Increase In Void Volume Fraction Research Articles

Analysis of the regeneration behavior of high temperature polymer electrolyte membrane fuel cells after hydrogen starvation

Two high temperature polymer exchange membrane fuel cells (HT-PEM FCs) were operated under repeated starvation/regeneration steps for 550 min (DPS-1) and 12 days (DPS-2). Concerning the investigation of the irreversible degradation under fuel (H2) starvation the focus was put on the electrochemical characteristics during regeneration, since during this step the system operates under normal conditions. Electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRT) analysis were used as electrochemical characterization methods. DRT analysis of selected impedance spectra during regeneration steps for the first seven days revealed an intensity increase of the charge transfer kinetics of the cathode (ca. 10 Hz) and the anode (ca. 100 Hz). From day 3 on, an additional peak at 300–800 Hz appeared likely pointing to the formation of surface oxides at the anode side. The EIS and DRT results were verified with Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM), Microcomputed Tomography (μ-CT) and Transmission Electron Microscopy (TEM). While TEM indicated advanced Pt particle agglomeration pointing to carbon corrosion, μ-CT measurements showed an increase of void volume fraction and a decrease of the tortuosity value. The combined results show that the anode gas outlet region of the FC is more degraded than the inlet region.

Determination of mechanical properties of FFF 3D printed material by assessing void volume fraction, cooling rate and residual thermal stresses

PurposeThis paper aims to present experimental and numerical analyses of fused filament fabrication (FFF) printed parts and show how mechanical characteristics of printed ABS-MG94 (acrylonitrile butadiene styrene) are influenced by the void volume fraction, cooling rate and residual thermal stresses.Design/methodology/approachPrinted specimens were experimentally tested to evaluate the mechanical properties for different printing speeds, and micrographs were taken. A thermo-mechanical finite element model, able to simulate the FFF process, was developed to calculate the temperature fields in time, cooling rate and residual thermal stresses. Finally, the experimental mechanical properties and the microstructure distribution could be explained by the temperature fields in time, cooling rate and residual thermal stresses.FindingsMicrographs revealed the increase of void volume fraction with the printing speed. The variations on voids were associated to the temperature fields in time: when the temperatures remained high for longer periods, less voids were generated. The Young's Modulus of the deposited filament varied according to the cooling rate: it decreased when the cooling rate increased. The influence of the residual thermal stresses and void volume fraction on the printed parts failure was also investigated: in the worst scenarios evaluated, the void volume fraction reduced the strength in 9 per cent, while the residual thermal stresses reduced it in 3.8 per cent.Originality/valueThis work explains how the temperature fields can affect the void volume fraction, Young's Modulus and failure of printed parts. Experimental and numerical results are shown. The presented research can be used to choose printing parameters to achieve desired mechanical properties of FFF printed parts.

Numerical investigation of low cycle fatigue mechanism in nodular cast iron

The failure mechanism in ductile cast iron under static loading is nucleation, growth, and coalescence of voids. It is known from the literature that elastic–plastic porous materials under strain controlled cyclic loading show an increase of void volume fraction called void ratchetting effect. From this observation, we suggest that void ratchetting is the relevant mechanism for low cycle fatigue in nodular cast iron as well. The low cycle fatigue process in the microstructure of ductile cast iron has been simulated cycle by cycle using unit cell models. The strain-life approach for fatigue is followed in this work. The matrix material behavior is modeled as an elastic–plastic one. From the simulations, void ratchetting is identified as a potential mechanism of low cycle fatigue in nodular cast iron which finally leads to failure. The lifetime of different load ratio and different matrix material behavior is extracted from the simulations. The simulated strain-life curves are compared with experimental data from literature.

Experimental study and modeling of damage of Al alloys using tensor theory

This article presents and evaluates experiments for the characterization and modeling of damage of structural aluminum and aluminum–magnesium alloys. Tensile tests were performed for specimens with artificial defects (voids) represented by different arrangements of pre-drilled micro-holes. The corresponding stress–strain curves were experimentally obtained. Plastic dilatation and deviatoric strain were determined both for the local zones with artificial defects and directly for meso-elements (i.e., material cells with artificial defects). A symmetric second-rank order tensor of damage was applied for a quantitative estimate of the material damage connected with the volume fraction and shape of micro-defects. The definition of this tensor is physically motivated, since its hydrostatic and deviatoric parts describe the evolution of damage connected with a change in volume fraction and shape of micro-defects, respectively. Such a representation of damage kinetics allows us to use two integral measures for the calculation of damage in deformed materials. The first measure determines damage related to an increase in void volume fraction (i.e., plastic dilatation). A critical amount of plastic dilatation corresponds to the moment of macro-fracture of the deformed metal. By means of experimental analysis, we can determine the function of plastic dilatation which depends on the strain accumulated by material particles under various stress and temperature-rate conditions of forming. The second measure accounts for the deviatoric strain of meso-elements, and is related to the change in their shapes. The critical deformation of ellipsoidal voids corresponds to the onset of their coalescence and to the formation of large cavernous defects. The second measure is considered as a criterion of micro-destruction due to formation of cavities in the deformed material. Based on the experimental data, some numerical modeling is realized for the investigated Al alloys to taken a change in stress triaxiality into account. It shows that a change in triaxiality toward smaller values results in an appreciable decrease of damage induced by strain. Both damage measures are important for the prediction of the meso-structure quality of metalware produced by metal forming techniques.

Microtomographic study of the evolution of microstructure during creep of an AlSi12CuMgNi alloy reinforced with Al 2O 3 short fibres

The evolution of the microstructure during creep of an AlSi12CuMgNi alloy with 15 vol% of Al 2O 3 short fibres is investigated by means of synchrotron microtomography. The results reveal a change of the morphology of the hybrid reinforcement composed of eutectic-Si, short fibres and intermetallic particles. This change takes place due to the diffusion induced coarsening of Si and a coarsening of the intermetallic particles. The interconnectivity of the hybrid reinforcement increases during long-term creep exposure reaching a degree of almost full interconnectivity after 6400 h of creep. The analysis of creep damage during secondary creep stage shows an increase of void volume fraction by a factor 2, while the number of voids per volume remains practically constant (void sizes larger than 8 voxels were analysed). An analysis of the voids’ location indicates that pores generated during processing of the composite grow, but no new pores are produced during the primary and secondary creep stages.

Three-dimensional micromechanical modeling of voided polymeric materials

A three-dimensional micromechanical unit cell model for particle-filled materials is presented. The cell model is based on a Voronoi tessellation of particles arranged on a body-centered cubic (BCC) array. The three-dimensionality of the present cell model enables the study of several deformation modes, including uniaxial, plane strain and simple shear deformations, as well as arbitrary principal stress states. The unit cell model is applied to studies on the micromechanical and macromechanical behavior of rubber-toughened polycarbonate. Different load cases are examined, including plane strain deformation, simple shear deformation and principal stress states. For a constant macroscopic strain rate, the different load cases show that the macroscopic flow strength of the blend decreases with an increase in void volume fraction, as expected. The main mechanism for plastic deformation is broad shear banding across inter-particle ligaments. The distributed nature of plastic straining acts to reduce the amount of macroscopic strain softening in the blend as the initial void volume fraction is increased. In the case of plane strain deformation, the plastic flow is observed to initiate across inter-particle ligaments in the direction of constraint. This particular mode of deformation could not have been captured using a two-dimensional, plane strain idealization of cylindrical voids in a matrix. The potential for localized crazing and/or cavitation in the matrix is addressed. It is observed that the introduction of voids acts to relieve hydrostatic stress in the matrix material, compared to the homopolymer. It is also seen that the predicted peak hydrostatic stress in the matrix is higher under plane strain deformation than under triaxial tension (with equal lateral stresses), for the same macroscopic stress triaxiality. The effect of void volume fraction on the macroscopic uniaxial tension behavior of the different blends is examined using a Considère construction for dilatant materials. The natural draw ratio was predicted to decrease with an increase in void volume fraction.

Effect of thermal treatments on microstructure and impact toughness of die cast Mg–Al–Mn alloys

A study of the effects of heat treatment on an Mg–Al–Mn alloy was carried out. Die cast AM60B alloy (Mg–6.0Al–>0.25Mn–<0.010Cu–<0.002Ni–<0.005Fe (wt-%)) specimens for microstructural investigation, tensile testing, and impact toughness testing, were produced using a multispecimen die in a high pressure, cold chamber apparatus. As cast specimens were studied either in their original condition or after they had been subjected to a direct aging treatment at 175°C. Solution treatment was also carried out, producing a T4 temper and a T6 temper by subsequent additional aging, and the resulting specimens examined. The investigation allowed evaluation of modifications to microstructural and mechanical properties produced by thermal treatments. In particular, the analysis of structure and solidification defect evolution showed that, despite an increase in void volume fraction and size induced by thermal treatments, a significant improvement of toughness during the crack growth process could be achieved with the appropriate tempers. Marked modifications to high strain rate loading conditions were detected, with improvements of total absorbed impact energy of up to 40% with respect to the as cast condition. This was associated with changes in fracture mechanisms, promoting a transition to a completely ductile mode in solution treated specimens.

Finite element analysis of damage evolution and the prediction of the limiting draw ratio in textured aluminum sheets

Void nucleation and growth models have been incorporated into an elasto-plastic finite element code together with an anisotropic fourth-order strain rate potential so that damage evolution during the deep drawing of textured aluminum sheets can be analyzed. The fourth-order strain rate potential is based on the Taylor model of crystal plasticity and therefore takes the presence of texture into account. The damage evolution is modeled in terms of void nucleation and growth during deformation. Strain-induced and stress-controlled nucleation models were employed in conjunction with the Cocks and Ashby growth model to calculate the increase in void volume fraction. The influence of plastic anisotropy on damage is discussed together with the roles of void nucleation and growth on damage evolution for cold rolled and cold rolled annealed aluminum sheets. It is shown that the growth of voids as opposed to their nucleation plays a very important role in damage formation during the development of localized necks. More attention should therefore be paid to void nucleation and growth in the analysis of localized necking and fracture during sheet metal forming.

Relationship between ductile crack initiation and void volume fraction

An investigation on ductile crack initiation in structural steel has been made, based on the concept of Gurson's yield function for porous material. First, the condition of ductile crack initiation in the uniform stress field has been investigated. The condition of ductile crack initiation under various stress triaxiality obtained from the tests on axisymmetric notched tensile specimens is well expressed by the condition of constant void volume fraction analytically obtained from Gurson's model. This result means that the condition of constant void volume fraction may be used as the criterion of ductile crack initiation. Secondly, the behavior of void growth and ductile crack initiation in the area near the notch tip under mode I and mode II loading has been investigated. Under mode I loading, the increase in void volume fraction around the notch with an increase in applied load agrees well with the behavior of porous material predicted by the finite element analysis based on Gurson's yield function, and the ductile crack initiation can be predicted by the concept of critical void volume fraction as in the case of uniform stress-strain field given above. The same criterion is not applicable to the crack initiation under mode II loading and further study is needed.