Fracture of Metal Foams: In‐situ Testing and Numerical Modeling

This paper is on a combined experimental/modeling study on the tensile fracture of open‐cell foams. In‐situ tensile tests show that individual struts can fail in a brittle or ductile mode, presumably depending on the presence of casting defects. In‐situ single strut tests were performed, enabling observation of deformation and fracture behavior and, in addition, serving as calibration for the proposed single‐strut model. The single strut model consists of beam elements to account for elasticity and plasticity, and of special‐purpose fracture elements to account for failure. The model is demonstrated for a characteristic loading configuration, combining stretching and bending.     

Al and Zn Foams Blown by an Intrinsic Gas Source

A method was developed to produce Al‐ and Zn‐based foams with a uniform distribution of small cells. Pre‐alloyed AlMg50 powder containing hydrogen was used as a replacement for the usual blowing agent TiH₂. AlMg50 powder released gas uniformly in the entire sample, caused the nucleation of a large number of cells and led to simultaneous growth that finally resulted in a uniform cell structure. The expansion behavior of these foams was studied by means of in situ X‐ray radioscopy. The macrostructure of the solidified foams was then analyzed through optical microscopy and X‐ray tomography and proved to be very uniform. The high strength of the foams was demonstrated by uni‐axial compression tests.     

Compressive Fatigue Behavior of Bovine Cancellous Bone and Bone Analogous Materials under Multi‐Step Loading Conditions

In this study the compressive cyclic behavior of bovine cancellous bone and three open‐cell metallic foams including AlSi7Mg foams (30 and 45 ppi) and CuSn12Ni2 foam (30 ppi) has been investigated. Multi‐step fatigue tests are carried out to study the deformation behavior under increasing compressive cyclic stresses. Short multi‐step tests, with steps of 300–500 cycles, are used to identify the cyclic yield stress (σ_cy) and the stress at failure (σ_fail). The residual strain accumulation, or cyclic creep, is observed during these tests. Long multi‐step tests, with 5000 cycles at selected stress ranges (0.4σ_cy, 0.6σ_cy, 0.8σ_cy, and σ_cy), are also carried out to study further the compressive fatigue behavior of the materials. Scanning electron microscopy (SEM) has been used to characterize the microstructure of the foams and the bone prior to and post mechanical testing. Particular attention is paid to the role of cyclic creep and buckling in the failure processes. The results show that residual strain accumulation seems to be the predominant driving force leading to failure of foams and bones. Although foams and bone fail by the same mechanism of cyclic creep, the deformation behavior at the transient region of each step is different for both materials. The maximum strain ε_max of foams decrease suddenly during the change of each step. This behavior may be explained by the rapidly developing microdamage in the cell struts that occur at the transient region of each step. Bones show more gradual decrease of ε_max, where microdamage may be accumulated progressively during the fatigue test.     

Syntactic Iron Foams – On Deformation Mechanisms and Strain‐Rate Dependence of Compressive Properties

Syntactic iron foams are produced by metal injection moulding from pure Fe powder and two grades of commercial glass microspheres. Mechanical performance of samples containing 5/10/13 wt% of microspheres is compared to unfilled reference material properties at strain‐rates covering 6 orders of magnitude, including Split Hopkinson Pressure Bar (SHPB) experiments. Complex mechanical behavior including strengthening effects of microspheres leading to a plateau strength level which is nearly independent of porosity as well as strain‐rate sensitivity of compressive properties are observed. Typical plateau onset stress levels exceed equivalent characteristics of most comparable cellular metallic materials, reaching between approximately 220 and 270 MPa under quasi‐static conditions, depending on amount and type of added microspheres. A qualitative explanation of significant events in the deformation sequence as reflected in the stress–strain‐curve is offered and discussed in the context of existing studies on syntactic foams. A course for further investigations to verify this hypothesis is suggested.     

In‐Situ Synchrotron X‐Ray Microtomography Studies of Microstructure and Damage Evolution in Engineering Materials

In materials science X‐ray microtomography has evolved as an increasingly utilized technique for characterizing the 3D microstructure of materials. The fundamentals of X‐ray microtomography experimental methods and the reconstruction and data evaluation processes are briefly described. A review of in‐situ synchrotron X‐ray microtomography studies in literature is given. Examples of recent work include in‐situ microtomography investiagtions of metallic foams, in‐situ studies of the sintering of copper particles, and in‐situ investigations of creep damage evolution in composites. Future perspectives of in‐situ X‐ray microtomography studies in materials science are outlined.     

Plastic Deformation of a Nano‐Precipitate Strengthened Ni‐Base Alloy Investigated by Complementary In Situ Neutron Diffraction Measurements and Molecular‐Dynamics Simulations

In situ neutron‐diffraction experiments at the spallation neutron source, simultaneously illuminating the diffraction of the matrix and the strengthening nano precipitates, allow the determination of their plastic deformation. An irreversible neutron‐diffraction‐profile evolution of the nano precipitates is observed. However, there is no conclusive trend of the nano‐precipitate peak‐width evolution subjected to the greater stress levels. Hence, in the present work, molecular‐dynamics simulations are applied to reveal the deformation mechanisms of the nano precipitate and its interaction with the surrounding matrix. The microstructure size, dislocation content, and structural parameters of the nano precipitates, quantified by X‐ray, transmission electron microscopy, and small‐angle neutron scattering, are used as the simulation input and reference. The simulation results show that there are two competing deformation mechanisms, which lead to the fluctuation of the nano‐precipitate‐diffraction widths, occurring during the higher plastic deformation stages.     

Characterization of close-celled cellular aluminum alloys

The deformation behaviour of two different types of aluminium alloy foam are studied under tension, compression, shear and hydrostatic pressure. Foams having closed cells are processed via batch casting, whereas foams with semi-open cells are processed by negative pressure infiltration. The influence of relative foam density, cell structure and cell orientation on the stiffness and strength of foams is studied; the deformation mechanisms are analysed by using video imaging and SEM (scanning electronic microscope). The measured dependence of stiffness and strength upon relative foam density are compared with analytical predictions. The measured stress versus strain curves along different loading paths are compared with predictions from a phenomenological constitutive model. It is found that the deformations of both types of foams are dominated by cell wall bending, attributed to various process induced imperfections in the cellualr structure. The closed cell foam is found to be isotropic, whereas the semi-open cell foam shows strong anisotropy.     

Compression‐compression fatigue performance of aluminium matrix composite foams reinforced by carbon nanotubes

The compression‐compression fatigue performance of carbon nanotube (CNT) reinforced aluminium matrix composite foams (AMCFs) were investigated. The ε‐N curves of AMCFs are composed of three stages (the elastic, strain hardening, and rapid accumulation stages), while the fatigue strain of AMCFs accumulates very rapidly in stage III compared with Al foams. The fatigue strength of AMCFs with CNT contents of 2.0, 2.5, and 3.0 wt% increases by 6%, 44%, and 102% than Al foams, respectively. Different from Al foams' deformation of layer‐by‐layer, the main failure modes of AMCFs are the brittle fracture and collapse of pores within significant shear deformation bands under fatigue loading. The uniform distribution of CNTs and good interfacial bonding of CNTs and Al matrix is the important factor for the improvement of fatigue properties of AMCFs.     

A Combined Analytic,Numeric, and Experimental Investigation Performed on NiTi/NiTiCu Bi‐Layer Composites under Tensile Loading

Adjusting mechanical behavior and controlling deformation parameters are significant tasks in designing shape memory components. In this paper, an analytical model describes the deformation behavior of NiTi/NiTiCu bi‐layer composites under tensile loading. Different deformation stages are considered based on single mechanical behavior at each stage. Closed‐form equations are derived for stress–strain variations of bi‐layer composites under uniaxial loading–unloading. Bi‐layer composites made via the diffusion bonding method from single layers of NiTi alloy with a composition of Ti‐50.7 at.% Ni, as an austenitic layer, and Ti‐45 at% Ni‐5 at% Cu, as a martensitic layer, are produced by the vacuum arc remelting technique. The tensile behavior of single‐ and bi‐layers is investigated by using loading–unloading experiments to find the nominal stress–strain curves. Numerical simulations are also done by employing Lagoudas constitutive model to simulate stress–strain diagrams. The solutions of the analytical method presented are validated by using the numerical simulations as well as the experimental results. With regard to the results obtained from the analytical modeling, the numerical simulations, and the experiments, it is evident that the bi‐layer composites with different thickness ratios provide adjustable mechanical behavior that can be considered in different application designs, for example, actuators equipped with shape memory components.

     

Dynamic crushing behavior of 3D closed-cell foams based on Voronoi random model

Dynamic crushing responses of three-dimensional cellular foams are investigated using the Voronoi tessellation technique and the finite element (FE) method. FE models are constructed for such closed-cell foam structures based on Voronoi diagrams. The plateau stress and the densification strain energy are determined using the FE models. The effects of the cell shape irregularity, impact loading, relative density and strain hardening on the deformation mode and the plateau stress are studied. The results indicate that both the plateau stress and the densification strain energy can be improved by increasing the degree of cell shape irregularity. It is also found that the plastic deformation bands appear firstly in the middle of the model based on tetrakaidecahedron at low impact velocities. However, the crushing bands are seen to be randomly distributed in the model based on Voronoi tessellation. At high impact velocities, the “I” shaped deformation mode is clearly observed in all foam structures. Finally, the capacity of foams absorbing energy can be improved by increasing appropriately the degree of cell shape irregularity.     

Behaviour of aluminium foam under uniaxial compression

Abstract

The deformation behaviour of different types of closed cell aluminium foam (Alulight, Alporas) was studied. Compression tests indicate that inhomogeneities in the density distribution might be the key factor in determining the mechanical behaviour of foams. Strengthening and softening of the foam can be related to the formation of deformation bands. Depending on the composition and the microstructure of the cell wall material, cells undergo either ductile or brittle collapse. A three dimensional finite element analysis, which has the capability to simulate the initial deformation of foam samples, is presented. Continuum mechanics methods are used to describe the behaviour of foam. Each foam sample is divided into subregions, according to their density distribution. Scaling laws are used to simulate the mechanical properties of the subregions. Experimental data are compared with results of the presented model. Very good agreement between experiments and modelling was found for the rather inhomogeneous Alulight material.     

Zur thermischen Ermüdung der Magnesium‐Basislegierung AZ91

Thermal fatigue of magnesium‐base alloy AZ91 Thermal fatigue tests of the magnesium‐base alloy AZ91 were carried out under total strain control and out‐of‐phase‐loading conditions in a temperature range between ‐50°C and +190°C. Specimens produced by a vacuum die casting process were loaded under constant total strain and uniaxial homogeneous stress. To simulate the influence of different mean stresses, experiments were started at different temperature levels, e.g. the lower, mean or upper temperature of the thermal cycle. The thermal fatigue behavior is described by the resulting stress amplitudes, plastic strain amplitudes and mean stresses as a function of the number of thermal loading cycles. Depending on the maximum temperature and the number of loading cycles, cyclic softening as well as cyclic hardening behavior is observed. Due to the complex interaction of deformation, recovery and recrystallization processes and as a consequence of the individual temperature and deformation history, thermal fatigue processes of the material investigated cannot be assessed using results of isothermal experiments alone. The upper temperatures or the resp. temperature amplitudes determine the total fatigue lifetime.     

Energy‐Absorbing Behavior of Aluminum Foams: Head Impact Tests on the A‐Pillar of a Car

Hundreds of compression tests have been performed in recent years to investigate the energy‐absorbing behavior of metal foams, which is mainly characterized by the “deformation plateau”. Plateau value and plateau length describe the basic behavior quite well and allow comparison of various foams, but in practice the geometry of the absorbing component and the assembly in the system with supports and panels also influence the behavior. The aim of this work was to evaluate the energy‐absorbing behavior of an aluminum foam absorbing element in an A‐pillar system of a real car. In cooperation with the Austrian car manufacturer Steyr–Daimler–Puch Fahrzeugtechnik, a Magna Steyr company, the deformation element in an A‐pillar of a passenger car was developed and tested. The geometry was given by the design of the steel frame and the cover panel of the pillar. The deformation elements were foamed at ARC Leichtmetallkompetenzzentrum Ranshofen GmbH (LKR) by the powder metallurgical process route. The head impact against the A‐pillar in overturning was simulated by the standard test procedure FMVSS 201u and the head injury criterion (HIC) was measured. The head impact tests were performed by impact of a free motion head (FMH) dummy against the A‐pillar in a complete car mounted in a special test rig. In a couple of test runs, the required HIC of less than 1000 could be achieved. The measured HIC values depend on the density and the structure of the aluminum foam, as well as on the deformation element geometry. These parameters were investigated and simulations concerning component behavior were carried out.     

Contact stress analysis on the X‐shape ring

A X‐ring is a loop of elastomer with X‐shaped cross‐section used as a mechanical seal or gasket. Such a X‐ring is designed to be seated in a groove and is compressed during assembly between two or more parts, creating a seal at the interface. The seal is designed to have line contact between the X‐ring and sealing faces. This allows a high local stress, able to contain high pressure, without exceeding the yield stress of the X‐ring shell body. This study aims to detect the contact stress and the deformed shape of a X–shaped ring shell under various compression conditions. For this analysis, four experiments were conducted to obtain material properties of the elastomer. The contact stress analysis is performed by applying material properties that we obtained through experiments. A contact stress analysis is carried out by finite element analysis.     

Effects of particulate reinforcement and strain‐rates on deformation and fracture behavior of Aluminum 6061‐T6 under high velocity impact

Aluminum matrix composites (AMC) exhibit an attractive combination of mechanical and physical properties such as high stiffness and low density, which favors their utilization in many structural applications. Thus, increasing the structural applications of AMC is the driving force for the need to adequately understand their deformation and failure mechanisms under various types of loading conditions. In this study, plastic deformation of alumina particle reinforced Aluminum 6061‐T6 matrix composite is investigated and compared to that of an un‐reinforced Aluminum 6061‐T6 alloy at high strain‐rates under compressive loading. Dynamic stress‐strain curves are obtained using direct impact Split Hopkinson Pressure Bar (SHPB). Particulate reinforcement increases the deformation resistance of the aluminum alloy at high strain‐rates. Strain localization along narrow adiabatic shear bands is observed in both the reinforced and un‐reinforced alloy. Whereas the microstructure of shear bands in un‐reinforced alloy showed finer grain size compared to that of the bulk material, the shear bands observed in the AMCs are darker than the bulk material and the reinforcing particles are observed to be more closely spaced along the shear bands.     

Process Control in Aluminum Foam Production Using Real‐Time X‐ray Radioscopy

Aluminum alloy foams were created by expanding foamable precursors containing a gas‐releasing blowing agent in a dense metallic matrix. The precursors were prepared in two different ways: either by hot‐compaction of powder mixtures or by thixocasting of billets obtained by cold compaction of powder blends. Foam evolution was visualized by means of real‐time X‐ray radioscopy with image frequencies ranging up to 18 Hz and spatial resolutions down to 10 μm. The difference in pore formation between the two processing routes could be studied. Rupture of cell walls during foam expansion could be visualized, critical rupture thickness measured, and the time‐scale of the rupture process estimated. By manufacturing foam precursors in which defects were incorporated deliberately, the question of the origin of very large pores in solid metal foams could be examined. By forced cooling of liquid metal foams while recording their structure, the importance of solidification‐induced changes of foam morphology was illustrated.     

X‐ray computed tomography quantification of damage in concrete under compression considering irreversible mode‐II microcracks

A method for X‐ray computed tomography quantification of damage in concrete under compression considering irreversible mode‐II microcracks is developed. To understand damage behaviour in concrete, a micromechanical analysis of damage under biaxial compression is conducted focusing on random micro‐defects in micro‐cells isolated from the representative volume element. Furthermore, for stress–strain response prediction, a quantification is developed concerning the behaviours of the dominant macrocracks in multiaxial compression. Specifically, two crack types are taken into account: mode‐I cracks and irreversible deformation cracks (including mode‐II microcracks). Furthermore, mode‐I cracks generate compression‐induced tensile load (transverse) area reduction and further stiffness degradation, whereas the latter contribute to the development of irreversible strains. Additionally, by investigating the development of gradually converging dominant cracks, the procedure for quantifying damage is competently executed. In addition, distinguished from other approaches, the quantified damage can be applied directly to constitutive models to produce stress–strain response highly agrees with experimental results.     

Effect of Hydroxyapatite Nanorods on the Fate of Human Adipose‐Derived Stem Cells Assessed In Situ at the Single Cell Level with a High‐Throughput,Real‐Time Microfluidic Chip

The fate of stem cells at the single cell level with limited communication with other cells is still unknown due to the lack of an efficient tool for highly accurate molecular detection. Moreover, the conditional sensitivity of biological experiments requires a sufficient number of parallel experiments to support a conclusion. In this work, a microfluidic single cell chip is designed for use with a protein chip to investigate the effect of hydroxyapatite (HAp) on the osteogenic differentiation of human adipose‐derived stem cells (hADSCs) in situ at the single cell level. By successfully detecting secretory proteins in situ, it is found that the HAp nanorods enhance osteogenic differentiation at the single cell level. In the chip, the single cell seeding approach confirms the osteogenic differentiation of the hADSCs, which endocytoses HAp, by reducing the influence of the factors secreted by neighboring differentiating cells. Most importantly, more than 7000 microchambers provide a sufficient number of parallel experiments for statistical analysis, which ensure a high level of repeatability of the HAp nanorod‐induced osteogenic differentiation. The microfluidic chip comprising single cell culture microchambers with in situ detection capability is a promising tool for research on cell behavior or cell fate at the single cell level.     

Phase‐field‐crystal study on deformation behavior of nanoscale monocrack system in the ductile‐to‐brittle transition region

Phase‐field‐crystal method is applied to study deformation behavior in the ductile‐to‐brittle transition region of the nanoscale monocrack system in this work. The influence of temperature, crystal orientation angle, and crack shape on the deformation behavior is investigated. Temperature can induce fracture mode change, while crystal orientation angle and crack shape can only affect the specific evolutionary behavior. In the ductile region, if the orientation of a vertex is approximately aligned with a certain close‐packed direction, crack extends shortly in cleavage mode at this vertex, which means cleavage crack propagation can be promoted in a particular range of crystal orientation angle. Additionally, the influence of crack shape is achieved by varying the orientation relationship between crack and lattice structure. In the brittle region, crystal orientation angle impacts on the specific cleavage evolution process, and crack shape can promote or hinder plastic deformation by affecting stress concentration.     

The mechanical response of Rohacell foams at different length scales

The quasi-static mechanical response of polymethacrylimide (PMI) foams of density ranging from 50 to 200 kg m⁻³ is investigated in order to provide experimental data to inspire and validate numerical constitutive models for the response of polymer foams. The macroscopic mechanical response is characterised by conducting quasi-static compression, tension, shear and indentation experiments, whereas microscopic deformation mechanisms are identified by conducting in situ SEM observations during static compression and tension tests; it is shown that foams of low density collapse by cell wall buckling while foams of high density undergo plastic cell-wall bending. As a result, both the elastic and plastic macroscopic response of the foam display a tension/compression asymmetry.