Pore Location Research Articles

Knowledge assisted machine learning to clarify pore influence on fatigue life of forging/additive hybrid manufactured Ti-17 alloy

Forging/additive hybrid manufactured Ti alloy parts suffer from relatively low fatigue life due to the existence of metallurgical defects in the transition zone, which also brings difficulty to fatigue life modeling. In this work, the synergistic effect of pore size and location on the rotating-bending fatigue life of hybrid manufactured Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti-17) samples was systematically investigated with the combination of machine learning approaches and physical knowledge. A machine learning framework with a back propagation neural network and generative adversarial network (GAN) was constructed and employed on sparse and limited datasets. A general and interpretable model was obtained with a high level of 90% confidence. In general, the fatigue life of hybrid manufactured Ti-17 alloys decreases with pore size and increases with its distance to surface. Specifically, critical sizes were obtained for near-surface and in-depth pores that have negligible influence on fatigue life of hybrid manufactured samples with respect to pore-free samples. The present work thus provides a systematic platform for the evaluation of the fatigue performance of hybrid manufactured titanium alloys.

Pore-based prediction of crack initiation life in very-high-cycle fatigue

The porosity of the material produced by additive manufacturing technology gives rise to a notable dispersion of the crack initiation life in the very-high-cycle fatigue regime. The crack initiation life in the very high cycle fatigue regime can be divided into the initial crack initiation life and early microcrack growth life. This paper proposed a model considering the effect of pore morphology and location to predict the initial crack initiation life. The average local stress in a grain near the pore is modified by considering the relationship between pore roundness, inclination, position, and stress concentration factor. The growth life of early microcrack is determined by integrating empirical formulas based on dislocation theory. Subsequently, the probability distribution of crack initiation life is obtained, which is in good agreement with the experimental results. The competition factor is proposed to quantitatively evaluate the tendency of crack initiation from the surface or the interior, taking into account the influence of local average stress and grain size. The predicted load corresponding to the shift in crack initiation position is in accordance with the experimental results.

Multiscale analysis of carbon/carbon composite pores based on X-ray computed tomography

Pore defects formed during the impregnation and carbonization of carbon/carbon (C/C) composites were investigated. X-ray computed tomography (XCT) was used to analyze the pore characteristics of C/C composites during liquid-phase impregnation and carbonization (LIC), highlighting the pore structural evolution from impregnation to carbonization and the resultant porous morphology. The porosity, pore location, pore size, and pore shape of the samples were subjected to quantitative analysis. At the micro-scale within the fiber bundles, the pores exhibited an ellipsoidal and elongated shape. In the impregnated samples, the pores were all isolated, and the ellipsoidal and elongated pores accounted for the majority of the pores, but their volume accounted for less than 9 % of the total pore volume. At the macro-scale, pores were predominantly distributed along the surfaces of fiber bundles and between layers, representing the primary form of pore distribution. Analysis of the carbonized samples showed that elongated pores within fiber bundles extended along both the fiber depth and radial directions. Macroscopic pores between fiber bundles and layers extended along the fiber bundle surfaces. The carbonization process caused the micro- and macro-pore spaces to interconnect to form connected pores, which accounted for 79 % of the total pore volume, while the ellipsoidal and elongated pores in the isolated pores accounted for less than 18 % of the total pore volume, forming a porous medium.

Identification of interfacial influence zone (IIZ) in clay rock-cement mortar binary based on the macro-pore distribution in space: An NMR and CT investigation

The interfacial influence zone (IIZ) is a weak link between cast-in-situ cement materials and bulk geological materials. Identifying IIZ is essential for studying binary structure’s mechanical properties. This paper aims to locate the IIZ of a clay rock-cement mortar (C-C) binary by macro-pore distribution in space using CT and NMR tests. First, the porous properties of clay rock and cement mortar were analyzed by the T2 spectrum of the NMR test. Mortar’s macro-pore volume is larger than clay rock’s. Second, the 3D pore-matrix models of these two materials were reconstructed., with larger porosity in mortar than clay rock, consistent with NMR tests. Therefore, the method of pore location using 3D reconstructed models based on CT images may be reliable. Finally, a pore-matrix model of the C-C binary was reconstructed, and the IIZ was identified by pore distribution. The IIZ shape is an approximately ellipsoidal area with thin ends and thick middle, with a max thickness of 8 mm.

Hygroscopic damage of fiber–matrix interface in unidirectional composites: A computational approach

The mechanical properties of fiber-reinforced composites are significantly affected by hygroscopic exposure. Recent experimental findings reveal that the diffusion of water molecules results in void formation at the fiber–matrix interface, contributing to ∼1% increase in porosity. This paper presents a computational micromechanics framework to analyze the damage behavior of unidirectional composites under hygroscopic aging and transverse loads. First, a mathematical relation for the porosity evolution with moisture content in the composite is obtained. Leveraging this correlation, the aged composite is modeled as a porous microstructure consisting of fiber, matrix, and voids at the fiber–matrix interface in finite element simulations. The fiber is considered elastic, and the matrix is modeled as an elastoplastic material using a pressure-dependent yield model to capture tension–compression asymmetry. A cohesive interface is considered between the fiber and matrix. Simulation results indicate that a void at the interface significantly influences plastic strain evolution and stress distribution, leading to early damage initiation and reduced strength. Previous works have indicated that matrix and inter-fiber voids do not have a significant impact on the strength of composites when their combined porosity is below 1%. On the contrary, this study demonstrates that a porosity of as low as 0.1% at the fiber–matrix interface can result in an 11% decrease in the transverse tensile strength of the composite. A parametric study is conducted to understand the fundamental effects of pore size, location, and relative position on the transverse tension, compression, and shear loading responses of a unidirectional composite. For a given moisture content, the predicted strength of the hygroscopic aged composite exhibits a lower and upper bound. The strengths observed in the experiments fall within the model-predicted range. Additionally, a computational method is proposed that incorporates the effects of porosity at the fiber–matrix interface into a modified cohesive strength. This method allows efficient computations and failure predictions for water or moisture-aged strength degradation of unidirectional composites.

Stochastic microstructure modeling and thermal conductivity of coal-based carbon foam

The mechanical, thermal and electrical properties and applications of coal-based carbon foam (CCF) are based on its porous structure. In the present study, a 3-D stochastic numerical program was used to model the microstructure of CCF considering its non-uniformly distributed elliptical pores (i.e., pore dispersity). The heuristic program derived from the foaming process reflected the heterogeneity of foam. Stochastic seeding was used to regulate the pore location dispersity caused by material unevenness and impurities, while the pore size dispersity was revealed by assigning dissolved gas to adjacent pores during pore growth. The structural similarity was confirmed by comparison between the established model and the actual foam in geometrical features, including ligament edge/wall thickness, pore size and ligament cross-section shape etc. The effective thermal conductivity (ETC) of the stochastic model is more sensitive to changes in porosity, compared to that of uniform model of previous literatures. The reason was suspected to be the longer heat transfer path and less cross-section variation of the ligaments of heterogeneous foam. Pore location and size dispersities were regulated in a stable range by program parameters, which indicated that the stochastic model accurately simulated the microstructural complexity of CCF.

Quantitative analysis of 3D pore characteristics effect on the ductility of HPDC Al–10Si-0.3 Mg alloy through X-ray tomography

Due to the uncertain fracture of the die-casting aluminum component (shock tower), X-ray tomography (XRT) coupled with digital image correlation (DIC) was implemented to understand various pore characteristics, specifically spatial distribution and morphology of porosity, and their influences on the ductility in thin-walled Al–10Si-0.3 Mg high pressure die casting. XRT analysis reveals that there was a great amount of porosity in the samples, which was heterogeneously distributed. DIC measurement indicates that porosity gave rise to strain concentration and reduced ductility. Finite element simulations were performed to reveal the combined effect of the pore size and location. Though the morphology and location of pore had nonnegligible impacts on ductility, the local porosity in fracture region played a dominant role in determining it. The local porosity in fracture region can't be obtained prior to tensile testing and consequently ductility can't be predicted based on it. It was found that, for the 27 studied samples, the local porosities in fracture regions were equal or close to the highest local porosities in the gauges, and the samples with higher level of the highest local porosity generally showed lower elongations. The range in which the ductility falls could be estimated according to the highest local porosity in the gauge, which can be achieved before tensile testing. The factors accounting for the difference between upper and lower bounds on ductility were also discussed.

How venom pore placement may influence puncture performance in snake fangs.

When designing experimental studies, it is important to understand the biological context of the question being asked. For example, many biological puncture experiments embed the puncture tool to a standardized depth based on a percentage of the total tool length, to compare the performance between tools. However, this may not always be biologically relevant to the question being asked. To understand how definitions of penetration depth may influence comparative results, we performed puncture experiments on a series of venomous snake fangs using the venom pore location as a functionally relevant depth standard. After exploring variation in pore placement across snake phylogeny, we compared the work expended during puncture experiments across a set of snake fangs using various depth standards: puncture initiation, penetration to a series of depths defined by the venom pore and penetration to 15% of fang length. Contrary to our hypothesis, we found almost no pattern in pore placement between clades, dietary groups or venom toxicity. Rank correlation statistics of our experimental energetics results showed no difference in the broad comparison of fangs when different puncture depth standards were used. However, pairwise comparisons between fangs showed major shifts in significance patterns between the different depth standards used. These results imply that the interpretation of experimental puncture data will heavily depend upon which depth standard is used during the experiments. Our results illustrate the importance of understanding the biological context of the question being addressed when designing comparative experiments.

The role of defect structure and residual stress on fatigue failure mechanisms of Ti-6Al-4V manufactured via laser powder bed fusion: Effect of process parameters and geometrical factors

Undoubtedly, one of the greatest revolutions in the last few decades has been the paradigm shift in design and manufacturing of components using additive manufacturing (AM) technologies. Despite the numerous benefits of metal AM, the presence of volumetric defects and the development of residual stress during manufacturing pose significant obstacles for producing load-bearing components critical for missions. Laser powder bed fusion (LPBF) has many process parameters, each potentially affecting volumetric defects and residual stress, which may lead to varying fatigue performance. In this paper, we comprehensively investigate the criticality of volumetric defects and residual stress on fatigue properties of AMed Ti-6Al-4V. Using X-ray computed tomography (XCT) and X-ray diffraction (XRD), we characterized volumetric defects and measured residual stress of the specimens, respectively. We performed extensive statistical analysis to correlate XCT, residual stress, and fatigue data, allowing us to identify the factors impacting components' fatigue life. Our observations indicate that residual surface stress plays a crucial role in the fatigue performance of components in the absence of defects at the “critical location”. Furthermore, we found that the pore location has a more significant impact than the defect size on fatigue fracture.

A pore pressure model and cracking strength analysis on the UO2 high burnup structure

Advanced nuclear reactors are currently moving toward long fuel cycles and high burnup level. In the case of high burnup, plenty of fission gas will be produced, which promotes creation of overpressure pores and fracture in the fuel. The aim of this paper is to establish the cracking strength evaluation model of UO2 fuel and to obtain the stress field of high burnup UO2 fuel. And the non-uniform distribution of pore size, non-uniform distribution of pore location and the pore pressure model in rod-type dispersion fuel are considered. Firstly, Weibull distribution is used to describe the pore size distribution. With the aid of the relationship between local burnup and mean pore radius, geometrical models of the UO2 high burnup structure under different burnup conditions are established. Secondly, a pore pressure model taking into account pore size distribution, the relationship between local burnup and mean pore radius and Ronchi’s equation of state is established. Based on literature data, this work further verifies the rationality of using Weibull distribution to describe pore size distribution. Finally, the effects of pore distribution, local burnup and temperature on the pore pressure and maximum tension stress in the fuel are analyzed, and the stress-dominated pore coarsening mechanism is studied by finite element method. The conclusion can be drawn that the large size pores are mainly formed by the continuous coalescence of the small size pores with the surrounding pores. The fuel is not easy to crack when the pore size and location distribution is uniform. Both burnup and temperature increase contribute to fuel cracking.

A modeling study of transport through composite membrane with support pore location distributing randomly based on 2D Voronoi tessellations

Composite membrane consisting of selective layer and support is the mainstream membrane configuration. Support has drawn increasing attention in recent years. In this work, transport through composite membrane with support pore location distributing randomly was studied based on 2D Voronoi tessellations and the corresponding membrane performance calculation method was developed. For the calculation, firstly, irregular Voronoi regions are transformed into regular polygons with fixed number of edges meanwhile maintaining the region area. Then, the membrane performance can be obtained by combining the computation results of regular support surface pore spatial distribution and random support surface pore spatial distribution formulas. Poisson Voronoi diagrams, one of the most representative Voronoi diagrams, were mainly used to describe the support surface pore spatial distribution. The results indicate the importance of the number of support surface pores, which determines the average area of Voronoi regions. Support surface pores should be as many as possible. Another important conclusion is that a certain randomness of support surface pore structure (size and location) may not exert an influence on the composite membrane performance. That is, when calculating the composite membrane performance, the support surface pore spatial distribution and surface pore size distribution can be considered as being regular.

Analysing the effect of defects on stress concentration and fatigue life of L-PBF AlSi10Mg alloy using finite element modelling

Additive manufacturing (AM) is a developing manufacturing technology, which provides excellent attributes compared to other manufacturing techniques. However, one of the critical challenges is the presence of defects that hinder the mechanical properties of the parts, particularly the fatigue life. Experimental understanding of fatigue is a cumbersome process. Therefore, numerical prediction based on specified conditions (such as porosity and applied load) can be an alternative to experimental analysis at the design stage of AM parts. In this study, elastic–plastic finite element analysis (FEA) is performed to obtain the stress distribution around pores, and their resultant effect on fatigue life for L-PBF (laser powder bed fusion) produced AlSi10Mg alloy samples. The stress field is calculated for both single and multiple pore models, where stress concentration is evaluated as a function of the pore’s location and its size. It is found that both pore location and size affect the stress field; however, location effects dominate over pore size. For the same remote applied stress level, the stress concentration around the pore increases with an increase in pore size, and the local maximum stress occurs near the pores that are closest to the surface. The current study also evaluates the relative effect of porosity fraction, average pore size, and location. It is found that the magnitude and sensitivity of stress concentration are hierarchically controlled by porosity location, pore size, and porosity density. A multi-scale finite element (FE) model is proposed based on inherent porosity data measured using Computed Tomography (CT) to predict overall fatigue life. The fatigue cycles are calculated using the rainflow counting algorithm in FE Safe using the stress–strain data obtained from the proposed FEA model. Using the proposed model, it is possible to generate S–N curves for any loading condition for a given porosity condition (porosity density and average pore size). The S–N curve results obtained from the FE model are compared to the experimental observations. The predicted fatigue life shows approximately 5% error with experimental results at high stress loading conditions. However, the proposed model overpredicts the fatigue life at low stress loading by almost 30%.

Mechanical properties and deformation behavior of equiatomic CoCrFeMnNi high-entropy alloy foam: A molecular dynamics study

In recent years, the aerospace and mechanical industries have employed metal foams for High Energy Capacity applications. However, the porous nature of such metallic foams results in a drop in tensile properties. High Entropy Alloys (HEAs) have been reported to overcome the strength-ductility trade-off as opposed to conventional metals and their alloys. Based on this, an equiatomic nano porous CoCrFeMnNi HEA is investigated using molecular dynamics simulation to elucidate the deformation behavior and mechanical performance at different atom distributions, percent porosity, and pore locations. The findings indicated that the mechanical properties are independent of atom distributions. However, with an increase in percent porosity, a decline in the ultimate tensile strength was observed. Up to 66% reduction in tensile strength was recorded. A similar trend was observed under temperature variations. By varying pore location, either within or on the structure, the pores within the foam structure led to a significant reduction in properties. The deformation mechanism of the HEA foam was established via analyses of the dislocation distributions, stacking faults, crack nucleation, and propagation. The findings offer new insights for material design to produce high-performance HEA foams and a comprehension of its failure mechanism under loading.

Study of the Seismoelectric Effect in Saturated Porous Media Using a Bundle of Capillary Tubes Model

The seismoelectric effect is the fundamental basis for seismoelectric logging. Most of the existing theories for the seismoelectric effect are based on the Pride theory, which adopts the assumption of a thin electric double layer and uses the volume-averaging method to derive the seismoelectric coupling equations; hence, the obtained electrokinetic coupling coefficient is not applicable to large-Debye-length cases. In addition, the Pride theory neglects the change in seepage velocity with the radial position of the pore when calculating the streaming current, which leads to an inaccurate reflection of the influence of pore size on the electrokinetic coupling coefficient. In this study, we proposed a flux-averaging method to solve the effective net residual charge density of porous media and further derived the electrokinetic coupling coefficient expressed by the effective net residual charge density. We also investigated the effect of formation parameters and compared the results with those calculated using the Pride theory. Since the proposed method is not limited by the thin electric double layer assumption, it is suitable for both small- and large-Debye-length cases. Moreover, we also carried out flume experiments to investigate the influence of salinity, where both thin and thick electric double layer cases were studied. The comparison between the results of the experiment and simulation verified the correctness of the proposed method. Furthermore, the proposed method took into account the variation in seepage velocity with pore location when solving for the streaming current; therefore, the influence of the pore size on the electrokinetic coefficient can be described more accurately.

Impedance Response of Ionic Liquids in Long Slit Pores

We study the dynamics of ionic liquids in a thin slit pore geometry. Beginning with the classical and dynamic density functional theories for systems of charged hard spheres, an asymptotic procedure leads to a simplified model which incorporates both the accurate resolution of the ion layering (perpendicular to the slit pore wall) and the ion transport in the pore length. This reduced-order model enables qualitative comparisons between different ionic liquids and electrode pore sizes at low numerical expense. We derive semi-analytical expressions for the impedance response of the reduced-order model involving numerically computable sensitivities, and obtain effective finite-space Warburg elements valid in the high and low frequency limits. Additionally, we perform time-dependent numerical simulations to recover the impedance response as a validation step. We investigate the dependence of the impedance response on system parameters and the choice of density functional theory used. The inclusion of electrostatic effects beyond mean-field qualitatively changes the dependence of the characteristic response time on the pore width. We observe peaks in the response time as a function of pore width, with height and location depending on the potential difference imposed. We discuss how the calculated dynamic properties can be used together with equilibrium results to optimise ionic liquid supercapacitors for a given application.

High cycle fatigue behavior of a selective laser melted Ti6Al4V alloy: Anisotropy, defects effect and life prediction

Selective laser melting (SLM) exhibits more and more utilization potentialities in today’s aviation industry. In order to ensure safe operation of SLM produced aero parts, it is important to have the fatigue performance well-characterized, and fatigue life prediction methods developed by considering the initial feature of this material. In this study, high cycle fatigue tests under stress ratios of −1, 0.1 and 0.5 at room temperature for the SLM Ti6Al4V alloy were carried out, and the fracture morphologies of fatigue specimens were observed. The results show that the subsurface pore is the main source of crack initiation for fatigue failure. By quantitative and statistical analysis of the pore characteristics in the crack source zone of all fractured specimens, it is revealed that the anisotropy and dispersion of fatigue performance are closely related to the difference of the shape, size, location and density of the pores in the crack source zone of specimens. The relationship between pore size and location, such as the ratio of radius to the distance to the specimen surface, plays an important role in estimating the fatigue life of the specimen. Aiming at the nature of defect-induced fatigue damage, the fatigue life of the SLM Ti6Al4V alloy was reasonably predicted using the fracture mechanics method based on the small-crack theory and the long crack growth data.

Atomistic Investigation of the Effects of Different Reinforcements on Al Matrix Composite

In this work, we studied the effects of different reinforcements on a metal matrix composite (MMC) using molecular dynamics (MD) simulations, where graphene was chosen as the two-dimensional (2D) material and diamond was selected as the three-dimensional (3D) material. Sintering and tensile processes were conducted on the MMC models containing reinforcements of various sizes, and the effects of reinforcements with the same surface area were compared. The results indicated that the 2D material was more beneficial for sintering at the heating stage, producing a higher-density structure. The volume of Al atoms fell from 752 to 736 nm3 as the graphene size in the composite system increased. However, a slight increase from 749 to 755 nm3 was observed when the diamond radius was small. Converted to relevant metrics in the experiments, the density of the composite reached 2.84 g/cc with a 3.3 wt.% addition of single-layer graphene (SLG) and 2.87 g/cc with a 15.4 wt.% addition of diamond, and the results were slightly higher than the experimental reports. Both SLG and diamond could reduce the number of arranged Al atoms from 43,550 to approximately 35,000, and bilayer graphene (BLG) with the largest size could further decrease the number of arranged atoms to nearly 30,000, implying that grain refinement could be obtained by increasing the surface area of reinforcements. Considering the scale of these models, the reinforcement size and pore location in the initial structures were deemed to have an impact on the mechanical properties. The composite with the largest proportion of SLG showed an increase of more than 1.6 GPa in tensile strength; however, BLG showed a significant drop of 1.9 GPa when stretched in the normal direction, as the large interlayer space acted as a large hole in tension. The diamond size did not appear to affect the strengthening effects. Nevertheless, the elongation values of composites with graphene were generally 35% higher than the Al-diamond composites.

High cycle fatigue analysis and modelling of cast Al–Si alloys extracted from cylinder heads considering microstructure characteristics

The high cycle fatigue behaviour of the Al–Si alloy extracted from a cylinder head is investigated through experiments and finite element simulations. Due to the complexity of the structure and the differences in cooling rates and local melt flow, the microstructure characteristics of the cylinder head studied exhibit high spatial heterogeneity. The scatter of the fatigue life of the alloy is high, which is significantly affected by the microstructure, especially by crack nucleation pores. The characteristics of the nucleation pores have a great effect on the dissipated inelastic energy (Wp) around the pores, which is used as the indicator for crack nucleation. Wp has the highest sensitivity to pore shape, followed by pore location and pore size. The nonlinear empirical relationship between Wp and pore characteristics is established with high reliability and accuracy, and the crack nucleation model is modified by considering pore characteristics. Based on the multistage fatigue model and the modified crack nucleation model, the fatigue life of cast Al–Si alloy extracted from cylinder heads can be predicted accurately within a feasible scatter band.

Effect of Pore Defect Size and Location on Damage Tolerance of Aluminum Alloy Piston and Fiber Ring Groove

The defects of high-power density piston aluminum alloy components involved in this paper include surface crack, internal crack, shrinkage cavity and cold shut. The service condition of piston components is 350°C-420°C, and the explosion pressure of piston crown is 28Mpa. The requirements for eddy current flaw detection of this component are in accordance with a and requirements in GB / T5126-2013 eddy current flaw detection standard, that is, it is not allowed to be greater than 0.12mm × 0.2mm × 3mm volumetric defects, and Ф1.0mm flat bottom hole equivalent point defect. For the piston components of 88kw / L high-power diesel engine, under the service conditions of temperature 350°C-420°C and piston top explosion pressure 28Mpa. Under the condition of thermal mechanical coupling, the typical NDT defects such as surface crack, internal crack, shrinkage cavity and cold shut of the first ring groove and internal position of the piston are studied. On scale, morphology and position sensitivity and their quantitative relationship. At the same time, numerical simulation analysis combined with relevant experimental verification is used to comprehensively analyze and evaluate the damage tolerance of defects, and scientifically evaluate the defects of different properties and sizes.

Effect of thermal induced porosity on high-cycle fatigue and very high-cycle fatigue behaviors of hot-isostatic-pressed Ti-6Al-4V powder components

• The regrowth of the residual pores in the as-HIPed Ti-6Al-4V powder component occurs after solution heat treatment. • Effect of thermal induced porosity on the very-high cycle fatigue behavior was firstly studied. • The competition of different failure types in the very-high cycle fatigue regime is related to the area density of pores (pore size >40 μm). The present work reports the effect of thermal induced porosity (TIP) on the high-cycle fatigue (HCF) and very high-cycle fatigue (VHCF) behaviors of hot-isostatic-pressed (HIPed) Ti-6Al-4V alloy from gas-atomized powder. The results show that the residual pores in the as-HIPed powder compacts present no obvious effect on the HCF life. The regrowth of the residual pores can be observed after solution heat treatment. The pore location ranks the most harmful for the fatigue life compared with the other initiating defects. The maximum stress intensity factors were calculated. The plastic zone size of fine granular area (FGA) is much less than the characteristic size of the microstructure, and the crucial size of the internal pores in this study is about 40 μm. The failure types of fatigue specimens in the VHCF regime were classified, and the competition of different failure types was described based on the modified Poisson distribution.