Experimental observations and micromechanical modeling of successive-damaging phenomenon in wood cells' tensile behavior

Single wood cells have complex tensile behavior. To gain insight into this complex functionality, the behavior of single wood tracheids was studied under controlled cyclic tensile loading. The cyclic tensile stress-strain curves show that beyond the yield point the tracheid undergoes permanent deformations and its rigidity increases. As in plasticity elastic (or visco-elastic) unloading takes place and energy is dissipated by permanent deformation. Consequently, single tracheids show a load-history dependent behavior. To understand the intervening mechanism in the process of elasto-plastic response of a wood tracheid, a micromechanical based model was developed. This model permits us to describe the influence of non-uniformity of microfibril angle (MFA) and other defects on the wood cell rigidity and to discuss different scenarios, which may occur during the tensile test. Successive damage of the hemicelluloses and lignin matrix and reduction of MFA as mainly responsible for elasto-plastic response of a wood cell were suggested. It should be noted that this paper is part of the research work which has been reported previously (Navi et al. in Wood Sci Technol 29:411-429, 1995; 36:447-462, 2002; Sedighi-Gilani et al. in Wood Sci Technol 39:419-430, 2005

Micromechanical approach to wood fracture by three-dimensional mixed lattice-continuum model at fiber level

To investigate the fracture behavior of wood, the porosity and heterogeneities of its microstructure should be taken into account. Considering these features of wood microstructure in a continuum-based model is still a difficult problem and the lattice model might be an alternative. In the developed mixed lattice-continuum model, the probable crack propagation volume was modeled by defining a three-dimensional lattice of different beam elements and the other regions were considered as continuum medium. Different beam elements of lattice represented the earlywood fibers, latewood fibers, ray cells and bonding medium between the fibers. The proposed model was used to investigate the mechanism of mode I fracture in a small notched wood specimen in RL orientation. The resulting pre-peak and softening curve and also the crack opening trajectory in both cross-section and longitudinal-section in model were in good agreement with the experimental observations. This model shows the importance of considering the three-dimensional and distributed propagation of microcracks and main cracks in fracture stability. It was also shown that in mode I fracture, RL orientation, the main crack propagates in the earlywood rin

MACRM: A Multi-agent Cluster Resource Management System

The falling cost of cluster computing has significantly increased its use in the last decade. As a result, the number of users, the size of clusters, and the diversity of jobs that are submitted to clusters have grown. These changes lead to a quest for redesigning of clusters' resource management systems. The growth in the number of users and increase in the size of clusters require a more scalable approach to resource management. Moreover, ever-increasing use of clusters for carrying out a diverse range of computations demands fault-tolerant and highly available cluster management systems. Last, but not the least, serving highly parallel and interactive jobs in a cluster with hundreds of nodes, requires high throughput scheduling with a very short service time. This research presents MACRM, a multi-agent cluster resource management system. MACRM is an adaptive distributed/centralized resource management system which addresses the requirements of scalability, fault-tolerance, high availability, and high throughput scheduling. It breaks up resource management responsibilities and delegates it to different agents to be scalable in various aspects. Also, modularity in MACRM's design increases fault-tolerance because components are replicable and recoverable. Furthermore, MACRM has a very short service time in different loads. It can maintain an average service time of less than 15ms by adaptively switching between centralized and distributed decision making based on a cluster's load. Comparing MACRM with representative centralized and distributed systems (YARN [67] and Sparrow [52]) shows several advantages. We show that MACRM scales better when the number of resources, users, or jobs increase in a cluster. As well, MACRM has faster and less expensive failure recovery mechanisms compared with the two other systems. And finally, our experiments show that MACRM's average service time beats the other systems, particularly in high loads

A micromechanical approach to the behaviour of single wood fibers and wood fracture at cellular level

Mechanical and fracture behaviors of wood are defined by the morphology and mechanical properties of wood fibers and their bonding medium. Parallel orientation of wood fibers makes them the most influential microstructural elements from the mechanical point of view. On the other hand, in wood fracture, the difference between the properties of fiber and bonding medium (which make weak cleavage plates) plays a more important role. Experiments show that the mechanical behavior of a single wood fiber under axial tension is complex, although the cause of this complexity has still not been clearly understood. In this thesis, in order to explain the mechanism underlying the mechanical behavior of wood fibers and the fracture of wood specimens at fiber level, a micromechanical approach has been used. Confocal laser scanning microscopy was used to investigate the pattern of the distribution of microfibrils in different wood fibers. It was shown that the microfibril angle within a single fiber is non-uniform and this non-uniformity in radial walls of earlywood fibers, which contain the bordered pits, is higher than tangential walls of earlywood fibers and also higher than in latewood fibers. Tensile and cyclic tensile tests on single spruce fibers were carried out and their non-linear and force-history dependent behaviors under axial tension were shown. It was found that the fiber behavior is affected by the range of microfibril angle non-uniformities and other defects. After a certain force limit, wood fiber undergoes irreversible strains and the elastic limit of the fiber increases in the tensile loading. To explain these results, a model based on the assumption of helical and non-uniform distribution of cellulose microfibrils in the fiber and damage of the hemicellulose and lignin matrix after yielding, was proposed. The model indicated that multi-damage and evolution of microfibrils in the damages segments are the main governing mechanisms of the tensile behavior of wood fiber. Difficulties of considering the porous and heterogeneous microstructure of wood in a continuum-based fracture model, led us to develop a mixed lattice-continuum model. The three-dimensional geometry of lattice, composed of different beam elements which represent the bonding medium and alternation of earlywood and latewood fibers, enabled us to detect the propagation of cracks in both cross sections and longitudinal sections at the fiber level. Model showed that in Mode I fracture, parallel to the fibers, the location of the developed crack and the resulting stress-strain curves have a good agreement with the experimental evidence

Microfibril angle non-uniformities within normal and compression wood tracheids

The pattern and extent of variation of microfibril angle (MFA) in normal and compression tracheids of softwood were investigated by using confocal laser scanning microscopy technique. All measurements support the idea that the orientation of microfibrils in single wood tracheids is not uniform. MFA of the radial wall of earlywood tracheids was highly non-uniform and had an approximately circular form of arrangement around the bordered pits (inside the border). Between the bordered pits the measured MFAs were less than the other parts of the tracheid. In the latewood tracheids MFA was less variable. The average orientation of simple pits in the crossfield region was consistent with the mean MFA of the tracheids; however some of the measurements showed a highly variable arrangement in the areas between the simple pits. In many cases the local measured MFAs of compression wood tracheids agreed with the orientation of natural helical cavities of compression wood. Comparing the measured results in different growth rings showed that MFAs in juvenile wood are generally larger than in perfect woo

Within-Fiber Nonuniformities of Microfibril Angle

The pattern and extent of variation of microfibril angle of macerated spruce fibers were investigated by confocal laser scanning microscopy. All measurements supported the idea that the orientation of the microfibrils is not uniform along the radial wall of earlywood fibers. Microfibrils had an approximately circular form of arrangement around the bordered pits (inside the border). Between the bordered pits, lower microfibril angles were measured than in the other parts of the fiber. This phenomenon was interpreted by assuming the existence of crossed microfibrils in these zones. Variation of microfibril angle in earlywood fibers was observed only in the vicinity of the bordered pits, not in the nonpitted zones and tangential walls. Within the latewood fibers, microfibril angle was approximately uniform, even close to the pitted areas. The average orientation of simple pits in the crossfield region was consistent with the mean microfibril angle of the fibers; however, some of the measurements showed a highly variable arrangement in the areas between the simple pits

Hygric properties of Norway spruce and sycamore after incubation with two white rot fungi

In this study, changes in the hygroscopic properties of two main wood species for violin making, Norway spruce and sycamore, after treatment with Physisporinus vitreus and Xylaria longipes were investigated. Swelling and moisture sorption capacity of wood at the growth ring scale were visually and quantitatively assessed by thermal neutron radiography analysis. It was demonstrated that the fungal treatment improved the dimensional stability of both Norway spruce and sycamore, but also increased their moisture adsorption capacity. Dynamic vapor sorption tests and measurements of the changes in dimensions of the specimens in the laboratory were in good agreement with the results of neutron radiography analysis. The main difference between the moisture sorption of the untreated controls and treated wood was observed at high relative humidity, e.g., above 75%. The contradictory behavior of the increased hygroscopicity and reduced swelling was explained by selective degradation of the chemical components and condensation of the moisture content gained in the capillary voids that developed in the cell walls during fungal decay

Effects of Y chromosome Microdeletions on Male Fertility

Background: In the process of human reproduction, spermatogenesis is one of the most important stages, which is controled by special genes on Y chromosome. Previous studies show that some infertile men have microdeletions on Y chromosome, which cause the reduction of sperm count. Three prominent spermatogenesis loci have been identified on the Y chromosome and entitled “azoospermia factors” (AZFa, b, and c). Hereby, this review article aimed to investigate the content of the Y chromosome microdeletions and their importance in male fertility. Methods: Data and information were collected on English-language articles from PubMed and MEDLINE databases. For Persian articles, Persian-language databases, including SID Scientific Database, IranMedex Medical Articles Database, IranDoc (Iran Scientific Information and Documents Research Institute), Magiran Publication Information, and MedLib were investigated. More than 50 articles on Y chromosome microdeletions and infertility published during 2000-2020 were studied. Results: Previous studies implicated that Y chromosome microdeletions in AZFa, AZFb, and AZFc regions are accompanied by defect in spermatogenesis, leading to oligo / azoospermia. Patients with AZFa and AZFb microdeletions present secretory azoospermia and do not have sperm in their seminiferous tubules. Complete AZFc deletion involves region b2/b4, which contains a total of 12 genes. Incomplete deletion of AZFc includes b1/b3, b2/b3 and gr/gr. The most common of which are gr/gr. In men with gr/gr deletion, sperm count and motility were lower than control group. Conclusion: Y chromosomal microdeletions emerged as the most frequent structural chromosome anomaly associated with the quantitative reduction of sperm. The development of assisted reproductive techniques (ART) like intra-cytoplasmic sperm injection (ICSI) and testicular sperm extraction (TESE) helps to bypass the natural barriers of fertilization

Impact of hepatitis B virus and hepatitis C virus infection on sperm parameters of infertile men

Background: Viral hepatitis is one of the health problems which have the effects on the health issues. It seems that hepatitis B virus (HBV) and hepatitis C virus (HCV) infection have negative impacts on the semen quality and male infertility rate. Objective: In this study, we aimed to evaluate the effects of HBV and HCV on sperm quality among Iranian infertile men referred to Royan Institute Reproductive Biomedicine Research Center between 2003 and 2014. Materials and Methods: This retrospective case-control study included 112 HBV positive infertile men and 47 HCV positive infertile men as case group and 112 HBV negative andHCVnegative matched infertile men as a control group. All semen analysis and viral parameters assessment was performed in the central laboratory with the same method and instruments. Results: Sperm count among infertile men with HBV and HCV infection was significantly lower than control group [the mean of the total sperm count 100.95 ± 118.59, 118.22 ± 141.18, 166.27 ± 151.25 (p < 0.001)]. Sperm motility was significantly decreased in HBV and HCV positive men in comparison to the control group [30.97 ± 25.88, 31.09 ± 28.72, 40.87 ± 23.37, respectively (p < 0.007)]. The percentage of normal sperm morphology was significantly higher in control group in comparison to HBV and HCV infected group [the mean of the normal semen morphology 3.23 ± 3.27, 3.70 ± 3.83, 4.51 ± 3.15 p < 0.015]. Although there is a significant decline in liquefaction time in the case group but the viscosity, semen volume, and PH of semen samples were similar in the both case and control groups. Conclusion: Our results suggest that HBV and HCV infection are associated with poor sperm quality