Creep Analysis of Thin-Walled Structures

From experimental modeling of shotcrete to numerical simulations of tunneling

A new incremental creep theory of isotropic materials based on the three invariants of stress tensor is proposed. In order to evaluate the constants in the constitutive equations the basic experiments are formulated. Special creep relations that contain less constants and invariants are derived. The approach has been applied to the problem of creep of anisotropic materials with different behaviour in tension and compression.

We improved and upgraded the apparatus that Fillers, Moonan, and Tschoegl used several years ago to investigate the influence of pressure and temperature on the mechanical properties of time dependent polymeric materials. The new apparatus can measure the volume and the shear relaxation moduli of solid polymer specimens, subjected to a combination of temperatures from −60°C to +110°C, and pressures from atmospheric to 360 MPa. The paper demonstrates the capabilities of the new apparatus from the scientific as well as engineering stand point. Shear relaxation measurements on poly(vinyl acetate) (PVAc) and styrene-butadiene rubber (SBR) are also presented. For PVAc we present also the bulk creep compliance, coefficient of thermal expansion and the bulk modulus.KeywordsReference TemperatureVinyl AcetateShift FactorLinear Variable Differential TransformerFractional Free VolumeThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

This paper, which is the first of two in a series, provides an overview of a viscoplastic constitutive model that accounts for time-dependent material deformation (e.g., creep, stress relaxation, etc.) in monolithic ceramics. Using continuum principles of engineering mechanics the complete theory is derived from a scalar dissipative potential function first proposed by Robinson (1978), and later utilized by Duffy (1988). Derivations based on a flow potential function provide an assurance that the inelastic boundary value problem is well posed, and solutions obtained are unique. The specific formulation used here for the threshold function (a component of the flow potential function) was originally proposed by Willam and Warnke (1975) in order to formulate constitutive equations for time-independent classical plasticity behavior observed in cement and unreinforced concrete. Here constitutive equations formulated for the flow law (strain rate) and evolutionary law employ stress invariants to define the functional dependence on the Cauchy stress and a tensorial state variable. This particular formulation of the viscoplastic model exhibits a sensitivity to hydrostatic stress, and allows different behavior in tension and compression.

As a processing technology of data and information, building information modeling (BIM) is often used for producing digital engineering model and promoting interoperability among participants involved in the Architecture, Engineering, and Construction (AEC) industry. The real values of BIM largely depend on effective information integrating and sharing among different stages or disciplines through the whole building lifecycle. To this day, although BIM has made a substantial contribution to the Chinese AEC industry, there are still some debates on its roles. Moreover, due to the complexity of structural design process, the weak link between structural model and BIM model makes its deep application even harder. To address these issues, this paper discusses the roles of BIM from the perspective of structural engineers through comparing with the traditional CAD. Then, the BIM structural model is studied by analyzing the model conformation, design mode, and Industry Foundation Classes (IFC)-format structural model. By comparing the differences between BIM physical model and structural mechanical model, an indirect method for the data transformation from BIM model to structural analysis model isproposed. An interface is developed based on the analysis of data formats and mapping rules. Three typical cases are adopted to demonstrate the data transfer efficiency by using the interface. The case study shows that the proposed method achieves higher efficiency for the data transformation from BIM model to structural analysis model.

In many applications thin-walled structures (beams, plates, pipes, pipe bends, etc.) operate at elevated temperatures. Under such conditions the behavior of metals and alloys is primarily determined by irreversible time-dependent creep and material deterioration processes. In order to estimate the long-term behavior it is important to understand the mechanisms of the time-dependent stress redistribution and damage growth, particularly in the zones of nozzles, pipe connections and welds. The widely used approach in modelling the creep-damage behavior is the continuum damage mechanics, which proposes constitutive equations for the creep strain rate tensor, evolution equations for damage variables, and states nonlinear initial-boundary value problems in order to perform a structural analysis, e.g. [7]. With the progress in the material science and continuum mechanics, many new constitutive models have been developed which include physically motivated state variables and are able to consider different effects in material behavior such as the damage induced anisotropy, stress state dependence of damage evolution, etc. These models can be incorporated into a commercial finite-element code in order to analyze time-dependent deformations and stresses in a thin-walled structure under a specific mechanical loading. As usual, an important step is to select a structural mechanics model or to specify the type of finite elements. One way is a “three-dimensional approach” which is based on balance equations of three-dimensional continuum.KeywordsSteady State CreepDamage ParameterTertiary CreepContinuum Damage MechanicTransverse Shear StressThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Coupled elastic–plastic damage model for a porous aggregate with an incompressible matrix displaying tension–compression asymmetry

An experimental assessment of grain size effects in the uniaxial straining of thin Al sheet with a few grains across the thickness

An effective compliance matrix Č is proposed to model the behavior of concrete based on phenomenological evidence and physical insight. Three parameters α, β, and γ are introduced in the effective compliance matrix Č. α and β are introduced to model the different behavior of concrete in tension and compression, while the third parameter γ is introduced to account for volumetric change. The predictive capability of the proposed elasto-damage model for uniaxial and multiaxial stress paths is investigated for uniaxial compression, biaxial compression, triaxial compression, uniaxial tension, and tension—compression— compression. The simulative capability of the model to capture the phenomenological behavior of concrete such as strain softening, stiffness degradation, biaxial strength envelope, volumetric dilatation, different behavior in tension and compression, and gain in strength under increasing confinement is reflected. The predicted results correlate well with the available experimental data.

A new approach is given to the theory of non-linear elastic materials which have different behaviour in tension and compression. Two applications are made to incompressible non-linear materials using general forms for the strain energy functions. The linear form of the theory is shown to be equivalent to that used by previous writers.

The influence of the choice of pharmacokinetic model on subsequent determination of covariate relationships in population pharmacokinetic analysis was studied using both simulated and real data sets. Simulations and data analysis were both performed with the program NONMEM. Data were simulated using a two-compartment model, but at late sample times, so that preferential selection of the two-compartment model should have been impossible. A simple categorical covariate acting on clearance was included. Initially, on the basis of a difference in the objective function values, the two-compartment model was selected over the one-compartment model. Only when the complexity of the one-compartment model was increased in terms of the covariate and statistical models was the difference in objective function values of the two structural models negligible. For two real data sets, with which the two-compartment model was not selected preferentially, more complex covariate relationships were supported with the one-compartment model than with the two-compartment model. Thus, the choice of structural model can be affected as much by the covariate model as can the choice of covariate model be affected by the structural model; the two choices are interestingly intertwined. A suggestion on how to proceed when building population pharmacokinetic models is given.

The classical theories of continuum mechanics — linear elasticity, viscoelasticity, plasticity and hydrodynamics — are defined by special constitutive equations. These can be understood to be asymptotic approximations of a quite general constitutive model, valid under restrictive assumptions for the stress functional or the input processes. The general theory of material behavior develops systematic methods to represent material properties in a context of physical evidence and mathematical consistency. According to experimental observations material behavior may be rate independent or rate dependent with or without equilibrium hysteresis. This motivates four different constitutive theories, namely elasticity, plasticity, viscoelasticity and viscoplasticity. Constitutive equations can be formulated explicitly as functionals. Then, the particular constitutive models correspond to continuity properties of these functionals, related to convenient function spaces. On the other hand, a system of differential equations may lead to an implicit definition of a stress functional. In this case additional variables are introduced, which are called internal variables. For these variables additional evolution equations must be formulated, specifying the rate of change of the internal variables in dependence on their present values and the strain (or stress) input. In the context of different models of inelastic material behavior the evolution equations have different mathematical characteristics. These concern the existence of equilibrium solutions and their stability properties. Rate independent material behavior is modelled by means of evolution equations, which are related to an arclength instead of the time as independent variable. It can be shown that the rate independent constitutive equations of elastoplasticity are the asymptotic limit of rate dependent viscoplasticity for slow deformation processes.

In all engineering material applications, material durability plays a crucial role. It is well-known that time-dependent materials creep visco-elastically. In addition, some experimental investigations indicate that time-dependent materials such as polymeric composites may undergo both damage and permanent deformation. The above features of material behavior are all life-limiting factors in that they can cause excessive dimensional distortions in addition to stiffness and residual strength degradations. The aim of this study is to establish a framework for modeling the foregoing features of time-dependent material behavior. Such a framework is the basis for performing durability analyses, which play a major role in engineering design.KeywordsDamage MechanicDamage VariableCreep DamageCreep Crack GrowthContinuum Damage MechanicThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Modeling of materials behavior at various temperatures of hot isostatically pressed superalloys

In a regular extrusion process for thermoplastic specimen, the material behavior in dies and its rheological properties have a major influence on the mechanical properties and the quality of the extruded product. This study outlines the multiphysical model, which has been adapted to address the specific issues associated with polyamide 12 polymer extrusion. A series of numerical simulations, based on the modeling of material behavior, have been conducted using the computational code Comsol Multiphysics®. These simulations have been applied to the study of polymer melt flows through an industrial extrusion die. The objective of this study is to analyze the influence of the imposed extrusion parameters on the physical field distribution and the material displacement within the die. The die swell at the exit of the extrusion die has been investigated as a function of the imposed pressure. The progression of the polymer flow front inside the extrusion tool and the swelling rates have been analyzed. In particular, the phenomenon of delayed swelling at high pressure has been highlighted at high pressure. Highlights Polymer swelling at industrial extrusion die exit is investigated. Polymer melt extrusion within industrial extrusion tool is numerically simulated. Flow front progression and extrudate swell are analyzed. Effect of extrusion pressure on instabilities and die swell is highlighted.