Strain Discontinuity Research Articles

Comparison of surface layers responses under heavy truck loadings on new and rehabilitated pavement with different interface bond conditions

The integrity and functionality of road pavements are vital for ensuring safe and efficient transportation networks. However, surface layers are prone to various forms of deterioration. This study investigates the responses of two pavement structures-one newly constructed and one rehabilitated-under traffic loading, through experimental tests conducted on an accelerated pavement testing facility (APT) and numerical simulations using the Viscoroute©2.0 software. Focus is placed on surface layer behavior, with an emphasis on understanding the behavior of interfaces between the asphalt layers. The findings show that the new pavement structure exhibits good bonding and continuous strain distribution, while the reinforced pavement structure exhibits partial sliding at the interface and strain discontinuity. For the new pavement structure, the strains are well predicted using a bonded interface, while for the rehabilitated pavement structure, the strains are predicted using a thin elastic interface layer with a low elastic modulus.

On discontinuous stress and strain evolutions in machining of dissimilar laser cladded workpiece

During subsequent machining of laser cladded workpiece, the stress transferred in depth direction is different with conventional cast workpiece due to the geometrical discontinuity, resulting in the stress difference and strain discontinuity at the interface between the cladding layer and substrate. Elastic and plastic deformation will occur in the cladding layer and substrate, respectively, which ultimately reduces the service life of the cladded workpiece. In this study, the stress and strain evolution in subsequent machining of laser cladded workpiece was investigated using an orthogonal cutting model by finite element method. A dimensionless thickness ratio between uncut chip thickness and cladding layer thickness was set and the maximum thickness ratio without equivalent plastic strain (PEEQ) discontinuity at the interface was determined. In addition, the results indicated that the maximum thickness ratio first decreased and then increased with the increase of cutting speed, while increases with the increase of the yield strength of substrate. The synergetic effects of thickness ratio, cutting speed and yield strength of substrate on the strain discontinuity at the interface were also investigated based on Taguchi method. The optimal combination of parameters is determined based on the regression model and divided into areas with considering machining efficiency as well. The map of cutting configurations facilitates the selection of proper machining parameters in order to estimate the strain discontinuity in machining laser cladded workpiece, ensuring a better service life of the workpiece.

Meso to macro connections to capture fatigue damage in cemented materials

Fatigue-induced damage is a common issue in cemented materials, involving the progressive formation and complicated propagation of fatigue cracks. These cracks typically localise on weak or fracture planes, leading to inhomogeneous deformation within the material. Thus, accurately predicting the fatigue phenomenon becomes challenging due to the material’s inhomogeneity and the complex evolution of cracks, from initiation to propagation and eventual failure. To address this challenge, this paper presents a constitutive model that accounts for strain discontinuity across fracture planes by employing kinematic enrichment. This enhancement facilitates interaction between the material responses of cracks and the outer bulk, thereby contributing to the overall macro behaviour of the materials. Moreover, the proposed model incorporates a new cohesive-frictional fatigue model that couples damage mechanics and bounding surface plasticity to describe the fatigue behaviour of fracture planes/cracks. Since the proposed model features a characteristic length scale, it exhibits size-dependent behaviour and helps overcome the issue of mesh dependence. The model's validity is demonstrated through its ability to capture nonlinear fatigue damage under constant/variable cyclic loading and to simulate the propagation of fatigue fracture process zones. Furthermore, the model effectively captures the significant influence of stress amplitudes on the fatigue lives of materials, making it an essential tool for predicting and mitigating fatigue-induced damage in cemented materials.

Enhanced cyclic stability and enlarged working temperature window of NiFeGa elastocaloric refrigerant via introducing strong texture and ductile interfacial precipitate

The poor mechanical properties will result in low fatigue life and limited working temperature window of elastocaloric effect (eCE) for ferromagnetic shape memory alloys with lower critical stress. In this work, a microstructure design strategy, i.e., simultaneously introducing strong texture and ductile interfacial γ-phase, was adopted to reduce the strain discontinuity near the grain boundary and enhance the grain boundary cohesion, further improving the mechanical properties and eCE performance. With this strategy, the compressive strength and strain of the prototype Ni54Fe19Ga27 alloy can exceed 1.5 GPa and 30.0% at room temperature, respectively. Furthermore, a wide working temperature window of 120 K with an adiabatic temperature change of −8.5 to −1.2 K and superior eCE fatigue life of 2 × 104 cycles with an adiabatic temperature change of ∼3 K have been achieved. This work is expected to provide an effective strategy to enhance the eCE performance of ferromagnetic shape memory alloys.

Impact of BN and WC Particulate Reinforcements on Mechanical Properties and Damage Development of Al-2048 Metal Matrix Composites

The primary goal of this work is to evaluate how BN and WC particles affect the mechanical properties and damage development behaviours of aluminum alloy-2048. Heat treatments for composites are also being investigated to improve mechanical properties. Tensile experiments reveal that BN particle reinforcement outperforms WC reinforcement in strength and flexibility for composites. T4 treatment, rather than traditional peak-aging treatment, is recommended for the composites (T6). The particle size distribution in 10v% WC/Al-2048 is the best in the three composites with the largest size of 16 µm and 80% of particles are from 6 to 10 µm. Tensile tests illustrate that 15v% BN/Al-2048 composite demonstrates a 9 and 14% rise in its ultimate tensile stress and 28 and 120% increases in its elongation. T4 heat treatment with an additional 0.6 percent prestrain can produce composites with the same UTS and 0.2 percent proof stress as T6 treatment, but the ultimate elongation below T4 treatment is over 100 percent more than that under T6 treatment. To make sense of the test results, an observation of the damage evolution behaviors of the reinforcing particles provides a concept that the composites’ strength is primarily determined by the balance between the reinforcing particles sharing the load and creating strain discontinuity in the matrix. The fraction of broken particles in the 20v% WC/Al-2048 composite as a function of strain is much higher than that in the 15v% BN/Al-2048 composite. Because of their tolerance for substantial strain at the interface, maximum K1c, and moderate thermal extension, BN elements can share a lot of loads and provide more excellent reinforcement than WC particles in terms of composite strength and flexibility.

A method for considering the influence of distinct casting stages in the flexural design of prestressed concrete cross sections

ABSTRACT Composite elements are structures of concrete, or other materials, constructed in different casting stages that act jointly under external loads. These elements are used when it is intended to combine the constructive advantages of precast structures with the monolithic behavior of cast-in-place structures. In regular civil engineering applications, such as the construction of a bridge or viaduct, the precast section is used as shoring before casting the slab in place. This process leads to imposed deformations prior to the ultimate limit state and a discontinuity in the specific strain of the composite cross section. This work proposes a methodology to design composite cross sections, built in two casting stages, evaluating the specific strain provided the construction process that can be easily implemented in precise computational routines. From applying the methodology on study-case numerical example, it is observed that the beam casted in two stages presents a factored moment resistance smaller than an identical beam casted in a single stage. However, further investigations should be conducted to assess the extent of this difference.

An adaptive shell element for explicit dynamic analysis of failure in laminated composites Part 1: Adaptive kinematics and numerical implementation

To introduce more fibre-reinforced polymers in cars, the automotive industry is strongly dependent on efficient modelling tools to predict the correct energy absorption in crash simulations. In this context, an adaptive modelling technique shows great potential. However, as the critical energy absorption in a crash occurs over a very short period of time, and since the deformation behaviour is very complex, car crash simulations are usually performed using explicit dynamic finite element solvers. Therefore, any practical adaptive technique must be adapted to an explicit setting in a software available to the automotive companies. In this paper, we propose an adaptive method for explicit finite element analysis and describe its implementation in the commercial finite element solver LS-DYNA. The method allows for both so-called weak discontinuities (discontinuities in strain), which are crucial for accurate stress and intralaminar damage predictions, and strong discontinuities (discontinuities in displacements), needed for a proper representation of growing delamination cracks. In particular, we detail the implementation of the proposed method into LS-DYNA and also how we propose to remedy the non-physical oscillations arising from the implementation of the adaptive scheme in a explicit dynamic setting. The paper is concluded with numerical examples where we demonstrate the potential for the adaptive approach and also perform a detailed study on its accuracy and stability.

The effect of microstructure on the relationship between grain boundary sliding and slip transmission in high purity aluminum

The relationship between grain boundary sliding (GBS) and slip transmission is investigated experimentally at grain boundaries in 99.99% aluminum with a through-thickness, coarse-grained microstructure deformed in tension at 190 °C. Using scanning electron microscope-enabled digital image correlation (SEM-DIC) and electron backscatter diffraction (EBSD), high-resolution strain fields and microstructural information were measured to examine the influence of microstructural neighborhoods on interactions between GBS and slip transmission and strain localization. Several distinct cases are presented that highlight important microstructural factors that govern deformation near grain boundaries. The findings include (1) direct transmission and GBS were anti-compatible and facilitated by opposing boundary types (low misorientation and high energy grain boundaries respectively); (2) increased GBS activity was correlated with decreased indirect transmission behavior; (3) GBS accommodation at triple junctions was enabled by intragranular plasticity; and (4) the local intragranular plastic strain discontinuity between grains determined the magnitude of GBS gradients. This work provides insight into the nature of these mechanisms and can be used to identify strain transfer criteria that can lead to improved GBS-sensitive crystal plasticity models.

Interface energy effect on effective elastic moduli of spheroidal particle-reinforced nanocomposites

By incorporating the interface energy effect into a classical micromechanics framework, effective elastic moduli of a composite material containing randomly distributed nanosized prolate spheroidal inhomogeneities are investigated in this paper. The effect of interface energy, which is usually neglected in classical micromechanics theories, becomes important when the size of the reinforcement phase in the composite enters the nanometer range. The interface energy effect is simulated by inducing interface stress on the zero-thickness membrane interface between the matrix and the inhomogeneities. The interfacial stress discontinuity equations are formulated in accordance with the equilibrium conditions on the idealized interface, from which the interfacial strain discontinuity is solved. Subsequently, the effective elastic moduli are derived based on the classical micromechanics homogenization approaches. Comparisons are made of the effective moduli under the current nanomechanical framework and under classical micromechanical theory. The effective moduli are shown to be dependent upon the size of the inhomogeneities and the interface properties when the interface energy effect is considered.

Incorporation of micro-cracking and fibre bridging mechanisms in constitutive modelling of fibre reinforced concrete

The formation and propagation of cracks under progressive loading in fibre reinforce concrete (FRC) are significantly influenced by fibre bridging mechanisms. Cracking and fibre bridging, governed by the FRC constituents and their properties, are two coupled and interacting phenomena that significantly affect the ductility and transition from diffuse to localised deformation. Constitutive modelling of FRC is challenging due to the high inhomogeneity and complex transition of deformations stages rooted from the difference in responses of cracked and intact material volumes coupled with cohesive resistance and fibre bridging of a crack. In this paper, a new approach to constitutive modelling of FRC is developed by enriching the constitutive structure to accommodate different responses of the crack, intact material and fibres. The strain discontinuity caused by cracks is accounted for via an enriched strain field which facilitates the introduction of the two interacting mechanisms, cohesive cracking and fibre bridging, in the constitutive model. The transition from diffuse to localised deformation is controlled by the fibre volume content and local deformation, via the density of active cracks. It is demonstrated that the proposed constitutive model is capable of describing the transition from diffuse to localised deformation associated with different macro responses under different loading conditions.

The Môn–Deemster–Ribblesdale fold–thrust belt, central UK: a concealed Variscan inversion belt located on weak Caledonian crust

Abstract The Ribblesdale fold belt, representing the Variscan inversion of the Bowland Basin, is a well-known geological feature of northern England. It represents a crustal strain discontinuity between the granite-underpinned basement highs of the northern Pennines and Lake District in the north, and the Central Lancashire High/southern Pennines, in the south. Recent seismic interpretation and mapping have demonstrated that the Ribblesdale fold belt continues offshore towards Anglesey via the Deemster Platform, beneath the Permo-Triassic sedimentary cover of the southern part of the East Irish Sea Basin. The Môn–Deemster fold–thrust belt (FTB) affects strata of Mississippian to late Pennsylvanian age. Variscan thrusts extend down into the pre-Carboniferous basement but apparently terminate at a low-angle detachment deeper in the crust, here correlated with the strongly sheared Penmynydd Zone exposed in the adjacent onshore. Up to 15% shortening is observed on seismic sections across the FTB offshore, but is greater in the strongly inverted onshore segment. Pre-Carboniferous thrusting post-dates formation of the Penmynydd Zone, and is probably of Acadian age, when basement structures such as the southward-vergent Carmel Head Thrust formed. Extensional reactivation of the Acadian structures in early Mississippian time defined the northern edge of the offshore Bowland Basin. The relatively late brittle structures of the Menai Strait fault system locally exhume the Penmynydd Zone and define the southern edge of the basin. The longer seismic records from the offshore provide insights to the tectonic evolution of the more poorly imaged FTB onshore.

Impact perforation of aluminium Cymat foam

The perforation behaviour of Cymat© aluminium foam at various impact loading rates was studied both experimentally and numerically. Perforation tests were performed with an inverse perforation setup using a split Hopkinson pressure bar (SHPB) system at speeds up to 40 m/s. Compared with a quasi-static test using the same specimen and clamping system, a significantly enhanced piercing force was found under impact loading. Numerical simulations of the perforation test were carried out using LS-DYNA code, and the material models available in this code were benchmarked. Good agreement was found between the experimental and simulated force/displacement curves when using the honeycomb material model with an appropriate failure criterion. The simulation revealed that a strain discontinuity front propagated ahead of the perforator when the impact velocity of the perforator exceeded 20 m/s. The significance of this numerical model is that it demonstrates that the main feature (average perforation force) can be reproduced with a rather simple pre-implemented material law for a homogeneous specimen. An analytical model using the concept of a shock front with a power law densification assumption is proposed to describe the enhancement in the impact piercing force.

Band-Gap Reduction in (BiCrO3)m/(BiFeO3)n Superlattices: Designing Low-Band-Gap Ferroelectrics

Author(s): Zhang, S; Xiao, HY; Peng, SM; Yang, GX; Liu, ZJ; Zu, XT; Li, S; Singh, DJ; Martin, LW; Qiao, L | Abstract: Ferroelectric BiFeO3 is promising for photovoltaic applications, especially in regard to the exploitation of ferroelectric photovoltaic effects for charge separation. However, its large band gap limits efficient sunlight absorption. Here, we demonstrate a new strategy to effectively tune the band gap of tetragonal BiFeO3 via superlattice structuring with the ferroelectric BiCrO3. The (BiCrO3)m/(BiFeO3)n superlattices are found to exhibit conventional ferroelectric properties, but low fundamental band gaps, smaller than either of the parent materials. First-principles calculations reveal that the unexpected band-gap reduction is induced by charge reconstruction due to lattice strain, octahedral distortion, and polarization discontinuity at the BiCrO3-BiFeO3 interfaces. Ultimately, these results provide a new strategy, in the form of superlattice structuring, which could open the door to the creation of efficient ferroelectric photovoltaics.

A refined layerwise finite element modeling of delaminated composite laminates with piezoelectric layers

A finite element model for static response and free vibration analysis of delaminated composite laminates with active layers is developed based on a coupled layerwise laminate theory. The proposed model assumes full layerwise variations for both mechanical displacements and the electric field, and more importantly, both strong and weak discontinuous functions are introduced into the displacement fields to model the displacement discontinuity induced by the delamination and strain discontinuity induced by the interface between the layers, respectively. This refined layerwise mechanics can give a more accurate description of displacement discontinuity. The formulation naturally includes the coupling and interactions between the composite laminates and piezoelectric layers. Various examples are investigated by the developed model and the results are compared with previously obtained alternative solutions or experimental results to verify the justification and accuracy of the present formulation. Finally, the effect of delamination on static response and vibration mode are studied in detail to provide beneficial information for the promising damage identification techniques.

The role of strain and polar discontinuity in magnetism in LaMnO3/SrTiO3/LaAlO3 (0 0 1) heterostructures

The role of strain and polar discontinuity in magnetism in LaMnO3/SrTiO3/LaAlO3 (0 0 1) heterostructures

Material softening and strain localization in spatial geometrically exact beam finite element method with embedded discontinuity

When some critical condition is reached at a material point of a solid body, a localized strain starts developing which makes the strain field discontinuous and highly accelerates local damaging of material. The present paper addresses this kind of strain localization in spatial geometrically exact beams. Here we propose a new beam finite element formulation which accounts for softening of material by applying the embedded strong strain discontinuity technology. The formulation is essentially an extension of the original strain-based formulation, upgraded such to allow for detecting the onset of strain localization and to introduce additional equations for evaluating singular strain peaks and jumps of displacements and rotations at the localized section in further deformation. The consistency condition that the equilibrium and the constitutive stress-resultants are equal is shown to be naturally suited for the implementation into the discontinuous formulation. The condition for the onset of strain localization at a beam cross-section is here related to the loss of uniqueness of the beam cross-sectional constitutive equations. If the condition for a unique inverse is violated, two solutions are possible for cross-sectional strains. In a subsequent deformation, one of the two solutions follows the softening regime of material. The discontinuous increments in strains, displacements and rotations at the softening cross-section are obtained from the equations of the structure supplemented by the consistency conditions of the softening cross-section.

Localization in an anisotropic planar aggregate of spheres

We present a micro-mechanical model that is able to predict localization in a sheared planar aggregate of spheres. We assume a non-linear contact law between interacting particles that deform differently from an affine deformation. Equilibrium determines this deviation. The aggregate is isotropically compressed and then sheared so anisotropy develops because of contacts deletion and a non-linear contact law. Because of anisotropy and fluctuations in particles deformation, the resulting macroscopic stiffness tensor, which relates increments in the average stress with increments in the average strain, is characterized by a lack of major symmetry, Aijkl ≠ Aklij . At given shear strain and coordination number it is possible to detemine a plane over which discontinuity in strain occurs; this is identified as localization.

Reinforced Concrete Finite Element Modeling based on the Discrete Crack Approach

The behavior of reinforced concrete elements is complex due to the nature of the concrete that is weak in tension. Among these complex issues are the initial cracking and crack propagation of concrete, and the bond-slip phenomenon between the concrete and reinforcing steel. Laboratory tested specimens are not only costly, but are limited in number. Therefore a finite element analysis is favored in combination to experimental data. The finite element technique involving the cracks inserting is one of the approaches to study the behavior of reinforced concrete structures through numerical simulation. In finite element modeling, the cracks can be represented by either smeared or discrete crack. The discrete crack method has its potential to include strain discontinuity within the structure. A finite element model (FEM) including the concrete cracking and the bond-slip was developed to simulate the nonlinear response of reinforced concrete structures.

Effects of connection flexibility in bridge girders under moving loads

AbstractThis paper looks at the problem of connection flexibility in steel‐concrete bridge girders under moving loads. The static action of the load changing location on the structure is considered. An analytical model of the girder is used assuming strain discontinuity at the steel‐concrete interface as a result of beam‐plate partial interaction. The effects of a flexible connection are characterized by the proposed index defined on the basis of the internal forces in the girder. This index can be calculated during loading tests on the basis of the neutral axis position at the section of the girder considered. Numerical analyses show that values of the index characterizing beam‐plate interaction depend on the position of the load on the structure and the function describing connection stiffness.

Configurational phases in elastic foams under lengthscale-free punching

We carry out experiments with brick-like specimens of elastic open-cell (EOC) foams of three relative densities. Individual specimens may be “tall” (height = width = depth) or “short” (2height=width=depth). We place each specimen on a supporting plate and use a lengthscale-free (wedge-shaped or conical) punch to apply forces downward along the specimen's height. Regardless of the type of specimen, the force–penetration curves remain linear, for the wedge-shaped punch, or quadratic, for the conical punch, up to a sizable penetration commensurate with the smallest lengthscale of the specimen. After that there is an abrupt, all-but-discontinuous change in stiffness: if the specimen is tall, the stiffness drops; if the specimen is short, the stiffness shoots up. To analyze these curious experimental results, we posit that EOC foams can be found in either of two configurational phases, here termed the low-strain phase and the high-strain phase, which share a two-dimensional interface (a surface of strain discontinuity). The analysis may be outlined as follows. In the first part of an experiment, there obtains a “similarity regime” in which the penetration of the punch and the radius of the interface are the only prevailing lengthscales (because the punch is lengthscale free). In this case, it is possible to show that the force–penetration curve must be linear, or quadratic, depending on whether the punch be wedge-shaped or conical, respectively. This prediction of the analysis is consistent with the experiments. In time, the similarity regime breaks down when the interface reaches one of the specimen's boundaries distal to the tip of the punch. If the specimen is tall, the soft, stress-free lateral boundary is reached first, and the stiffness must drop; if the specimen is short, the hard boundary in contact with the supporting plate is reached first, and the stiffness must shoot up. These predictions too are consistent with the experiments. To provide direct empirical evidence of the interface, we use a digital-image correlation method. Lastly, we run computational simulations of all the experiments, using finite elements and the skeleton-and-bubble model of EOC foams. The computational results are in good accord with the experimental ones, and they allow us to carry out a detailed validation of the analysis. Our findings evince the cardinal role of configurational phases in the mechanics of EOC foams.