Article

Fictional mechanism explanations: clarifying explanatory holes in engineering science

Authors:
To read the full-text of this research, you can request a copy directly from the author.

Abstract

This paper discusses a class of mechanistic explanations employed in engineering science where the activities and organization of nonstandard entities (voids, cracks, pits…) are cited as core factors responsible for failures. Given the use of mechanistic language by engineers and the manifestly mechanistic structure of these explanations, I consider several interpretations of these explanations within the new mechanical framework (among others: voids should be considered as shorthand expressions for other entities, voids should be reduced to lower-level mechanisms, or the explanations are simply abstract mechanistic explanations). I argue that these interpretations fail to solve several philosophical problems and propose an account of fictional mechanism explanations instead. According to this account, fictional mechanism explanations provide descriptions of fictional mechanisms that enable the tracking of counterfactual dependencies of the physical system they model by capturing system constraints. Engineers use these models to learn about and understand properties of materials, to build computational simulations of their behaviour, and to design new materials.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the author.

ResearchGate has not been able to resolve any citations for this publication.
Article
Full-text available
I argue that fictional models, construed as models that misrepresent certain ontological aspects of their target systems, can nevertheless explain why the latter exhibit certain behaviour. They can do this by accurately representing whatever it is that that behaviour counterfactually depends on. However, we should be sufficiently sensitive to different explanatory questions, i.e., ‘why does certain behaviour occur?’ vs. ‘why does the counterfactual dependency invoked to answer that question actually hold?’. With this distinction in mind, I argue that whilst fictional models can answer the first sort of question, they do so in an unmysterious way (contra to what one might initially think about such models). Moreover, I claim that the second question poses a dilemma for the defender of the idea that fictions can explain: either these models cannot answer these sorts of explanatory questions, precisely because they are fictional; or they can, but in a way that requires reinterpreting them such that they end up accurately representing the ontological basis of the counterfactual dependency, i.e., reinterpreting them so as to rob them of their fictional status. Thus, the existence of explanatory fictions does not put pressure on the idea that accurate representation of some aspect of a target system is a necessary condition on explaining that aspect.
Article
Full-text available
A model based on discrete unit events coupled with a graph search algorithm is developed to predict intergranular fracture. The model is based on two hypotheses: (i) the key unit event associated with intergranular crack propagation is the interaction of a grain boundary crack with a grain boundary segment located at an angle with the initial crack plane; and (ii) for a given crack path, the overall crack growth resistance can be calculated using the crack growth resistance of a collection of unit events. Next, using a directed graph containing the connectivity of grain boundary junctions and the distances between them, and crack deflection versus crack growth resistance data, a directed graph in the J-resistance space is created. This graph contains information on the crack growth resistance for all possible crack paths in a given grain microstructure. Various crack growth resistance curves are then calculated including those corresponding to: (i) a local resistance minimum; (ii) a global minimum; and (iii) for verification, a path specified by microstructure-based finite element calculations. The results show that the proposed method based on discrete unit events and graph search can predict the crack path and the crack growth resistance for cracks that propagate from one grain boundary junction to another. The proposed computationally inexpensive model can be used to design material microstructures with improved intergranular fracture resistance, and/or to assess the overall crack growth resistance of materials with a known distribution of grain morphology.
Article
Full-text available
Two modes of plastic flow localization commonly occur in the ductile fracture of structural metals undergoing damage and failure by the mechanism involving void nucleation, growth and coalescence. The first mode consists of a macroscopic localization, usually linked to the softening effect of void nucleation and growth, in either a normal band or a shear band where the thickness of the band is comparable to void spacing. The second mode is coalescence with plastic strain localizing to the ligaments between voids by an internal necking process. The ductility of a material is tied to the strain at macroscopic localization, as this marks the limit of uniform straining at the macroscopic scale. The question addressed is whether macroscopic localization occurs prior to void coalescence or whether the two occur simultaneously. The relation between these two modes of localization is studied quantitatively in this paper using a three-dimensional elastic-plastic computational model representing a doubly periodic array of voids within a band confined between two semi-infinite outer blocks of the same material but without voids. At sufficiently high stress triaxiality, a clear separation exists between the two modes of localization. At lower stress triaxialities, the model predicts that the onset of macroscopic localization and coalescence occur simultaneously. © 2015 The Author(s) Published by the Royal Society. All rights reserved.
Article
Full-text available
Film cooling technology is developed to enhance the temperature resistant of nickel-base single crystal alloy blade. The shape, dimension, and arrangement of cooling holes impact the blade strength and life grievously. In this paper, the influences of holes arrangement on creep characteristic of cooling holes in the plate sample are investigated. The constitutive model for creep considering both cavitation and degradation damage is developed to predict the creep behavior of cooling holes. Results show that there are stress interferences among cooling holes. The distance and radius of the cooling holes impact the creep behavior of cooling holes seriously. Decreasing horizontal distance of the holes results in creep time reducing. On the contrary, increasing the vertical distance of the holes makes the creep time reduced.
Article
Full-text available
Observational studies show that voids are prominent features of the large scale structure of the present day Universe. Even though their emerging from the primordial density perturbations and evolutionary patterns differ from dark matter halos, N-body simulations and theoretical models have shown that voids also merge together to form large void structures. In this study, following Sheth & van de Weygaert (2004), we formulate an analytical approximate description of the hierarchical void evolution of growing voids by adopting the halo merging algorithm given by Lacey & Cole (1993) in the Einstein de Sitter (EdS) Universe. To do this, we take into account the general volume distribution of voids which consists of two main void processes: merging and collapsing. We show that the volume distribution function can be reduced to a simple form, by neglecting the collapsing void contribution since the collapse process is negligible for large size voids. Therefore, the void volume fraction has a contribution only from growing voids. This algorithm becomes the analogue of the halo merging algorithm. Based on this growing void distribution, we obtain the void merging algorithm in which we define and formulate void merging and absorption rates, as well as void size and redshift survival probabilities and also failure rates in terms of the self similar and currently favored dark energy dominated cold dark matter models in the EdS Universe.
Article
Full-text available
An important failure mechanism in ductile metals and their alloys is by growth and coalescence of microscopic voids. In structural materials, the voids nucleate at inclusions and second-phase particles by decohesion of the particle–matrix interface or by particle cracking. Void growth is driven by plastic deformation of the surrounding matrix. Early micromechanical treatments of this phenomenon considered the growth of isolated voids. Later, constitutive equations for porous ductile solids were developed based on homogenization theory. Among these, the most widely known model was developed by Gurson for spherical and cylindrical voids.
Article
Full-text available
The concept of mechanism is analyzed in terms of entities and activities, organized such that they are productive of regular changes. Examples show how mechanisms work in neurobiology and molecular biology. Thinking in terms of mechanisms provides a new framework for addressing many traditional philosophical issues: causality, laws, explanation, reduction, and scientific change.
Article
Full-text available
After a decade of intense debate about mechanisms, there is still no consensus characterization. In this paper we argue for a characterization that applies widely to mechanisms across the sciences. We examine and defend our disagreements with the major current contenders for characterizations of mechanisms. Ultimately, we indicate that the major contenders can all sign up to our characterization. KeywordsMechanism–Explanation–MDC–Glennan–Bechtel–Astrophysical mechanism
Article
Background: Identification of damage at an early stage is crucial for critical structural components. Objective: Creep induced micro-voids in heat treated polycrystalline pure copper are experimentally and numerically characterized. Methods: This is accomplished by the use of non-linear ultrasonic waves. Numerically, the study is carried out on an elastic material with randomly located micro-voids. The finite difference in time domain method is used. Results: Experimentally it is found that material damage due to micro-voids that are developed before 40% creep life may effectively be detected with nonlinear ultrasonic waves in the low power regime. Increase in second order nonlinearity parameter with percentage creep life is observed along with the corresponding increase in micro-void concentration. Optical metallography and micro-hardness measurements were used to corroborate the experimental results. For the numerical studies, void dimensions selected for study are much lower than the probing wave length. Concentration of micro-voids is varied from 0.01% to 3%. The second order nonlinearity parameter showed a slight increase in the initial stages and an abrupt increase with higher micro-void concentration in simulations. This agrees with experimental trends. Conclusion: The feasibility of detecting micro-voids in the early stages of creep using nonlinear ultrasonic waves is experimentally demonstrated and a simple model with voids is numerically studied to show the sensitivity of the second order nonlinearity parameter to micro voids.
Article
The inter-hole interference of nickel-based single crystal film-cooling holes specimen during creep deformation were investigated by both experiment and crystal plastic finite element method. The results demonstrated that inter-hole interference promoted the evolution of microstructure, accelerated formation of inter-hole slip zone and accumulation of plastic strain. Based on the above findings, considered the inter-hole interference effect, the existing McClintock model was modified by stress triaxiality, and improved McClintock model provided a relatively good prediction of deformation of film-cooling holes at complex stress state. The influence of inter-hole interference on the fracture failure mode is further studied, of which is mainly achieved by the plastic slip zone between the holes. During fracture progress, it is seen that micro voids and cracks coalesced in this region. Comparing the stripe plastic slip zone observed in experiments and the fracture failure mode of film cooling hole specimen predicted by the finite element method, it was found that the predicted potential fracture paths showed excellent agreement with the experimental results.
Article
One of the most confounding controversies in the ductile fracture community is the large discrepancy between predicted and experimentally observed strain-to-failure values during shear-dominant loading. Currently proposed solutions focus on better accounting for how the deviatoric stress state influences void growth or on measuring strain at the microscale rather than the macroscale. While these approaches are useful, they do not address a significant aspect of the problem: the only rupture micromechanisms that are generally considered are void nucleation, growth, and coalescence (for tensile-dominated loading), and shear-localization and void coalescence (for shear-dominated loading). Current phenomenological models have thus focused on predicting the competition between these mechanisms based on the stress state and the strain-hardening capacity of the material. However, in the present study, we demonstrate that there are at least five other failure mechanisms. Because these have long been ignored, little is known about how all seven mechanisms interact with one another or the factors that control their competition. These questions are addressed by characterizing the fracture process in three high-purity face-centered cubic (FCC) metals of medium-to-high stacking fault energy: copper, nickel, and aluminum. These data demonstrate that, for a given stress state and material, several mechanisms frequently work together in a sequential manner to cause fracture. The selection of a failure mechanism is significantly affected by the plasticity-induced microstructural evolution that occurs before tearing begins, which can create or eliminate sites for void nucleation. At the macroscale, failure mechanisms that do not involve cracking or pore growth were observed to facilitate subsequent void growth and coalescence processes. While the focus of this study is on damage accumulation in pure metals, these results are also applicable to understanding failure in engineering alloys.
Article
Considering the effects of numbers and drill processes of film cooling holes, the effect of a multiaxial stress state on the creep behavior of Ni-based single-crystal superalloy was investigated. The thin-wall plate specimens with film cooling holes, including multiholes and a single central hole, have been designed. The drilling processes are electro stream machining (ESM), electrical discharge machining (EDM) and laser drilling (LD). The results show that creep lives were longer in the thin-wall specimens with a single central hole and shorter in specimens with multi-holes due to the multihole interference effect. Under the same stress and temperature, the specimens with multiholes using the LD process have the longest creep lives compared with the other two processes. To predict the creep life of a specimen with film cooling holes, a creep damage constitutive model under the crystal plasticity framework was proposed by adding geometric structure variables β and m. Coupling with the modified skeletal point stress method (SPSM), the creep rupture lives of the specimens with film cooling holes were well predicted. The error of creep life between the finite element (FE) calculation and experimental results is within 28.03%.
Article
This manuscript presents a new phase field model for simulating galvanic and pitting corrosion phenomena in metallic materials. The Laplace equation is employed to approximate the electric potential distribution, which determines the phase evolution by relating the anodic current density to the interface kinetics parameter. While the anode is assumed to be nonpo-larizable, the nonlinear polarization behavior including the diffusion-limited kinetics is considered as boundary condition on the cathode. Several numerical examples are presented to verify the accuracy of the proposed model. We also demonstrate the application of this model for simulating coupled galvanic-pitting corrosion processes in a hybrid joint and an aluminum composite material under varying environmental conditions. The last example simulates the corrosion of a steel wire, which shows the feasibility of incorporating homogeneous chemical reactions and polarization behavior on the anode into the proposed model.
Book
Scientists studying the burning of stars, the evolution of species, DNA, the brain, the economy, and social change, all frequently describe their work as searching for mechanisms. Despite this fact, for much of the twentieth century philosophical discussions of the nature of mechanisms remained outside philosophy of science. The Routledge Handbook of Mechanisms and Mechanical Philosophy is an outstanding reference source to the key topics, problems, and debates in this exciting subject and is the first collection of its kind. Comprising over thirty chapters by a team of international contributors, the Handbook is divided into four Parts: Historical perspectives on mechanisms The nature of mechanisms Mechanisms and the philosophy of science Disciplinary perspectives on mechanisms. Within these Parts central topics and problems are examined, including the rise of mechanical philosophy in the seventeenth century; what mechanisms are made of and how they are organized; mechanisms and laws and regularities; how mechanisms are discovered and explained; dynamical systems theory; and disciplinary perspectives from physics, chemistry, biology, biomedicine, ecology, neuroscience, and the social sciences. Essential reading for students and researchers in philosophy of science, the Handbook will also be of interest to those in related fields, such as metaphysics, philosophy of psychology, and history of science. © 2018 selection and editorial matter, Stuart Glennan and Phyllis Illari. All rights reserved.
Article
Despite widespread evidence that fictional models play an explanatory role in science, resistance remains to the idea that fictions can explain. A central source of this resistance is a particular view about what explanations are, namely, the ontic conception of explanation. According to the ontic conception, explanations just are the concrete entities in the world. I argue this conception is ultimately incoherent and that even a weaker version of the ontic conception fails. Fictional models can succeed in offering genuine explanations by correctly capturing relevant patterns of counterfactual dependence and licensing correct inferences. Using the example of Newtonian force explanations of the tides, I show how, even in science, fiction can be a vehicle for truth.
Article
A yield criterion is developed which unifies void growth and void coalescence theories. Standard void growth theory assumes that plastic flow is diffuse, if not prevalent everywhere within the matrix of the elementary cell considered. On the other hand, void coalescence theory assumes states of post-localized plasticity whereby plastic flow is restricted to intervoid ligaments. The new theory accommodates both scenarios through some appropriate choice of microscopic velocity fields. An important implication for actual evolution problems is a seamless transition from void growth to void coalescence. This is in contrast with previous hybrid approaches whereby abrupt transitions are associated with the presence of unavoidable corners in the effective yield surface. More generally, the new criterion is applicable to describe yielding in porous metal plasticity for both low and high void volume fractions.
Article
What distinguishes good explanations in neuroscience from bad? This book constructs and defends standards for evaluating neuroscientific explanations that are grounded in a systematic view of what neuroscientific explanations are: descriptions of multilevel mechanisms. In developing this approach, it draws on a wide range of examples in the history of neuroscience (e.g., Hodgkin and Huxley's model of the action potential and LTP as a putative explanation for different kinds of memory), as well as recent philosophical work on the nature of scientific explanation.
Article
In this paper I apply the mechanistic account of explanation to engineering science. I discuss two ways in which this extension offers further development of the mechanistic view. First, functional individuation of mechanisms in engineering science proceeds by means of two distinct sub types of role function, behavior function and effect function, rather than role function simpliciter. Second, it offers refined assessment of the explanatory power of mechanistic explanations. It is argued that in the context of malfunction explanations of technical systems, two key desiderata for mechanistic explanations, ‘completeness and specificity’ and ‘abstraction’, pull in opposite directions. I elaborate a novel explanatory desideratum to accommodate this explanatory context, dubbed ‘local specificity and global abstraction’, and further argue that it also holds for mechanistic explanations of malfunctions in the biological domain. The overall result is empirically-informed understanding of mechanistic explanation in engineering science, thus contributing to the ongoing project of understanding mechanistic explanation in novel or relatively unexplored domains. I illustrate these claims in terms of reverse engineering and malfunction explanations in engineering science.
Article
The aim of this investigation is to establish a universal, accurate and efficient fracture criterion for ductile metals. First, new experiments including pure torsion, uniaxial tension followed by torsion and non-proportional biaxial compression on the Al 2024-T351 alloy are presented. These experimental results, along with published data on same material by Stoughton and Yoon (2011), are used to establish a phenomenological fracture criterion using the magnitude of stress vector and the first invariant of stress tensor. The results are compared to, and shown better than, the maximum shear stress fracture criterion proposed by Stoughton and Yoon, J2 fracture criterion and the Xue–Wierzbicki fracture criterion. Moreover, the proposed fracture criterion is used to correlate the ductile fracture data of another aluminium alloy published by Brünig et al. (2008).
Article
Ductile failure is governed by the evolution of micro-voids within a material. The micro-voids, which commonly initiate at second phase particles within metal alloys, grow and interact with each other until failure occurs. The evolution of the micro-voids, and therefore ductile failure, depends on many parameters (e.g., stress state, temperature, strain rate, void and particle volume fraction, etc.). In this study, the stress state dependence of the ductile failure of Al 5083-H116 is investigated by means of 3-D Finite Element (FE) periodic cell models. The cell models require only two pieces of information as inputs: (1) the initial particle volume fraction of the alloy and (2) the constitutive behavior of the matrix material. Based on this information, cell models are subjected to a given stress state, defined by the stress triaxiality and the Lode parameter. For each stress state, the cells are loaded in many loading orientations until failure. Material failure is assumed to occur in the weakest orientation, and so the orientation in which failure occurs first is considered the critical orientation. The result is a description of material failure that is derived from basic principles and requires no fitting parameters. Subsequently, the results of the simulations are used to construct a homogenized material model, which is used in a component-scale FE model. The component-scale FE model is compared to experiments and is shown to over predict ductility. By excluding smaller nucleation events and load path non-proportionality, it is concluded that accuracy could be gained by including more information about the true microstructure in the model; emphasizing that its incorporation into micromechanical models is critical to developing quantitatively accurate physics-based ductile failure models.
Article
There is a growing recognition that fictions have a number of legitimate functions in science, even when it comes to scientific explanation. However, the question then arises, what distinguishes an explanatory fiction from a nonexplanatory one? Here I examine two cases—one in which there is a consensus in the scientific community that the fiction is explanatory and another in which the fiction is not explanatory. I shall show how my account of “model explanations” is able to explain this asymmetry, and argue that realism—of a more subtle form—does have a role in distinguishing explanatory from nonexplanatory fictions.
Article
Explanatory unification—the urge to “explain much by little”—serves as an ideal of theorizing not only in natural sciences but also in the social sciences, most notably in economics. The ideal is occasionally challenged by appealing to the complexity and diversity of social systems and processes in space and time. This article proposes to accommodate such doubts by making a distinction between two kinds of unification and suggesting that while such doubts may be justified in regard to mere derivational unification (which serves as a formal constraint on theories), it is less justified in the case of ontological unification (which is a result of factual discovery of the actual degree of underlying unity in the world).
Article
In this article I assess Alisa Bokulich’s idea that explanatory model fictions can be genuinely explanatory. I draw attention to a tension in her account between the claim that model fictions are explanatorily autonomous, and the demand that model fictions be justified in order for them to be genuinely explanatory. I also explore the consequences that arise from Bokulich’s use of Woodward’s account of counterfactual explanation and her abandonment of Woodward’s notion of an intervention. As it stands, Bokulich’s account must be deemed unworkable.
Article
Instances of negative causation—preventions, omissions, and the like—have long created philosophical worries. In this paper, I argue that concerns about negative causation can be addressed in the context of causal explanation generally, and mechanistic explanation specifically. The gravest concern about negative causation is that it exacerbates the problem of causal promiscuity—that is, the problem that arises when a particular account of causation identifies too many causes for a particular effect. In the explanatory context, the problem of promiscuity can be solved by characterizing the phenomenon to be explained as a contrast between two or more events or non-events. This contrastive strategy also can solve other problems that negative causation presents for the leading accounts of mechanistic explanation. Along the way, I argue that to be effective, accounts of causal explanation must incorporate negative causation. I also develop a taxonomy of negative causation and incorporate each variety of negative causation into the leading accounts of mechanistic explanation.
Article
Cup-and-cone fracture in single-phase ductile metals appears to originate at holes formed by drawing away of material from non-metallic inclusions, as suggested by Tipper. In copper, the holes expand under the triaxial stresses in the neck and coalesce in a macroscopic fissure; in α iron fine cracks are formed by the stress concentrated at the holes. In coarsegrained material shear cracks are formed on the surface of the neck.Pure polycrystalline aluminium separates at the neck of a tensile specimen by slipping-off along a plane of shear. This is thought to be the usual mode of failure in materials in which work-hardening has been exhausted.
Article
In this paper I argue that holes are not objects, but should instead be construed as properties or relations. The argument proceeds by first establishing a claim about angles: that angles are not objects, but properties or relations. It is then argued that holes and angles belong to the same category, on the grounds that they share distinctive existence and identity conditions. This provides an argument in favour of categorizing holes as one categorizes angles. I then argue that a commitment to the existence of properties to be identified with holes provides sufficient resources to account for true claims about holes.
Article
The cold expansion of fastener holes in aircraft components is a standard technique to improve fatigue life. There is concern that the beneficial residual stresses arising from cold expansion may relax due to creep. This paper describes experimental measurement and finite element prediction of cold expansion residual stresses and their redistribution after creep, with and without additional mechanical load. Experimental measurements of near-surface stresses used an X-ray diffraction technique while average through-the-thickness stresses were measured using a new method based on Sachs' boring. This new method allowed non-axisymmetric residual stresses to be measured, as was the case when creep relaxation occurred with mechanical load. Axisymmetric and three dimensional finite element analyses were used to predict the stress distribution through the thickness of the component showing good agreement with measurement. Creep relaxation of residual stress does indeed occur, but some benefit of the cold expansion remains, particularly when creep relaxation is combined with the application of mechanical load.
Article
This book develops a manipulationist theory of causation and explanation: causal and explanatory relationships are relationships that are potentially exploitable for purposes of manipulation and control. The resulting theory is a species of counterfactual theory that (I claim) avoids the difficulties and counterexamples that have infected alternative accounts of causation and explanation, from the Deductive-Nomological model onwards. One of the key concepts in this theory is the notion of an intervention, which is an idealization of the notion of an experimental manipulation that is stripped of its anthropocentric elements. This notion is used to provide a characterization of causal relationships that is non-reductive but also not viciously circular. Relationships that correctly tell us how the value of one variable Y would change under interventions on a second variable Y are invariant. The notion of an invariant relationship is more helpful than the notion of a law of nature (the notion on which philosophers have traditionally relied) in understanding how explanation and causal attribution work in the special sciences.
Article
The physical and/or intrinsic connection approach to causation has become prominent in the recent literature, with Salmon, Dowe, Menzies, and Armstrong among its leading proponents. I show that there is a type of causation, causation by disconnection, with no physical or intrinsic connection between cause and effect. Only Hume-style conditions approaches and hybrid conditions-connections approaches appear to be able to handle causation by disconnection. Some Hume-style, extrinsic, absence-relating, necessary and/or sufficient condition component of the causal relation proves to be needed.
Article
A general theory of void nucleation in irradiated metals is developed via the nodal line/critical point formalism of Poincaré. The theory reduces to several simple subcases, depending on the experimental conditions, in particular the damage rate, temperature, and gaseous and non-gaseous impurity concentrations. The most important of these subcases are: 1. (1) Spontaneous (i.e. no activation barrier) void nucleation driven by inert gas. 2. (2) Heterogeneous nucleation on metastable gas:vacancy clusters. 3. (3) Heterogeneous nucleation on clusters of non-gaseous impurities. 4. (4) Homogeneous nucleation with or without the assistance of mobile, surface active impurities. Simple nucleation rate equations are presented for each of these cases. Equations for void number density are derived for the various steady state nucleation cases on the basis of a simple void growth equation. The predictions of void number densities are found in reasonable agreement with measurements on pure Ni, type 316 stainless steel, and on the Nimonic alloy, PE-16. The theory also reproduces the shape of the void number density vs 1 T curves-including the activation energies.
Article
FOR metals undergoing slow deformation at elevated temperatures, it has been shown that nucleation of voids by condensation of lattice vacancies requires either a very high concentration of excess vacancies (approximately 100 times the equilibrium value1), or very high concentration of stress2. It appears that such concentrations of vacancies are never approached even at high rates of deformation3, so that voids must form at points where sufficiently large stresses develop. It has been suggested that ledges on a sliding grain boundary may provide these sites for nucleation.
Article
The failure analysis of the 70 MW gas turbine first stage blade made of nickel-base alloy Inconel 738LC is presented. The blades experience internal cooling hole cracks in different airfoil sections assisted by a coating and base alloy degradation due to operation at high temperature. A detailed analysis of all elements which had an influence on the failure initiation was carried out, namely: loss of aluminium from coating due to oxidation and coating phases changing; decreasing of alloy ductility and toughness due to carbides precipitation in grain boundaries; degradation of the alloy gamma prime (γ′) phase (aging and coarsening); blade airfoil stress level; evidence of intergranular creep crack propagation. It was found that the coating/substrate crack initiation and propagation was driven by a mixed fatigue/creep mechanism. The coating degradation facilitates the crack initiation due to thermal fatigue.The substrate intergranular crack initiation and propagation were due to a creep mechanism which was facilitated by grain boundary brittleness caused by formation of a continuous film of carbides on grain boundaries, the degradation of γ′ due to elongation (rafting) and coalescence, and high thermomechanical stress level.
Article
Cottrell and Bilby's theory of the locking of dislocations is extended. The interaction between impurity atoms and dislocations, both edge and screw, is estimated considering the dilation and shear strain associated with carbon atoms in α-iron and using the observed crystallographic relations between the dislocation line, the slip plane and the lattice. It is found that for both types of dislocations the maximum interaction energy is about 0.75 eV and the saturation concentration near the dislocation about 6 atomic per cent. Carbon atoms occupy preferentially three equivalent positions, 120 degrees apart, around a screw dislocation. Many of them are in the slip plane around an edge dislocation. The elastic energy of a screw dislocation is reduced up to 20 per cent in the presence of a carbon atmosphere.
Article
Two distinct mechanisms of crack initiation and advance by void growth have been identified in the literature on the mechanics of ductile fracture. One is the interaction a single void with the crack tip characterizing initiation and the subsequent void by void advance of the tip. This mechanism is represented by the early model of Rice and Johnson and the subsequent more detailed numerical computations of McMeeking and coworkers on a single void interacting with a crack tip. The second mechanism involves the simultaneous interaction of multiple voids on the plane ahead of the crack tip both during initiation and in subsequent crack growth. This mechanism is revealed by models with an em-bedded fracture process zone, such as those developed by Tvergaard and Hutchinson. While both mechanisms are based on void nucleation, growth and coalescence, the inferences from them with regard to crack growth initiation and growth are quantitatively different. The present paper provides a formulation and numerical analysis of a two-dimensional plane strain model with multiple discrete voids located ahead of a pre-existing crack tip. At initial void volume fractions that are sufficiently low, initiation and growth is approximately represented by the void by void mechanism. At somewhat higher initial void volume fractions, a transition in behavior occurs whereby many voids ahead of the tip grow at comparable rates and their interaction determines initiation toughness and crack growth resistance. The study demonstrates that improvements to be expected in fracture toughness by reducing the population of second phase particles responsible for nucleating voids cannot be understood in terms of trends of one mechanism alone. The transition from one mechanism to the other must be taken into account.
Article
The motion of a hole in an infinite viscous body is given for simple shear with superimposed hydrostatic tension. An estimate is made for a plastic body. The holes close under pressure and shear, but reach a steady-state eccentricity and orientation with tension and shear. A criterion for fracture in shear bands, based on coalescence of neighboring holes, indicates an exponential reduction in fracture strain with tension for plasticity. A simple equation approximating the fracture criterion is presented for problems of practical interest.Unter Voraussetzung eines unbegrenzt grossen viskosen krpers wird die Verngerung und Bewegung eines Loches unter einfacher Schubbeanspruchung mit berlagerter hydrostatische Spannung dargestellt.Fr einen plastischen Krper wird eine Abschtzung durchgefhrt. Die Lcher schliessen sich unter Druck and Schub, erreichen abet bei Spannung and Schub eine stationre Exzentrizitt and Orientierung. Ein Kriterium fr den Btuch in Schubzonen, herrhrend vom Zusammenschluss benachbarter Lcher zeigt bei plastischen Krpern snit wachsender Spannung eine exponentielle Verringerung der erforderlichen Bruchbeanspruchung.Fr Probleme von praktischem Interesse wird eine einfache Gleichung, die das Bruchkriterium annhernd wiedergibt vorgestellt.Le mouvement d' un trou dans un corps visqueux et infini est donn pour le cas du tondage simple avec la tension hydrostatique superpose. Les trous se ferment sous la pression et le tondage, mais avec la tension et le tondage ils attaignent une excentricit et une orientation d' tat fixe. Un critere pour la fracture dans les bandes tondues ce qui est fond sur la coalescence des trous voisins indique pour la plasticit qu'il y a une reduction exponentielle dans la deformation ncessaire a fracturer avec tension. On prsent une equation simple qui approxime le critere de fracture pour les problmes d' intrt pratique.
Article
The effect of microscopic voids on the failure mechanism of a ductile material is investigated by considering an elastic-plastic medium containing a boubly periodic array of circular cylindrical voids. For this voided material under uniaxial or biaxial plane strain tension the state of stresses and deformations is determined numerically. Bifurcation away from the fundamental state of deformation is analysed with special interest in a repetitive pattern that represents the state of deformation inside a shear band. Both in the fundamental state and in the bifurcation analysis the interaction between voids and the details of the stress distribution around voids are fully accounted for. Comparison is made with the shear band instabilities predicted by a continuum model of a ductile porous medium. Based on the numerical results an adjustment is suggested for the approximate yield condition in this model of dilatant, pressure sensitive plastic behaviour.
Article
Correctly identifying the mechanism responsible for a failure is a major step in failure analysis. Today, human experts normally perform this task. In the problem-solving process, human experts often recall similar cases to help identifying the mechanism involved. This has motivated the use of case-based reasoning to develop a computerized system for failure-mechanism identification in this study. Major issues and the methods applied are discussed. To determine its accuracy, the system is subsequently evaluated using historical cases, which are classified into two categories: standard and exceptional. The test results show that 100% accuracy can be achieved for standard cases, and that exceptional cases also attain accuracy as high as 71.25%. It is thus concluded that case-based reasoning is a viable approach for the identification of failure mechanisms.
Article
The effects of pitting corrosion on the fatigue behavior of bare 7075-T6 aluminum alloy were investigated. Pitting corrosion decreased the fatigue lives by a factor of about 6 to 8. The fatigue lives were also calculated assuming an equivalent initial flaw corresponding to pits of average and maximum dimensions. The measured fatigue lives generally agreed with the predictions using the average rather than the maximum pit size as the initial crack size. This result could be explained by the pit size distributions offering a significantly larger population of pits near the average size. This work has demonstrated the promise of standardized spray tests for obtaining quantitative measures of corrosion that can be used as inputs in analytical models for fatigue life prediction for evaluating integrity of aircraft structures.
Article
The nucleation and growth of voids during tensile straining has been studied in spheroidized carbon-steels. These results, and others from the literature, are compared with a simple theory of void nucleation and growth.RésuméNous avons étudié la germination et la croissance de cavités au cours de la déformation en traction d'acier à carbone sphéroïdal. Nous comparons ces résultats, ainsi que d'autres résultats de la litérature, avec une théorie simple de la germination et de la croissance des cavités.ZusammenfassungKeimbildung und Wachstum von Hohlräumen wurde unter Zugbelastung an weichgeglühten Kohlenstoffstählen untersucht. Die Ergebnisse und weitere aus der Literatur werden mit einer einfachen Theorie der Keimbildung und des Wachsens von Hohlräumen verglichen.
Article
Upper-bound methods are used to obtain close approximations to the constraint factor in a three-dimensional model of the intervoid matrix at incipient plastic limit-load failure. This is the condition where homogeneous deformation can no longer continue throughout a ductile porous solid and all subsequent plastic deformation becomes concentrated in the intervoid matrix over a single sheet of microvoids; thus producing a ductile-fracture surface. The results are obtained for square-prismatic unit cells, which can be assembled to give a continuous intervoid matrix and a simple empirical expression is fitted to the results to give a closed-form expression for the constraint factor, for use in subsequent work on the mechanics of ductile fracture. The upper-bound results for the plastic limit-loads are used to illustrate, on a quantitative basis, the truncation of the Berg-Gurson dilational-plastic yield locus, which was illustrated in previous work only on a qualitative basis.