Changes In Chemical Concentrations Research Articles

Crack velocity functions and thresholds in brittle solids

A unifying treatment of environment-sensitive crack velocity functions for intrinsically brittle solids is presented. The formalism is soundly based on the concept of thermal activation barriers, but is phenomenological in that it does not attempt to identify the explicit underlying physical and chemical processes responsible for these barriers. Equations prescribing the v-G (crack velocity versus mechanical energy release rate) characteristics at specified chemical concentrations (partial pressures) and temperatures are thereby presented. These equations incorporate the familiar three velocity regions into a composite function: region I, chemically assisted fluctuations over stress-enhanced energy barriers; region III, similar but in the absence of environmental species; region II, a connecting flow-limited transport branch. In addition, the equations include provision for healing and repropagation branches in unloading-reloading cycles. Central to the argument is the assertion that zero-velocity thresholds are quiescent configurations, defined by appropriate Dupré work of adhesion terms, W. These W terms serve as reference baselines for the entire v-G function, such that changes in chemical concentration (relative humidity) or interface type (virgin versus healed) may be considered in terms of simple curve shifts along the G axis. Data for selected brittle solids, principally mica but also glass and sapphire, in moist environments are used to illustrate the formalism. Für umgebungsabhängige Riß-Geschwindigkeits-Funktionen von spröden Materialien wird eine vereinheitlichende Betrachtungsweise vorgestellt. Die Vorgehensweise gründet sich vor allem auf das Modell der thermischen Aktivierungsschwellen, ist aber in dem Sinne phänomenologisch, als daß auf die tatsächlichen physikalischen und chemischen Prozesse, die für diese Schwellen verantwortlich sind, nicht eingegangen wird. Es werden Gleichungen aufgestellt, die den v-G Verlauf (Rißausbreitungsgeschwindigkeit gegen mechanische Energiefreisetzungsrate) bei bestimmten Konzentrationen (Partialdrücken) und Temperaturen ausdrücken. Diese Gleichungen beschreiben die bekannten drei Bereiche der Rißausbreitungsgeschwindigkeit in einer einzigen Funktion: Bereich I, chemisch unterstützte thermische Fluktuationen über spannungsabhängige Energieschwellen hinweg; Bereich III, analog, doch ohne den Einfluß des umgebenden Mediums; der die Bereiche I und III verbindende Bereich II in welchem das umgebende Medium dem Riß nur noch teilweise folgen kann. Zusätzlich schließen die Gleichungen das Ausheilen von Rissen und das erneute Fortschreiten bei Entlastungs-Wiederbelastungs-Zyklen ein. Grundlegend für alle Überlegungen ist die Annahme, daß der Punkt, an dem die Rißausbreitungsgeschwindigkeit Null wird, einen Zustand darstellt, der durch eine entsprechende Dupré-Adhäsionsarbeit, W, definiert ist. Diese W-Terme dienen als Bezugslinie für die gesamte v-G Funktion, so daß Änderungen der Konzentration (relative Feuchtigkeit) oder des Oberflächentyps (nicht angegriffen gegen ausgeheilt) als einfache Verschiebung der Kurve entlang der G-Achse betrachtet werden können. Die Vorgehensweise wird anhand der Daten ausgewählter spröder Materialien in feuchter Atmosphäre, hauptsächlich Glimmer aber auch Glas und Saphir, exemplarisch dargestellt. On présente ici un traitement unificateur des fonctions de vitesse de fissures sensibles à l'environnement dans le cas de solides intrinsèquement fragiles. Le formalisme est basé sur le concept des barrières d'activation thermique et est phénomènologique dans le sens où il ne tente pas d'expliquer les phénomènes physiques et chimiques sous-jacents qui sont responsables de ces barrières. On a établi des équations donnant les caractéristiques v-G (vitesse des fissures en fonction du taux de libération d'énergie mécanique) pour des concentrations chimiques (pressions partielles) et des températures déterminées. Ces équations incorporent les trois domaines de vitesse courants en une fonction composite: domaine I, fluctuations assistées chimiquement au delà des barrières d'énergie activées par les contraintes; domaine III, similaire mais avec absence d'espèces extérieures; domaine II, branche de transport de raccordement à flux limité. Ces équations incluent de plus la prévision de branches d'atténuation et de repropagation des cycles de charge-décharge. L'argument principal est que les seuils de vitesse nulle sont des configurations de repos, définies par des termes d'adhérence de Dupré, W. Ces termes W servent de base de référence pour la fonction v-G complète, de sorte que les variations de concentration chimique (humidité relative) ou de type d'interface (vierge ou traitée) puissent être considérées comme de simples décalages des courbes le long de l'axe G. On donne, pour illustrer ce formalisme, des résultats concernant des matériaux fragiles sélectionnés, principalement le mica mais également le verre et le saphir, en environnement humide.

Some theoretical and field aspects of reinjection in geothermal reservoirs

Theoretical and field aspects of reinjection into geothermal reservoirs is discussed using examples from various geothermal fields. Reinjection effects can be observed in terms of pressure maintenance, changes in chemical concentrations, and temperature decline. Pressure support due to injection is observed in many liquid‐dominated fields, and in two‐phase field pressure, effects are often felt through mobility changes. In most geothermal fields where injection is employed, changes in chemical concentrations of the produced fluids have been observed, due to the higher salinity of the injected fluids. In some cases the returns of the injected fluids observed through chemical changes have resulted in temperature declines at the producing wells. In those cases the fluid flow is probably concentrated through a few fractures or faults with insufficient surface area for heat exchange with the host rock.

Aragonite-calcite transformation based on Pennsylvanian molluscs

A mineralogical, microstructural and geochemical approach was used to trace the postdepositional alteration of molluscs and to quantify the biogenic aragonite to calcite transformation in the Boggy Formation (mid-Desmoinesian) of Oklahoma. The results show that with progressive alteration, primary biogenic aragonite was gradually replaced by diagenetic low-Mg calcite. During the initial phase of alteration, the nacreous tablets of the shell layers fused, and with progressive postdepositional diagenesis, they were replaced by either fine- or coarsegrained calcite. During this diagenetic transformation, no significant changes in chemical concentrations were noted for Ba, Cu, Ni, Pb, and Zn; whereas Mg, Mn, and Fe increased by factors of about 14 to 25 relative to their depositional concentrations. Concurrently, Na, Sr, δ 18 O, and δ 13 C concentrations decreased by factors ranging from 2.5 to 10. Under similar diagenetic conditions, matrix samples were more altered than were contemporaneous fossils, and it is postulated that grain size/surface area was a major control on the degree of alteration of carbonate grains. The matrix of the Boggy Formation has retained sufficient geochemical information to suggest an original aragonitic limemud precursor of nonmolluscan affinity. With the geochemical methods and quantified trends, the least-altered mollusc samples of the Boggy Formation, consisting of only aragonite, can be identified. Geochemical data of the most-altered samples suggest that the diagenetic fluids which affected Boggy sediments and fossils were similar in composition to meteoric water. Moreover, the dissolving aragonite, through diffusion-controlled dissolution, determined the elemental as well as isotopic chemistry of the diagenetic calcite in the altered shells. It is postulated that similar diagenetic waters and processes affected sedimentary units of the Breathitt (Pennsylvanian, Kentucky), Brush Creek (Pennsylvanian, Pennsylvania-Ohio), Lukow (Jurassic, Poland), Bear Paw-Lea Park (Cretaceous, western Canada), and Nugssuaq (Cretaceous, Greenland) Formations which contain well-preserved aragonitic molluscan faunas.

Chemical spill exposure assessment.

POSSM, the PCB On-Site Spill Model, is a contaminant transport model developed to predict environmental concentrations associated with a chemical spill. The model predicts daily changes in chemical concentrations on a spill site (e.g., in soil and on vegetation) and losses of chemical due to volatilization, surface runoff/soil erosion, and leaching to groundwater. Spill areas consisting of soil/vegetation and/or an impervious surface (e.g., asphalt and concrete) can be analyzed, as can different spill cleanup practices. POSSM is used to analyze exposure levels associated with a hypothetical capacitor spill. While the model was developed for PCB spills, it is generally applicable to a number of organic compounds.