Lattice Discrete Particle Model (LDPM) for failure behavior of concrete. II: Calibration and validation

The Lattice Discrete Particle Model (LDPM) formulated in the preceding Part I of this study is calibrated and validated in the present Part II. Calibration and validation is performed by comparing the results of numerical simulations with experimental data gathered from the literature. Simulated experiments include uniaxial and multiaxial compression, tensile fracture, shear strength, and cycling compression tests.     

Lattice Discrete Particle Modeling (LDPM) of Alkali Silica Reaction (ASR) deterioration of concrete structures

A large number of structures especially in high humidity environments are endangered by Alkali–Silica Reaction (ASR). ASR leads to the formation of an expansive gel that imbibes water over time. The gel expansion causes cracking and consequent deterioration of concrete mechanical behavior in the form of strength and stiffness reduction. In the recent past, many research efforts were directed towards evaluation, modeling and treatment of ASR effects on structures but a comprehensive computational model is still lacking. In this paper, the ASR effect is implemented within the framework of the Lattice Discrete Particle Model (LDPM), which simulates concrete heterogeneous character at the scale of coarse aggregate pieces. The proposed formulation, entitled ASR-LDPM, allows precise and unique modeling of volumetric expansion; expansion anisotropy under applied load; non-uniform cracking distribution; concrete strength and stiffness degradation; alkali ion concentration effect; and temperature effects of concrete subjected to ASR. In addition, a unique advantage of this formulation is its ability to distinguish between the expansion directly related to ASR gel expansion and the one associated with cracking. Simulation of experimental data gathered from the literature demonstrates the ability of ASR-LDPM to predict accurately ASR-induced concrete deterioration.     

Lattice modelling of size effect in concrete strength

This paper uses a recently improved lattice network model to study the size effect in the strength of plain concrete structures. The several improvements made to the lattice network model are: (i) tension softening of the matrix phase is included in the material modelling; (ii) the structural response is modelled by incrementing the deformation rather than the load. This eliminates the need for introducing arbitrary scaling parameters in the beam element failure criteria and; (iii) a square rather than a triangular lattice beam network is found to be adequate for modelling concrete, thus greatly reducing the computational time.The improved square lattice network has been used to simulate the complete load-deformation response of notched three-point bend beams of different sizes with a view to checking the validity of several size effect models available in the literature. Lattice simulation was found to identify microcracking, crack branching, crack tortuosity and bridging, thus allowing the fracture process to be followed until complete failure. The improved lattice model predicted smooth structural response curves in excellent agreement with test results.The simulated nominal strengths also correlated very well with the test results, apart from that for the smallest beams (depth 38.1 mm). However, even in the relatively broad range of sizes (1:8) of the test beams, there was no clear evidence that one size effect model is superior to the other. In fact, rather surprisingly the test data would appear to be equally well described by all the available size effect models. The lattice simulations however indicated a trend which is better predicted by the multifractal scaling model.     

Meso-structural study of concrete fracture using interface elements. I: numerical model and tensile behavior

A recently developed FE-based mesostructural model for the mechanical behavior of heterogeneous quasi-brittle materials is used systematically to analyze concrete specimens in 2D. The numerical model is based on the use of zero-thickness interface elements equipped with a normal-shear traction-separation constitutive law representing non-linear fracture, which may be considered a mixed-mode generalization of Hillerborg’s “Fictitious Crack Model.” Specimens with 4 × 4 and 6 × 6 arrays of aggregates are discretized into finite elements. Interface elements are inserted along the main lines in the mesh, representing potential crack lines. The calculations presented in this paper consist of uniaxial tension loading, and the continuum elements themselves are assumed to behave as linear elastic. In this way, the influence of various aspects of the heterogeneous geometry and interface parameters on the overall specimen response has been investigated. These aspects are aggregate volume fraction, type of arrangement and geometry, interface layout, and values of the crack model parameters chosen for both the aggregate-aggregate and matrix-aggregate interfaces. The results show a good qualitative agreement with experimental observations and illustrate the capabilities of the model. In the companion second part of the paper, the model is used to represent other loading states such as uniaxial compression, Brazilian test, or biaxial loading.     

A Microstructure-based Simulation Environment on the Basis of an Interface Enhanced Particle Model

The present contribution introduces enhanced discrete element simulation methodologies (DEM) with a special focus on a microstructure-based model environment. Therewith, it is possible to represent the failure of cohesive granular materials like concrete, ceramics or marl in a qualitative as well as quantitative manner. Starting from a basic polygonal two-dimensional particle model for non-cohesive granular materials, more complex models for cohesive materials are obtained by inclusion of beam or interface elements between corresponding particles. In particular, we will introduce an interface enhanced DEM methodology where a standard ingredient of computational mechanics, namely interface elements, are combined with the particle methodology contained in the DEM. The last step in the series of increasing complexity is the realization of a microstructure-based simulation environment which utilizes the interface enhanced DEM methodology. With growing model complexity a wide variety of failure features of cohesive as well as non-cohesive geomaterials can be represented. Finally, the plan of the paper is enriched by the validation of the newly introduced and re-developed discrete models with regard to qualitative and quantitative aspects.     

On the Use of a Lattice Model for Analyzing of In-Plane Vibration of Thin Plates

In this paper, a novel approach for simulating in-plane vibration of thin plates is proposed. It is based on the spectral element method (SEM) used within a lattice modeling framework. First, derivation of a frequency dependent dynamic stiffness matrix for a spectral beam element is presented. Then, the lattice modeling concept is introduced. In the model, the two-dimensional plate is discretized as a set of (one dimensional) spectral beam elements connected at the ends. The proposed approach is then used for modal analysis of rectangular plates of different aspect ratios (1 and 2) and boundary conditions (completely free and clamped). Simulated natural frequencies and modal shapes are compared to results available in the literature. It was found that the proposed model can reasonably reproduce low natural frequencies (in most cases within 10%) and modal shapes. Future work will focus on the use of the model as an aid in non-destructive testing of structures.     

Investigating particle-level dynamics to understand bulk behavior in a lab-scale Agitated Filter Dryer (AFD) using Discrete Element Method (DEM)

Agitated filter drying (AFD) is a complex physical-thermal separation process which involves isolating solutes from its mother liquor. In agro-chemical and pharmaceutical industry, filter-dryers are used for sequestering active ingredients (AIs) and key intermediates from the wet cake after the crystallization step. During the agitated drying phase, the mechanical agitation of the wet cake, implemented to enhance heat and mass transport, has been commonly observed to result in formation of undesired agglomerates that require further processing. Only relatively few experimental and computational studies of the effects of operating parameters and material properties on the drying and agglomeration growth kinetics have been described in the literature. In absence of robust predictive models, the go-to solution in order to avoid the agglomeration behavior of AIs has been to use minimal agitation which is not only suboptimal but also significantly increases the drying times.The simulation of drying and agglomeration behaviors in AFD is particularly challenging because the agitated drying processes are mechanistically governed by simultaneous heat, mass and momentum transfer equations. In addition, the behavior of agglomeration growth and drying pathway varies significantly with the physical properties of the residual solvents in the cake as well as the operating conditions of the agitated dryer. A comprehensive modeling approach to simulate both drying and agglomeration behavior in AFDs through implementation of mechanistic Discrete Element Modeling (DEM) simulations with coupled granular liquid bridge cohesion model, heat conduction model and evaporation kinetics is presented. Additionally, in-depth analysis of particle scale behavior which is responsible for drying and agglomerate growth kinetics are also studied with respect to different scaling criteria is also presented.     

Meso-structural study of concrete fracture using interface elements. II: compression,biaxial and Brazilian test

In the previous companion paper, a recently proposed meso-mechanical model using fracture-based zero-thickness interfaces was used to analyze 2D concrete specimens subject to uniaxial tension, to compare and discuss the results with respect to well-known experimental behavior, and to study the influence of composition and parameters on the overall behavior. In this second part paper, the model is used to simulate additional situations including uniaxial compression for conventional and high-strength concrete, Brazilian test including effects of platen size, and biaxial tests. The results show that the simulations are equally realistic in this wider range of material behavior, always within the limitation imposed by its two dimensional character.     

Tensile failure of concrete at high loading rates: New test data on strength and fracture energy from instrumented spalling tests

For the numerical prediction of the response of concrete structures under extreme dynamic loading, like debris impact and explosions, reliable material data and material models are essential. TNO-PML and the Delft University of Technology collaborate in the field of impact dynamics and concrete modelling. Recently, TNO-PML developed an alternative Split Hopkinson Bar test methodology which is based on the old principle of spalling, but equipped with up-to-date diagnostic tools and to be combined with advanced numerical simulations. Data on dynamic tensile strength and, most important, on fracture energy at loading rates up to 1000 GPa/s are obtained. The paper describes the test and measurement set-up, presents the new test data and the analysis of the test results. In addition, a rate-dependent softening curve is given which is based on the integrated findings so far.     

周期信号各参数的离散采样测量法（一）

描述用离散采样测量周期信号的有效值，失真度及相位等参数的简易且准确度很的测量方法，从数学上证明这些方法的正确性，并指出测量不确定度的来源。     

Weibull Probability Model for Fracture Strength of Aluminium (1101)-Alumina Particle Reinforced Metal Matrix Composite

In recent times, conventional materials are replaced by metal matrix composites (MMCs) due to their high specific strength and modulus. Strength reliability, one of the key factors restricting wider use of composite materials in various applications, is commonly characterized by Weibull strength distribution function. In the present work, statistical analysis of the strength data of 15% volume alumina particle (mean size 15 μm) reinforced in aluminum alloy (1101 grade alloy) fabricated by stir casting metho...     

Analysis-of-Marginal-Tail-Means (ATM): A Robust Method for Discrete Black-Box Optimization

Abstract

We present a new method, called analysis-of-marginal-tail-means (ATM), for effective robust optimization of discrete black-box problems. ATM has important applications in many real-world engineering problems (e.g., manufacturing optimization, product design, and molecular engineering), where the objective to optimize is black-box and expensive, and the design space is inherently discrete. One weakness of existing methods is that they are not robust: these methods perform well under certain assumptions, but yield poor results when such assumptions (which are difficult to verify in black-box problems) are violated. ATM addresses this by combining both rank- and model-based optimization, via the use of marginal tail means. The trade-off between rank- and model-based optimization is tuned by first identifying important main effects and interactions from data, then finding a good compromise which best exploits additive structure. ATM provides improved robust optimization over existing methods, particularly in problems with (i) a large number of factors, (ii) unordered factors, or (iii) experimental noise. We demonstrate the effectiveness of ATM in simulations and in two real-world engineering problems: the first on robust parameter design of a circular piston, and the second on product family design of a thermistor network.     

Lattice vibrations of materials for lithium rechargeable batteries. VI: Ordered spinels

Raman scattering spectra have been investigated to evaluate the local structure of lithiated oxides used as electrode materials for lithium-ion batteries. We report the analysis of the vibrational spectra of ordered spinel phases including the partially delithiated λ-Li_0.5Mn₂O₄ ( SG), the partial charge-ordered LiMn₂O₄ orthorhombic form (Fddd SG) and the LiNi_0.5Mn_1.5O₄ substituted oxide (P4₁32 SG). Analysis of spectroscopic data is performed using the classical factor group theory and the vibration features are compared with those of the ordered lithium ferrite -LiFe₅O₈ and the normal spinels LiMn₂O₄ and LiNi_0.5Mn_1.5O₄ (Fd3m SG), and the inverse spinel LiNiVO₄.     

Assessment of Mechanical Properties of Cohesive Particulate Solids. Part 1: Particle Contact Constitutive Model

The fundamentals of cohesive particulate solids' consolidation and flow properties using a reasonable combination of particle and continuum mechanics are explained. By means of the model "stiff particles with soft contacts," the combined influence of elastic, elastoplastic, and viscoplastic repulsion in particle contacts is derived. As a result, contact normal force displacement F N (h K ) and adhesion force models F H (F N ) are presented that describe the instantaneous and time consolidation behavior at characteristic (averaged) particle contacts. The approach has been shown to be effective for the data evaluation of a very cohesive titania nanopowder (surface diameter d S = 200 nm , solid density 𝜌 s = 3870 kg/m 3 ).     

Assessment of Mechanical Properties of Cohesive Particulate Solids. Part 1: Particle Contact Constitutive Model

The fundamentals of cohesive particulate solids' consolidation and flow properties using a reasonable combination of particle and continuum mechanics are explained. By means of the model "stiff particles with soft contacts," the combined influence of elastic, elastoplastic, and viscoplastic repulsion in particle contacts is derived. As a result, contact normal force displacement F N (h K ) and adhesion force models F H (F N ) are presented that describe the instantaneous and time consolidation behavior at characteristic (averaged) particle contacts. The approach has been shown to be effective for the data evaluation of a very cohesive titania nanopowder (surface diameter d S = 200 nm , solid density ρ s = 3870 kg/m 3 ).     

Experimental studies on shear failure of freeze-bonds in saline ice: Part I. Set-up, failure mode and freeze-bond strength

The strength of freeze-bonds in thin saline ice has been investigated through two series (in 2008 and 2009) of experiments in the Hamburg Ship Model Basin (HSVA) as a function of the normal confinement (σ), the submersion time (Δt) and the initial ice temperature (T_i). The freeze-bonds were mostly formed in a submerged state, but some were also formed in air. The experimental set-up was improved in the 2009 experiments. In 2008 a ductile-like failure mode dominated (78%), whereas in 2009 the brittle-like dominated (93%). We suggest that this is a combined ice and test set-up effect. The 2009 experimental procedures allowed for careful sample handling giving higher strength and it was softer. Both these things should provoke a more brittle-like force-time response. The average freeze-bond strength in brittle-like samples was around 9 kPa while in ductile-like samples was around 2 kPa. The maximum freeze-bonds strength were measured for short submersion times, from 1 to 20 min, and reached a maximum value of 30 kPa.A Mohr-Coulomb like failure model was found appropriate to represent the freeze-bond shear strength as function of the normal confinement. Saline freeze-bonds in saline water had cohesion/friction angle around 4 and 1.4 kPa/25° for the brittle- and ductile-like samples respectively, which fitted well with previously published data.A bell-shape dependence for τ_c vs. Δt was found, which agreed with the predictions by Shafrova and Høyland (2007). We suggest that this is essentially a freeze-bond porosity effect and propose three phases in time with subsequent cooling, heating and equilibrium to account for this trend. Qualitative experiments showed that the submersion time and the initial ice temperature were strongly coupled.To account for the connection between contact time, block dimensions and ice properties and the freeze-bond strength, dimensionless number were used. Fourier scaling was more appropriate than Froude scaling to scale freeze-bonds.The freeze-bonding made in air developed fast (in less than 30 s) when the ice was cold and dry, but no freeze-bonding occurred for the same contact times when the ice was warm and wet.     

A statistical approach for the assessment of reliability in ceramic materials from ultrasonic velocity measurement: Cumulative Flaw Length Theory

A primary objective of statistical fracture approach is to predict the probability of failure of a component for an arbitrary stress state when the failure statistics are known. This study introduces the fundamentals and application of a new approach to characterize the mechanical behaviour of high temperature ceramic materials, including refractory materials, by coupling non-destructive methods, in particular ultrasonic velocity measurement, and the Batdorf statistical fracture theory. A new approach, termed Cumulative Flaw Length Theory (CFLT), has been developed for the case of macroscopically homogeneous isotropic materials containing randomly oriented microcracks uniformly distributed in a location subjected to non-uniform multiaxial stresses. A function representing the number of cracks per unit volume is estimated based on the histograms of ultrasonic velocity measurements. This function is used without additional assumptions to determine the probability of fracture under an arbitrary stress condition. Two different cordierite-mullite high temperature ceramic materials were characterized under the assumptions of this theory to provide experimental evidence to support the model.     

Measurement of the in situ ply fracture toughness associated with mode I fibre tensile failure in FRP. Part I: Data reduction

The fracture toughness associated with fibre tensile failure was measured for a T300/920 laminated carbon/epoxy material system using the compact tension specimen configuration. Six methods of data reduction were investigated for calculation of the toughness with the aim of finding the best technique, in terms of reproducibility of results and simplicity. The calculated fracture toughnesses were found to correlate well, though varying amounts of scatter were produced by each method. An optimum method was proposed that does not rely on the use of an optically measured crack length.     

Fracture behavior of 9% nickel high-strength steel at various temperatures: Part I. Tensile tests

Fracture behavior of a 9% nickel 1000 MPa grade high-strength steel was investigated with tensile tests at various temperatures. Four critical stresses were found, which determined the fracture behaviors at various temperatures. Various fracture behaviors could be classified into three categories: (1) at −196 °C, a longitudinal crack initiated from the center of the necking region and propagated along the tensile direction to the regions close to both ends of the necking, it then changed the orientation and developed into two transverse cracks which propagated into opposite directions on two separated cross-sections. (2) In the range of −30 °C to 20 °C, the fracture surfaces were composed of typical center-fibrous-initiation region, middle shear-radical region and outer shear-lip region. (3) In the range of −150 °C to −60 °C, the middle shear-radiation region showed a very rough pattern with several convex ridges. Fracture mechanisms were analyzed by combining various fracture morphologies with FEM-calculated results of stress and strain.     

Fracture assessment of damaged square hollow section (SHS) K-joint using BS7910:2005

A pre-cracked square hollow section K-joint was tested under static loads up to failure. It is found that the load-displacement curves are in good agreement with the finite element results. Ductile tearing was observed to initiate from the crack front parallel to the chord side wall where fracture toughness is smaller. Using plastic collapse load obtained via twice elastic compliance technique and fracture toughness obtained from crack tip opening displacement, the two fracture parameters K_r and L_r are plotted on the standard failure assessment diagram. It shows a conservative assessment for the cracked K-joint subjected to brace end axial loads.