Global Deformation Behavior Research Articles

Linking local and heterogeneous deformation behavior to global deformation of materials by in-situ experimental techniques

Plastic deformation of metallic materials is essentially heterogeneous and localized. In the case of dislocation slip, for example, slip of dislocations occurs on particular crystallographic planes and in particular directions, and dislocations are multiplied from dislocation sources located on limited slip planes. Heterogeneous deformation is also induced by the constraint of adjacent grains in polycrystalline materials and appears in different length scales in the form of different slip patterns within each grain, deformation bands, and so on. To achieve higher strengths, the microstructures of structural metallic materials have become more and more complicated and are often composed of different phases (or domains) having different strength levels. In such complicated microstructures, deformation becomes even more heterogeneous. Therefore, it is important, especially in high strength metals, to link local and heterogeneous deformation aspects to the global deformation behavior, in order to fundamentally understand the nature of deformation and then to design advanced materials for performing desired mechanical properties. Conventional material testing, such as tensile tests can determine the global deformation behavior of materials, but the obtained stress-strain curves show only the average material response, in which local information is hidden. Local and heterogeneous characteristics of deformation have been observed by various kinds of microscopy, but it is typically difficult to correlate such local and heterogeneous features with the global and average materials response, especially during deformation. Recently, significant progress has been made in various kinds of in-situ experimental techniques, and a breakthrough in deepening the fundamental understanding of deformation of materials is expected. Such techniques include in-situ neutron/synchrotron diffraction during deformation, in-situ deformation observations in scanning and transmission electron microscopy, nano-indentation, and digital image correlation (DIC) during deformation. In the current paper, some recent results obtained in the author’s group linking local and heterogeneous characteristics to the global deformation response for several metallic materials are introduced, illustrating the use of state-of-the-art techniques.

Local and global deformation behaviour in rolled pure magnesium sheets at room temperature under different strain rates

In this paper, deformation behaviour of a rolled pure Mg sheet at room temperature was explored at quasi-static strain rates (10−4, 10−3 and 10−2·s−1) under the context of microstructural and textural observation. Tensile tests with an aid of digital image correlation (DIC) were performed along a rolled (RD) and transverse (TD) direction resulting in ~3%, ~4% strain and fracture. The microstructure and texture of the initial and deformed samples were characterised by EBSD and OM measurements. Mechanical properties including r-value and strain rate sensitivity (SRS) were evaluated and analysed with twinned area fraction. The results showed an obvious anisotropy and rate dependence in mechanical properties and deformation modes. The higher flow stress in TD samples was effectively elucidated by the Schmid factor analysis. The origin of the early fracture in RD samples was discussed with respect to the SRS and strain distributions where RD samples displayed a steep and narrowed strain localisation. Interestingly, TD samples showed a reversed dependence of twin activity at the fractured samples and total elongation on strain rate. Instead of tension twinning, basal <a> slip induced width strain in TD samples is likely to promote homogeneous lateral deformation which may eventually lead to the higher elongation in TD samples.

Damage Deformation of Flexure‐Yielding Steel‐Reinforced Concrete Coupling Beams: Experimental and Numerical Investigation

SRC coupling beams offer many significant advantages, including the reduction in section depth, reduced congestion at the wall boundary region, improved degree of coupling for a given beam depth, and improved deformation capacity. In this paper, 7 half‐scale flexure‐yielding SRC coupling beams designed according to Chinese approach have been tested under cyclic loads. Detailed parameters such as aspect ratios, steel reinforcement ratios, and steel flange‐web ratios were systematically studied, and the damage behavior of SRC coupling beams were presented in this paper. The test results show that the aspect ratio, steel ratio, and steel flange‐web ratio have great influence on the damage behavior of SRC coupling beams. Three‐dimensional nonlinear finite element models were constructed and benchmarked through comparison with test results for both global and local damage deformation behavior. Based on the material damage and strength degradation, four performance levels were defined and corresponding chord rotation limits were obtained through the verified numerical analysis.

Cyclic plastic behavior of unidirectional SiC fibre-reinforced copper composites under uniaxial loads: An experimental and computational study

Silicon carbide continuous fiber-reinforced copper matrix composites (Cu/SiCf) offer huge potential as heat sink material for high-heat-flux applications at elevated temperatures (>300 °C) owing to the beneficial combination of strong, stiff and refractory SiC fibers with a highly conductive and ductile copper matrix. As high-heat-flux components are normally subjected to large temperature fluctuations combined with thermal strain variations, degradation of the Cu/SiCf composites caused by fatigue damages under cyclic loads may affect the structural integrity of the component. In this study, the robustness of the Cu/SiCf composites under cyclic loads at different temperatures was evaluated by means of uniaxial cyclic loading tests. The stability of the cyclic deformation hysteresis curves was used as a measure of material’s durability. The experimentally observed global deformation behavior was interpreted with the help of a micromechanics-based theoretical model. It was shown that the Mori-Tanaka type mean field theory could describe the cyclic loading behavior of the composite quite well with a good fitting quality between the measured and simulated saturated cyclic curves. The effects of fiber content, applied strain range and test temperature are discussed.

A multiscale experimental approach for correlating global and local deformation response in woven composites

A multiscale experimental approach is introduced that facilitates correlating globally applied stress to the local material response at yarn scale. Using a high-resolution optical digital image correlation, low and high strain zones in off-axis woven composite samples loaded in uniaxial tension are identified and the corresponding full-field strains are quantified. The local strain response quantified over low and high strain domains are used along with the continuum scale deformation to establish a correlation between global and local deformation behavior as a function of fiber orientation. In addition, global stress-local strain curves are extracted for low and high strain domains at different fiber orientation angles. A simple plasticity model in conjunction with the obtained stress-strain curves is implemented and utilized to develop a phenomenological model that enables correlating global stress with the locally developed strain at different locations in a composite specimen subjected to off-axis tensile load.

Hierarchical heterogeneity and an elastic microstructure observed in a metallic glass alloy

Heterogeneities in a Zr-based bulk metallic glass (BMG) were investigated using dynamic modulus mapping on a nanoindentation platform, revealing a complex elastic microstructure consisting of interpenetrating locally stiff and compliant regions with characteristic feature lengths on the order of 100 nm. The unique microstructures were observed in as-cast, annealed and laser-processed materials. Surprisingly, at various mapping locations across the cast sample cross sections, the elastic microstructures displayed directional alignment of the features with the mold surface. The results introduce an unprecedented spatial regime of elastic variations in monolithic BMGs that may contribute towards a more complete understanding of structure-property relationships in these materials, especially with regards to global deformation behavior.

Behaviour of Prestressed Concrete Under Drop Impact Loading

Structures are subjected to impacts loads of low to high intensity such as developed from falling of heavy object on floors, vehicle accident with bridge columns, missile attacks during war and terrorist blasts etc. The time duration under such incident is very low, an order of few milliseconds; therefore local and global deformation behavior of structures was of great importance in past research. Moreover, under impact loading the material behavior is entirely different than under static loading conditions. Therefore, in this study, impact tests have been carried out on prestressed concrete plates to identify their behavior when subjected to drop weight impact. The prestressed concrete plates of size 0.8 m x 0.8 m and thickness 100mm were subjected to drop impact by 242.8kg hammer. These plates having identical material and geometrical properties except prestressing force applied for initial stressing. The compressive strength of concrete used for casting of all plates was 40N/mm2. The impact force, deflection and failure modes of the plate specimens have been studied. It was observed that with increase in applied prestressing force impact resistance capacity of plate has been increased and magnitude of peak deflection has been reduced.

Impact Simulations Using a Quasi‐Static Contact Submodel in Reduced Flexible Multibody Systems

AbstractFor an efficient approximation of the local and global deformation behavior, impact simulations using reduced elastic models can be performed. Modally reduced models require a large number of eigenmodes to capture all local deformation effects. For this reason, the use of static shape functions in the contact area is recommended. However, due to high eigenfrequencies introduced by the static shape functions, the time integration is no longer efficient. Hence, this work presents an approach to reduce the influence of these high frequencies on the time integration. (© 2016 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Modelling the damage and deformation process in a plastic bonded explosive microstructure under tension using the finite element method

Modelling the deformation and failure processes occurring in polymer bonded explosives (PBX) and other energetic materials is of great importance for processing methods and lifetime storage purposes. Crystal debonding is undesirable since this can lead to contamination and a reduction in mechanical properties. An insensitive high explosive (PBX-1) was the focus of the study. This binary particulate composite consists of (TATB) filler particles encapsulated in a polymeric binder (KELF800). The particle/matrix interface was characterised with a bi-linear cohesive law, the filler was treated as elastic and the matrix as visco-hyperelastic. Material parameters were determined experimentally for the binder and the cohesive parameters were obtained previously from Williamson et al. (2014) and Gee et al. (2007) for the interface. Once calibrated, the material laws were implemented in a finite element model to allow the macroscopic response of the composite to be simulated. A finite element mesh was generated using a SEM image to identify the filler particles which are represented as a set of 2D polygons. Simulated microstructures were also generated with the same size distribution and volume fraction only with the idealised assumption that the particles are a set of circles in 2D and spheres in 3D. The various model results were compared and a number of other variables were examined for their influence on the global deformation behaviour such as strain rate, cohesive parameters and contrast between filler and matrix modulus. The overwhelming outcome is that the geometry of the particles plays a crucial role in determining the onset of failure and the severity of fracture in relation to whether it is a purely local or global failure. The model was validated against a set of uniaxial tensile tests on PBX-1 and it was found that it predicted the initial modulus and failure stress and strain well.

Global and local deformation behavior and mechanical properties of individual phases in a quenched and partitioned steel

Third generation advanced high strength steels produced via quenching and partitioning (Q&P) treatment are receiving increased attention. A 0.25C–3Mn–1.5Si–0.023 Al steel was subjected to Q&P processing (with varying partitioning temperature and time) resulting in the formation of complex multi-phase microstructures. The effect of Q&P parameters on the microstructure and morphology of microconstituents was analyzed. Mechanical properties of the material and of its individual microconstituents were studied via tensile testing and nanoindentation on individual microconstituents, which were identified a priori by electron back-scattered diffraction analysis. Special attention is paid to the effect of the morphology of retained austenite on its transformation stability. In situ tensile tests and digital image correlation analysis were performed to study deformation behavior of the Q&P processed steel at the micro-scale with respect to the local microstructure. The effect of local microstructure and properties of individual phases on the degree of strain partitioning is discussed.

On the stress-state dependent plasticity of brittle metallic glasses: Experiment, theory and simulation

In this work, the stress-state dependent plasticity of brittle Ti-based metallic glass was investigated by experiment, mechanics theory and simulation systematically. With different settings of arc-shaped edges, various stress gradients could be generated artificially. The systematical work showed that the global plastic deformability of a metallic glass depended strongly on the stress states under compressive loadings. By introducing the Mohr–Coulomb yielding criterion and the associated flow rule, we proposed an elastic–plastic model to analyze the deformation behaviors of all specimens. Combined with the experimental results and the numerical results by finite element method (FEM), it was demonstrated that the present simulation model could suitably describe the plastic deformation process of the specimens, and the different stress states might have a vital significance on the global plastic deformation behavior. The current results and analytical model could provide an approach to understand the further plastic deformation mechanism of brittle metallic glasses with different stress states.

Characterisation of the Local Deformation Behaviour of Engineering Plastics Rolls

Abstract: Previous research on the ‘Characterization of the global deformation behaviour of engineering plastics rolls’ investigated the deformation behaviour of loaded Polyoxymethylene (POM) and Polyetheretherketone (PEEK) rolls regarding global considerations only. This study concentrates on the analysis of the local strain distribution in these rolls. For this purpose, the former component rolling tests were repeated and images of the front side of the rolls were taken. A full‐field strain analysis of the images was conducted for the static stage and the rolling process. The different behaviour of POM and PEEK under the investigated loading conditions, which has been discovered previously, could be confirmed in the local scale. Moreover, the results permit conclusions concerning the material behaviour under these loading conditions: Whilst POM shows typical viscoelastic properties, the behaviour of PEEK can be interpreted as elastic‐plastic combined with time dependence of the irreversible deformation part.

Deformation Behavior of Ultra-Thin Metal Foils in Strip Drawing Friction Test

This study focuses on the surface deformation of thin metal foils caused by friction. To clarify the effect of the relative ratio of surface plastic region to the foil thickness on global deformation behavior, strip drawing tests for ultra thin metal foil with 20μm and 100μm thickness were conducted As a result, different surface deformation and elongation behavior under the same friction condition were observed in different thicknesses. Aided by finite element analysis of the friction test, the contribution of the deformation caused by friction to the foil elongation was investigated and the importance of the friction on material deformation in metal foil forming was demonstrated.

Characterization of the global deformation behaviour of engineering plastics rolls

In real service application of polymer rolls as rolling elements, the time-dependent deformation behaviour leads to a negative influence on the running smoothness after a long-lasting static load. By means of two polymeric materials (POM and PEEK) with different viscoelastic properties over a wide temperature range the influence of material properties on the running smoothness was investigated. A combination of static and dynamic tests as well as finite element simulations were performed. Observed differences in the behaviour of both materials in the service could be reflected well by this work. POM revealed creep flattening and a running performance which could be modelled well by viscoelastic material laws. By contrast, PEEK showed a different performance and its behaviour could not be represented adequately by viscoelastic material models.

Effect of local wall thinning on the collapse behavior of pipe elbows subjected to a combined internal pressure and in-plane bending load

The objective of this study was to investigate the effect of local wall thinning on the collapse behavior of pipe elbows subjected to a combined internal pressure and in-plane bending load. This study evaluated the global deformation behavior and collapse moment of the elbows, which contained various types of local wall-thinning defects at their intrados or extrados, using three-dimensional elastic–plastic finite element analysis. The analysis results showed that the global deformation behavior of locally wall-thinned elbows was largely governed by the mode of the bending and the elbow geometry rather than the wall-thinning parameters, except for elbows with considerably large and deep wall thinning that showed plastic instabilities induced by local buckling and plastic collapsing in the thinned area. The reduction in the collapse moment with wall-thinning depth was considerable when local buckling occurred in the thinned areas, whereas the effect of the thinning depth was small when ovalization occurred. The effects of the circumferential thinning angle and thinning length on the collapse moment of elbows were not major for shallow wall-thinning cases. For deeper wall-thinning cases, however, their effects were significant and the dependence of collapse moment on the axial thinning length was governed by the stress type applied to the wall-thinned area. Typically, the reduction in the collapse moment due to local wall thinning was clearer when the thinning defect was located at the intrados rather than the extrados, and it was apparent for elbows with larger bend radius.

Finite element modelling and testing of an injection moulded generic tow reinforced structure

The deformation and fracture behaviour of a generic injection moulded structure reinforced by a robotically placed commingled glass fibre/polypropylene UD-tow (uni-directional) was simulated by finite elements. The suitability of modelling hybrid structures using features available within commercial finite element programs and standard material properties was assessed. Two different 2D modelling approaches based on shell and beam elements, which were computationally less intensive, were evaluated against 3D solid element models in both bending and flexure. It was shown that all three approaches predict the global deformation behaviour in both tension and bending compared with experimental data. Differences between the 2D and 3D approaches were mainly in the predicted stress distribution around the load introduction points.

Fracture analyses and experimental results of crack growth under general mixed mode loading conditions

In this paper detailed results of 3D finite element (FE) and mixed mode analyses of different fracture specimens are presented and discussed. Special interest is taken in 3D and mode coupling effects to be found in strain energy release rate (SERR) results along crack fronts, in particular adjacent to corners, where a crack front intersects a free surface of a specimen. It will be shown that these effects stay small if they are related to Poisson’s ratio but that they can also be considerably pronounced if they are related to the global deformation behaviour of the specimen. The computational fracture analysis is based on the calculation of separated energy release rates (SERRs) by the aid of the modified virtual crack closure integral (MVCCI)-method in order to calculate the local SERR-distributions along the crack front. Furthermore some qualitative experimental results will show the influence of these variable mixed mode I, II and III loading conditions along the crack front on crack initiation and on the further development of 3D crack growth in the specimens.

設計圧を越える1/4PCCVモデル内圧試験の全体挙動解析

NUPEC has been planning the ultimate strength test of 1/4 uniform scale PCCV imaging severe accident. The pre-test analysis will be compared with measured data. This paper deals with several techniques to obtain physically reasonable analysis results with better convergence up to 3.8 times design pressure.The pre-test analysis was performed using axisymmetric global modeling based on these techniques. Concrete cracking governed the global behavior of PCCV. That is, the reduction of concrete stress due to cracking caused the changes in stress distribution of rebars and tendons and resultant global deformation behavior.

Prediction of the large-strain mechanical response of heterogeneous polymer systems: local and global deformation behaviour of a representative volume element of voided polycarbonate

The mechanical behaviour of voided polycarbonate has been predicted by using a detailed finite element model of the microstructure and an accurate elasto–viscoplastic model for the glassy polymeric matrix material. On the microstructural level a spatially periodic plane strain matrix with irregularly distributed voids has been considered. The voids represent low-modulus non-adhering rubbery particles under negative pressure. The constitutive model for the homogeneous parts of the material reflects the typical yield and post-yield behaviour of glassy polymers: strain rate and history dependent yield, intrinsic strain softening and subsequent strain hardening. The finite element simulations show that the irregular void distribution causes a radical change in deformation behaviour. In particular the macroscopic strain softening disappears. This transformation in macroscopic behaviour originates from the arbitrary order in which local shear bands between the randomly distributed voids are formed and subsequently harden. In the averaged overall mechanical response the individual unstable yield and post-yield behaviour of the local shear bands is evened out, resulting in an overall stable macroscopic deformation behaviour. This mechanism is believed to be primarily responsible for the toughness enhancement of heterogeneous polymer systems through the addition of easily cavitating rubbery particles.

Combined micro‐ and macromechanical considerations of layered composite shells

AbstractThis contribution deals with a computational treatment of the non‐linear behaviour of composite sandwich shells with extremely thin face layers and a relatively thick orthotropic core, and multi‐layered fiber reinforced polymer or metal matrix composite shells. Special emphasis is laid on algorithms which allow the consideration of local effects, like face layer wrinkling in sandwich shells and progressive damage or microplasticity in fibre composites, by semi‐analytical approaches implemented into a finite element formulation. This leads to the advantage that the shell structure must be discretized only up to an element mesh density which is sufficient for describing the global deformation and stability behaviour.