Micro-mechanical analyses of the effect of stress triaxiality and the Lode parameter on ductile damage and failure

The paper deals with the effect of different stress states on plastic deformations, damage and fracture of ductile materials. To be able to model these effects a continuum damage model has been introduced taking into account the dependence of stress-state on the constitutive equations. The model is based on the introduction of damaged and fictitious undamaged configurations. All parameters appearing in the constitutive equations are stress-state-dependent which can be characterized by the stress intensity, the stress triaxiality and the Lode parameter. Only experiments are not adequate enough to determine all constitutive parameters. Thus, additional series of three-dimensional micro-mechanical simulations of representative volume elements have been performed to get more insight in the complex damage mechanisms. These simulations cover a wide range of stress triaxialities and Lode parameters in tension, shear and compression domains. After all, the results from the micro-mechanical simulations are used to suggest the damage equations and to identify corresponding parameters. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stress-triaxiality-dependent modeling of ductile damage and fracture behavior

The paper deals with the effect of stress triaxiality on the inelastic behavior of aluminum alloys. The proposed continuum model takes into account stress-triaxiality-dependence of the yield condition as well as of the damage criterion and the fracture condition with different branches corresponding to different micro-mechanisms. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Local and non‐local ductile damage and failure modelling at large deformation with applications to engineering

The numerical analysis of ductile damage and failure in engineering materials is often based on the micromechanical model of Gurson [1]. Numerical studies in the context of the finite‐element method demonstrate that, as with other such types of local damage models, the numerical simulation of the initiation and propagation of damage zones is strongly mesh‐dependent and thus unreliable. The numerical problems concern the global load‐displacement response as well as the onset, size and orientation of damage zones. From a mathematical point of view, this problem is caused by the loss of ellipticity of the set of partial di.erential equations determining the (rate of) deformation field. One possible way to overcome these problems with and shortcomings of the local modelling is the application of so‐called non‐local damage models. In particular, these are based on the introduction of a gradient type evolution equation of the damage variable regarding the spatial distribution of damage. In this work, we investigate the (material) stability behaviour of local Gurson‐based damage modelling and a gradient‐extension of this modelling at large deformation in order to be able to model the width and other physical aspects of the localization of the damage and failure process in metallic materials.     

Strain induced damage of ductile materials under compression

The problem of ductile damage and failure prediction at the upsetting of cylindrical specimens with artificial voids is solved by taking a change in stress triaxiality into account. It is shown that such a more accurate assessment leads to a greater shift of stress triaxialities into a range of negative values as compared to their averaged values. At such values of stress triaxiality the material can be subjected to compression without failure under arbitrarily large deformations due to healing of micro-defects. The constitutive equations of a recently developed tensorial framework for ductile damage [5] are applied to modelling. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Micromorphic homogenisation and its application to a model of ductile damage

Heuristic gradient extensions of classical damage models are established to overcome their spurious mesh dependency. In contrast, an extension of Gurson's model of ductile damage towards the microdilatational theory is presented through a straight-forward homogenization. FEM simulations prove the regularization capabilities of the model. (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Three-dimensional cell model analysis of ductile damage under dynamic loading condition

The paper deals with the damage and fracture behavior of ductile metals under dynamic loading conditions. The in [1–3] presented phenomenological continuum damage and fracture model, which takes into account the rate- and temperature-dependence of the material, provides reasonable results of experiments with high strain rates while the identification of the corresponding material parameters results difficult from the available experimental data. This lack of information can be resolved by micro-mechanical numerical simulations of void containing unit-cells. In this context results of dynamic micro-mechanical simulations are presented which can be used to study the damage effects on the micro-scale and to validate the rate-dependent continuum damage model. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Computer modeling of dynamics of damage cumulation in the ductile matrix of a fiber-reinforced composite

Conclusions On the basis of the present results together with other previously obtained data [12, 14, 15, 18, 19] it can be assumed that the problem of deformation and failure [20, 21], which is often reduced to explaining the problem of whether failure is the result of deformation or its cause, has resulted from attempts to transfer the macroconsiderations of continuum to the atomic discrete level where the strain (shear accumulation) and failure (cumulation of ruptures) are interrelated to such an extent that their contraposition is of artificial nature. For example, the fluctuation rupture of the bond on the free boundary of the crystal causes formation of a dislocation and shearing [15], and shearing in the form of twinning leads to rupture of bonds and formation of a crack nucleus (see Fig. 1c). For analysis of failure, it is at present insufficient to use only dislocation [22] or only kinetic [21] considerations. As mentioned in [20], in the physics of failure it is necessary to examine all the objects examined in the physics of plasticity and, in addition to this, even more complicated objects. Attempts to simplify the problem proved to be unsuccessful and, consequently, the correct method of solving the problems of the physics of strength should be based on developing the methods of physics of strength up to analysis of atomic mechanisms of damage cumulation [20]. Molecular dynamics represents such a method of examining failure which makes it possible to change to the atomic level [10].Translated from Mekhanika Kompozitnykh Materialov, No. 6, pp. 973–976, November–December, 1987.     

Unit-cell simulations of damage and failure in PRMMCs

The purpose of this work is damage and failure modeling in multiphase metallic materials via unit-cell simulations and homogenization methods. To this end, we investigate such behaviour in particle-reinforced metal matrix composites (PRMMCs). Taking into account the processes of void nucleation (due, e.g., to particle debonding and/or cracking) and growth, we examine the effect of phase composition, particle sizes and distributions, as well as the nature of the particle/matrix interface, on damage and failure in the unit cell. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Micro-mechanical study of interactions between polymers and filler aggregates

Filled rubber-like materials exhibit a complex, history-dependent hysteretic behavior which is mostly due to damage of micro-structures inside the rubber matrix. In this paper, we study the contribution of filler aggregates inside the elastomer to this damage behavior. To this end, a recently proposed multi-scale model of single aggregates [1] is applied. The network decomposition concept adopted there is further extended here to an additional network [1] which takes into account elasticity of filler aggregates and polymer chains in their vicinity. This network is described by means of a 3D statistical volume element (SVE) obtained by homogenization over the aggregate size distribution within the rubber matrix. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analysis of the effect of impact damage on the strength characteristics of a composite

Theoretical and experimental investigations into the loss of the static tensile strength of a KMKU-2M.120. E01 composite after low-speed impacts of different energy have been carried out. All calculations are performed by the method of finite elements. The drop in the strength as a function of impact energy is estimated according to two criteria: the criterion of an equivalent hole (the upper limit of loss of strength) and the criterion of concentration (the lower limit of loss of strength). The results obtained agree closely with experimental data and therefore can be recognized as reliable.     

Small-sample studies on right censored data with discrete failure times

It has been already noticed that the classical Greenwood formula may not be an unbiased estimator for the variance of the Kaplan–Meier Product Limit estimator (PLE). However, a rigorous proof for such a suggestion has not been available. In this paper, we investigate some small-sample properties of the PLE and show that the Greenwood formula strictly underestimates the variance of the PLE. Besides, some existing estimators for the variance of the PLE are also discussed.     

Study of the effect of various factors on the low-temperature strength of polyethylene with graft polyacrylonitrile

Low-density polyethylene film has been subjected to graft polymerization with acrylonitrile. The effect of the graft polymerization conditions and subsequent treatment on the strength of the grafted film has been investigated. The strength was measured at -196°C. The grafted film behaves like a heterogeneous material in which the unoriented polyacrylonitrile macrostructures play the part of filler particles. The strength of the grafted film is additively composed of the strengths of the polyethylene and the polyacrylonitrile and the change in strength after stretching, heating, and other treatments is determined by the changes in the degree of orientation of the polyethylene.V. I. Lenin Belorussian State University, Minsk. Translated from Mekhanika Polimerov, No. 4, pp. 589–593, July–August, 1973.     

Seismic response of secondary structures mounted on ductile frames

This paper deals with the response of vibration prone non–structural components (or secondary structures) mounted on regular generic plane frames (i.e. primary structures) subjected to ground–accelerations generated by ordinary earthquakes. The response of these multi–degree–of–freedom (MDOF) systems is approximated utilizing constant–ductility floor response spectra of two–degree–of–freedom (2DOF) primary–secondary systems and validated by the response derived by a fully coupled analysis of the entire primary–secondary system. Furthermore, results of coupled and decoupled analyses are set in contrast. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigating the effect of various nanoparticle and base liquid types on the nanofluids heat and fluid flow in a microchannel

In this paper, heat transfer and pressure drop of different nanofluid types in a two-dimensional microchannel is investigated numerically. To do this, an Eulerian–Eulerian two-phase model is used for nanofluid simulation and the governing equations are solved using a finite volume method. Nine different nanoparticles and three different base liquid types (water, ethylene glycol and engine oil) are considered. Heat transfer and pressure drop of different nanofluid types are compared at Re = 100 and 1% volume concentration for different nanoparticles and at constant inlet velocity for different base liquids. Numerical results show an almost equal pressure drop for all the nanoparticles dispersed in water, while, the heat transfer coefficient is highest for water–diamond and is the lowest for water–SiO₂ nanofluids. Also, the pressure drop for water-based nanofluid is very lower than the others and the heat transfer coefficient is the highest for water-based nanofluids.     

Testing the no-treatment effect based on a possibly misspecified accelerated failure time model

The accelerated failure time model is a useful alternative to the Cox proportional hazard model. We investigate whether or not a misspecified accelerated failure time model provides a valid test of the no-treatment effect in randomized clinical trials. We show that the minimum dispersion statistic based on rank regression by Wei et al. (1990) must be modified in order to conduct valid tests under misspecification, whereas the resampling-based methods by Jin et al. (2003) are valid without any modification. Numerical studies are conducted to examine the small sample behavior of the modified minimum dispersion statistic and the resampling-based method. Finally, an illustration is given with a dataset from a clinical trial.     

An investigation of the effect of educational background on performance in simulation studies

We might expect a link between educational background, in terms of degree subject studied, and performance on simulation studies. We study this link by comparing the performance of student groups from computer science, statistics and business on a simulation task. The analysis covers their overall performance and their performance on different elements of the task. Surprisingly, we are unable to find much difference in the performance of the students, leading us to conclude that educational background has little effect, at least for a relatively straightforward modelling task. The implications for organisations employing simulation modellers and for their educational needs are discussed.