与聚苯乙烯嵌段的分子量比较接近的增容剂分子 ,具有更好的增容效果 ,其增容的共混体系具有典型的韧性特征。     

The effect of weathering on residual stresses and mechanical properties in injection-moulded semi-crystalline polymers

Injection-moulded bars made from polypropylene (PP), polyacetal (POM) and nylon 6,6 (N6, 6), have been weathered outdoors in Jeddah, Saudi Arabia, for varying periods. In POM and N6, 6 the residual stresses became tensile near the surface, though this happened even with unexposed bars. Prolonged ageing or weathering of PP caused the residual stresses to fall to very low values. With POM and N6, 6 significant surface damage developed on weathering, and multiple surface cracks opened up during uniaxial tensile testing. With N6,6 these developed into the form of diamond-shaped ductile fracture cavities, and the failure mechanism appeared quite different to that obtained with unexposed mouldings. With weathered PP no significant surface damage was visible even in the scanning electron microscope, but prolonged weathering caused a change in the failure mechanism, with fracture usually occurring without necking and drawing with specimens weathered for two years or more.     

Erhöhung der Dauerfestigkeit geschmiedeter Pleuel durch Kugelstrahlen unter Vorspannung

Increasing the operating strength of forged conrods by shot-peening under pre-load The level of residual stress at the surface of the standard conrod is dependent on the residual stresses induced by blast-cleaning and the subsequent production sequence. As a result of the production sequence, both tensile and compressive residual stresses occur in the fracture-susceptible zones. This leads to a wide spread of tolerable load, which must be taken into consideration when designing the conrod. By carrying out shot-peening under pre-load, residual compressive stresses to within the material's yield limit can be applied at the surface of the conrod in the direction of load transfer. This not only increases the conrod's average strength, but also displaces the fracture point to below the surface of the component. Consequently, the life expectancy of a conrod shot-peened under pre-load is calculated from the point of fracture within the conrod shaft. In this way the weakening effect of surface flaws down to a depth of 0,3 mm can be compensated for to a large extent. This is the decisive factor in the small scatter value of the conrods shot-peened under pre-load.     

脆性塑料改性PVC体系中相界面的作用

将ＰＶＣ体系分别与ＰＶＣ／ＰＳ、ＰＶＣ／ＰＭＭＡ体系相对比,观察到两类不同体系的拉伸及冲击性能具有相当的一致性。认为在用脆性塑料改性ＰＶＣ时,分散相对基体间的界面起很重要的作用,实验表明具有核／壳结构的分散相粒子对ＰＶＣ的影响主要取决于分散相壳层与基体间界面的性质。     

Development of carbon fibres from pitches modified with polymers

Polymethylmethacrylate (PMMA), polystyrene (PS) and polycarbonate (PC) were incorporated into the pitch with an objective to increase the tensile strength and tensile modulus of the pitch fibres (green) and to retain or to improve the mechanical properties of the carbonised fibres. It was found PMMA addition to the pitch increases the strength or flexibility of the green fibres but results in the formation of highly porous carbon fibres. The incorporation of PS and PC improves the tensile strength and tensile modulus both at the green and carbonised levels. The enhancement of mechanical properties is better in the case of carbon fibres derived from PC modified pitches.     

Effect of solvent and cosolvent on friction welding properties between part of PMMA with PVC

The effect of distilled water, methanol, ethanol, acetone and cosolvent (comprising ethanol and distilled water, the volume fraction of ethanol in the cosolvent was 10V_f% and 20V_f%) on the friction welding properties between parts of PMMA with PVC is investigated. Treatment with those solvents, 20V_f% cosolvent and 10V_f% cosolvent promote about 360%, 280% and 110% more bonding strength than without any solvent treatment. The tensile fracture morphology of welded PMMA with PVC has three zones: the non-plasticized central zone (Fud), the plasticized peripheral zone (Fpl), and the partly plasticized zone between Fud and Fpl (Fpd). Comparing tensile fracture morphology and residual solvent with bonding strength, it has larger Fpd regions, no Fud in the central part and less residual solvent is responsible for better bonding strength.     

Scanning electron microscopy studies on tensile,tear and abrasion failure of plasticized poly (vinyl chloride) and copolyester thermoplastic elastomers

Scanning electron microscopic observations on the tensile, tear and abrasion failure of plasticized poly (vinyl chloride) (PVC) and copdyester (Hytrel 40D) thermoplastic elastomers showed that the patterns developed on the fracture surfaces could be correlated with the strength and type of failure of these materials. Hytrel was found to undergo ductile failure whereas PVC showed shear fracture under tensile stress. The higher tear and abrasion resistances of Hytrel than those of PVC were also manifested by the difference in fracture patterns on the failure surfaces of these samples.     

A study on the mechanical stress relieving and safety assessment without post-weld heat treatment

For full welded body valve, the temperature of grommet cannot exceed 150 °C in order to prevent it from damaging and assure the tightness and the service life of valve. Therefore, post-weld heat treatment (PWHT) cannot be used to relieve the residual stresses. In this study, the effect of the mechanical stress relieving (MSR) treatment on the residual stresses was studied by the finite element method and experimental work. A pressure and time diagram of MSR treatment was established. A two-dimensional axisymmetric finite element model was used to simulate the residual stresses field. Before and after MSR treatment, the residual stresses on the outer surface were measured by the blind hole drilling method. Finally, the fracture toughness behaviors of weld zone (WZ) and heat affected zone (HAZ) were investigated in terms of crack tip opening displacement (CTOD) according to BS7448 and DNV-OS-C401 fracture toughness tests standards. The safety of the valve in active service was assessed without PWHT. Through comparison and analysis, the axial residual stresses and the hoop residual stresses on the outer surface of valve are mainly tensile. The peak value of tensile stress occurs nearer to the outer surface of the valve. MSR treatment can decrease the peak value of axial residual stresses and hoop residual stresses on the outer surface obviously and make the residual stresses distribution more uniform. The safety of the valve in active service is reliable without PWHT.     

Polymer films with surfaces unmodified and modified by non-thermal plasma as new substrates for cell adhesion

The surface properties of biomaterials, such as wettability, polar group distribution, and topography, play important roles in the behavior of cell adhesion and proliferation. Gaseous plasma discharges are among the most common means to modify the surface of a polymer without affecting its properties. Herein, we describe the surface modification of poly(styrene) (PS) and poly(methyl methacrylate) (PMMA) films using atmospheric pressure plasma processing through exposure to a dielectric barrier discharge (DBD). After treatment the film surface showed significant changes from hydrophobic to hydrophilic as the water contact angle decreasing from 95° to 37°. All plasma-treated films developed more hydrophilic surfaces compared to untreated films, although the reasons for the change in the surface properties of PS and PMMA differed, that is, the PS showed chemical changes and in the case of PMMA they were topographical. Excellent adhesion and cell proliferation were observed in all films. In vitro studies employing flow cytometry showed that the proliferation of L929 cells was higher in the film formed by a 1:1 mixture of PS/PMMA, which is consistent with the results of a previous study. These findings suggest better adhesion of L929 onto the 1:1 PS/PMMA modified film, indicating that this system is a new candidate biomaterial for tissue engineering.     

Tensile and tear failure of plasticized poly (vinyl chloride)/epoxidized natural rubber miscible blends

A study on the tensile and tear failure of miscible blends of plasticized poly (vinyl chloride), PVC, and epoxidized natural rubber (ENR) was carried out. Fractographs were taken in a scanning electron microscope in order to obtain an insight into the failure mechanism of the samples. It was found that low ENR blends failed by shearing in tensile testing, and they exhibited irregular stick-slip deformation and characteristics like vulcanizates in tear testing. Blends of plasticized PVC and ENR were more flexible than the individual components. As a result of the geometrical effect the tear fracture surface has rougher features than the tensile fracture surface. Sinusoidal wavy tear patterns in the form of a typical sine wave and rectified half sine wave of the fracture surface are characteristic features of tear resistant non-vulcanizates of rubber-like materials. Moreover, the study revealed that there were no features of phase separation of PVC and ENR due to incompatibility at any of the fracture surfaces examined.     

Residual stresses in short-glass-fibre-reinforced thermoset injection mouldings

Residual stress distributions have been measured in injection mouldings made from glass-fibre-reinforced phenolic and polyester (dough moulding compound (DMC)) thermosets. The results were inconsistent and this is believed to correspond to variability within the materials rather than to measurement error. The phenolic composite mouldings tended to have compressive residual stress near to the surface and tensile stress in the interior but examples were found in which this sense was reversed. Post-curing the phenolic composite mouldings caused the residual stress magnitudes to increase and reduced the variability in the observed stress distributions. Two DMCs were investigated. The stresses were generally lower in magnitude than those observed in the phenolic composite mouldings; examples in which the residual stress was compressive near the surface and those in which it was tensile were found with almost equal probability. © 1998 Kluwer Academic Publishers     

Residual stress effect on impact properties of Gr/Al metal matrix composite

The effects of residual stress on the impact properties of the unidirectionally reinforced P 100 Gr/6061 Al metal matrix composites with different thermal histories have been investigated using an instrumented impact test method and scanning electron microscopy. The cantilever impact generally causes tensile failure at the notch and compressive loading on the opposite side of the specimen. The specimens with yield tensile matrix residual stresses have planar fracture surfaces and low impact energy due to the contribution of tensile residual stress. The specimens with small residual stresses have moderate impact energy because debonding between fibre and matrix or fibre/matrix separation also serves as an additional mechanism for energy absorption. The specimens with higher compressive matrix residual stresses have the largest maximum load of all the specimens with the same matrix treatment. The specimen with matrix compressive yield residual stress has the maximum impact energy owing to a stepwise fracture surface. It can be concluded that good impact properties of composite materials can be obtained by choosing a suitable thermal history to modify the deleterious tensile matrix residual stress.     

In situ observation of crack propagation in ESP (engineered stress profile) glass

Important features of the ESP (engineered stress profile) glasses are the crack arrest and multiple cracking phenomena that occur even in an unstable stress field. In this work a detailed “in situ” observation of crack observation and analysis was performed with the aim to examine crack propagation in detail and relate it to the residual stress field produced by ion exchange and to the final mechanical performances of the material. The results showed that the peculiar residual stress field with a maximum below the surface is responsible for the formation of a multitude of stable cracks on the tensile surface of the glass that evolved into through-thickness flaws. The propagation within the material is limited by the increasing compressive residual stress, which also leads to kinking of the cracks in a direction parallel to the surface. The observed fracture phenomena are also responsible for a shielding effect that makes the measured failure resistance of ESP glass larger than predicted by simplistic single crack models.     

On the generality of discontinuous fatigue crack growth in glassy polymers

Fatigue fracture surface characteristics of five commercially available amorphous polymers [poly(methylmethacrylate) (PMMA), polycarbonate (PC), poly(vinyl chloride) (PVC), polystyrene (PS), and polysulphone (PSF)] as well as bulk-polymerized PMMA prepared over a wide range of molecular weights were studied to determine if common mechanisms of fatigue crack propagation prevail among these glassy polymers. In those polymers with viscosity-average molecular weight ¯M _v2×10⁵, the macroscopic appearance of the fracture surface showed the presence of a highly reflective mirror-like region which formed at low values of stress intensity and high cyclic test frequencies (100 Hz). The microscopic appearance of this region revealed that many parallel bands exist oriented perpendicular to the direction of crack growth and that the bands increase in size with K. In all instances, the crack front advanced discontinuously in increments equal to the band width after remaining stationary for hundreds of fatigue cycles. Electron fractographic studies verified the discontinuous nature of crack extension through a craze which developed continuously with the load fluctuations. By equating the band size to the Dugdale plastic zone dimension ahead of the crack, a relatively constant yield strength was inferred which agreed well with reported craze stress values for each material. At higher stress intensity levels in all polymers and all values of ¯M _v, another series of parallel bands were observed. These were also oriented perpendicular to the direction of crack growth and likewise increased in size with the range in stress intensity factor, K. Each band corresponded to the incremental advance of the crack during one load cycle, indicating these markings to be classical fatigue striations.     

The role of T-stress in brittle fracture for linear elastic materials under mixed-mode loading

The purpose of this paper is to revisit the maximum tensile stress (MTS) criterion to predict brittle fracture for mixed mode conditions. Earlier experimental results for brittle fracture of polymethylmethacrylate (PMMA) using angled cracked plates are also re-examined. The role of the T -stress in brittle fracture for linear elastic materials is emphasized. The generalized MTS criterion is described in terms of mode I and II stress intensity factors, K _I and K _II and the T- stress (the stress parallel to the crack), and a fracture process zone, r _c . The generalized MTS criterion is then compared with the earlier experimental results for PMMA subjected to mixed mode conditions. It is shown that brittle fracture can be controlled by a combination of singular stresses (characterized by K ) or non-singular stress ( T -stress). The T -stress is also shown to have an influence on brittle fracture when the singular stress field is a result of mode II loading.     

The effect of specimen size on the mechanical behaviour associated with crazing

Since crazes generally nucleate at the surface it is expected that the size of the specimen, as described by the ratio of surface area to volume, should affect the mechanical behaviour of polymers which deform primarily by crazing. The stress relaxation curves and the stress-strain curves of PS, PMMA, PTFE, and PC were measured in liquid nitrogen for specimens of different size which were machined from the same rod. The predicted size effect was observed in that the smaller (6.4mm diameter) specimens stress-relaxed faster and the stress to produce a given amount of craze deformation was lower than for the larger (12.7 mm diameter) specimens. The range of the tensile strength from 0 to size is also presented based on the stress to nucleate the first craze and on the tensile strength that is observed when no crazing occurs