Mechanical Response Of Material Research Articles

The Effect of Microcrystals on ZrF4-BaF2-LaF3-AlF3-NaF Glass Fiber Fracture

The mechanical response of materials to an applied force is determined by the inherent microstructure properties of the material.Microcrystals formed during heating and fiber drawing represent inherent flaws, which play a strong role in the mechanical properties as well as the handling ability of optical waveguides, in particular ZBLAN (ZrF4–BaF2–LaF3–AlF3–NaF) fibers. The property of the fiber to flex and bend is useful for handling and positioning the fiber in a desired application. A series of ZBLAN fibers were bent to specific radii of curvature to determine the applied curvature at which fiber fracture occurs. The fibers were first subjected to a tailored temperature arrangement for a designated amount of time to induce varying amounts of crystal formation. The fiber failure was documented and photographed with the aid of optical microscopy and scanning electron microscopy. Fracture mechanics analysis was used to characterize the impact of crystal size on the failure of the ZBLAN fiber. The results of this assessment show the impact of thermal degradation on ZBLAN fibers as well as suggest an optimum fiber drawing take-up reel diameter.

Asphalt Concrete Layer to Support Track Slab of High-Speed Railway

Asphalt concrete (AC) is a traditional material that is used to construct highway pavement and offers the advantages of good bearing capacity, waterproofing, shock absorption, and noise reduction. In recent years, AC also has been used to construct railway infrastructure as part of the rapid development of high-speed rail in China. The study presented in this paper focused on the material composition and mechanical response of railway asphalt concrete (RAC) on the basis of mechanistic models and laboratory experiments. According to the material test results, the air voids of the material RAC-25 used for railway infrastructure should be controlled from 1% to 3%. The optimal asphalt content should be obtained from parameter tests. The recommended asphalt content should be less than 5.8%. Load-bearing infrastructure with the use of RAC-25 to replace partially the surface layer of subgrade, which was a stabilized layer on top of the subgrade, may yield a better performance. This design was No. 1 asphalt concrete railway substructure (ACRS-1). On the basis of the test results, the model with a 12-cm depth of graded gravel subgrade surface layer replaced by RAC-25 showed much better capacity and stability than other models. By comparison, the model reinforced by RAC-25 showed a higher K30 value, less deformation, good temperature sensitivity, and better loading performance. In summary, the ACRS-1 reinforced by RAC-25 was found to be a suitable type of structure for high-speed railway infrastructure.

Specimen Inertia in high strain rate tensile testing

It is well established that measurements of the dynamic mechanical response of materials are susceptible to errors due to the inertia of the specimen, which causes stresses in addition to the intrinsic material strength. A number of authors have derived equations for these stresses in compression experiments; these equations can be used as a guideline for good specimen design. However, no such equations have been presented in the literature for the equivalent effects in tensile experiments. This paper begins by considering and rationalising the equations available for compression, before producing a set of equations which can be used in design of specimens for, e.g., tensile Hopkinson bar experiments.

Thermo‐mechanical couplings in elastomers – experiments and modelling

Elastomers take an important role in many industrial applications. In the automotive industries for example, elastomers are used in various bearings, where they inhibit vibration propagation and thereby significantly enhance driving performance and comfort. Several models have been developed to simulate the material's mechanical response to various stresses and strains a component may undergo during its lifetime. So far, these models are commonly developed under isothermal conditions. In this contribution it is shown that the mechanical properties significantly depend on the temperature and that the material heats up under large dynamic deformations. Therefore, an elastomer's behaviour is not described sufficiently with an isothermal approach, a detailed thermo‐viscoelastic modelling is required. In this contribution, the behaviour of elastomers is experimentally investigated in order to gain informations about the time‐ and temperature‐dependent mechanical properties. We perform different tests on a natural rubber to emphasize the temperature dependence of the equilibrium stress‐strain relation as well as the time‐dependent behaviour in relaxation tests. As it is necessary for parameterising a material model, thermal tests are carried out to determine the specific heat capacity, the thermal expansion coefficient and the thermal conductivity. In a second step, we introduce a material model which is able to represent the temperature‐dependent viscoelastic material behaviour including large deformations, as well as the self‐heating of the material. The model's mechanical parameters are identified on tension tests. In first FE calculations, the applicability of the introduced model is proven by depicting the experimental results of several tension tests at different temperatures. Besides these validations, the self‐heating under dynamic load, depending on the loads amplitude and frequency as well as the surrounding temperature is calculated.

Interdependencies Between Mechanical Properties and Microstructural Development During One-Dimensional Shock Loading

The mechanical response of materials under shock loading conditions is governed, as indeed it is at lower strain-rates by events occurring at the microstructural level. However, since shock loading experiments occur under conditions of one-dimensional strain (rather than the more familiar onedimensional stress of quasi-static loading conditions), the generation of useful, quantitative microstructural information is a non-trivial exercise. In a simple plate impact experiment, releases from the edge of the target assembly travel into the centre, taking the target into a complex three-dimensional strain-state, rendering meaningful microstrutural analysis difficult or even impossible. As a consequence, the target assembly must contain a series of momentum trapping rings around the region of interest, the interfaces of which will open up during the passage of release waves, which prevents their subsequent transmission. A typical recovery target assembly is shown in figure 1, based upon the work of Gray and his colleagues [1-2].

Elastic precursor decay in S-200F beryllium

We have performed a series of plate impact experiments on vacuum hot-pressed (VHP) S-200F Be at peak shock stresses between 2.1 and 23.0 GPa to gain insight into the dynamic strength (Hugoniot elastic limit (HEL)), equation-of-state, and damage behavior of this technologically important material. In this paper we focus on our VISAR observations of the evolution of elastic precursor amplitude with Be target thickness in a series of plate impact experiments conducted in both transmission and reverse geometry. We observe monotonic decay in precursor amplitude with run distance for sample thicknesses between 4 and 8 mm and present the HEL values obtained from these experiments. We will discuss the observed precursor decay with respect to the relative roles of twinning and dislocation-mediated slip in the overall dynamic material mechanical response.

PFEM-based modeling of industrial granular flows

The potential of numerical methods for the solution and optimization of industrial granular flows problems is widely accepted by the industries of this field, the challenge being to promote effectively their industrial practice. In this paper, we attempt to make an exploratory step in this regard by using a numerical model based on continuous mechanics and on the so-called Particle Finite Element Method (PFEM). This goal is achieved by focusing two specific industrial applications in mining industry and pellet manufacturing: silo discharge and calculation of power draw in tumbling mills. Both examples are representative of variations on the granular material mechanical response—varying from a stagnant configuration to a flow condition. The silo discharge is validated using the experimental data, collected on a full-scale flat bottomed cylindrical silo. The simulation is conducted with the aim of characterizing and understanding the correlation between flow patterns and pressures for concentric discharges. In the second example, the potential of PFEM as a numerical tool to track the positions of the particles inside the drum is analyzed. Pressures and wall pressures distribution are also studied. The power draw is also computed and validated against experiments in which the power is plotted in terms of the rotational speed of the drum.

Multiscale Analysis of Processing‐Microstructure‐Mechanical Behavior Interrelationships of UFG Microalloyed Steels

The processing difficulties and enhanced ductility of UFG (ultrafine‐grained) microalloyed steels are currently one of the most stimulating challenges due to problems in practical applications as structural materials. A key challenge is the ability to represent the real behavior of such modern structural material using multiscale approach. In this study, various processing routes and nonlinear combined hardening model were applied to capture strain path effects that play major role in the deformation process of UFG microalloyed steels. Special emphases are given on the inhomogeneity of the accumulated energy of deformation and resulted microstructure evolution. The hardening model was implemented into a multiscale constitutive model within a crystal plasticity framework. The proposed idea provides a bridge between the dislocation substructure and fine precipitates interactions at the nano‐ and microscale, local stress concentrations on the grain boundaries at the microscale, intragranular stresses at the mesoscale, and material mechanical response at the macroscale. Based on experimental results, in order to fully cover all the analysis scales, the multiscale modeling approach is proposed. The results presented in this study show that a combination of continuum mechanics and dislocation theory can be used successfully to map dislocation strengthening and to build a rheological model for UFG microalloyed steels.

Evaluation of the mechanical behaviour of Telemark ski boots: Part II – structural analysis

This work reports the evaluation of the mechanical behaviour of Telemark ski boots, by means of an integrated approach, considering polymeric material characterisation, reported in a previous study (Part I), and numerical structural analysis, reported in this study (Part II). Telemark boots entail a complex procedure for analysis of the mechanical response, with regard to both material assumption and overall structural behaviour, and represent a reference problem within sky footwear in consideration of this peculiar complexity. A visco-elastic constitutive model is formulated to describe the material mechanical response defined in accordance with experimental test performed. Solid models define the morphology of the ski boots and represent the basis for numerical modelling. Specific boundary conditions are assumed to mimic the binding effect. The numerical analysis leads to an interpretation of the global and local responses of a specific Telemark ski boot, considering material properties and structural conformation. The results provide valid information on the ski boot mechanical response, taking into account also the effects of temperature on material response and strain rate with regard to loading conditions. The evaluation of variations in shape and material of the different components can be performed representing a fundamental support for the design.

A condensed variational model for thermo-mechanically coupled phase transformations in polycrystalline shape memory alloys

AbstractWe derive an energy-based material model for thermomechanically coupled phase transformations in polycrystalline shape memory alloys. For the variational formulation of the model, we use the principle of the minimum of the dissipation potential for nonisothermal processes for which only a minimal number of constitutive assumptions has to be made. By introducing a condensed formulation for the representative orientation distribution function, the resulting material model is numerically highly efficient. For a first analysis, we present the results of material point calculations, where the evolution of temperature as well as its influence on the mechanical material response is investigated.

A multiscale approach for modeling progressive damage of composite materials using fast Fourier transforms

Composite materials possess a highly complex material behavior, and thus advanced simulation techniques are necessary to compute their mechanical response. In this regard, especially modeling failure and progressive damage presents a challenging task. Conventional macro mechanical methods and even closed form estimates are in many cases not sufficient to predict the appropriate mechanical material response. Full-field simulations must be resorted to, but these are known to be very expensive from the computational point of view. In this contribution we propose a more efficient multiscale approach similar to FE2. Nonlinear material effects caused by progressive damage behavior are captured directly on the discretized material level using simple isotropic continuum damage laws. In contrast to conventional FE2 methods which use the Finite Element Method (FEM) to solve both scales numerically, the fine scale problem (material level) is rewritten in an integral form of Lippmann–Schwinger type and solved efficiently using the fast Fourier transformation (FFT). The calculation is carried out on a regular voxel grid that can be obtained from 3D images like tomographies. The fine scale problem is integrated in a standard Finite Element framework which is used to solve the macroscopic BVP (component level). In the work at hand, the scale coupling technique and the computation of the macroscopic tangent are described, and in some numerical examples the convergence behavior of the macroscopic Newton algorithm is investigated. Thereby the simulations were considered until localization and softening on the material scale occurred. It is shown that the proposed method presents an effective way to determine the exact physical macroscopic response considering arbitrary microstructures and loading conditions.

Deformation and damage due to drying-induced salt crystallization in porous limestone

This paper presents a computational model coupling heat, water and salt ion transport, salt crystallization, deformation and damage in porous materials. We focus on crystallization-induced damage. The theory of poromechanics is employed to relate stress, induced by crystallization processes or hygro-thermal origin, to the material's mechanical response. A non-local formulation is developed to describe the crystallization kinetics. The model performance is illustrated by simulating the damage caused by sodium chloride crystallization in a porous limestone. The results are compared with experimental observations based on neutron and X-ray imaging. The simulation results suggest that the crystallization kinetics in porous materials have to be accurately understood in order to be able to control salt damage. The results show that the effective stress caused by salt crystallization depends not only on the crystallization pressure but also on the amount of salt crystals, which is determined by the spreading of crystals in the porous material and the crystallization kinetics.

Symmetry analysis and invariance relations in creep

A novel and rational approach based on Lie analysis is proposed to investigate the mechanical behaviour of materials exhibiting experimental master curves. Our approach relies on the idea that the mechanical response of materials is associated with hidden symmetries; the general objective of this contribution is to reveal those symmetries from measurements and to construct constitutive laws from them. This approach provides a priori two ways of formulating constitutive laws from data as well as the possibility of predicting new master curves and material charts. The first part of the paper is devoted to the presentation of the general methodology. Afterwards, the strategy is applied to the uniaxial creep and rupture behaviour of a Chrome-Molybdene alloy (9Cr1Mo) at different temperatures and stress levels. Constitutive equations for creep and rupture master responses are identified for this alloy and validated on experimental data.

Dynamic mechanical behavior of 6061 al alloy at elevated temperatures and different strain rates

The compressive stress-strain relationships of 6061Al alloy over wide temperatures and strain rates are investigated. The dynamic impact experiments are performed using an improved high temperature split Hopkinson pressure bar apparatus. The experimental results are compared with those obtained by the modified Johnson-Cook constitutive model. It is found that the dynamic mechanical behavior depends sensitively on temperature under relatively low strain rates or on strain rate at relatively high temperatures. The good agreement indicates that it is valid to adopt the parameter identification method and the constitutive model to describe and predict the mechanical response of materials.

Material characterisation using high-temperature Split Hopkinson pressure bar

In order to characterise the mechanical response of materials in manufacturing processes, such as wire and bar rolling involving very high strain rates, temperatures and level of straining, an experimental device is presented. The device is suitable for testing at strain rates up to approximately 4000s−1, temperatures up to 1200°C (≈1500K) and strains around 0.5. It is based on the classical Split Hopkinson pressure bar and is complemented with an inductive heating source for achieving requested temperatures. By keeping the specimen separated from the Hopkinson bars just until an instant before impact (50ms) considerable cooling and temperature gradients in the specimen are avoided. Three steel grades, two stainless steels and a high-speed steel, were tested. Four different material models whose parameters were fitted to the obtained experimental data were used for mechanical characterisation: two empirically based and two physically based. Overall, one of the physically based models showed the best agreement between experimental results and the predicted flow stresses.

Dynamic uniaxial extension of elastomers at constant true strain rate

Elastomers are widely used for damping components in various industrial contexts because of their remarkable dissipative properties: they can bear severe mechanical loading conditions, i.e., high strain rates and large strains. Depending on the strain rate, the mechanical response of these materials can vary from purely rubber-like to glassy. In the intermediate strain rate range (1-100/s), uniaxial extension experiments are classically conducted at constant nominal strain rate. We present here a new experimental methodology to investigate the mechanical response of soft materials at constant true strain rate in the intermediate strain rate range. For this purpose, the displacement imposed on the specimen by the tensile machine is an exponential function of time. A high speed servo-hydraulic machine is used to perform experiments at strain rates ranging from 0.01 to 100/s. A specific specimen is designed in order to achieve a uniform strain field (and thus a uniform stress field). Furthermore, an instrumented aluminium bar is used to measure the applied force; which overcomes the difficulties due to dynamic effects. Simultaneously, a high speed camera enables the measurement of strain in the sample using a point tracking technique. Finally, the method is applied to determine the stress-strain curve of an elastomer for both loading and unloading responses up to a stretch ratio λ = 2.5; the influence of the true strain rate on both stiffness and dissipation of the material is then discussed.

Interfacially Driven Deformation Twinning in Bulk Ag-Cu Composites

Interfaces and interface/defect interactions increasingly dominate the mechanical response of materials as the dimensions of the grains decrease to the nanoscale. Recently, we reported unusually profuse deformation twinning in Ag-Cu layered eutectic composites with bilayer thicknesses in the submicron regime (~200 nm–400 nm) at room temperature and low strain rates. Using atomistic simulations and dislocation theory, we propose that the Ag-Cu interface facilitated deformation twinning in Cu by permitting the transmission of twinning partials from Ag to Cu. In this way, twins in Ag can provide an ample supply of twinning partials to Cu to support and sustain twin growth in Cu during deformation. Interface-driven twinning as revealed by this study suggests the exciting possibility of altering the roles of dislocation slip and twinning through the design of heterophase interface structure and properties.

Overview: Damage resistance of graded ceramic restorative materials

Improving mechanical response of materials is of great interest in a wide range of disciplines, including biomechanics, tribology, geology, optoelectronics, and nanotechnology. It has been long recognized that spatial gradients in surface composition and structure can improve the mechanical integrity of a material. This review surveys recent results of sliding-contact, flexural, and fatigue tests on graded ceramic materials from our laboratories and elsewhere. Although our findings are examined in the context of possible applications for next-generation, graded all-ceramic dental restorations, implications of our studies have broad impact on biomedical, civil, structural, and an array of other engineering applications.

Shear Deformation of Diamond Crystals: Topology Analysis of Electron Density

Defects in crystals such as surfaces, interfaces, and grain boundaries significantly affect mechanical responses of materials under deformation in microscopic scale. While locally-distributed energy and stress obtained by ab-initio method are considered to be useful tools for capturing the local effects of defects inside crystals, topological analysis of electron density proposed by Bader is another way to approach such local effects. Estimating the changes of electron density or its curvatures on some specific points such as bond midpoints, we can obtain the characteristics of the local electronic structure as well as by the partial density of states or the bond order. In this study, we have performed the topological analysis and the PDOS analysis on shear deformation of diamond crystals, carbon, Si, and Ge. Using the results, we conclude that the stress responses depend on the balance between the bonding strength and the angular dependence of sp3 configuration.

Thermo-mechanical strain gradient plasticity with energetic and dissipative length scales

Plasticity in heterogeneous metallic materials of small volumes is governed by the interactions of the dislocations in the bulk and at the interfaces. The temperature distribution and accumulated heat in the medium, due to the high rate of loading, can also strongly affect the mechanical properties in micro and nanosystems. In this paper, the framework of higher-order strain gradient plasticity with interfacial energy effect is used to investigate the coupling of thermal and mechanical responses of materials in small scales and fast transient process. In order to accurately describe the size effect exhibited in micro/nano structured materials and to address the strengthening and hardening mechanisms, the theory is developed based on the decomposition of the mechanical state variables into energetic and dissipative counterparts. This, in turn, endowed the constitutive equations to have both energetic and dissipative gradient length scales ℓen and ℓdis respectively. The effect of the material microstructural interface between two materials is also incorporated into the proposed formulation. Hence four material length scales are introduced: two for the bulk and the other two for the interface. In addition, the temperature distribution in the system due to conversion of plastic work into heat and the partial dissipation of heat due to fast transient time, is included into the model using a generalized heat equation. The microstructural interactions (i.e. photon–electron coupling) and the effect of fast transients is considered by incorporating two time scales into the heat equation. The developed framework is solved numerically by using a thin film on an elastic substrate under biaxial straining. Furthermore, the obtained results are compared with experimental data.