The high temperature mechanical characteristics of superplastic 3 mol% yttria stabilized zirconia

A detailed study was undertaken to characterize the deformation behavior of a superplastic 3 mol% yttria-stabilized tetragonal zirconia (3YTZ) over a wide range of strain rates, temperatures and grain sizes. The experimental data were analyzed in terms of the following equation for high temperature deformation: Image Full-size image ∞ σn d−pexp(−Q/RT), where Image Full-size image is the strain rate, σ is the flow stress, d is the grain size, Q is the activation energy, R is the gas constant, T is the absolute temperature, and n and p are constants termed the stress exponent and the inverse grain size exponent, respectively. The experimental data over a wide range of stresses revealed a transition in stress exponent. Deformation in the low and high stress regions was associated with n not, vert, similar 3 and p not, vert, similar 1, and n not, vert, similar 2 and p not, vert, similar 3, respectively. The transition stress between the two regions decreased with increasing grain size. The activation energy was similar for both regions with a value of not, vert, similar 550 kJ mol−1. Microstructural measurements revealed that grains remained essentially equiaxed after the accumulation of large strains, and very limited concurrent grain growths occurred in most experiments. Assessment of possible rate controlling creep mechanisms and comparison with previous studied indicate that in the n not, vert, similar 2 region, deformation occurs by a grain boundary sliding process whose rate is independent of impurity content. Deformation in the n not, vert, similar 3 region is controlled by an interface reaction that is highly sensitive to impurity content. It is concluded that an increase in impurity content increases yttrium segregation to grain boundaries, which enhances the rate of the interface reaction, thereby decreasing the apparent transition stress between the n not, vert, similar 2 and n not, vert, similar 3 regions. This unified approach incorporating two sequential mechanisms can rationalize many of the apparently dissimilar results that have been reported previously for deformation of 3YTZ.

The influence of trace impurities on the mechanical characteristics of a superplastic 2 mol% yttria stabilized zirconia

In contrast to metallic alloys, the mechanical characteristics of superplastic ceramics are very sensitive to minor changes in levels of trace impurities. In the present study, the mechanical behavior of a 2 mol% yttria stabilized tetragonal zirconia was studied in tension and compression in two batches of material, with small variations in levels of trace impurities, to examine the influence of stress axis and impurity content on the deformation behavior. The mechanical properties of the material were characterized in terms of the expression: (epsilon)over dot proportional to sigma(n) where (epsilon)over dot is the strain rate, sigma is the stress and n is termed the stress exponent. The mechanical behavior of the ceramic was identical in tension and compression, for a material with a given level of impurity. The high purity specimens exhibited a transition from a stress exponent of similar to 3 to similar to 2 with an increase in stress, whereas the low purity material displayed only n similar to 2 behavior over the entire stress range studied. Detailed high resolution and analytical electron microscopy studies revealed that there was no amorphous phase at interfaces in both batches of material; however, segregation of Al at interfaces was detected only in the low purity material. The observed transition in stress exponents can be rationalized in terms of two sequential mechanisms: grain boundary sliding with n similar to 2 and interface reaction controlled grain boundary sliding with n similar to 3. The transition from n similar to 3 to similar to 2 occurred at lower stresses with an increase in the grain size and a decrease in the purity level.

Finite Element Solution Of Surface-Tension Driven Flows In Laser Surface-Melting

Lasers are very efficient in heating localized regions and hence they find a wide application in surface treatment processes. The surface of a material can be selectively modified to give superior wear and corrosion resistance. In laser surface-melting and welding problems, the high temperature gradient prevailing in the free surface induces a surface-tension gradient which is the dominant driving force for convection (known as thermo-capillary or Marangoni convection). It has been reported that the surface-tension driven convection plays a dominant role in determining the melt pool shape. In most of the earlier works on laser-melting and related problems, the finite difference method (FDM) has been used to solve the Navier Stokes equations [1]. Since the Reynolds number is quite high in these cases, upwinding has been used. Though upwinding gives physically realistic solutions even on a coarse grid, the results are inaccurate. McLay and Carey have solved the thermo-capillary flow in welding problems by an implicit finite element method [2]. They used the conventional Galerkin finite element method (FEM) which requires that the pressure be interpolated by one order lower than velocity (mixed interpolation). This restricts the choice of elements to certain higher order elements which need numerical integration for evaluation of element matrices. The implicit algorithm yields a system of nonlinear, unsymmetric equations which are not positive definite. Computations would be possible only with large mainframe computers.Sluzalec [3] has modeled the pulsed laser-melting problem by an explicit method (FEM). He has used the six-node triangular element with mixed interpolation. Since he has considered the buoyancy induced flow only, the velocity values are small. In the present work, an equal order explicit FEM is used to compute the thermo-capillary flow in the laser surface-melting problem. As this method permits equal order interpolation, there is no restriction in the choice of elements. Even linear elements such as the three-node triangular elements can be used. As the governing equations are solved in a sequential manner, the computer memory requirement is less. The finite element formulation is discussed in this paper along with typical numerical results.

A pattern adaptive thinning algorithm

A simple sequential thinning algorithm for peeling off pixels along contours is described. An adaptive algorithm obtained by incorporating shape adaptivity into this sequential process is also given. The distortions in the skeleton at the right-angle and acute-angle corners are minimized in the adaptive algorithm. The asymmetry of the skeleton, which is a characteristic of sequential algorithm, and is due to the presence of T-corners in some of the even-thickness pattern is eliminated. The performance (in terms of time requirements and shape preservation) is compared with that of a modern thinning algorithm.

An optimal design for screening trials

Yao, Begg, and Livingston (1996, Biometrics 52, 992-1001) considered the optimal group size for testing a series of potentially therapeutic agents to identify a promising one as soon as possible for given error rates. The number of patients to be tested with each agent was fixed as the group size. We consider a sequential design that allows early acceptance and rejection, and we provide an optimal strategy to minimize the sample sizes (patients) required using Markov decision processes. The minimization is under the constraints of the two types (false positive and false negative) of error probabilities, with the Lagrangian multipliers corresponding to the cost parameters for the two types of errors. Numerical studies indicate that there can be a substantial reduction in the number of patients required.

Decision-theoretic designs for dose-finding clinical trials with multiple outcomes

A decision-theoretic framework is proposed for designing sequential dose-finding trials with multiple outcomes. The optimal strategy is solvable theoretically via backward induction. However, for dose-finding studies involving k doses, the computational complexity is the same as the bandit problem with k-dependent arms, which is computationally prohibitive. We therefore provide two computationally compromised strategies, which is of practical interest as the computational complexity is greatly reduced: one is closely related to the continual reassessment method (CRM), and the other improves CRM and approximates to the optimal strategy better. In particular, we present the framework for phase I/II trials with multiple outcomes. Applications to a pediatric HIV trial and a cancer chemotherapy trial are given to illustrate the proposed approach. Simulation results for the two trials show that the computationally compromised strategy can perform well and appear to be ethical for allocating patients. The proposed framework can provide better approximation to the optimal strategy if more extensive computing is available.

An extension of the continual reassessment method using decision theory

The primary goal of a phase I trial is to find the maximally tolerated dose (MTD) of a treatment. The MTD is usually defined in terms of a tolerable probability, q*, of toxicity. Our objective is to find the highest dose with toxicity risk that does not exceed q*, a criterion that is often desired in designing phase I trials. This criterion differs from that of finding the dose with toxicity risk closest to q*, that is used in methods such as the continual reassessment method. We use the theory of decision processes to find optimal sequential designs that maximize the expected number of patients within the trial allocated to the highest dose with toxicity not exceeding q*, among the doses under consideration. The proposed method is very general in the sense that criteria other than the one considered here can be optimized and that optimal dose assignment can be defined in terms of patients within or outside the trial. It includes as an important special case the continual reassessment method. Numerical study indicates the strategy compares favourably with other phase I designs.

Recipient v russkom razgovore : o raspredelenii funkcij mezhdu otvetami da, nu i tak

The study investigates actions by recipients in spontaneous Russian conversations by focusing on DA, NU and TAK, when they are used as responses to the main speaker's larger on-going turn. The database for the study consists of some 7 hours of spontaneous conversations. The participants of the conversations come from different parts of Russia. The use of DA, NU and TAK was analyzed by applying the method of ethnomethodological conversation analysis from the point of view of the type of the context, the sequential placement of the response and its manner of production. The particles were analyzed both in contexts in which they responded to an informing and in affective contexts. The particles NU and TAK were used by the speakers almost exclusively in informing contexts, whereas DA was the central response type in affective contexts. DA was also the most common response to information with affective implications. The information, to which the particle NU provided as response, was often unspesific and projected a spesification or explanation by its speaker as the next action. DA and TAK, by contrast, treated the information as one that could be followed and was sufficient in its local context. As a response to parenthetical information NU responded to information that was only loosely connected with the mainline of talk. The particle DA, by contrast, was used as a response to such parenthetical information, which was more crucial for the larger on-going activity. Only NU was used as a response that invited the main speaker to continue a turn that she or he had offered as possibly complete. NU was also used by the recipient after her or his own contribution as a continuer. In affective contexts, DA expressed, depending on its more spesific context, not only agreement but also other functions, such as giving up arguing or prior knowledge on the topic being discussed. In addition DA responses were used to display empathy and identification with the state of affairs expressed by the co-participant. NU, by contrast, was seldom used as a response to a turn that expressed affect. When it was used in affective contexts, it displayed agreement with the co-participant or just registered an assessment by her or him.

Mauwake reference grammar

This dissertation provides a synchronic grammatical description of Mauwake, a Papuan (Trans-New Guinea) language of about 2000 speakers on the North Coast of the Madang Province in Papua New Guinea. The theoretical background is that of Basic Linguistic Theory (BLT), used extensively in analysing and writing descriptive grammars. The chapters from morphology to clause level are described from form to function; in the later chapters the function is taken more often as the starting point. Any theory-specific terminology is kept to the minimum and formalisms have been avoided in accordance with BLT principles. Mauwake has a classic 5-vowel system and 14 consonant phonemes. With its simple phonology it is a typical representative of the Madang North Coast languages. For a Papuan language there are relatively few morphophonological alternations. Nouns are either alienably or inalienably possessed. There is no obligatory number marking in nouns or noun phrases. Pronouns have several different forms: five for case and three for other functions. The dative pronouns are treated as [+human] locatives, and they have also grammaticalised as possessives. The verbal morphology is agglutinative and mainly suffixal. Unusual features include two distributive suffixes, and the interaction of the derivational benefactive and the inflectional beneficiary suffixes. The applicative suffix has either transitivising or causative but not benefactive function. The switch-reference system distinguishes between simultaneous and sequential action, as well as same or different subject in relation to the following clause. There are several verbs denoting coming and going, and they may combine with one of three prefixes to indicate bringing and taking. Mauwake is a nominative-accusative type language, and the basic constituent order in a clause is SOV. Subject and object are the only syntactic arguments. There is no indirect object, but a clause can have two or even three objects. A nominalised clause with a finite verb functions as a relative clause or a complement clause; one with a nominalised verb has several different functions. Functional domains described include modality, negation, deixis, quantification, possession and comparison. As there are four negators, Mauwake has more variation in negative expressions than is usual in Papuan languages. Clause chaining is the preferred strategy for joining clauses into sentences, but coordination and subordination of finite clauses are also common. The form of a complement clause depends on whether it is of the fact, action or potential type. Tail-head linkage is used as a cohesive device between sentences. The discourse-level features described are topic and focus.

A Bayesian decision approach for sample size determination in phase II trials

Stallard (1998, Biometrics 54, 279-294) recently used Bayesian decision theory for sample-size determination in phase II trials. His design maximizes the expected financial gains in the development of a new treatment. However, it results in a very high probability (0.65) of recommending an ineffective treatment for phase III testing. On the other hand, the expected gain using his design is more than 10 times that of a design that tightly controls the false positive error (Thall and Simon, 1994, Biometrics 50, 337-349). Stallard's design maximizes the expected gain per phase II trial, but it does not maximize the rate of gain or total gain for a fixed length of time because the rate of gain depends on the proportion: of treatments forwarding to the phase III study. We suggest maximizing the rate of gain, and the resulting optimal one-stage design becomes twice as efficient as Stallard's one-stage design. Furthermore, the new design has a probability of only 0.12 of passing an ineffective treatment to phase III study.

Optimal designs for evaluating a series of treatments

Several articles in this journal have studied optimal designs for testing a series of treatments to identify promising ones for further study. These designs formulate testing as an ongoing process until a promising treatment is identified. This formulation is considered to be more realistic but substantially increases the computational complexity. In this article, we show that these new designs, which control the error rates for a series of treatments, can be reformulated as conventional designs that control the error rates for each individual treatment. This reformulation leads to a more meaningful interpretation of the error rates and hence easier specification of the error rates in practice. The reformulation also allows us to use conventional designs from published tables or standard computer programs to design trials for a series of treatments. We illustrate these using a study in soft tissue sarcoma.

Bias reduction using stochastic approximation

The paper studies stochastic approximation as a technique for bias reduction. The proposed method does not require approximating the bias explicitly, nor does it rely on having independent identically distributed (i.i.d.) data. The method always removes the leading bias term, under very mild conditions, as long as auxiliary samples from distributions with given parameters are available. Expectation and variance of the bias-corrected estimate are given. Examples in sequential clinical trials (non-i.i.d. case), curved exponential models (i.i.d. case) and length-biased sampling (where the estimates are inconsistent) are used to illustrate the applications of the proposed method and its small sample properties.

Error bounds for calculation of the Gittins indices

For a wide class of semi-Markov decision processes the optimal policies are expressible in terms of the Gittins indices, which have been found useful in sequential clinical trials and pharmaceutical research planning. In general, the indices can be approximated via calibration based on dynamic programming of finite horizon. This paper provides some results on the accuracy of such approximations, and, in particular, gives the error bounds for some well known processes (Bernoulli reward processes, normal reward processes and exponential target processes).

Gittins indices and constrained allocation in clinical trials

We explore the use of Gittins indices to search for near optimality in sequential clinical trials. Some adaptive allocation rules are proposed to achieve the following two objectives as far as possible: (i) to reduce the expected successes lost, (ii) to minimize the error probability at the end. Simulation results indicate the merits of the rules based on Gittins indices for small trial sizes. The rules are generalized to the case when neither of the response densities is known. Asymptotic optimality is derived for the constrained rules. A simple allocation rule is recommended for one-stage models. The simulation results indicate that it works better than both equal allocation and Bather's randomized allocation. We conclude with a discussion of possible further developments.

Quantifying progressive anterior overgrowth in the thoracic vertebrae of adolescent idiopathic scoliosis patients

Study design Anterior and posterior vertebral body heights were measured from sequential MRI scans of adolescent idiopathic scoliosis (AIS) patients and healthy controls. Objective To measure changes in vertebral body height over time during scoliosis progression to assess how vertebral body height discrepancies change during growth. Summary of background data Relative anterior overgrowth has been proposed as a potential driver for AIS initiation and progression. This theory proposes that the anterior column grows faster, and the posterior column slower, in AIS patients when compared to healthy controls. There is disagreement in the literature as to whether the anterior vertebral body heights are proportionally greater than posterior vertebral body heights in AIS patients when compared to healthy controls. To some extent, these discrepancies may be attributed to methodological differences. Methods MRI scans of the major curve of 21 AIS patients (mean age 12.5 ± 1.4 years, mean Cobb 32.2 ± 12.8º) and between T4 and T12 of 21 healthy adolescents (mean age 12.1 ± 0.5 years) were captured for this study. Of the 21 AIS patients, 14 had a second scan on average 10.8 ± 4.7 months after the first. Anterior and posterior vertebral body heights were measured from the true sagittal plane of each vertebra such that anterior overgrowth could be quantified. Results The difference between anterior and posterior vertebral body height in healthy, non-scoliotic children was significantly greater than in AIS patients with mild to moderate scoliosis. However there was no significant relationship between the overall anterior-posterior vertebral body height difference in AIS and either severity of the curve or its progression over time. Conclusions Whilst AIS patients have a proportionally longer anterior column than non-scoliotic controls, the degree of anterior overgrowth was not related to the rate of progression or the severity of the scoliotic curve.