Analysis of damage and fracture mechanisms in ductile metals under non-proportional loading paths

The paper discusses biaxial experiments and corresponding numerical simulations to analyze the effect of non-proportional loading paths on damage and fracture behavior of ductile metals. Newly developed specimens are taken from thin metal sheets and are tested under different biaxial loading conditions covering a wide range of stress states. In this context, an anisotropic continuum damage model is presented based on yield and damage conditions as well as on evolution laws for plastic and damage strain rates. Different branches of the damage criteria are taken into account corresponding to various damage and failure processes on the micro-level depending on stress triaxiality and Lode parameter. Experiments with biaxially loaded specimens have been performed. Results for proportional and corresponding non-proportional loading histories are discussed. During the experiments strain fields in critical regions of the specimens are analyzed by digital image correlation (DIC) technique while the fracture surfaces are examined by scanning electron microscopy (SEM). Numerical simulations of the experiments have been performed and numerical results are compared with experimental data. In addition, based on the numerical analyses stress distributions in critical parts of specimens are detected. The results demonstrate the efficiency of the new specimen’s geometries covering a wide range of stress states in the shear/tension and shear/compression regime as well as the effect of loading history on damage and fracture behavior in ductile metal sheets

Modeling of ductile damage using numerical analyses on the micro-scale

The presentation deals with a continuum damage model which has been generalized to take into account the effect of stress state on damage criteria as well as on evolution equations of damage strains. It is based on the introduction of damaged and corresponding undamaged configurations. Plastic behavior is modeled by a yield criterion and a flow rule formulated in the effective stress space (undamaged configurations). In a similar way, damage behavior is governed by a damage criterion and a damage rule considering the damaged configurations. Different branches of the damage criterion are considered corresponding to various damage mechanisms depending on stress intensity, stress triaxiality and the Lode parameter. Experiments with carefully designed specimens are performed and the test results are used to identify basic material parameters. However, it is not possible to determine all parameters based on these tension and shear tests. To be able to get more insight in the complex damage behavior under different loading conditions, additional series of micro-mechanical numerical analyses of void containing unit cells have been performed. These finite element calculations on the micro-level cover a wide range of stress triaxialities and Lode parameters in the tension, shear and compression domain. The numerical results are used to show general trends, to develop equations for the stress-statedependent damage criteria, to propose evolution equations of damage strains, and to identify parameters of the continuum model

Species richness and stand diversity in relation to site and succession of forests in Sarawak and Brunei (Borneo)

Site conditions in the Mixed Dipterocarp forests of Sarawak and Brunei are characterised by the absence of prolonged drought. This permits minor site differences to become ecologically fully effective. In the kerangas forests, drought conditions occur frequently and regularly. An extremely variable water regime and uniformity of chemical and physical soil conditions are typical of kerangas peat bogs and deltaic peatswamp forest sites. Consequently, fewer niches are available and the number of species is less on these sites than in Mixed Dipterocarp forest. Upland kerangas sites are intermediate in this respect. About 850 tree species (428 genera) are recorded from kerangas forests. It is estimated that the total number of tree species in all forest types in Sarawak and Brunei is between 2 500 and 3 000, and 2 800 and 3 300 respectively. The positions, shapes, and variation coefficients of the species-area lines are strongly influenced by conditions of soil and physiography. The diversity index of McINTOSH and the average information content indicate a high degree of diversity among trees of stands on sites with balanced water regime. Low values occur in late phases of peat development and on sites with frequent droughts or with alternating dry and waterlogged conditions. The differences are accentuated if the basal area is substituted for number of individuals in the index. The shape of the dominance-diversity curves according to WHITTAKER indicates that on favourable sites a great number of environmental factors determines the diversity of ecological niches. On less favourable sites, niche space appears to be restricted by environmental severity, and a tendency to dominance and reduced species diversity is evident. The values of an integrating index of diversity are related to soil-landform units which reflect ecological gradients primarily related to the water regime. The trend of the index values agrees with the trend of structural complexity expressed as estimated aerodynamic surface roughness. A synoptic assessment of the various indicators of diversity suggest the important role which environmental factors play in shaping ecological niche-spaces in the equatorial lowland forests

New approach for determination of strain rate sensitivity of mild steel dc01 under stack compression and uniaxial tensile test

Deformation under uniaxial tensile loading with using Digital Image Correlations (DIC) is the easiest way to analyze the material behavior in sheet metal forming. In order to determine the plastic parameters such as hardening, anisotropy and strain rate sensitivity at higher strain level, equi-biaxial stress state is prerequisite. As reported in the literature, Bulge tests are frequently used for this purpose, but in this work, stack compression test is used as an alternative. In this experiment, deformation in the middle layer where the friction effect is the lowest was monitored using two pairs of DIC systems in rolling and transversal directions. Uniaxial tensile tests as well as stack compression tests were performed on mild ferritic steel DC01 at different strain rates, from 0.001 −1 to 10 −1. Strain rate sensitivity parameter was investigated at different level of strains for both experiments and strain rate sensitivity profiles were obtained. Results show a decrease of material strain rate sensitivity with increasing the true strain

Collective exchange processes reveal an active site proton cage in bacteriorhodopsin

Proton translocation across membranes is vital to all kingdoms of life. Mechanistically, it relies on characteristic proton flows and modifications of hydrogen bonding patterns, termed protonation dynamics, which can be directly observed by fast magic angle spinning (MAS) NMR. Here, we demonstrate that reversible proton displacement in the active site of bacteriorhodopsin already takes place in its equilibrated dark-state, providing new information on the underlying hydrogen exchange processes. In particular, MAS NMR reveals proton exchange at D85 and the retinal Schiff base, suggesting a tautomeric equilibrium and thus partial ionization of D85. We provide evidence for a proton cage and detect a preformed proton path between D85 and the proton shuttle R82. The protons at D96 and D85 exchange with water, in line with ab initio molecular dynamics simulations. We propose that retinal isomerization makes the observed proton exchange processes irreversible and delivers a proton towards the extracellular release site

The reference cube: A new ballistic model to generate staining in firearm barrels.

After contact shots to the head biological traces can be found inside firearm barrels. So far silicone coated, gelatin filled box models were used to generate such staining according to the triple contrast method (mixture of acrylic paint, barium sulfate and blood sealed in a thin foil bag). This study was conducted to develop a transparent ballistic model allowing contact shots. Gelatin filled polyethylene bottles with and without a silicone coat were tested in comparison to non-covered gelatin blocks. Finally, thin foil bags of 5 cm × 5 cm dimension were glued on a synthetic absorbent kitchen wipe on top of which 1 L 10% gelatin solution was molded to create blocks of 8.5 cm length. A kitchen wipe with a paint pad on its inside formed the front of the cube. Three contact shots each with a 9 mm Luger pistol and a .38 special revolver were performed on all model variations. The staining was documented by endoscopy and swabs gathered from both ends of the barrel were analyzed by quantitative PCR. Reliable staining was achieved using the front covered gelatin block with comparable results to the silicone coated box model used before. For further research using ballistic models to simulate a human head a symmetric form of the gelatin block such as a cube is recommended

New 2D-Experiments and Numerical Simulations on Stress-state-dependence of Ductile Damage and Failure

AbstractThe paper deals with a series of new experiments and corresponding numerical simulations to be able to study the effect of stress state on damage and failure behavior of ductile metals. The material behavior is modeled by a continuum approach based on free energy functions defined in damaged and corresponding fictitious undamaged configurations leading to elastic material laws which are affected by damage. Inelastic behavior of ductile materials is modeled by continuum plasticity and continuum damage model, respectively. The present approach takes into account the effect of stress state on damage and failure conditions expressed in terms of the stress intensity, the stress triaxiality and the Lode parameter. Previous studies have shown that it will not be possible to propose the stress-state-dependent functions for damage and failure criteria only based on tests with uniaxially loaded specimens. Therefore, new experiments with carefully designed and two-dimensionally loaded specimens have been developed. Corresponding numerical simulations of these tests show that they cover a wide range of stress states allowing validation of stress-state-dependent functions for the damage criterion and evolution laws for the damage strains

NhaD type sodium/proton-antiporter of Halomonas elongata: a salt stress response mechanism in marine habitats?

BACKGROUND: Sodium/proton-antiporters (Nha) are known to play an important role in pH- and Na(+)-homeostasis. In microorganisms several types with different capacity, affinity and selectivity for Na(+ )and Li(+ )exist. The homeostasis system of E. coli, NhaA and NhaB, is well researched, but the function of other types of Na(+)/H(+)-antiporters like NhaD is yet to be fully understood. Since several antiporters play an important role at various points in the physiology of higher organisms, one can speculate that the main functions of some of those procaryotic antiporters differ from pH- and Na(+)-homeostasis. RESULTS: This study investigates the function and regulation of a gene encoding for a NhaD type antiporter which was discovered in the halophilic eubacterium Halomonas elongata. The deduced primary amino acid sequence of the abovementioned gene showed more than 60% identity to known antiporters of the NhaD type from Alkalimonas amylolytica, Shewanella oneidensis and several other marine organisms of the γ-Proteobacteria. Evidence was found for a dual regulation of H. elongata NhaD expression. The gene was cloned and expressed in E. coli. Antiporter deficient NaCl and LiCl sensitive E. coli mutants EP432 and KNabc were partially complemented by a plasmid carrying the H. elongata nhaD gene. Surprisingly the LiCl sensitivity of E. coli strain DH5α having a complete homeostasis system was increased when NhaD was co-expressed. CONCLUSION: Since NhaD is an antiporter known so far only from halophilic or haloalcaliphilic Proteobacteria one can speculate that this type of antiporter provides a special mechanism for adaptation to marine habitats. As was already speculated – though without supporting data – and substantiated in this study this might be active Na(+)-import for osmoregulatory purposes

Pair-Reaction Dynamics in Water: Competition of Memory, Potential Shape, and Inertial Effects

When described by a one-dimensional reaction coordinate, pair-reaction rates in a solvent depend, in addition to the potential barrier height and the friction coefficient, on the potential shape, the effective mass, and the friction relaxation spectrum, but a rate theory that accurately accounts for all of these effects does not exist. After a review of classical reaction-rate theories, we show how to extract all parameters of the generalized Langevin equation (GLE) and, in particular, the friction memory function from molecular dynamics (MD) simulations of two prototypical pair reactions in water, the dissociation of NaCl and of two methane molecules. The memory exhibits multiple time scales and, for NaCl, pronounced oscillatory components. Simulations of the GLE by Markovian embedding techniques accurately reproduce the pair-reaction kinetics from MD simulations without any fitting parameters, which confirms the accuracy of the approximative form of the GLE and of the parameter extraction techniques. By modification of the GLE parameters, we investigate the relative importance of memory, mass, and potential shape effects. Neglect of memory slows down NaCl and methane dissociation by roughly a factor of 2; neglect of mass accelerates reactions by a similar factor, and the harmonic approximation of the potential shape gives rise to slight acceleration. This partial error cancellation explains why Kramers’ theory, which neglects memory effects and treats the potential shape in harmonic approximation, describes reaction rates better than more sophisticated theories. In essence, all three effects, friction memory, inertia, and the potential shape nonharmonicity, are important to quantitatively describe pair-reaction kinetics in water

Complex loading and simulation of acoustic thickness shear mode resonator

During the last decades thickness shear mode resonators (TSM, QCM) have been object of comprehensive research. Many approaches were made to describe the behavior and physical effects when loaded. We present a physical model that describes the TSM in the full frequency range, including overtones for a large variety of loadings (e.g. gases, liquids or solid materials). By using an automated curve fit algorithm, absolute values for the loaded material (e.g. thickness, viscosity) can be extracted. The model has been validated with a large number of experiments including liquids with complex viscosities, biomolecule interactions, electrochemisty or vacuum deposition techniques. Additionally, the appearance of layer resonances have been predicted and verified. Layer resonances are remarkable because they appear at even-numbered overtones, which have been considered to be impossible