Weak spots

It has been shown that the ultimate tensile strength, Su, for brittle materials depends upon the size of the speeimen and will deerease with inereasing dimensions, sinee the probability of having weak spots is inereased. This is termed the size effeet. This size effeet was investigated by Weibull (1951) who suggested a statistieal fune-tion, the Weibull distribution, deseribing the number and distribution of these flaws. The relationship below models the size effeet for deterministie values of Su (Timoshenko, 1966). 

Figure 1 Schematic drawing of PVC molecules with the chlorine atoms represented by circles. Dark areas represent the weak spots formed by clusters of partially negatively charged chlorine atoms.

As indicated above, when a positive direct current is impressed upon a piece of titanium immersed in an electrolyte, the consequent rise in potential induces the formation of a protective surface film, which is resistant to passage of any further appreciable quantity of current into the electrolyte. The upper potential limit that can be attained without breakdown of the surface film will depend upon the nature of the electrolyte. Thus, in strong sulphuric acid the metal/oxide system will sustain voltages of between 80 and 100 V before a spark-type dielectric rupture ensues, while in sodium chloride solutions or in sea water film rupture takes place when the voltage across the oxide film reaches a value of about 12 to 14 V. Above the critical voltage, anodic dissolution takes place at weak spots in the surface film and appreciable current passes into the electrolyte, presumably by an initial mechanism involving the formation of soluble titanium ions. 

Fluidised bed Most suitable for small articles Produces thick films. Varying metal gauge could produce uneven films and weak spots... 

The mechanism of inhibition by the salts of the long chain fatty acids has been examined . It was concluded that, in the case of the lead salts, metallic lead was first deposited at certain points and that at these points oxygen reduction proceeded more easily, consequently the current density was kept sufficiently high to maintain ferric film formation in addition, any hydrogen peroxide present may assist in keeping the iron ions in the oxide film in the ferric condition, consequently the air-formed film is thickened until it becomes impervious to iron ions. The zinc, calcium and sodium salts are not as efficient inhibitors as the lead salts and recent work has indicated that inhibition is due to the formation of ferric azelate, which repairs weak spots in the air-formed film. This conclusion has been confirmed by the use of C labelled azelaic acid, which was found to be distributed over the surface of the mild steel in a very heterogeneous manner. ... 

If infra-red heating or any other radiation curing method is employed, areas which are shaded from the rays or are outside the area of greatest flux density, cannot dry as hard as the fully irradiated surfaces, and may form weak spots susceptible to mechanical damage and consequent corrosion. 

Consider an experiment in which liquid carbon dioxide is introduced into an otherwise evacuated glass tube, which is then sealed (Figure 9.4, p. 232). At 0°C, the pressure above the liquid is 34 atm, the equilibrium vapor pressure of C02(Z) at that temperature. As the tube is heated, some of the liquid is converted to vapor, and the pressure rises, to 44 atm at 10°C and 56 atm at 20°C. Nothing spectacular happens (unless there happens to be a weak spot in the tube) until 31°C is reached, where the vapor pressure is 73 atm. Suddenly, as the temperature goes above 31°C, the meniscus between the liquid and vapor disappears The tube now contains only one phase. 

According to Hosemann-Bonart s model8), an oriented polymeric material consists of plate-like more or less curved folded lamellae extended mostly in the direction normal to that of the sample orientation so that the chain orientation in these crystalline formations coincides with the stretching direction. These lamellae are connected with each other by some amount of tie chains, but most chains emerge from the crystal bend and return to the same crystal-forming folds. If this model adequately describes the structure of oriented systems, the mechanical properties in the longitudinal direction are expected to be mainly determined by the number and properties of tie chains in the amorphous regions that are the weak spots of the oriented system (as compared to the crystallite)9). 

Even with everything under control, an analyst is well-advised to keep his eyes open so he will have an idea of what artifacts could turn up, and can plan to keep irregularities in check. The list of items in Table 4.44 could turn up in the checklist of any GMP-auditor worth his salt a corresponding observation would probably trigger his suspicion that there might be further weak spots. The table is given here to provide the reader with an idea of the human and technical factors that can influence the quality of results, and to permit a search for examples that fit a certain category. 

The weak spots, or Achilles heels, of the body occur where the skin ends and mucous epithelial tissues begin (mouth, anus, eyes, ears, nose and urinogenital tract). These mucous membranes present a much more favourable environment for microbial growth than the skin, in that they are warm, moist and rich in nutrients. Such membranes. 

All diffraction spots in the aurichalcite mineral calcined at 400°C (Figure 4b) could be attributed to ZnO, except for four weak spots with a d-spacing of 0.192 nm. These spots matched well the Cu0 ll2 reflections with a reported value of d = 0.195 nm (6). Crystalline CuO was found to be present at this stage by XRD, as seen from Figure 3c. 

Use this book in addition to your regular chemistry text. In several places we will refer you to your text for tables, and so on, that were too extensive to put in this book. We have tried to concentrate on those topics that we find most students have trouble. Pay particular attention to the Utterly Confused About... section of each chapter. These topics are the ones that we find are the most difficult for students to master. The Test Yourself section will give you an opportunity to see if you understand the concepts and should point out your weak spots for additional study. You will have to work and study hard to do well, but we are confident that this book will help you master the material and lift that fog of confusion. 

Historical and isolation studies of weak spot have given varying results. In some cases those spots were found to be sterile, and in others superficial occurrence of both Colletotrichum and Phoma was demonstrated. In many cases unidentified fungus mycelium could be found in petiole tissue and in leaf tissue clear down into the palisade cells. These may have been, therefore, demonstrations of one of the reasons why leaves dropped prematurely. The problem is still not completely settled, but many workers now consider the good effect from tonic sprays due to control of weak pathogens. 

The pitting potential is a stochastic parameter. The potential de-CTeases as the area inaeases, because the chance of there being a weak spot for pit initiation is greater on a large surface than on a small surface. 

Fig. 6.1. Model of crack fiber interaction in a simple composite, (a) In the uncracked composite, the fiber is gripped by the matrix, (b) A matrix crack is halted by the fiber. Increasing the load allows the crack to pass around the fiber without breaking the interfacial bond, (c) Interfacial shearing and lateral contraction of the fiber result in debonding and a further increment of crack extension, (d) After considerable debonding the fiber breaks at some weak spot within the matrix and further crack extension occurs, (e) The broken fiber end must be pulled out against the frictional grip of the matrix if total separation of the composite is to occur. After Harris (1980).

The fracture strengths of polymers are generally lower than those of metals and ceramics. The mode of failure for thermosets is generally referred to as the materials being brittle. Cracks, related to bond breakage, occur at points of excess stress. These create weak spots and may lead to fracture if the applied stress, appropriate to create bond breakage, continues. 

The presence of weak spots and structural non-uniformities in the agglomerates were considered to be determining factors in the break-up process [116,117]. 

Lit.5 mp 160-162° (dec.). TLC shows a weak spot for ditosylate, and 1H NMR integration (aromatic region) shows this impurity to be 8-9%. The physical... 

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