binaural weighting of monaural spectral cues for sound localization for human listeners cues for vertical plane localization are provided by direction dependent pinna filtering this study quantified listeners weighting of the spectral cues from each ear as a function of stimulus lateral angle interaural time difference itd and interaural level difference ild subjects indicated the apparent position of headphone presented noise bursts synthesized in virtual auditory space the synthesis filters for the two ears either corresponded to the same location or to two different locations separated vertically by 20deg weighting of each ear s spectral information was determined by a multiple regression between the elevations to which each ear s spectrum corresponded and the vertical component of listeners responses the apparent horizontal source location was controlled either by choosing synthesis filters corresponding to locations on or 30deg left or right of the median plane or by attenuating or delaying the signal at one ear for broadband stimuli spectral weightin

the influence of pinnae based spectral cues on sound localization the role of pinnae based spectral cues was investigated by requiring listeners to locate sound binaurally in the horizontal plane with and without partial occlusion of their external ears the main finding was that the high frequencies were necessary for optimal performance when the stimulus contained the higher audio frequencies e g broadband and 4 0 khz high pass noise localization accuracy was significantly superior to that recorded for stimuli consisting only of the lower frequencies 4 0 and 1 0 khz low pass noise this finding was attributed to the influence of the spectral cues furnished by the pinnae for when the stimulus composition included high frequencies pinnae occlusion resulted in a marked decline in localization accuracy numerous front rear reversals occurred moreover the ability to distinguish among sounds originating within the same quadrant also suffered performance proficiency for the low pass stimuli was not further degraded under conditions of pinnae occlusion in locating the 4 0 khz high pass noise when both neither or only one ear was occluded the data demonstrated unequivocally that the pinna based cues of the near ear contributed powerfully toward localization accuracy

sensitivity of post critical states to changes in design parameters abstract sensitivity of post buckling paths is studied in the context of the general theory of elastic stability of discrete structural systems it is assumed that the sensitivity of the critical state itself has been computed and a formulation is developed to account for sensitivity of the curvature of the post critical states when there are changes in design parameters a linear fundamental path is considered explicit expressions are obtained for the sensitivities and they take the form of perturbation expansions only first order sensitivity of post critical paths has been developed a simple example of an angle section column with deformable cross section illustrates that although a critical state may be insensitive to changes in certain design parameters the post critical response may be highly sensitive in the first example presented an axially loaded angle section column the postbuckling response even changes from stable to unstable depending on the values of the design parameter considered

segmentation and classification of range images the recognition of objects in three dimensional space is a desirable capability of a computer vision system range images which directly measure 3 d surface coordinates of a scene are well suited for this task in this paper we report a procedure to detect connected planar convex and concave surfaces of 3 d objects this is accomplished in three stages the first stage segments the range image into surface patches by a square error criterion clustering algorithm using surface points and associated surface normals the second stage classifies these patches as planar convex or concave based on a non parametric statistical test for trend curvature values and eigenvalue analysis in the final stage boundaries between adjacent surface patches are classified as crease or noncrease edges and this information is used to merge compatible patches to produce reasonable faces of the object s this procedure has been successfully applied to a large number of real and synthetic images four of which we present in this paper

sensitivity analysis of coincident critical loads with respect to minor imperfection abstract a new formulation is presented for sensitivity analysis of a coincident critical load factor only symmetric elastic structures subjected to symmetric loads are considered and sensitivity coefficients are found for a symmetric design modification which corresponds to a minor imperfection it is shown that the formulation for sensitivity analysis of multiple linear buckling load factor can be successfully combined with that of nonlinear buckling to develop a formulation for coincident nonlinear buckling load factor the proposed formulation is verified by analytical examples of simple spring bar systems

continuum topology optimization of buckling sensitive structures twoformulationsforcontinuum topologyoptimizationofstructurestakingbucklingconsiderationsinto account are developed implemented and compared in thee rst the structure undergoing a specie ed loading is modeled as a hyperelastic continuum at e nite deformations and is optimized to maximize the minimum critical buckling load in the second the structure under a similar loading is modeled as linear elastic and the critical buckling load is computed with linearized buckling analysis specie c issues addressed include usage of suitable mixing rules a node based design variable formulation techniques for eliminating regions devoid of structural material from the analysis problem and consistent design sensitivity analysis the performance of the formulations is demonstrated on the design of different structures when problems are solved with moderate loads and generous material usage constraints designs using compression and tension members are realized alternatively when fairly large loads together with very stringent material usage constraints are imposed structures utilizing primarily tension members result issues that arise when designing very light structures with stringent material usage constraints are discussed along with the importance of considering potential geometrical instabilities in the concept design of structural systems

direct computation of qualitative 3 d shape and motion invariants structure from motion often refers to the computation of three dimensional structure from a matched sequence of images however a depth map of a surface is difficult to compute and may not be a good representation for storage and recognition given matched images it is shown that the sign of the normal curvature in a given direction at a given point in the image can be computed from a simple difference of slopes of line segments in one image using this result local surface patches can be classified as convex concave cylindrical hyperbolic saddle point or planar at the same time the translational component of the optical flow from which the focus of expansion can be computed is obtained

virtual localization improved by scaling nonindividualized external ear transfer functions in frequency this study examined virtual sound localization in three conditions that differed according to the directional transfer functions dtfs that were used to synthesize the virtual targets the own ear and other ear conditions used dtfs measured from listeners own ears and those measured from other subjects respectively the scaled ear condition employed other ear dtfs that were scaled in frequency to minimize the mismatch between spectral features in the listener s and the other subject s dtfs all measures of localization error typically were lowest in the own ear condition in other ear conditions all error measures tended to increase in proportion to the inter subject differences in dtfs when spectral features in an other ear set of dtfs fell systematically lower in frequency than in a listener s own dtfs low frontal targets typically were reported as low in the rear and high rear targets were reported as high in front when spectral features in a set of dtfs fell systematically higher in frequency than in a listener s own dtfs elevation judgements showed an upward bias in the scaled ear condition all measures of performance tended to improve relative to the other ear condition in the majority of cases frequency scaling more than halved the penalty for use of another subject s dtfs

sequential optimal truss generator for frequency ranges abstract the concept of an ordered set of optimum designs is introduced here and all the design variables and behavioral variables belonging to the optimum designs are represented by successive piecewise taylor series a most natural direct and efficient way of generating or sweeping out all the optimum designs sequentially in the set is devised the procedure is started with the eigenvalue analysis on the truss defined by the set of all the minimum cross sectional areas neither any further eigenvalue analysis nor application of any optimization technique is required in the proposed procedure it is demonstrated that the proposed method is efficient not only for an ordered set of optimum designs of a large truss associated with the single lowest eigenvector but also for that associated with multiple lowest eigenvectors

review of options for structural design sensitivity analysis part 1 linear systems the paper reviews options for structural design sensitivity analysis including global finite differences continuum derivatives discrete derivatives and computational or automated differentiation the objective is to put these different approaches to design sensitivity analysis in the context of accuracy and consistency computational cost and implementation options and effort linear static analysis and transient dynamic analysis are reviewed in a separate appendix special attention is paid to the semi analytical method a future paper will address design sensitivity analysis in nonlinear structural problems

individual differences in external ear transfer functions reduced by scaling in frequency this study examined inter subject differences in the transfer functions from the free field to the human ear canal which are commonly know as head related transfer functions the directional components of such transfer functions are referred here to as directional transfer functions dtfs the dtfs of 45 subjects varied systematically among subjects in regard to the frequencies of spectral features such as peaks and notches inter subject spectral differences in dtfs were quantified between 3 7 and 12 9 khz for sound source directions throughout the coordinate sphere for each pair of subjects an optimal frequency scale factor aligned spectral features between subjects and thus minimized inter subject spectral differences frequency scaling of dtfs reduced spectral differences by a median value of 15 5 across all pairs of subjects and by more than half in 9 5 of subject pairs optimal scale factors showed a median value of 1 061 and a maximum of 1 38 the optimal scale factor between any pair of subjects correlated highly with the ratios of subjects maximum interaural delays sizes of their external ears and widths of their heads

second order sensitivity analysis in vibration and buckling problems a perturbation approach for the first and second order sensitivity analysis of eigenvalue problems as they arise in buckling and vibration of structural systems is presented a particular feature introduced in the formulation is the dependence of all matrices that model the problem not only on the design parameter but also on a pre critical state thus the sensitivity of the response of the fundamental path in buckling problems or a pre stressed state in vibration problems are introduced in the analysis two techniques are considered in this work the direct and the adjoint methods first and second order sensitivity equations for eigenvalues and eigenvectors are obtained and it is shown that second order sensitivity of an eigenvalue can be accomplished by using only one adjoint problem

design sensitivity of post buckling states including material constraints this paper reports theoretical studies on the design sensitivity of problems with geometrically non linear behavior leading to buckling and post buckling of thin walled structural members for the class of problems considered buckling occurs in the form of a stable bifurcation and it is assumed that changes in the design parameters do not break the bifurcation behavior the specific focus of the research is the first yield or first failure of the material as part of the sensitivity study of equilibrium states along the post critical path the investigation employs a discrete model of a structure in terms of generalized coordinates suitable for finite element analysis and a single load parameter and perturbation techniques to classify the critical state and to approximate the post critical path the problem of material behavior is modeled by means of constraints on the post critical path based on a yield criterion for simplicity the presentation uses the von mises yield criterion but other more complex criteria such as those employed in composite materials first ply failure can also be represented two forms of the constraints are formulated and the problem of sensitivity with respect to changes in a design parameter is discussed a simple example of a circular plate is presented to illustrate the use of the formulation for the sensitivity with respect to a single design parameter

optimization of imperfection sensitive symmetric systems for specified maximum load factor a method is presented for optimum design of a symmetric structure which reaches an unstable bifurcation point as the load factor is increased reduction of the maximum load level due to the antisymmetric imperfection is considered and a straightforward algorithm is proposed for calculating the magnitude of reduction of the load factor corresponding to the most critical mode of antisymmetric imperfection the sensitivity coefficients of the bifurcation load factor are calculated by using the interpolation method developed by the authors and an approximate formulation is presented for sensitivity analysis of the maximum load factor it is shown in the examples that the errors in the sensitivity coefficients do not lead to any significant difference in the optimum design

surface signatures an orientation independent free form surface representation scheme for the purpose of objects registration and matching this paper introduces anew free form surface representation scheme for the purpose of fast and accurate registration and matching accurate registration of surfaces is a common task in computer vision the proposed representation scheme captures the surface curvature information seen from certain points and produces images called surface signatures at these points matching signatures of different surfaces enables the recovery of the transformation parameters between these surfaces we propose using template matching to compare the signature images to enable partial matching another criterion the overlap ratio is used this representation scheme can be used as a global representation of the surface as well as a local one and performs near real time registration we show that the signature representation can be used to recover scaling transformation as well as matching objects in 3d scenes in the presence of clutter and occlusion applications presented include free form object matching multimodal medical volumes registration and dental teeth reconstruction from intraoral images

conditions for symmetric antisymmetric and zero stiffness bifurcation in view of imperfection sensitivity and insensitivity abstract in general it is not clear how to design structures such that they are ab initio imperfection insensitive i e without modifications of the original design after the diagnosis of imperfection sensitivity symmetry and antisymmetry of bifurcation paths representing qualitative properties of a structure not only simplify the postbuckling analysis but also have an influence on the behavior of real i e imperfect structures the special case of a zero stiffness postbuckling path incorporates both symmetry and antisymmetry mathematical definitions of the three categories symmetric antisymmetric and zero stiffness equilibrium paths are given it is shown that symmetry as well as antisymmetry causes the vanishing of specific coefficients in an asymptotic series expansion following from koiter s initial postbuckling analysis thereupon methods of checking for the three categories of bifurcation behavior are discussed finally the three categories are investigated in terms of necessary and or sufficient conditions for imperfection insensitivity for instance a horizontal tangent of the postbuckling path at the bifurcation point is required for imperfection insensitivity four examples illustrate non symmetric symmetric and antisymmetric bifurcation as well as zero stiffness postbuckling behavior for the first two examples the approach of increasing the stiffness results in the conversion of the system from imperfection sensitivity into insensitivity remarkably this happens without changing the prebuckling behavior and the buckling load of the structure the third example demonstrates that imperfection sensitivity is an inevitable implication of antisymmetric bifurcation

robust segmentation of primitives from range data in the presence of geometric degeneracy this paper addresses a common problem in the segmentation of range images we present methods for the least squares fitting of spheres cylinders cones and tori to 3d point data and their application within a segmentation framework least squares fitting of surfaces other than planes even of simple geometric type has rarely been studied our main application areas of this research are reverse engineering of solid models from depth maps and automated 3d inspection where reliable extraction of these surfaces is essential our fitting method has the particular advantage of being robust in the presence of geometric degeneracy i e as the principal curvatures of the surfaces being fitted decrease the results returned naturally become closer and closer to those surfaces of simpler type i e planes cylinders cones or spheres which best describe the data many other methods diverge because in such cases various parameters or their combination become infinite

segmentation through variable order surface fitting the solution of the segmentation problem requires a mechanism for partitioning the image array into low level entities based on a model of the underlying image structure a piecewise smooth surface model for image data that possesses surface coherence properties is used to develop an algorithm that simultaneously segments a large class of images into regions of arbitrary shape and approximates image data with bivariate functions so that it is possible to compute a complete noiseless image reconstruction based on the extracted functions and regions surface curvature sign labeling provides an initial coarse image segmentation which is refined by an iterative region growing method based on variable order surface fitting experimental results show the algorithm s performance on six range images and three intensity images

curvature augmented tensor voting for shape inference from noisy 3d data improves the basic tensor voting formalism to infer the sign and direction of principal curvatures at each input site from noisy 3d data unlike most previous approaches no local surface fitting partial derivative computation nor oriented normal vector recovery is performed in our method these approaches are known to be noise sensitive since accurate partial derivative information is often required which is usually unavailable from real data also unlike approaches that detect signs of gaussian curvature we can handle points with zero gaussian curvature uniformly without first localizing them in a separate process the tensor voting curvature estimation is non iterative does not require initialization and is robust to a considerable amount of outlier noise as its effect is reduced by collecting a large number of tensor votes qualitative and quantitative results on synthetic and real complex data are presented

consistent tangent operators for rate independent elastoplasticity abstract it is shown that for problems involving rate constitutive equations such as rate independent elastoplasticity the notion of consistency between the tangent stiffness operator and the integration algorithm employed in the solution of the incremental problem plays a crucial role in preserving the quadratic rate of asymptotic convergence of iterative solution schemes based upon newton s method within the framework of closest point projection algorithms a methodology is presented whereby tangent operators consistent with this class of algorithms may be systematically developed to wit associative j 2 flow rules with general nonlinear kinematic and isotropic hardening rules as well as a class of non associative flow rules are considered the resulting iterative solution scheme preserves the asymptotic quadratic convergence characteristic of newton s method whereas use of the socalled elastoplastic tangent in conjunction with a radial return integration algorithm a procedure often employed results in newton type of algorithms with suboptimal rate of convergence application is made to a set of numerical examples which include saturation hardening laws of exponential type

a general procedure for the direct computation of turning and bifurcation points we address the formulation and numerical implementation of general purpose algorithme suitable for the direct computation of stability points in non linear solid mechanics the effectiveness of the resulting algorithm is illustrated by numerical simulation that include bifurcation and loss of stability in elastoplasticity non linear rods and non linear three dimensional shells

spectro temporal factors in two dimensional human sound localization this paper describes the effect of spectro temporal factors on human sound localization performance in two dimensions 2d subjects responded with saccadic eye movements to acoustic stimuli presented in the frontal hemisphere both the horizontal azimuth and vertical elevation stimulus location were varied randomly three types of stimuli were used having different spectro temporal patterns but identically shaped broadband averaged power spectra noise bursts frequency modulated tones and trains of short noise bursts in all subjects the elevation components of the saccadic responses varied systematically with the different temporal parameters whereas the azimuth response components remained equally accurate for all stimulus conditions the data show that the auditory system does not calculate a final elevation estimate from a long term order 100 ms integration of sensory input instead the results suggest that the auditory system may apply a multiple look strategy in which the final estim

design sensitivity analysis and optimization for nonlinear buckling of finite dimensional elastic conservative structures the purpose of this review paper is to summarize the existing methods of design sensitivity analysis and optimization of elastic conservative finite dimensional systems with respect to nonlinear buckling behavior difficulties related to geometrical nonlinear singular behaviors are discussed in detail characteristics of optimized structures are demonstrated in reference to snapthrough behavior hill top branching and degenerate critical points a new optimization result of a flexible truss that fully utilizes the snapthrough behavior is also presented

refined semi analytical design sensitivities efficient structural optimization routines require availability of gradient information semi analytical sa design sensitivities are rather popular as they combine ease of implementation with computational efficiency their main drawback however is their well known inaccuracy problem for shape design sensitivities it was found that the inaccuracies are especially unacceptable for slender structures and become more pronounced when relatively large rigid body motions can be identified for individual finite elements based on these observations the authors recently developed a refined sa method taking full advantage of analytical differentiation of rigid body modes the present article presents a sound and unified formulation of refined semi analytical rsa design sensitivities for linear linearized buckling geometrically nonlinear and limit point analyses numerical results are presented in order to demonstrate the efficiency of the proposed method it is concluded that the refined sa method possesses the advantages of the traditional sa method whereas it does not exhibit its unacceptable inaccuracies

semi analytical static nonlinear structural sensitivity analysis the paper discusses two uses of the semi analytical method for nonlinear sensitivity analysis first the application to noncritical response is analyzed it is shown that the semi analytical method is equivalent to a particular form of the overall finite difference approach it is concluded that the difference between overall finite differences and the semi analytical approach is blurred for the case of nonlinear static response next the application of the semi analytical method to calculating sensitivities of limit loads is discussed a formulation that is easy to implement in general purpose finite element programs is derived three examples are used to demonstrate the application of the formulation and to explore its accuracy

parameter sensitivity of inelastic buckling and post buckling response this paper is concerned with recent sensitivity results for buckling and post buckling problems the article starts with an overview of the fem based approach to inelastic structural mechanics systems with singular points along the equilibrium path the existing sensitivity techniques are reviewed their inherent drawbacks are indicated and a new methodology is suggested the discretized formalism is mainly employed throughout but a continuum formulation appropriate for this class of problems is also given numerical studies illustrate the presentation

design sensitivity analysis of elastoplastic structures an iterative and incremental algorithm is presented for the design sensitivity analysis dsa of elastoplastic structures geometrical non linearity is included in the formulation and the structure may be subjected to a cyclic loading the key concept is discussed using an elastoplastic truss it is shown that the design sensitivity coefficients dscs are path dependent and discontinuous at the time at which yielding takes place in a member yield time in the proposed algorithm the yield time is considered to be a function of the design variables in this way the discontinuity in the dscs at the material transition points can be overcome in a simple and routine way incremental response analysis and dsa are carried out simultaneously using the newton raphson type iterative procedure since the proposed algorithm is completely consistent with the analysis procedure it can be implemented into an existing code for structural analysis application to distributed parameter structures with kinematic and or isotropic hardening is discussed using the von mises yield condition and the elastic predictor radial return method dsa is carried out for a ten bar truss with a piecewise linear constitutive relation it is shown that the dscs found using the proposed method agree quite well with those calculated by the central difference method finally an alternate method without any iteration for dsa is proposed and the results from the two methods are compared

characterization of external ear impulse responses using golay codes this report explains the use of a complementary series golay codes for probing the impulse response of the external ear the codes are used to measure both the resonance of the human ear canal using a sealed sound delivery system and to measure the transfer function of the pinna using a free field source with two series of 512 binary numbers the improvement in signal to noise ratio over a single impulse approaches the theoretical value of 30 1 db 10 log 2 512 this technique has many of the same properties as maximal length sequences m r schroeder j acoust soc am 66 497 500 1979 but it has the added advantage that the sequence length is an integer power of two and is therefore particularly convenient to use with modern fourier transform techniques

the influence of duration and level on human sound localization the localization of sounds in the vertical plane elevation deteriorates for short duration wideband sounds at moderate to high intensities the effect is described by a systematic decrease of the elevation gain slope of stimulus response relation at short sound durations two hypotheses have been proposed to explain this finding either the sound localization system integrates over a time window that is too short to accurately extract the spectral localization cues neural integration hypothesis or the effect results from cochlear saturation at high intensities adaptation hypothesis while the neural integration model predicts that elevation gain is independent of sound level the adaptation hypothesis holds that low elevation gains for short duration sounds are only obtained at high intensities here these predictions are tested over a larger range of stimulus parameters than has been done so far subjects responded with rapid head movements to noise bursts in the two dimensional frontal space stimulus durations ranged from 3 to 100 ms sound levels from 26 to 73 db spl results show that the elevation gain decreases for short noise bursts at all sound levels a finding that supports the integration model on the other hand the short duration gain also decreases at high sound levels which is in line with the adaptation hypothesis the finding that elevation gain was a nonmonotonic function of sound level for all sound durations however is predicted by neither model it is concluded that both mechanisms underlie the elevation gain effect and a conceptual model is proposed to reconcile these findings

conversion from imperfection sensitive into imperfection insensitive elastic structures i theory abstract a qualitative improvement of the initial postbuckling behavior of imperfection sensitive elastic structures is their conversion into imperfection insensitive structures by means of modifications of the original design such a conversion is restricted to symmetric bifurcation koiter s initial postbuckling analysis is applied in the framework of the fem to deduce mathematical relations for the transition from imperfection sensitivity to insensitivity which may be achieved by additional supports of the structure this conclusion as well as several other conclusions from the theoretical investigation reported in part i of this paper are corroborated by the results from a comprehensive numerical investigation documented in part ii of this work

sensitivity analysis of bifurcation load of finite dimensional symmetric systems three methods are presented for sensitivity analysis of bifurcation load factor of finite dimensional conservative symmetric systems subjected to a set of symmetric proportional loads in the first method a conventional method with diagonalization is utilized to derive an explicit formula of sensitivity coefficients corresponding to a minor imperfection next a new concept is introduced to find the sensitivity coefficients of the load factor displacements and the eigenmodes under fixed lowest eigenvalue of the tangent stiffness matrix based on this concept a method is presented for finding approximate sensitivity coefficients of the buckling load factor finally a direct method is presented to find the accurate sensitivity coefficients of the bifurcation load factor displacements at buckling and the buckling mode of a symmetric system note that different formula should be used for sensitivity analysis of a limit point load factor in the examples the proposed three methods are compared in view of accuracy of the results and simplicity in coding

design sensitivity analysis and optimization of nonlinear structuralresponse using incremental procedure this paper describes procedures for design sensitivity analysis of nonlinear structural response using the incremental solution scheme the total and updated lagrangian approaches are discussed sensitivity analyses for the stress strain and displacement constraints are developed in the optimization process the buckling load constraint is also imposed numerical aspects of design sensitivity analysis are discussed three numerical examples are solved to illustrate the design sensitivity analysis and optimization process the numerical procedures are stable and work well they can be applied to more complex structures to optimize them

bifurcation hierarchy of symmetric structures abstract a group theoretic method for the analysis of bifurcation behavior of regular polygonal symmetric structures is described possible bifurcation paths and points of these structures are categorized in terms of dihedral and cyclic groups which express the symmetry in particular we offer a complete description of those double bifurcation points which occur due to group symmetry the type the number and the stability of bifurcation paths branching at these points are determined by deriving bifurcation equations the existence of a potential function plays a substantial role for the existence of bifurcation paths as a result of these all possible bifurcation process can be known a priori as a natural consequence of bifurcation hierarchy before actual numerical analysis an implementation of this method in numerical analysis shows its validity and usability

global segmentation and curvature analysis of volumetric data sets using trivariate b spline functions this paper presents a method to globally segment volumetric images into regions that contain convex or concave elliptic iso surfaces planar or cylindrical parabolic iso surfaces and volumetric regions with saddle like hyperbolic iso surfaces regardless of the value of the iso surface level the proposed scheme relies on a novel approach to globally compute bound and analyze the gaussian and mean curvatures of an entire volumetric data set using a trivariate b spline volumetric representation this scheme derives a new differential scalar field for a given volumetric scalar field which could easily be adapted to other differential properties moreover this scheme can set the basis for more precise and accurate segmentation of data sets targeting the identification of primitive parts since the proposed scheme employs piecewise continuous functions it is precise and insensitive to aliasing

experiments in curvature based segmentation of range data this paper focuses on the experimental evaluation of a range image segmentation system which partitions range data into homogeneous surface patches using estimates of the sign of the mean and gaussian curvatures the authors report the results of an extensive testing program aimed at investigating the behavior of important experimental parameters such as the probability of correct classification and the accuracy of curvature estimates measured over variations of significant segmentation variables evaluation methods in computer vision are often unstructured and subjective this paper contributes a useful example of extensive experimental assessment of surface based range segmentation

sensitivity analysis for frictional contact problems in the augmented lagrangian formulation direct differentiation method of sensitivity analysis is developed for frictional contact problems as a result of the augmented lagrangian treatment of contact constraints the direct problem is solved simultaneously for the displacements and lagrange multipliers using the newton method the main purpose of the paper is to show that this formulation of the augmented lagrangian method is particularly suitable for sensitivity analysis because the direct differentiation method leads to a non iterative exact sensitivity problem to be solved at each time increment the approach is applied to a general class of three dimensional frictional contact problems and numerical examples are provided involving large deformations multibody contact interactions and contact smoothing techniques

edge region based segmentation of range images in this correspondence we present a new computationally efficient three dimensional 3 d object segmentation technique the technique is based on the detection of edges in the image the edges can be classified as belonging to one of the three categories fold edges semistep edges defined here and secondary edges the 3 d image is sliced to create equidepth contours edcs three types of critical points are extracted from the edcs a subset of the edge pixels is extracted first using these critical points the edges are grown from these pixels through the application of some masks proposed in this correspondence the constraints of the masks can be adjusted depending on the noise present in the image the total computational effort is small since the masks are applied only over a small neighborhood of critical points edge regions furthermore the algorithm can be implemented in parallel as edge growing from different regions can be carried out independently of each other

structural design optimization of shallow frame structures undergoing large deflections an optimization algorithm for structural design against instability is developed for shallow beam structures undergoing large deflections the algorithm is based on the maximization of the limit load under specified volume constraint the analysis for obtaining the limit load involves coupling of axial and bending deformations and is based on the nonlinear finite element analysis using the displacement control technique the optimization is carried out using both the sequential quadratic programming sqp and optimality criterion oc techniques and the results are compared for the sqp technique the sensitivity derivatives of the critical load factor are calculated using the adjoint method based on the information obtained from the nonlinear buckling analysis a shallow plane arch illustrates the structural design optimization methodology and the results are compared with those in the literature it is shown that a design based on the generalized eigenvalue problem linear buckling gives an optimum limit load less than the initial limit load whereas the optimization using the nonlinear buckling analysis obtains a larger value for the optimum limit load compared to the initial limit load it has also been demonstrated that the optimum results obtained using oc technique are in good agreement with those obtained through sqp technique however the computational time for the oc is significantly lower than that of sqp which requires search techniques and sensitivity of the limit load for its successful completion and termination

elastic postbuckling analysis via finite element and perturbation techniques part 1 formulation the main equations for the equilibrium stability and critical state analysis of discrete elastic systems are presented following the works of thompson but in such a way that the original set of generalized coordinates and loads are preserved in the total potential energy this introduces differences in the resulting equations in bifurcation analysis but does not introduce any new feature regarding the physics of the problem the new formulation is approximated by means of a standard finite element approach based on interpolation of displacements in which the derivatives of the potential energy are approximated the terms retained are those of moderately large rotation theory the energy analysis is finally related to the more conventional finite element notation in terms of stiffness matrices and it is shown how in such a way it can be included in present day codes part 2 of the paper deals with applications to the analysis of shells of revolution using a semi analytical approximation two cases are presented in detail bifurcation in axisymmetric and in asymmetric modes and the results show good correlation with those of other authors the influence of load and geometric imperfections is evaluated

efficient handling of stability problems in shell optimization by asymmetric worst case shape imperfection the paper presents an approach to shape optimization of proportionally loaded elastic shell structures under stability constraints to reduce the stability related problems a special technique is utilized by which the response analysis is always terminated before the first critical point is reached in this way the optimization is always related to a precritical structural state the necessary load carrying capability of the optimal structure is assured by extending the usual formulation of the optimization problem by a constraint on an estimated critical load factor since limit points are easier to handle the possible presence of bifurcation points is avoided by introducing imperfection parameters they are related to an asymmetric shape perturbation of the structure during the optimization the imperfection parameters are updated to get automatically the worst case pattern and amplitude of the imperfection both the imperfection parameters and the design variables are related to the structural shape via the design element technique a gradient based optimizer is employed to solve the optimization problem three examples illustrate the proposed approach copyright 2007 john wiley sons ltd

design sensitivity analysis of non linear structures in regular and critical states abstract a general design sensitivity formulation is introduced for non linear elastic structures reaching critical equilibrium states a discretized formulation is considered first and the sensitivity analysis is discussed for regular and critical states including analysis of post critical behavior next a variational approach is presented for beam and surface structures for which generalized stress and strain operator formulation is applied the formulae for sensitivity of bifurcation loads are derived in terms of adjoint fields which are shown to be equivalent to post buckling fields in the case of symmetric bifurcation

sensitivity analysis and optimization corresponding to a degenerate critical point sensitivity coefficients of a critical load factor corresponding to a degenerate critical point is shown to be unbounded even for a minor imperfection excluding very restricted case where the imperfection does not have direct effect on the lowest eigenvalue of the stability matrix this fact leads to serious difficulty in obtaining optimum design under nonlinear stability constraints the optimum design problem is alternatively formulated with constraint on the lowest eigenvalue of the stability matrix and the sensitivity formula for the lowest eigenvalue is presented the existence of a degenerate critical point and accuracy of the sensitivity coefficient are discussed through the example of a four bar truss

remarkable postbuckling paths analyzed by means of the consistently linearized eigenproblem in addition to the determination of load levels at critical points of stability problems frequently the postbuckling behavior is of interest for non linear problems the so called consistently linearized eigenvalue problem is a suitable tangent linearization method which facilitates determination of stability limits the solution process of the eigenproblem is significantly simplified by appropriate coordinate transformations within this process characteristic shapes of eigenvalue curves allow identification of bifurcation buckling modes snap through modes and hilltop buckling modes mathematical properties of the eigenvalue curves are addressed and conclusions regarding the shape of postbuckling paths are drawn considerations also touch upon the conversion from imperfection sensitivity into insensitivity the theoretical findings are corroborated by examples dealing with a von mises truss and a similar discrete system showing a remarkable postbuckling behavior such as a zero stiffness equilibrium path for both systems the same approach of stiffness increase allows conversion from imperfection sensitivity into insensitivity copyright 2008 john wiley sons ltd

recovery of individual head related transfer functions from a small set of measurementsa head related transfer functions hrtfs vary with individuals and in practice measuring hrtfs with high directional resolution for each individual is tiresome based on a basis functions representation of hrtfs the present work proposes a method for recovering individual hrtfs from a small set of measurements the hrtfs are represented by a combination of a small set of spatial basis functions sbfs with frequency and individual dependent weights the sbfs are derived by applying spatial principal component analysis to a baseline hrtf dataset with high directional resolution the individual weights for any subject outside the dataset are estimated from measurements at a few source directions and then the hrtfs with high directional resolution are recovered by combining the sbfs and the individual weights in an illustrative case the sbfs derived from a baseline dataset that includes 20 subjects are used to recover the hrtf magnitudes for six subjects outside the baseline dataset results show that individual hrtf magnitudes can be recovered from measurements at 73 directions with a mean signal to distortion ratio of 19 db the proposed method is also applicable to recovering head related impulse responses the results of psychoacoustic experiments indicate that in most cases the recovered and measured hrtfs are indistinguishable