A fully implicit multi-axial solution strategy for direct ratchet boundary evaluation : implementation and comparison

Ensuring sufficient safety against ratcheting is a fundamental requirement in pressure vessel design. However, determining the ratchet boundary using a full elastic plastic finite element analysis can be problematic and a number of direct methods have been proposed to overcome difficulties associated with ratchet boundary evaluation. This paper proposes a new lower bound ratchet analysis approach, similar to the previously proposed Hybrid method but based on fully implicit elastic-plastic solution strategies. The method utilizes superimposed elastic stresses and modified radial return integration to converge on the residual state throughout, resulting in one Finite Element model suitable for solving the cyclic stresses (Stage 1) and performing the augmented limit analysis to determine the ratchet boundary (Stage 2). The modified radial return methods for both stages of the analysis are presented, with the corresponding stress update algorithm and resulting consistent tangent moduli. Comparisons with other direct methods for selected benchmark problems are presented. It is shown that the proposed method evaluates a consistent lower bound estimate of the ratchet boundary, which has not previously been clearly demonstrated for other lower bound approaches. Limitations in the description of plastic strains and compatibility during the ratchet analysis are identified as being a cause for the differences between the proposed methods and current upper bound methods

A fully implicit multi-axial solution strategy for direct ratchet boundary evaluation : theoretical development

Ensuring sufficient safety against ratchet is a fundamental requirement in pressure vessel design. Determining the ratchet boundary can prove difficult and computationally expensive when using a full elastic-plastic finite element analysis and a number of direct methods have been proposed that overcome the difficulties associated with ratchet boundary evaluation. Here, a new approach based on fully implicit Finite Element methods, similar to conventional elastic-plastic methods, is presented. The method utilizes a two-stage procedure. The first stage determines the cyclic stress state, which can include a varying residual stress component, by repeatedly converging on the solution for the different loads by superposition of elastic stress solutions using a modified elastic-plastic solution. The second stage calculates the constant loads which can be added to the steady cycle whilst ensuring the equivalent stresses remain below a modified yield strength. During stage 2 the modified yield strength is updated throughout the analysis, thus satisfying Melan’s Lower bound ratchet theorem. This is achieved utilizing the same elastic plastic model as the first stage, and a modified radial return method. The proposed methods are shown to provide better agreement with upper bound ratchet methods than other lower bound ratchet methods, however limitations in these are identified and discussed

Relationships between hornblende K-Ar ages, chemical composition and hydrogen isotopes, Connemara, western Ireland: evidence for a massive extinct hydrothermal system

Major element compositional analyses, K-Ar ages, deltaD parts per thousand and delta O-18 parts per thousand values for 30 zoned and unzoned hornblendes from Dalradian amphibolites and metagabbros. mostly in north Connemara are reported. Although the cooling ages are expected from previous U-Pb zircon studies to be c. 475-450 Ma the results obtained are from 556 +/- 6 to 410 +/- 9 Ma with an average of 470 Ma. Fluid movements. probably at 275 +/- 15 degreesC, i.e. much below Ar closure temperature for hornblende. erratically reset the ages, as is shown by a negative correlation of hornblende deltaD and age and a wide scatter of ages even within 2 m. The changes were implemented by deltaD exchange between fluid and hornblendes in which ionic porosity. Z, influenced the loss of Ar and possibly its gain from the fluid to give the excess Ar found in some samples. Z is controlled by hornblende chemical composition. High Mg, Si and Mg/Fe and low Fe, Al, Ti, Na and particularly low K, amphiboles giving low Z values retained Ar more firmly and gained Ar more readily than compositions which had higher Z values, which gave younger ages. These range down to c, 400 Ma. being the age of the intrusion of the Galway Granite suite that initiated the fluid circulation. The scatter of ages is a consequence of incomplete equilibration, mainly because of the slow deltaD exchange rate below 350 degreesC and partly because the fluid permeated erratically in different areas and down cracks of all kinds, promoting irregular Ar movement. The meteoric fluid circulated through Connemara. the Galway Granite and at least some of the contiguous Silurian sediments of the South Mayo trough. These overlying sediments may have contributed to the water circulated in north Connemara which was slightly less negative deltaD than in central Connemara. For hornblende K-Ar ages to be a reliable measure of times of uplift and cooling, they need to be demonstrated to be free from the influence of hot fluids by showing no correlation of age with deltaD

Sémiotiques du texte et de l’image

Je me préoccupe dans cet article d’un problème soulevé il y a plus de deux cents ans dans le Laocoön, qui oppose entre les signes picturaux et textuels une différence de support et de dimensionnalité. Or, aucune sémiotique ou sémiologie « rigoureuse » ne se permettrait le luxe d’avoir deux définitions distinctes du signe, l’une qui s’appliquerait aux signes linguistiques, textuels, et l’autre réservée à des signes picturaux. Ce problème a ainsi fait l’objet au cours de ce siècle de différentes tentatives de réponses dont deux qui m’intéressent plus particulièrement. La première s’inscrit dans l’optique du tournant linguistique ; la seconde est la sémiotique de Peirce, dont la composante la plus pertinente est la logique des icônes. C’est le problème posé dans ce contexte par la métaphore, qui servira d’illustration des différences fondamentales qu’on peut constater entre les deux approches.My concern in this article is to review a problem raised over two hundred years ago in Lessing’s Laocoon, which drew attention to differences in support and dimensionality between pictorial and textual signs. Obviously no serious semiotic or semiological theory can allow itself the luxury of two definitions of its principal unit, the sign, one for pictures and one for texts. Over the course of the century we have witnessed several attempts to address the problem raised by Laocoon, of which two are of particular interest. The first is a theoretical outcome of the so-called “ linguistic turn ”, the second is Peirce’s logic of the icon. The problem raised by metaphor in this context is used to illustrate fundamental differences in the two approaches to the problem

Fond et forme dans l'image allégorique

Il y a vingt ans, l'historien de l'art américain Craig Owens publiait en deux parties une étude visant à réhabiliter l'allégorie dans un contexte postmoderniste. Cette étude est devenue depuis un passage obligé de toute réflexion sur ce mode. S'inspirant très largement d'un texte de Walter Benjamin consacré au drame baroque allemand, Owens soutenait entre autres que l'allégorie est résolument tournée vers le passé avec, pour figures caractéristiques, la ruine isolée et la mélancolie et que sa structure spécifique peut se concevoir comme le « supplément » derridien.S'appuyant sur deux séries de photographies allégoriques contemporaines, le présent article cherche à replacer certaines des thèses d'Owens dans un cadre sémiotique peircien en s'intéressant tout particulièrement à sa tentative de caractériser le mode par son contenu et à la thèse du passéisme mélancolique qui lui serait spécifique.Twenty years ago, the American art historian Craig Owens published a two-part study on allegory that has since become a necessary reference in any discussion concerning the allegorical mode. Drawing heavily on Benjamin's thesis regarding German Baroque tragic drama, Owens claimed that allegory was resolutely linked to the past, that its principal figures were the ruin and melancholy, and that its specific structure could be conceived in terms of the Derridean supplement.Using two series of contemporary photographic allegories, this article seeks to examine some of Owens' claims from a Peircean perspective. It will specifically focus on Owen's attempt to characterize the allegorical mode by its content, and will examine the association of allegory with the past, with melancholy and the ruin

Iconisme et structure de l'objet des signes linguistiques

Recherche sur les données qu'on peut admettre dans une explication des phénomènes linguistiques. Il est question des déterminants de l'ordre des termes dans la phrase, ce qui vaut également pour les groupes nominal et verbal. S'agissant d'iconisme, l'étude se place dans le cadre de la théorie peircienne du signe iconique, plus précisément dans un courant de la linguistique contemporaine qui en est issue, à savoir la théorie de l'iconicité.Research centered on pertinent data accepted to explain linguistic phenomena. The focus is on determining factors which affect the order of terms within sentences and within nominal and verbal clauses as well. Dealing with iconicity, the study is based on the Peircian theory of the iconic sign and more precisely belongs to a contemporary trend of linguistics which derives from the Peircian theory, namely the theory of iconicity

A generalised method for ratchet analysis of structures undergoing arbitrary thermo-mechanical load histories

A novel approach is presented based upon the Linear Matching Method framework in order to directly calculate the ratchet limit of structures subjected to arbitrary thermo-mechanical load histories. Traditionally, ratchet analysis methods have been based upon the fundamental premise of decomposing the cyclic load history into cyclic and constant components respectively, in order to assess the magnitude of additional constant loading a structure may accommodate before ratcheting occurs. The method proposed in this paper, for the first time, accurately and efficiently calculates the ratchet limit with respect to a proportional variation between the cyclic primary and secondary loads, as opposed to an additional primary load only. The method is a strain based approach and utilises a novel convergence scheme in order to calculate an approximate ratchet boundary based upon a predefined target magnitude of ratchet strain per cycle. The ratcheting failure mechanism evaluated by the method leads to less conservative ratchet boundaries compared to the traditional Bree solution. The method yields the total and plastic strain ranges as well as the ratchet strains for various levels of loading between the ratchet and limit load boundaries. Two example problems have been utilised in order to verify the proposed methodology

A constitutively consistent lower bound, direct shakedown and ratchet method

Strathclyde theses - ask staff. Thesis no. : T13793When a structure is subject to cyclic loads there is a possibility of it failing due to ratchet or incremental collapse. In many engineering structures the demonstration of non-ratcheting behaviour is a fundamental requirement of the design and assessment process. Whilst it is possible to use incremental finite element analysis to simulate the cyclic response for a given load case to demonstrate shakedown or ratchet, it does not yield any information on the safety factor. In addition, there are several practical problems in using this approach to determine whether or not a component has achieved shakedown. Consequently several direct methods which find the loads at the shakedown and ratchet boundaries have been developed in the past 3 decades. In general, lower bound methods are preferred for design and assessment methodologies. However, to date, the lower bound methods which have been proposed for shakedown and ratchet analysis have not been fully reliable and accurate. In this thesis a lower bound shakedown and ratchet method which is both reliable and accurate is proposed. Previously proposed elastic plastic lower bound ratchet methods are revisited and modified to understand the limitations in current methods. From this, Melan's theorem is reinterpreted in terms of plasticity modelling and shown to have the same form as a non-smooth multi yield surface plasticity model. A new shakedown method is then proposed based on the non-smooth multi yield surface plasticity model. The new shakedown method is extended using a two stage process to determine the ratchet boundary for cyclic loads in excess of the alternating plasticity boundary. Two simplified variants of the ratchet method are also proposed to decrease the computational expense of the proposed ratchet method. Through several common benchmark problems the proposed methods are shown to give excellent agreement with the current upper bound methods which have been demonstrated to be accurate. The flexibility of the shakedown method is demonstrated by extending the method to incorporate temperature dependent yield, hardening and simplified non-linear geometric effects.When a structure is subject to cyclic loads there is a possibility of it failing due to ratchet or incremental collapse. In many engineering structures the demonstration of non-ratcheting behaviour is a fundamental requirement of the design and assessment process. Whilst it is possible to use incremental finite element analysis to simulate the cyclic response for a given load case to demonstrate shakedown or ratchet, it does not yield any information on the safety factor. In addition, there are several practical problems in using this approach to determine whether or not a component has achieved shakedown. Consequently several direct methods which find the loads at the shakedown and ratchet boundaries have been developed in the past 3 decades. In general, lower bound methods are preferred for design and assessment methodologies. However, to date, the lower bound methods which have been proposed for shakedown and ratchet analysis have not been fully reliable and accurate. In this thesis a lower bound shakedown and ratchet method which is both reliable and accurate is proposed. Previously proposed elastic plastic lower bound ratchet methods are revisited and modified to understand the limitations in current methods. From this, Melan's theorem is reinterpreted in terms of plasticity modelling and shown to have the same form as a non-smooth multi yield surface plasticity model. A new shakedown method is then proposed based on the non-smooth multi yield surface plasticity model. The new shakedown method is extended using a two stage process to determine the ratchet boundary for cyclic loads in excess of the alternating plasticity boundary. Two simplified variants of the ratchet method are also proposed to decrease the computational expense of the proposed ratchet method. Through several common benchmark problems the proposed methods are shown to give excellent agreement with the current upper bound methods which have been demonstrated to be accurate. The flexibility of the shakedown method is demonstrated by extending the method to incorporate temperature dependent yield, hardening and simplified non-linear geometric effects