Environmental stress cracking of PVC and PVC-CPE

The failure of unnotched PVC and PVC-CPE was studied in a number of vapour and liquid environments. The failure mechanisms observed are related to craze initiation and logarithmic craze growth. A model to explain the origin of the logarithmic craze growth is presented. The failure mechanism in air and in benzene and toluene vapours is ductile, whereas it is brittle in n-hexane, n-decane and ethanol vapours, n-octane/benzene mixtures and natural gas condensate. Equations to describe the time to failure for both failure mechanisms are derived. The ductile failure is ascribed to plasticization and/or plastic deformation around the craze. Brittle failure is thought to arise from crazes which reach a critical size.     

Environmental stress cracking of PVC and PVC-CPE

The craze initiation of PVC and PVC-CPE in a number of vapour and liquid environments was studied. The observed decrease in the craze-initiation stress could be ascribed to the absorption of components by the PVC and the PVC-CPE matrix. The magnitude of the absorption depends on the activity of the components and on their interaction with the PVC matrix. The activity of the components was defined according to thermodynamics. The interaction of the components can be obtained from measurements of the angle of contact, though it is preferred to quantify this interaction by absorption experiments. A simple model is presented to predict the long-term craze-initiation stress of PVC in environments.     

Environmental stress cracking of PVC and PVC-CPE

The fracture toughness of Polyvinylchloride (PVC) and PVC modified with 10% chlorinated polyethylene (PVC-CPE) was studied in vapour and in liquid environments by crack growth measurements on single-edge notch specimens under three-point bending at 23°C. In addition, some results obtained in air at lower temperatures are presented. The fracture toughness is quantified by a stress intensity factor leading to failure after a given loading period. It is shown that for a given slow crack growth rate at 23 °C, the environment hardly affects the fracture toughness of PVC. In contrast, the slow crack growth in PVC-CPE at 23 °C is accelerated by the presence of benzene vapour, n-octane/benzene mixtures and gas condensate. A decrease in temperature results in an increase in fracture toughness, both for PVC and for PVC-CPE. A Dugdale model to describe the craze ahead of the crack was used to analyse the observed changes in fracture toughness.     

The main mechanisms of stress corrosion cracking in natural gas trunk lines

Stress corrosion cracking in a pipe material presents a most critical hazard to natural gas trunk lines. The paper addresses the causes (mechanism) of crack formation and growth. Understanding of these issues will enable a proper implementation of methods for early identification, diagnostic, and prediction of this type of failure. A most comprehensive explanation of a joint influence of internal and external factors on the corrosion cracking can be provided by means of the probabilistic local electrochemical corrosion theory. It is based on the probabilistic nature of initiation of a local corrosion damage. A probabilistic approach is used to develop a concept of stress corrosion cracking. The factors responsible for the formation of local damage nuclei have been determined. The pipe manufacturing operations are shown to have a certain effect on the pipeline susceptibility to stress corrosion cracking. Modeling has been performed for the corrosion processes that may occur in a gas trunk line section subjected to a stress corrosion cracking hazard.     

Environmental stress cracking of polycarbonate catheter connectors

A catheter connector allows medical staff to supply a range of fluids to ill patients using a single intravenous line. In the case of premature babies, such a line provides nutrition in the form of a synthetic milk (TPN) and antibiotics to prevent infection. The connectors are made from several alternative polymers, but polycarbonate is used widely. However, the material is susceptible to environmental stress cracking (ESC) from a variety of fluids. The case study shows how the first versions of one design were poorly moulded and cracked during service, allowing bacteria to enter the line and so infect the baby. The baby contracted meningitis, but lived although suffering brain damage. Evidence was produced which showed that it was a widespread problem in the mid-1990s in UK hospitals. The investigation suggested that poor design and manufacture were the root cause of the problem.     

Environmental stress cracking behaviour of urethane methacrylate based resins

Two kinds of cross-linked urethane methacrylate resins have been investigated using three-point bend tests to determine their environmental stress cracking (ESC) behaviour in a range of liquids (water, sodium hydroxide, ethylene glycol, acetonitrile, acetic acid, acetone, tetrahydrofuran, toluene, 3,5,5-trimethylhexanol and petrol). The resins were found to undergo ESC in organic liquids only, and the critical strains, _c, and critical stresses, _c, have been related to the solubility parameters, , of the liquid environments. The most severe ESC was observed in solvents with –19–20 MPa^1/2, corresponding to minimum points in the plots of _c and _c against . Generally, the resin with the higher cross-link density had a greater resistance to ESC, but the effect of liquid diffusion complicated the situation and was found to play an important role in the ESC behaviour of these materials. The results confirmed that liquid diffusion into the resins lowered the critical strain (and stress), leading to earlier failure. In the case of the lower cross-link density resin, very fast diffusion was found to cause softening. However, it was noted that liquid diffusion can also blunt crazes and cracks.     

Environmental stress cracking of glassy polymers and solubility parameters

The use of solubility parameters to predict critical stress ( _c ^* ) or strains (_c) for environmental cracking/crazing in several glassy polymers (e.g. PMMA, PPO, PS, PVC, PSF and PC) is re-examined. It is shown that the enthalpic component ( _H ) of the Flory-Huggins semi-interaction parameter () does not always give a good correlation between _c and _H even though solvent molar volume and polymer-solvent molecular interactions have already been considered. Re-analysis of available experimental data using Gent's theory shows that there is a general trend for _c ^* (or _c) to increase with . These results, therefore, support Gent's proposed mechanism of environmental stress crazing/cracking. It is finally concluded that unless a definite relationship can be established between _c or _c ^* with _H it is not possible to predicta priori _c or _c ^* , given the empirical solubility parameters of a solvent. Unfortunately, there are not many such relationships as discovered in this paper.     

A focus on different factors affecting hydrogen induced cracking in oil and natural gas pipeline steel

In this research, hydrogen induced cracking (HIC) behavior of an API X70 pipeline steel has been studied. In order to create HIC cracks, an electrochemical hydrogen charging experiment was carried out on X70 steel by using 0.2 M sulfuric acid and 3 g/l ammonium thiocyanate for 8 h. Moreover, SEM, EDS and EBSD techniques were used to characterize the as-received (AR) steel and investigate the different aspects of HIC phenomenon as well. The results showed that the inclusions and precipitates which play a key role in HIC phenomenon have been distributed randomly through the cross-section of tested steel. However, the concentration of them was higher at the center of cross-section than other areas. All HIC cracks initiated and propagated through the center of thickness where center segregation of elements has occurred. It is also observed that HIC cracks were initiated from several special types of inclusions and precipitates such as manganese sulphide and carbonitride precipitates. EBSD results showed that the dominant local texture of center of thickness in RD-TD plane was {001}//ND and {111}//ND. Moreover, HIC cracks propagate through differently oriented grains where the local texture is random.     

High-pH inclined stress corrosion cracking in Australian and Canadian gas pipeline X65 steels

High-pH stress corrosion cracking is a form of environmental degradation of gas pipeline steels. The crack path is intergranular by nature and typically perpendicular to the maximum applied (hoop) stress (i.e. perpendicular to the pipe outer surface). Some unusual instances of cracks have been observed in Canadian and Australian X65 pipes, where cracks grow away from the perpendicular for considerable distances. This paper presents a comparative study in terms of crack morphology, mechanical properties and crystallographic texture for these Australian and Canadian pipe steels. It is shown that the crack morphologies are quite similar, the main difference being the angle at which the cracks propagate into the material. This difference could be explained by the different through-wall texture and grain aspect ratio measured in the two materials. The interdependency of crack tip plasticity, crack tip electrochemistry and anisotropy in microstructural texture seems to heavily affect the resulting inclined crack path.     

Effects of hydrogen on stress cracking in unheated thermal powder station pipes

Summary Reasons are examined for damage to unheated bends in thermal power station pipes made of steel 20 and steel 12Kh1MF. Corrosion and thermal fatigue are accompanied by creep and by a substantial contribution from the development of brittle cracks on account of hydrogen occluded by the steel from the two-phase or one-phase medium acting on the inner surfaces of the tubes.Translated from Fiziko-khimicheskaya Mekhanika Materialov, Vol. 27, No. 1, pp. 31–36, January–February, 1991.     

使用三氧化钨薄膜之苯气体传感器

本文利用微机电(Micro-Electro-Mechanical Systems,MEMS)制程制作成三氧化钨薄膜苯气体传感器,其结构包含三氧化钨(WO3)感测层、白金(Pt)指叉式电极(包括加热及感测两部分).利用RF溅渡方式沉积出三氧化钨薄膜,再由退火处理来得到较多的孔洞结构来提升传感器的灵敏度,并使用蒸镀(E-beam)方式制作白金指叉式电极.再将微型传感器置于气体检测箱在200℃、250℃及300℃不同操作温度下观察苯浓度与感测层电阻值间的变化关系.     

Chemical stress cracking of acrylic fibres

The generation of periodic microscopic transverse cracks in oriented acrylic fibres immersed in hot alkaline hypochlorite solution is described in detail and shown to be a variety of chemical stress cracking. It is greatly accelerated by external tensile stress, high fibre permeability, moderate fibre orientation, and water-plasticization. The proposed mechanism for bond cleavage involves cyclization of nitrile groups (similar to the prefatory reaction in pyrolysis of acrylic fibres), followed immediately by N-chlorination and chain scission. Mechanical retractile forces (internal or external) then cause chain retraction and crack growth. Despite the remarkable regularity of the crack pattern, which typically resembles a series of stacked lamellae, the process is independent of any such underlying fibre morphology. The cracking process does, however, appear to be a sensitive indicator of residual latent strain in the fibre, which may persist even after high-temperature annealing.     

Tribological properties of Zr-C:H:Nx% coatings with different amounts of nitrogen addition

A closed field unbalanced magnetron (CFUBM) sputtering system is used to deposit Zr-C:H:N_x% coatings with nitrogen contents ranging from 0 to 29 at.% on AISI M2 steel disks. The microstructures of the various coatings are analyzed using Raman spectrometry, while the hardness and adhesion strength are measured via nanoindentation tests and scratch tests, respectively. The tribological properties of the coatings are tested against AISI 52100 steel balls under loads of 0.5-10 N using a ball-on-disk wear tester. The wear testing results reveal that the Zr-C:H:N_12% coating has excellent tribological properties, including a low wear depth, a low friction coefficient and an extended lifetime.     

Hydrogen-assisted stress cracking of high-strength wheel bolts

Early failures occurred with two sizes of wheel bolts used for attaching front wheels and dual rear wheels to heavy truck hubs. Failure resulted in fracture of the bolts, and was a response to the material and process specification which produced a steel microstructure highly susceptible to hydrogen-assisted stress cracking. The microstructure resulted from an alloy steel that was carburized, heat treated, and then zinc plated. This combination of material and processing produced a high-strength SAE Grade 8 bolt (equivalent to an ISO 10.9 grade) with a hard, brittle case and an anodic zinc coating. A slight misalignment of the wheel bolt coupled with a ball seat mounting design for the wheel nuts created a combined axial and bending stress that exceeded the threshold for hydrogen-assisted stress cracking.