Variable Mechanical Stress Research Articles

Vascular Cambium—Between the Hammer and the Anvil: A Tensile Stress Hypothesis on the Mechanism of Radial Growth of Broadleaved Trees

The vascular cambium is the main lateral meristem responsible for the secondary growth of trees. There are a number of explicit and implicit assumptions behind this statement which allow questions to be raised about the mechanism underlying the radial growth of trees. Based on the hypothesis of the diurnal strains of plant organs, it is anticipated that the process of radial growth can be understood as an adaptation to the cyclically changing mechanical stress in the radial direction generated by the phloem during the 24 h day cycle. This qualitative hypothesis treats cambium as a tissue subjected to nighttime stretching and daytime compression in the radial direction. The proposed osmo-mechanical hypothesis of the radial growth of vascular cambium links the daily change in water status and the considerable daily strains in the xylem and phloem with the radial net expansion of a tree trunk. We highlight transpiration as a major factor influencing the secondary growth of woody plants. Thus, we indirectly relate all the biotic (e.g., insect infestation, fungi infections, injuries, shadowing, intra- and interspecies competition, parasitism, symbiosis, etc.) and abiotic (e.g., humidity, water availability, wind, injuries, shadowing, day length in a vegetative season, altitude, temperature, insolation, etc.) processes influencing transpiration with radial growth. In the proposed hypothesis, we also infer that differences in the strains in phloem and xylem are the direct source of tensile stress, tensile stress relaxation, compressive stress, and compressive stress relaxation in the vascular cambium. This sequence appears to be crucial in understanding both the process of the radial growth of trees and the formation of differential wood patterns, within the same genotype as well as in different genotypes. It also provides arguments for the discussion on the mechanisms regulating processes in the vascular cambium. It points out the important role of the variable mechanical stresses in the radial, circumferential, and axial directions and their interference in the development of this lateral meristem. Thus, this hypothesis supports the concept of the epigenetic and systemic regulation of intrinsic wood patterns and tree forms by environmental factors. The hypothesis is focused exclusively on broadleaved trees and symplastic growth. This limitation of the scope is due to a concern for clarity. In this form, the hypothesis provides an alternative explanation for a pure process of radial growth and paves the way for a better interpretation of such phenomena as earlywood and latewood formation. At the same time, this approach to the vascular cambium provides answers to many questions related to the generation of the mechanical conditions necessary for the occurrence of intrusive growth between tangential cell walls; this is of fundamental importance for fusiform initials readjustment, vessel element and fibre formation, ring-porous wood formation, etc.

Improving the Fatigue Behaviour of PURMAL S-70 Polyurethane Paint

Oxidation is one of the main degradation mechanisms that affects most industrial components and occurs as a result of a chemical reaction by which an oxygen atom is added to the molecule of an organic substance or compound. Light, in particular the ultraviolet (UV) component, activates the process of degradation of organic materials, favouring a series of oxidation reactions that occur when the component is in contact with oxygen in the air or water (especially salt water).Given that a number of components work in variable mechanical stress conditions (vibrations caused by the technological process itself or generated by repeated starts / shutdowns of equipment), the protective coatings aim to improve the physico-chemical properties of the surfaces to reduce the degradation, as well as to improve the fatigue behaviour, knowing that the fatigue cracks are initiated from local stress concentrators caused by the surface imperfections of the components. Paper presents the experimental results on fatigue behaviour of the deposited layers, after exposure to UV radiation. In the experimental program, a 2 mm thick AlMg3 sheet (EN AW-5754) was used as substrate. The Aluminium oxide layer naturally created on the metal surface, prevents the effect of oxygen and atmospheric pollutants, but this layer is unstable in corrosive environments. Thus, for a qualitative protection, protective organic layers are used. In experiments, the polyurethane paint PURMAL S 70, RAL 9010 (PUR) and the same paint but aditivate with graphene oxide (PUR + GO) were compared. Prior to fatigue testing, the painted specimens were subjected to accelerated artificial UV-aging process with ultraviolet radiation for 72 hours. Axial fatigue tests were performed with a frequency of 40 Hz, in pulsating mode, with R = 0.053 and with decreasing loads, starting from 0.85% of the tensile strength (Rm), up to 0.5% Rm. Analysis of S‑N curves revealed that, in the field of oligocyclic fatigue there are no significant differences between the fatigue results obtained on PUR-protected specimens, compared to those protected with PUR + GO, instead in the case of polycyclic fatigue, the paint with addition of 1% graphene oxide has improved fatigue resistance.

Coumarins, psoralens, and quantitative 1H-NMR spectroscopy for authentication of lemon (Citrus limon [L.] Burm.f.) and Persian lime (Citrus × latifolia [Yu.Tanaka] Tanaka) juices

Mutual adulterations of lemon and lime juices may be detected using coumarins and psoralens as markers. Poor manufacturing practices or legal but mechanically intense processing of lemons were recently suspected to lead to false accusations of deliberate adulterations with lime juices due to potentially unspecific markers. Therefore, we studied coumarin and psoralen profiles in carefully dissected flavedo, albedo, and endocarp of lime and lemon as well as in juices produced under variable mechanical stresses at laboratory and pilot plant scale. Although the marker herniarin was detectable in juices from lime and harshly extracted lemons at low levels, isopimpinellin, bergapten and the herein proposed, tentatively assigned 5-geranyloxy-8-methoxypsoralen represented unambiguously lime-specific markers. Coumarin and psoralen data also allowed differentiating juices produced at differing degrees of mechanical stress. The latter was also possible using quantitative 1H-NMR spectroscopy, which yielded best results when combined with HPLC data on coumarins and psoralens. In the future, the reported approach may be used for establishing a robust database prior to being used in industrial practice.

Mechanical-stressing measurements of formation energy of single Shockley stacking faults in 4H-SiC

The formation energy of single Shockley stacking faults (1SSFs) in thermo-equilibrium is crucial for validating the anomalous expansion mechanism of 1SSF induced by forward current injection in 4H-SiC bipolar devices. As a function of variable mechanical stress externally applied to a plate sample of 4H-SiC, we systematically measured the ultraviolet intensity thresholds demarcating photo-induced expansion and contraction of 1SSFs. The experimental results on two types of 1SSF differing in the Schmid factor of mobile partial dislocations achieving 1SSF expansion/contraction showed a value of 4.7 ± 1.6 mJ m−2, significantly smaller than the reported value of 14 ± 2.5 mJ m−2 obtained by transmission electron microscopic measurements.

Device for Cell Culture under Variable Mechanical Stress by Controlling Deformation on its Substrate

Mechanotransduction can enhance cell cultures when its substrate is subjected to dynamic strain. This paper presents the design and implementation of a device that applies variable mechanical stress on a flexible biocompatible substrate. An embedded system implements a closed-loop PID control that acts on a proportional valve, which regulates the negative pressure in a chamber, generated by a miniature vacuum pump. The top cover of the chamber is a membrane that stretches according to the generated vacuum and is used for cell culture. The elastic modulus of the produced membrane was 260 kPa, similar to soft biological tissue. The deformation of the membrane was quantified by image processing and a linear pressure-deformation relationship was achieved (r and gt; 0.99). The controller was designed based on the identified valve's dynamic and it allowed variable deformations on the membrane with an amplitude of 20 % and a frequency of 2 Hz, selected by the user. The implemented device is PC independent and has a cost, size and weight almost two orders of magnitude lower than a similar commercial version.

Breath-by-breath oxygen uptake during running: Effects of different calculation algorithms.

What is the central question of this study? Breath-by-breath gas exchange analysis during treadmill exercise can be disturbed by different breathing patterns depending on cadence, and the flow sensor might be subjected to variable mechanical stress. It is still unclear whether the outcomes of the gas exchange algorithms can be affected by running at different speeds. What is the main finding and its importance? Practically, the three investigated breath-by-breath algorithms ('Wessel', 'expiration-only' and 'independent breath') provided similar average gas exchange values for steady-state conditions. The 'independent breath' algorithm showed the lowest breath-by-breath fluctuations in the gas exchange data compared with the other investigated algorithms, both at steady state and during incremental exercise. Recently, a new breath-by-breath gas exchange calculation algorithm (called 'independent breath') was proposed. In the present work, we aimed to compare the breath-by-breath O2 uptake ( ) values assessed in healthy subjects undergoing a running protocol, as calculated applying the 'independent breath' algorithm or two other commonly used algorithms. The traces of respiratory flow, O2 and CO2 fractions, used by the calculation algorithms, were acquired at the mouth on 17 volunteers at rest, during running on a treadmill at 6.5 and 9.5kmh-1 , and thereafter up to volitional fatigue. Within-subject averages and standard deviations of breath-by-breath were calculated for steady-state conditions; the data of the incremental phase were analysed by means of linear regression, and their root mean square was assumed to be an index of the breath-by-breath fluctuations. The average values obtained with the different algorithms were significantly different (P<0.001); nevertheless, from a practical point of view the difference could be considered 'small' in all the investigated conditions (effect size<0.3). The standard deviations were significantly lower for the 'independent breath' algorithm (post hoc contrasts, P<0.001), and the slopes of the relationships with the corresponding data yielded by the other algorithms were <0.70. The root mean squares of the linear regressions calculated for the incremental phase were also significantly lower for the 'independent breath' algorithm, and the slopes of the regression lines with the corresponding values obtained with the other algorithms were<0.84. In conclusion, the 'independent breath' algorithm yielded the least breath-by-breath O2 uptake fluctuation, both during steady-state exercise and during incremental running.

On the Dislocation Contribution to the Dielectric Loss in the Materials with Piezoelectric Properties

A variable electric field is applied to a crystal. This field gives rise – through the piezoelectric coupling – to the variable mechanical stresses. Then the dislocations in the crystal will be driven by Peach-Koehler force and will start moving, dissipating the external field energy. Connection of the electric field energy dissipated per unit time with the internal friction is found. The case of resonant loss (Granato-Lucke model) is considered. The loss related to this mechanism to be at frequencies of megahertz range. The relaxation processes being responsible for the Bordoni and Hasiguti peaks also are considered. The use of obtained equations makes it possible to distinguish the dislocation contribution to both dielectric loss and dielectric dispersion and, therefore, to derive additional information about the crystal structure in a sufficiently simple way in terms of only one method.

Hybrid Systems-in-Foil—Combining the Merits of Thin Chips and of Large-Area Electronics

This paper reports on the status of a comprehensive ten-year research and development effort toward hybrid system-in-foil (HySiF). In HySiF, the merits of high-performance integrated circuits on ultra-thin chips and of large-area and discrete electronic component implementation are combined in a complementary fashion in and on a flexible carrier substrate. HySiF paves the way to entirely new applications of electronic products where form factor, form adaptivity and form flexibility are key enablers. In this review paper the various aspects of thin-chip fabrication and embedding, device and circuit design under impact of unknown or variable mechanical stress, and the on- and off-chip implementation of sensor, actuator, microwave, and energy supply components are addressed.

Enhancing the Matrix Addressing of Flexible Sensory Arrays by a Highly Nonlinear Threshold Switch.

The increasing need for smart systems in healthcare, wearable, and soft robotics is creating demand for low-power sensory circuits that can detect pressure, temperature, strain, and other local variables. Among the most critical requirements, the matrix circuitry to address the individual sensor device must be sensitive, immune to disturbances, and flexible within a high-density sensory array. Here, a strategy is reported to enhance the matrix addressing of a fully integrated flexible sensory array with an improvement of 108 fold in the maximum readout value of impedance by a bidirectional threshold switch. The threshold switch shows high flexibility (bendable to a radius of about 1 mm) and a high nonlinearity of ≈1010 by using a nanocontact structure strategy, which is revealed and validated by molecular dynamics simulations and experiments at variable mechanical stress. Such a flexible electronic switch enables a new generation of large-scale flexible and stretchable electronic and optoelectronic systems.

Altered Cellular Kinetics in Growth Plate according to Alterations in Weight Bearing

PurposeTo examine the effects of change in weight bearing on the growth plate metabolism, a simulated animal model of weightlessness was introduced and the chondrocytes' cellular kinetics was evaluated.Materials and MethodsUnloading condition on the hind-limb of Sprague-Dawley rats was created by fixing a tail and lifting the hind-limb. Six rats aged 6 weeks old were assigned to each group of unloading, reloading, and control groups of unloading or reloading. Unloading was maintained for three weeks, and then reloading was applied for another one week thereafter. Histomorphometry for the assessment of vertical length of the growth plate, 5-bromo-2'-deoxyuridin immunohistochemistry for cellular kinetics, and biotin nick end labeling transferase-mediated deoxyuridine triphosphate-biotin nick end labeling (TUNEL) assay for chondrocytes apoptosis in the growth plate were performed.ResultsThe vertical length of the growth plate and the proliferative potential of chondrocytes were decreased in the unloading group compared to those of control groups. Inter-group differences were more significant in the proliferative and hypertrophic zones. Reloading increased the length of growth plate and proliferative potential of chondrocytes. However, apoptotic changes in the growth plate were not affected by the alterations of weight bearing.ConclusionAlterations in the weight bearing induced changes in the chondrocytic proliferative potential of the growth plate, however, had no effects on the apoptosis. This may explain why non-weight bearing in various clinical situations hampers normal longitudinal bone growth. Further studies on the factors for reversibility of chondrocytic proliferation upon variable mechanical stresses are needed.

Single Molecule Measurements of Catch Bond Formation in Cadherin Cell-Adhesion Proteins

We resolve the molecular interactions by which cadherin cell adhesion proteins tune the mechanical properties of cell-cell junctions. Cells in multicellular organisms undergo constant rearrangements; during tissue formation and wound healing, cells divide, change positions and tug on their neighbors. While the genes that orchestrate these process have been studied extensively, we do not know nearly enough about how cell-cell contacts resist physical force. At the molecular level, cells are held together by adhesion proteins, most commonly by the cadherin family of proteins. Cadherins are essential for tissue formation and for maintaining tissue integrity. Using single molecule force measurements, we demonstrate that cadherin bonds become stronger as cells are pulled apart. This counter-intuitive data is the first demonstration that cadherin’s form catch bonds; the proteins grip strongly in the presence of an external force but detach when force is removed. Our results suggest a molecular mechanism for regulating cell-cell adhesion under conditions of variable mechanical stress.

Magnetic indication of the stress-induced martensitic transformation in ferromagnetic Ni–Mn–Ga alloy

A quantitative study of the stress-induced martensitic transformation in Ni 49.7Mn 29.1Ga 21.2 magnetic shape memory alloy has been carried out in two different ways: the first way is based on the measurements of saturation magnetization under variable mechanical stress and the second one is founded on the quantitative theoretical treatment of experimental stress–strain loops. A functional dependence between the volume fraction of transformed martensite and applied stress has been determined from both magnetization and strain values. A quantitative agreement between the functions determined in two different ways has been observed, and hence, the effectiveness of the magnetic indication of the stress-induced martensitic transformations has been proved. This method can be used to monitor stress-induced transformations in martensitic films, needles and small specimens.

Direct observation of catch bonds involving cell-adhesion molecules.

Bonds between adhesion molecules are often mechanically stressed. A striking example is the tensile force applied to selectin-ligand bonds, which mediate the tethering and rolling of flowing leukocytes on vascular surfaces. It has been suggested that force could either shorten bond lifetimes, because work done by the force could lower the energy barrier between the bound and free states ('slip'), or prolong bond lifetimes by deforming the molecules such that they lock more tightly ('catch'). Whereas slip bonds have been widely observed, catch bonds have not been demonstrated experimentally. Here, using atomic force microscopy and flow-chamber experiments, we show that increasing force first prolonged and then shortened the lifetimes of P-selectin complexes with P-selectin glycoprotein ligand-1, revealing both catch and slip bond behaviour. Transitions between catch and slip bonds might explain why leukocyte rolling on selectins first increases and then decreases as wall shear stress increases. This dual response to force provides a mechanism for regulating cell adhesion under conditions of variable mechanical stress.

Variation in proteoglycan metabolism by articular chondrocytes in different joint regions is determined by post-natal mechanical loading

In this study we investigated the hypothesis that cartilage from defined regions of ovine stifle joints, which were subjected to differing mechanical stresses, contained phenotypically distinct chondrocyte populations. Chondrocyte phenotypes were identified by the relative biosynthesis of the proteoglycans (PGs) aggrecan, biglycan and decorin. Articular cartilage (AC) from adult and neonatal ovine stifle joints were examined. Cells were cultured as both full-depth AC explants and in alginate beads after their isolation from the AC matrix. When chondrocytes from the various topographical regions of adult ovine knee joints were cultured as explants they demonstrated a consistent difference with regard to the metabolism of aggrecan and decorin. Significantly, this topographically-dependent phenotypic expression of PGs was preserved when the chondrocytes were cultured in alginate beads. In adult joints, chondrocytes from the central region of the tibial plateau not covered by the meniscus, which is subjected to high mechanical loads in-vivo, synthesized less aggrecan but more decorin than cells from regions covered by the meniscus. When chondrocytes from identical AC regions of neonatal ovine joints were cultured as explants, no topographical difference in aggrecan nor decorin metabolism could be detected. The results of this study, in association with the existing literature, lead us to propose that post-natal mechanical loading of AC could select for chondrocyte clones or induce a lasting modulation of chondrocyte phenotypic expression in different joint regions. Such cellular changes could result in the synthesis of PG populations that confer properties to AC most suited to resist the variable mechanical stresses in the different joint regions. This study serves to emphasize the importance of using cartilage from identical joint areas when examining PG metabolism by chondrocytes. Further investigation into the relationship between mechanical loading, regional chondrocyte phenotype selection and the response of these cells to anabolic and catabolic factors may provide important insights into the focal nature of AC degeneration in osteoarthritis.

Fatigue of gas-turbine blades in random vibrations

The critical parts of modern machines are subjected to a complex combination of loads. With improvement in machines the degree of stress of the parts and assemblies increases. Therefore, to evaluate the reliability it is necessary to study in more detail the structure and amount of the effective variable mechanical stresses and the resistance to these stresses of machine parts. Many critical aircraft parts and assemblies (the rotating blades of helicopters, aircraft airfoils and skins, and the blades and other parts of aircraft gas-turbine engines) are subjected to random loads which may be characterized as narrow-band random oscillatory processes [i].

Dynamic stress state of blades of gas-turbine-engine turbocompressors

In order to evaluate the safety factor and fatigue life of turbocompressor blades, the authors study the loading process and the subsequent establishment of a rate of fatigue damage accumulation. This was done by accounting for the variable mechanical stresses and constructing histograms depicting their distribution. The process of blade vibration was studied by means of spectral analysis. The authors show that it is possible to calculate equivalent harmonic stresses which are satisfactorily consistent with the actual histograms of the double amplitudes of the acting variable mechanical stresses.