Unloading Step Research Articles

Characterizing and Tailoring the Substrate Profile of a γ-Glutamyltransferase Variant.

Xenobiology is an emerging field that focuses on the extension and redesign of biological systems through the use of laboratory-derived xenomolecules, which are molecules that are new to the metabolism of the cell. Despite the enormous potential of using xenomolecules in living organisms, most noncanonical building blocks still need to be supplied externally, and often poor uptake into cells limits wider applicability. To improve the cytosolic availability of noncanonical molecules, a synthetic transport system based on portage transport was developed, in which molecules of interest "cargo" are linked to a synthetic transport vector that enables piggyback transport through the alkylsulfonate transporter (SsuABC) of Escherichia coli. Upon cytosolic delivery, the vector-cargo conjugate is enzymatically cleaved by GGTxe, leading to the release of the cargo molecule. To deepen our understanding of the synthetic transport system, we focused on the characterization and further development of the enzymatic cargo release step. Hence, the substrate scope of GGTxe was characterized using a library of structurally diverse vector-cargo conjugates and MS/MS-based quantification of hydrolysis products in a kinetic manner. The resulting substrate tolerance characterization revealed that vector-amino acid conjugates were significantly unfavored. To overcome this shortcoming, a selection system based on metabolic auxotrophy complementation and directed evolution of GGTxe was established. In a directed evolution campaign, we improved the enzymatic activity of GGTxe for vector-amino acid conjugates and revealed the importance of residue D386 in the cargo unloading step.

"Impact of hydride composite materials on thermochemical hydrogen compression"

The formation of a hydride from a metal or intermetallic alloy is an exothermic reversible chemical reaction. The thermodynamic properties governing hydride formation facilitate their thermochemical applications, such as hydrogen compression. Thermal transport inside the materials is of highest importance for system dynamics and its productivity. In this contribution, we demonstrate a two-stage thermochemical hydrogen compressor using the AB5 alloy LaNi5 in the first stage and an AB2 alloy TiMn2 (Hydralloy®) in the second stage. To evaluate the impact of the thermal conductivity of the materials, the productivity of the compressor was compared for pure metal hydride compacts (MH) and metal hydride composite materials (MHC). Compared to pure MH pellets, the MHC used in this study contain expanded natural graphite (ENG), a secondary phase with highest thermal conductivity. As the radial thermal conductivity of the MHC increased, the time required for the loading, temperature change and unloading steps was successfully reduced by 400%. Productivity was increased by over 320 % from 14 NlH2/(kgMaterial*h) to 44 NlH2/(kgMaterial*h). Overall, MHC have the potential to simplify handling, reactor design and reduce investment costs for thermochemical compression systems. Thus, MHC have highest impact and potential for thermochemical applications.

Genesis, mechanism, and avoidance of cosmetic coating defects – An industrial case study

Film-coated tablets as solid oral dosage forms are a well-accepted way of administering drugs but are not without specific challenges during manufacturing. One relevant criterion of the final product is the visual integrity and therewith, the absence of cosmetic optical defects such as edge chipping. The aim of the present study was to examine the origin of those edge chipping defects, which were observed during commercial manufacturing of film-coated tablets, and to provide recommendations for process optimization to reduce the defect occurrence. The unraveling of the herein described phenomenon necessitated an interplay of in-depth material characterization, discrete element modeling (DEM) as well as an in-house developed optical measurement system for the automated quantification of tablet defects. As a result of this investigation, the automatic unloading step after the tablet coating process was identified as the most critical step for the occurrence of chipping defects and a replacement by manual unloading was proposed to reduce the defect propensity. The recommended optimization was subsequently confirmed in several manufacturing runs and a reduction of defect propensity by a factor of 5 was observed, highlighting the relevance and the impact of the performed thorough investigation.

Micro-Mechanical Fracture Investigations on Grain Size Tailored Tungsten-Copper Nanocomposites

Tungsten-copper composites are used in harsh environments because of their superior material properties. This work addresses a tungsten-copper composite made of 20 wt.% copper, which was subjected to grain refinement by high-pressure torsion, whereby the deformation temperature was varied between room temperature and 400 °C to tailor the grain size. Deformation was performed up to microstructural saturation and verified by hardness measurement and scanning electron microscopy. From the refined nanostructured material, micro-cantilever bending beams with cross-sections spanning from 5 × 5 to 35 × 35 µm2 were cut to examine possible size effects and the grain size influence on the fracture behavior. Fracture experiments were performed in situ inside a scanning electron microscope by applying a quasi-static loading protocol with partial unloading steps. Inspection of the fracture surfaces showed that all cantilevers failed in an inter-crystalline fashion. Nevertheless, remaining coarser tungsten grains impacted the resultant fracture toughness and morphology. Cantilevers fabricated from the 400 °C specimen exhibited a fracture toughness of 220 ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm $$\\end{document} 50 Jm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\frac{{\ ext{J}}}{{{\ ext{m}}}^{2}}$$\\end{document}. For the room temperature cantilevers, a fracture toughness of 410 ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm $$\\end{document} 50 Jm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\frac{{\ ext{J}}}{{{\ ext{m}}}^{2}}$$\\end{document} was observed, which declined to 340 ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm $$\\end{document} 30 Jm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\frac{{\ ext{J}}}{{{\ ext{m}}}^{2}}$$\\end{document} for cantilevers < 10 × 10 µm2, confirming a size effect. The increased fracture toughness is attributed to the delamination-like structures formed in the room temperature sample.

Primary and secondary consolidation characteristics of a high plasticity overconsolidated clay in compression and swelling

For a reliable prediction of the time-development of soil deformations, a detailed characterization of both primary and secondary deformations is required. Nevertheless, in the current engineering practice the estimation of the primary and secondary deformations relies on the assumption of a constant coefficient of consolidation and a constant creep index respectively. However, high plasticity overconsolidated clays have reportedly complex stiffness and permeability relationships that challenge the validity of those assumptions. Analytical solutions of the one dimensional consolidation have been proposed, accounting for variation in permeability and oedometer modulus. Nevertheless, the methods assume a simplified log-linear and linear relationship with void ratio and effective stress for the two parameters, respectively. One-dimensional oedometer and constant rate of strain testing were employed to investigate the time-deformation development of a particular high plasticity overconsolidated clay. A dependency of the time until the end of primary deformations on the stress path was observed, with the majority of the unloading steps extending well beyond the 24 h typical duration of a load increment. It was revealed that the coefficient of consolidation generally increases during consolidation, and it may vary by more than one order of magnitude at the early stages of the consolidation process. This variation has been attributed to a transient effect noticed in the permeability, which has not been previously reported. The linear relationship between primary and secondary deformation indices was confirmed as an upper bound. In an effort to better characterize the time dependent behaviour a new index, the primary time deformation index, was defined as the slope at the inflection point of the time-curves. This parameter was found to be linearly related to the creep index, presenting thus a potential of better predictions of the duration and magnitude of consolidation deformations for the engineering practice.

Nanomechanical assessment of tribological behavior of TiN/TiCN multi-layer hard coatings deposited by physical vapor deposition

This research shows different hard Ti-based coatings deposited over steel substrates using the physical vapor deposition (PVD) method. The graded coating of the three produced Ti, TiN and TiCN were assessed using phase composition analysis and spectroscopy study. The microstructure characterization was studied by X-ray Photoelectron spectroscopy (XPS), Scanning Electron Microscopy (SEM), and transmission electron microscopy (TEM). The results show that integrating Ti interlayer with subsequent TiN/TICN layers can be considered a suitable way to yield hard Ti-based coating with enhanced mechanical and tribological behavior due to nitride and carbonitride phases formed from the breakdown of carbon and nitrogen atoms. The elastic modulus and hardness of all layers were identified by nanoindentation and utilized to achieve yield strength. Later, the curvature of the unloading step and elastic properties are used for finding flow behavior parameters in terms of work hardening factors (n and K). The attained findings extracted from the load–displacement curve of both nanoindentation and nano scratch were analyzed to examine the microscopic performance of all three Ti, TiN, and TiCN coatings. Scratch mechanisms analysis in all coatings shows that the main failures in samples are caused by tensile stress stemming from Hertzian cracks. However, as the force increases, the coating surface undergoes an incremental deformation, and failure mode varies to interfacial spallation.

Estimation of soil porosity in mine tailing using parameters from instrumented oedometer test

In the geological field, unexpected tailing failures have recently occurred and caused critical damage. The objective of this study was to evaluate the porosity of mine tailing using compressibility and elastic wave velocities from an instrumented oedometer test. Three samples (MS1, MS2, MS3) were obtained from the mine tailing at different depths, and their properties were assessed. The oedometer test was conducted from 0 to 1280 kPa for MS1, MS2 (with various initial stresses), and MS3, and the elastic wave velocities were measured during each loading and unloading step. The specific gravity, X-ray fluorescence, and X-ray diffraction test results revealed a portion of hematite (Fe2O3) in all the samples. According to the instrumented oedometer test, the compressibility and elastic wave velocities of the samples varied with respect to the sampling depth and stress conditions. In particular, the α factor and β exponent from the shear wave velocity followed the empirical relationship of general soil types. Empirical porosity evaluation methods reliably estimated the sample porosity at shallow depths by using the parameters from the instrumented oedometer tests. Thus, the in situ porosity at a mine tailing can be effectively evaluated if the profiling of elastic velocities in the field is available.

Increasing the Strain Resistance of Si/SiO2 Interfaces for Flexible Electronics.

Understanding the changes that occur in the micro-mechanical properties of semiconductor materials is of utmost importance for the design of new flexible electronic devices, especially to control the properties of newly designed materials. In this work, we present the design, fabrication, and application of a novel tensile-testing device coupled to FTIR measurements that enables in situ atomic investigations of samples under uniaxial tensile load. The device allows for mechanical studies of rectangular samples with dimensions of 30 mm × 10 mm × 0.5 mm. By recording the alternation in dipole moments, the investigation of fracture mechanisms becomes feasible. Our results show that thermally treated SiO2 on silicon wafers has a higher strain resistance and breaking force than the SiO2 native oxide. The FTIR spectra of the samples during the unloading step indicate that for the native oxide sample, the fracture happened following the propagation of cracks from the surface into the silicon wafer. On the contrary, for the thermally treated samples, the crack growth starts from the deepest region of the oxide and propagates along the interface due to the change in the interface properties and redistribution of the applied stress. Finally, density functional theory calculations of model surfaces were conducted in order to unravel the differences in optic and electronic properties of the interfaces with and without applied stress.

Application of the Small Punch Creep-Recovery Test (SPCRT) for the Estimation of Large-Amplitude Viscoelastic Properties of Polymers.

The Small Punch Creep-Recovery Test (SPCRT) is a novel miniature test used to estimate the viscoelastic properties of polymers and biomaterials. The current investigation related to the SPCRT is limited to Finite Element Method (FEM) simulations and experimental tests on PVC. The aim of this investigation was focused on: (i) extending the experimental tests to other polymers with dissimilar viscoelastic properties; (ii) deepening the influence of non-linear viscoelastic properties in the estimation capabilities of the SPCRT; and (iii) developing a numerical methodology to estimate and take into account the viscoelastic recovery produced during the unloading step of compressive creep-recovery tests (CCRT) and SPCRTs. The experimental tests (CCRTs and SPCRTs) were done on polyethylene PE 500, polyoxymethylene POM C, nylon PA 6, and polytetrafluoroethylene (PTFE), with a range of creep loads, in the case of CCRTs, in the whole elastic regime and the surroundings of the yield strength of each material. The experimental results confirmed that the SPCRT was an accurate and reliable testing method for linear viscoelastic polymers. For a non-linear viscoelastic behavior, SPCRT estimated the viscoelastic properties obtained from CCRTs for creep loads near the yield strength of the polymer, which corresponded with large-amplitude viscoelastic properties in dynamic creep testing. In order to consider the viscoelastic recovery generated in the unloading step of CCRTs and SPCRTs, a Maxwell-Wiechert model with two branches was used, simulating the different steps of the experimental tests, and solving numerically the differential equation of the Maxwell-Wiechert model with the Runge-Kutta-Fehlberg (RKF) numerical method. The coefficients of the elements of the Maxwell-Wiechert model were estimated approaching the straining curve of the recovery step of the simulation with the same curve registered on each experimental test. Experimental CCRTs with different unloading times demonstrated that the use of this procedure derived in no influence of the unloading step time in the viscoelastic properties estimation.

Identification of Acoustic Emission Signatures and Their Capability to Assess the Damage of Reinforced Concrete Beam under Incremental Cyclic Loading

Acoustic emission (AE) has been widely used for damage detection and characterization in reinforced concrete (RC) structures. However, there is little work focusing on correlation of AE signatures and physical phenomena involved within the failure process of RC beams. In this paper, AE monitoring was performed on five small-scale RC beams that were subjected to an incremental cyclic loading regimen. Correlation of AE activity with in situ crack progression was performed by comparing AE signatures with a series of real-time images of crack development. Based on the combination of AE response and development of flexural and shear crack patterns, four phases named microcrack formation, macrocrack initiation, crack opening/closing, and near failure were recognizable. Meanwhile, AE cumulative energy (CE) analysis during loading and unloading demonstrated its potential for damage detection and qualification. The test results show crack initiation and propagation are accompanied with significantly greater AE activity in comparison to existing crack reopening or closing; and during the unloading step when the RC beams are near failure, the calculated CE is greater than that for initial crack appearance. In addition, AE event localization method and waveform analysis were performed to aid in identifying the source mechanisms of meaningful events. Crack propagation events were associated with burst type signals. During unloading step near failure, signals of high amplitude and long rise time are located along the reinforcement indicating a potential loss of bond as the reinforcement interacts with its surrounding concrete. The results showed that the combination of each analysis type aided in determining whether AE generated during loading/unloading of RC can be a meaningful parameter in monitoring damage of RC structures.

Impact of unloading kinematics on the occurrence of capping during the production of pharmaceutical tablets

Capping is a common defect that can occur during the manufacturing of pharmaceutical tablets. Several studies showed that decreasing the unloading speed of the manufacturing cycle plays a role in the occurrence of such defects. Following this idea, we study in this work the influence of the unloading step on capping using a compaction simulator.Measuring the die wall pressure made it possible to detect precisely that tablets capped just after the unloading (some milliseconds only). To evaluate the impact of the unloading speed on capping, we developed a two-step unloading phase controlled by three manufacturing parameters. It was possible to mitigate capping by decreasing the speed at which the contact between the punches and the tablet was lost. Capping seemed due to dynamical effects related to the release of the axial pressure. The modification of the unloading step to mitigate capping led to significant changes in tablet density but no clear trends were found for the residual die-wall pressure and tablet strength.This work made it possible to improve the understanding of capping. Moreover, the two-step unloading cycle gave a new idea for possible modifications that could be done on rotary presses in order to mitigate capping.

Assessment of mechanical elasticity of soils based on confined compression tests

Prevention of soil compaction requires examining soil mechanical stability. This is usually done by applying the basic concept of the pre-compression stress σp, assuming a fully elastic behavior for soils being loaded by stresses lower than σp and a mixture of elastic-plastic behavior being loaded by higher stresses. However, a small cumulative plastic deformation may occur even when the applied stresses is not beyond σp being indeterminable by current pe-compression test procedures which is not accounting for unloading steps. Therefore, we investigated an alternative approach to simultaneously determine σp and mechanical elasticity of soil by introducing unloading steps (referred to as release compression, RC). Results were compared to the conventional procedure with continuously increasing loads (continuous compression, CC). In the case of RC, soil settlement was measured both after each loading (RC-L) and after each unloading (RC-U) phases between two consecutive pressures providing two load displacement curves. Oedometer tests were conducted with three different soils comprising different clay contents ranging from 75 to 454 g kg−1. Remolded soil was filled into steel rings and compressed under pre-determined loads in a loading frame providing test samples with known pre-compression stresses of 60 and 120 kPa. The results revealed that σp values determined under two different applied loading paths (RC vs. CC) were significantly different. Both testing approaches resulted in an overestimation of σp values based on stress-strain curves which was more pronounced for RC-L compared to CC paths. However, the results from RC-U paths were closer to the CC method than the RC-L path suggesting that if an unloading step is used, RC-U curve should be used to determine σp values. The results also showed that soil with higher clay contents comprised the lowest σp value determined from the oedometer tests among all examined soils. From release compression test, we were also able to determine the elasticity of the sample for each loading stress. Among the examined soil samples, the clay soil (with higher clay content) showed the highest elasticity index being less susceptible to soil deformation.

Anisotropic damage evolution and modeling for a nickel-based superalloy built by additive manufacturing

The objective of the present work is to characterize the effects of microstructural features, including porosity, precipitates, and to quantify their contributions to the damage evolution in the Inconel 718 alloy built by selective laser melting (SLM). 3D morphology of porous defects in the SLM alloy obtained from X-ray micro-computed tomography before and after hot isostatic pressing (HIP) were analyzed and classified based on the pore size, shape, and distribution. The unmelt region pores with the large size and irregular shape are found serving as crack sources and responsible for the anisotropic damage evolution. The inter-dendritic precipitated phases and twin boundaries were observed and preferred to be crack initiation sites and propagation paths, which further promote microstructural damage evolution. The macroscopic anisotropic damage of Inconel 718 alloy was evaluated through the variations of the material stiffness measurement from tensile tests with multiple unloading steps. A microstructure-dependent damage model was introduced to quantitatively describe the contributions of the microstructures o the evolution of macroscopic material degradation. The total damage was decomposed into three components based on the microstructural mechanisms and gives meaningful evolution laws of the microstructural characteristics. The material stiffness degradation processes of SLM can be predicted by the present model with good agreement.

Experimental investigation on statistical characteristics of cell voltage distribution for a PEMFC stack under dynamic driving cycle

The proton exchange membrane fuel cell (PEMFC) stack consists of individual cells in series. Its operating life is subjected to performance of the weakest cell because of the short-board effect, thus voltage uniformity during dynamic long-running process is significant to its durability. In this work, based on a 1044 h aging experiment on a 6.55 kW PEMFC stack under dynamic driving cycle, voltage uniformity is analyzed. In a single cycle, voltage uniformity becomes worse with the increase of loading current and there are some local maxima of voltage coefficient variation (Cv) at the moment of loading or unloading step. Cv value at higher current is greater and increases faster with cycles. At the end of experiment, Cv at 135 A is more than 6%. Besides, skewness (Sk) is used to evaluate the skew direction and degree of cell voltage data in a cycle. In most cycles, Sk values at 34.22 A are above 0 and Sk values at 59.70 A and 135 A are below 0. After the Box-Cox transformation, which is used to improve symmetry of data and reduce Sk, the cell voltage data have passed the verifications of normal fitting, probability-probability plot and quantile-quantile plot. Therefore, it is found that cell voltage data tend to obey skewed normal distribution, which is of positive significance for improving voltage uniformity and durability of PEMFC stack.

Architected material analogs for shape memory alloys

Architected material analogs for shape memory alloys

A new large strain approach for predicting tunnel deformation in strain-softening rock mass based on the generalized Zhang-Zhu strength criterion

The deformation capacity of tunnel will be overestimated by small strain theory especially in high geostress and heavily squeezing rocks, the calculated wall convergence of tunnel may even exceed the excavated radius. Based on the Generalized Zhang-Zhu (GZZ) strength criterion, a new large strain solution for tunnel excavated in strain-softening rocks under high geostress is proposed by a semi-analytical approach. During calculation, the material nodes in the plastic zone of surrounding rock are generated automatically in each unloading step. The locations of these nodes are updated after each step and the governing equations are established on the updated state. The proposed procedure allows one to predict the rock stresses, strains, and deformations in a rational and rigorous manner, and it is validated by numerical simulation. Results show that the ground displacement calculated by proposed approach is in good agreement with that obtained by GeoFBA3D, which indicates that the proposed approach is capable in predicting the large deformation of surrounding rock. As long as the unloading radial pressure of each step δp is sufficient small, i.e., the number of material nodes in the plastic zone exceeds 40, the computational error can be neglected. Compared to HB strength criterion, the GZZ criterion can take into account the intermediate principal stress, which is beneficial to tunnel stability. The applicable conditions for large strain theory and small strain theory are given by parametric studies. Parametric studies show that the calculation error produced by small strain theory can reach 30% when the deformation of surrounding rock exceeds about 10%R0, therefore, a large strain theory is necessary for calculation.

A comparative study of force fields for predicting shape memory properties of liquid crystalline elastomers using molecular dynamic simulations

Molecular dynamic (MD) simulation techniques are increasingly being adopted as efficient computational tools to design novel and exotic classes of materials for which traditional methods of synthesis and prototyping are either too costly, unsafe, and time-consuming in laboratory settings. Of such class of materials are liquid crystalline elastomers (LCEs) with favorable shape memory characteristics. These materials exhibit some distinct properties, including stimuli responsiveness to heat or UV and appropriate molecular structure for shape memory behaviors. In this work, the MD simulations were employed to compare and assess the leading force fields currently available for modeling the behavior of a typical LCE system. Three force fields, including Dreiding, PCFF, and SciPCFF, were separately assigned to model the LCE system, and their suitability was validated through experimental results. Among these selected force fields, the SciPCFF produced the best agreement with the experimentally measured thermal and viscoelastic properties compared to those of simulated steady-state density, transition temperature, and viscoelastic characteristics. Next, shape fixity (Rf) and shape recovery (Rr) of LCEs were estimated using this force field. A four-step simulated shape memory procedure proceeded under a tensile mode. The changes in molecular conformations were calculated for Rf and Rr after the unloading step and the reheating step. The results revealed that the model LCE system exhibits characteristic behaviors of Rf and Rr over the thermomechanical shape memory process, confirming the suitability of selected force field for use in the design and prediction of properties of typical LCE class of polymers.

Experimental Research on Permeability of Coal and Rock Mass considering Postpeak Failure

In most mining areas of China, coal seams are characterized by low gas pressure, poor permeability, and high gas adsorption capacity, all of which have brought considerable difficulties to coal seam mining and coalbed methane (CBM) extraction. According to the multiyear scientific research and production practice of China, gas is migrated in quantity only after the coal body is directly mined, and the surrounding rocks deform and fracture under the mining influence. Thus, the key to effective control of gas migration and the coal and CBM comining technology lies in investigating the gas resolution, permeation, migration, and accumulation laws in the coal seams under the unloading confining pressure during mining. The MTS815.02 rock mechanics testing system and its supporting equipment are combined to test the permeability characteristics of coal and rock mass (postpeak fractured coal and sandstone specimens) under the loading and unloading of confining pressure using the steady method, and then, the permeation laws of the fractured coal and rock mass are obtained. Results show that after the postpeak rock crack propagation reaches a stable state, the confining pressure gradually increases, and the gas permeability presents an approximately linear reduction; in the postpeak unloading phase, the opening and coalescence degree of rock cracks gradually increase as the deformation extends. Thus, permeability reaches a peak value. The strain softening phase follows, where the cracks are closed and permeability declines to a certain extent. Moreover, the unloading step size of confining pressure has bearing on gas permeability. Specifically, as the unloading step size of confining pressure decreases, the change of gas permeability increases in stability.

Fatigue crack growth determination based on cyclic plastic zone and cyclic J‐integral in kinematic–isotropic hardening materials with considering Chaboche model

AbstractFor many materials, plastic deformations under cyclic loadings differ from monotonic loading. Cyclic plastic zone with considering the strain hardening effects becomes complicated during unloading step. In this research, the effects of nonlinear kinematic and isotropic hardening behaviours on the cyclic plastic reaction around the crack tip for different conditions are investigated. For study of various hardening characteristics, as‐received and annealed copper are tested under same symmetric cyclic loadings. The results indicate considerable isotropic hardening behaviour in annealed material. A Chaboche nonlinear hardening model was used to determine the hardening parameters. The cyclic plastic zone around the crack tip in C(T) specimen was measured by back stresses. The cyclic plastic zones are specified via variations of the back stresses in a cycle. The cyclic plastic zones predicted by Chaboche model are smaller than those for Irwin model because of the hardening effects. Also, Irwin model for prediction of the cyclic plastic zone size is modified with considering the cyclic plastic effects. According to the results, the cyclic plastic zone around the crack tip is almost constant in the same load range, and load ratio (R) has a slight effect on this zone. Whereas, for constant load range, cyclic J‐integral (∆J) is different for various R values. For studying of the fatigue crack growth, two parameters of ∆J and cyclic plastic zone area were evaluated. These parameters can apply in fracture mechanics especially in elastic–plastic conditions with considering the cyclic plastic responses.

Mechanical characterization of powder materials: A general approach detailed for cemented carbides

Material parameter curves in an advanced material model describing compaction of spray dried cemented carbide powder are determined successfully based on a general approach for material characterization of powder materials. Pressing forces from a production machine and equivalent finite element (FE) calculations are used in inverse modelling. A pressing method that includes multiple unloading steps is used. The material model is of Drucker Prager CAP kind and friction between powder and pressing tool is modelled as a function of normal pressure. The results are verified with density gradient measurements using a neutron source. The method is proven to be robust and the results show good agreement between experiment and simulation. Effects that have not been captured numerically previously are captured due to the high accuracy of material characterization. The present approach is detailed for tungsten carbide powders but is valid for other powder materials when properly calibrated for constitutive and frictional effects in the same manner as outlined here.