Skorohod-Olevsky viscous sintering model sensitivity to temperature distribution during the sintering process

U ovom radu proučavan je uticaj neuniformnosti temperaturnog polja po uzorku na rezultate simulacije procesa sinterovanja. Kao konstitutivni model ponašanja materijala upotrebljen je modifikovan fenomenološki Skorohod-Olevski viskozni model sinterovanja. Poređeni su rezultati simulacije dobijeni korišćenjem uniformne raspodele temperature, sa rezultatima koji koriste temperaturno polje dobijeno simulacijom zagrevanj auzorka. Poređenja se odnose na uzorke različiti hveličina, kako bi se ispitao različiti nivo neuniformnosti. Dobijeni rezultati u ovoj studiji pokazuju da postoje situacije u kojima je potrebno uzeti u obzir neuniformnost raspodele temperature po uzorku, odnosno da pojednostavljenje modela korišćenjem uniformne raspodele dovodi do neprihvatljivih grešaka.This paper investigates the influence of temperature field non-uniformity on sintering simulation results using the Skorohod-Olevsky viscous sintering model. As a difference to previous studies, here a thermal transient analysis is performed to provide a detailed temperature field over the component within sintering time. Results obtained using uniform temperature distribution are compared to those obtained using a nonuniform distribution derived from a transient thermal analysis. Results are compared for different geometry sizes, that lead to different temperature non-uniformity levels. The study has shown that the temperature nonuniformity cannot always be neglected and should be considered as a possible source of modeling error

Skorohod-Olevsky viscous sintering model sensitivity to temperature distribution during the sintering process

U ovom radu proučavan je uticaj neuniformnosti temperaturnog polja po uzorku na rezultate simulacije procesa sinterovanja. Kao konstitutivni model ponašanja materijala upotrebljen je modifikovan fenomenološki Skorohod-Olevski viskozni model sinterovanja. Poređeni su rezultati simulacije dobijeni korišćenjem uniformne raspodele temperature, sa rezultatima koji koriste temperaturno polje dobijeno simulacijom zagrevanj auzorka. Poređenja se odnose na uzorke različiti hveličina, kako bi se ispitao različiti nivo neuniformnosti. Dobijeni rezultati u ovoj studiji pokazuju da postoje situacije u kojima je potrebno uzeti u obzir neuniformnost raspodele temperature po uzorku, odnosno da pojednostavljenje modela korišćenjem uniformne raspodele dovodi do neprihvatljivih grešaka.This paper investigates the influence of temperature field non-uniformity on sintering simulation results using the Skorohod-Olevsky viscous sintering model. As a difference to previous studies, here a thermal transient analysis is performed to provide a detailed temperature field over the component within sintering time. Results obtained using uniform temperature distribution are compared to those obtained using a nonuniform distribution derived from a transient thermal analysis. Results are compared for different geometry sizes, that lead to different temperature non-uniformity levels. The study has shown that the temperature nonuniformity cannot always be neglected and should be considered as a possible source of modeling error

Assessment of residual stresses and mechanical characterization of materials by "hole drilling" and indentation tests combined and by inverse analysis

Hole Drilling (HD) tests are frequently employed as “quasi-non-destructive” experiments, for assessments of residual stresses in metallic components of power plants and of other industrial structures. With respect to the present broadly standardized HD method, the following methodological developments are proposed and computationally validated in this paper: assessments of elastic and plastic parameters by indentation exploiting the hole generated by HD tests; employment of “Digital Image Correlation” (DIC) for full-field displacement measurements, instead of the strain measurements by gauge “rosettes” usually adopted so far; transitions from experimental data to sought parameters by inverse analyses based on computer simulations of both tests and on minimizations of a “discrepancy function”. Interactions between the two experiments are here investigated, besides the elastic parameters transition from indentation (IND) to HD test interpretation. The main advantage achievable by the procedure proposed herein is reduction of additional “damage” and cost due to usual experimental procedures for diagnosis of structural components (e.g. frequently adopted “small punch” experiments or laboratory tension tests)

Inverse Structural Analyses on Small Punch Tests, with Model Reduction and Stochastic Approach

Integrity and durability of metal structures at present can be economically assessed by quasi-non-destructive experiments such as Small Punch Tests already envisaged by international industrial codes. In this communication the following innovative provisions are presented and proposed in view of their contributions to accuracy and economy of diagnostic analyses of structural components: the test is simulated by a finite element computer code; parameters in a popular elastic-plastic material model are computed by inverse analysis; diverse parameters identification procedures are comparatively employed; Proper Orthogonal Decomposition is employed for preliminary model reduction in order to make parameters identification more economical for multiple engineering applications; Kalman filter approach is adopted for stochastic back-analysis in structural diagnosis

Residual stresses estimation by “small punch” experiment

Semi-non-destructive Small Punch (SP) tests, at present frequently employed for mechanical characterization of structural metals in plant components [1], are here employed also for assessment of stresses. The innovative procedure proposed herein belongs to the structural inverse analysis methodology (see e.g. [2]) and can be outline d as follows. The sample removal from an in-service component for SP tests is exploited as external action altering the residual stress state possibly present in the location considered. Full-field measurements by Digital Image Correlation of consequent displacements in the surrounding surface are input for inverse analysis. The inverse analysis leading to deterministic estimation of the plane stress tensor is based on: computer simulations by finite element modelling of the sample removal test, Proper Orthogonal Decomposition (POD) [3] for model reduction apt to simplify the computational operations, discrepancy function minimization with employment of the elasticity parameters provided by the subsequent SP test. Such minimization is performed by a Trust Region Algorithm made fast and economical through preliminary POD. The purpose and practical advantage of the proposed method consist of no more need of Hole Drilling or other tests [4], additional to SP tests with further “damage”, for residual stress estimation and mechanical characterization of the material: namely, the back-analysis procedure proposed herein implies exploitation of present standardized SP test for estimations of both stresses and elastic-plastic properties of materials

Structural diagnoses by synergy of experimental, computational and material mechanics.

Mechanics of continua and its computational methods (e.g. by finite element discretization) make at present possible assessments of the safety margins of structures under extreme loads. Reliability of such results, concerning structures possibly deteriorated in service, clearly requires reliable input data concerning present damages, primarily concerning material properties and pre-existing stress states. Experiments, in laboratory or in situ, can provide the above data, but should be designed in order to reduce consequent damages, costs and times. Transition from results provided by optimized tests to parameters to employ for overall safety analyses requires solution of an inverse analysis problem based on test simulations and centred on the minimization of a “discrepancy function” which quantifies the difference between measurements and their counterparts provided by test simulations, with searched parameters as unknown variables. The above minimization often concerns non-convex objective function and non-linear constraints and, therefore, should be carried out by a suitably selected algorithm (“trust region” mathematical programming, genetic algorithm or artificial neural network). Kalman filters can be employed when probability density distributions of the resulting parameters estimates may be computed as consequences of uncertainties in measurements and in other input data. The present communication purpose is to provide a brief survey of the above methodology, by reference to some recent research results achieved by our team. The following subjects have been selected. (a) A typical procedure is superficially outlined step-by-step by an example concerning brittle fracture: indentation test in situ, finite element simulation, model reduction by “proper orthogonal decomposition”, parameters identification. (b) Back-analysis estimation of both residual stresses and inelastic material properties by standard “small punch” tests on metallic structural components of thermo- or hydro-electrical power plants. (c) Double purposes similar to those in (b), but achievable by a novel “hole drilling” method either in concrete dams or in geological layers crossed for hydrocarbons extraction. The latter procedure might be considered in the current large research project motivated by the Gulf of Mexico disaster and promoted and coordinated jointly by the National Academies, of Engineering and of Science, in the USA

Parameter identification in elastoplastic material models by Small Punch Tests and inverse analysis with model reduction

Small Punch Tests (SPT) are, at present, frequently employed for diagnostic analyses of metallic structural components and are considered in codes of practice because the damage generated by miniature specimen extraction is small (“quasi-non-destructive” tests). This paper contains a description of the following contributions for improvement in the state-of-the-art of SPT practice: assessment of material parameters through inverse analysis, made faster and more economical by employing model reduction through a Proper Orthogonal Decomposition (POD) procedure. The methodology is developed to assess material parameters entering into diverse constitutive models, thus resulting in a more flexible identification framework, capable of addressing different kinds of material behaviour. Within the paper, real experimental data are used and comparisons of computed stress–strain curves with experimentally measured ones, through typical tensile tests, show an excellent agreement

Assessment of both residual stresses and material properties for structural diagnosis “in situ”

The assessments of parameters in inelastic constitutive material models and of residual stresses are at present a research subject of growing importance in several industrial fields either for diagnosis of possibly damaged plants in service or for control of structural component productions. Residual stresses (RS) in metal structures, particularly in hydro- and thermoelectric power plants, are frequently assessed by “quasi-non-destructive” Hole Drilling Tests (HDT), also required by various standards and codes, e.g. [1], and frequently dealt with in related recent literature, e.g. [2]. The research results presented in this congress can be outlined as follows (some details will appear in [3] while a survey of the novel methodological context is available, e.g., in [4]). The RS state to assess is modelled so that the parameters to identify govern it linearly and, hence, inverse analyses can be performed fast “in situ” through a pre-generated matrix, after modelling the HDT by a finite element procedure. Displacements caused by the HDT are measured “full-field” by Digital Image Correlation (DIC) cameras (no longer by strain gauges). Sensitivity analyses are performed in order to optimize the selection of DIC monitored points and the RS modelling. In order to reduce further “damages”, required material properties (usually elastic moduli alone) are assessed by an indentation test (IND) either on the bottom or on the edges of the hole. Inverse analyses by a deterministic approach (a general synthesis in [4]) both on HDT and IND are designed as for interactions between the two tests and are computationally tested for their optimization

Influence of Humidity and Temperature on Mechanical Properties of Corrugated Board - Numerical Investigation

Paper is a material whose mechanical properties are highly dependent on humidity and temperature, naturally building the relationship between the stiffness and strength of corrugated board and changing weather conditions. In this paper, attention is focused on the dependence of the physical properties of the cardboard on changes in humidity and temperature, which undergo dynamic fluctuations both during the production of corrugated board and during its storage. Two techniques were used to test this effect, namely numerical homogenization and global sensitivity analysis. Both methods were implemented to determine the theoretical relationships between the change in humidity and/or temperature in each layer of corrugated board and its global bending, compression, and shear stiffness. The procedure was used to analyze different types of 5-ply and 3-ply cardboard. The obtained results allowed the authors to build a complete map of the relationship between the change in humidity of selected layers and the strength characteristics of the full assembly