Modeling of critical strain for dynamic recrystallization of niobium microalloyed steels

The critical strain for dynamic recrystallization (DRX) is most important in designing rolling schedules for the refinement of grain size by boundary-induced transformation mechanisms. Modeling of the critical strain for DRX from the stress-strain curves obtained from hot compression was physically built in this paper. The stress-strain behaviour of materials during hot deformation should be a combination of work-hardening and recrystallization softening. Before DRX occurred, the stress-strain behaviours could be described by a constitutive equation in which basic strain hardening and the effect of strain rate and temperature on stress-strain behaviour are included. Once DRX was promoted, obvious deviation between the experimental and calculated stress-strain curves appeared, which denoted the critical strain for DRX. The modeling in this work could be used not only to accurately calculate the critical strain for DRX but also to analyze the dynamic softening behaviours during hot deformation. To validate the calculated results, the stress-strain database was analyzed in the H beam sample deformed at 1000C with a strain rate of 0.1/s, and a critical strain of 0.22 was obtained by this novel method as an example. The calculated result is in good agreement with the experimental data obtained by micrographical observations.

VIRTUAL HULL MONITORING: CONTINUOUS FATIGUE ASSESSMENT WITHOUT ADDITIONAL INSTRUMENTATION

A novel technique to monitor hull stresses using data currently collected on most ships is explored. This technique, referred to herein as virtual hull monitoring, uses global position signals, measured or numerically-modelled wave data, and a database of calculated stress transfer functions. This enables monitoring of short-term stress states and corresponding fatigue damage accumulation for many structural locations, either onboard or at a central location, for an entire fleet. The components, benefits, and limitations of this proposed technique are discussed. Wave buoy and strain gauge measurements from a full-scale naval vessel trial are used in comparisons with hindcast wave data and the calculated stress spectra for one structural location. Close agreement between the wave data sources and corresponding stress spectra warrants further examination of virtual hull monitoring. 

Effect of Stress Shadow Caused by Multistage Fracturing from Multiple Well Pads on Fracture Initiation and Near-Wellbore Propagation from Infill Wells

Summary Multistage fracturing with multiwell pads (MSFMP) is an essential technology for the efficient development of unconventional oil and gas reservoirs, but the reservoir area between two well pads is often not stimulated. Fracture initiation and near-wellbore propagation from infill horizontal wells drilled with different azimuth from the optimal azimuth in the unstimulated area is poorly understood, largely because of the stress shadow (or induced stress) caused by MSFMP. In this study, we propose an integrated method for calculating the stress shadow caused by MSFMP and then determine optimal completion parameters for infill horizontal wells in the unstimulated connecting area between two well pads. First, we develop a theoretical stress shadow model caused by MSFMP on the basis of the dislocation theory. Considering two extreme cases, fully open and completely closed propped fractures, the range of stress shadow in the unstimulated area after MSFMP of 20 horizontal wells in Platform H of tight reservoirs in the Changqing Oilfield, China, is considered as an example. Second, we import the calculated stress shadow into a 3D perforated fracturing model that is built based on the discrete lattice method. Then, we investigate the influence of perforation technology, horizontal wellbore azimuth, phase angle, and injection rate on fracture initiation and near-wellbore propagation. Our results show that this model is capable of calculating stress shadow at any position and then can be used to optimize the fracturing interval for the middle unstimulated area. We find that appropriate perforation and fracturing parameters significantly decrease the complexity of near-wellbore fractures. The models and results presented in this paper provide a new method and new insight for quantifying and optimizing fracture initiation and propagation for infill horizontal wells to maximize reservoir stimulation efficiency.

Strength analysis of warship hull’s bottom loaded by the pressure wave from a non-contact explosion of sea mine explosion of sea mine

The study was based on the analysis of stamina of steel flat bottom section of transport warships, burdened by the spherical pressure wave from the non-contact explosion of TNT at a distance of 20 m under the keel. This study aims to determine the TNT mass required to break the hull. The task was solved by finite element method (FEM) explicite using CAE program [1], in which the hull’s bottom was modelled as thin shell space. The hull’s burden with pressure wave was modelled as a pressure impulse specified by the formula introduced by T.L. Geers, K.S. Hunter and R.S. Price [2]. To describe the material properties, considering high-speed strain, the Johnson-Cook model was used [3]. Therefore, the main goal of the hereby paper is to present how to correctly model the impact of large, concentrated masses of the ship’s equipment on its hull. The study presents the results of the calculated stress and strain states of the analysed section of the construction of the hull.

Tensile Characteristics of Bio-Composite Material Reinforced with Corn Skin

The main focus of the present work was to study corn skin as reinforcement of polyester bio-composite (CSPCs). The effect of reinforcement type, i.e. short fibers and discontinuous chips, on the tensile properties was studied. The corn skin materials were chemically treated with NaOH and added as reinforcement of polyester bio-composite using the hand lay-up fabrication method. Tensile tests were carried out according to ASTM D3039. The tensile strength characteristics of stress and modulus showed a different behavior between the two types of reinforcement due to a slight difference in specimen thickness, which affected the calculated stress and modulus values. Furthermore, from a physical properties point of view, the larger surface area of CSC compared to CSF, which still contains a lignin layer after the treatment with NaOH, could decrease the interfacial bonding between polyester as the matrix and CSC as the reinforcement. The tensile damage characteristics showed brittle behavior, propagataing perpendicular to the loading direction. Matrix cracking and interfacial debonding were identified as the main two damage modes of the CSF bio-composite and the CSC bio-composite, where the final failure was dominated by fiber pull out and chip fracture.

Usefulness of Coulomb Static Stress Changes in Earthquake Hazard Studies: An Example from the Lake Van Area, Eastern Turkey

It has been globally documented over different tectonic environments that Coulomb static stress changes caused by a mainshock can promote or demote stresses along the neighboring faults and thus triggers or delays following seismicity. In the present study Coulomb stress changes of the earthquakes in the Lake Van area are calculated using available data and the likely source faults. The calculated stress change maps demonstrate that the large earthquakes in the Lake Area are mostly stressed by the preceding earthquakes, suggesting earthquake rupture interactions. It is further suggested that Coulomb stress maps could be used for constraining the likely locations of the future large earthquakes and in the earthquake hazard mitigation studies.

A Study on the Shape Selection of Mechanical Fastening for the Repair of Fighter Wing

A study on optimal shape selection of a mechanical fastening for the repair of crack defect of ROK Air Force F-5 fighter wing was conducted. The crack defect occurred in the spar of the wing, and the technical manual does not specify the repair method. However, ROK Air Force decided to develop a repair technology for this defect in consideration of various logistic conditions. Three repair shapes for the proper repair were devised and the finite element analysis was performed to examine the structural safety of these three connection members. As a result of the structural safety review, two connection members except one were structurally safe with safety margins over zero because the calculated stress values were at or below the yield strength level. Therefore, two connection members were determined to be able to use for repair under the condition that the aircraft operated within the design limit load. The results of this study would be very useful if the same defect occurs in long-term aircraft operated by the ROK Air Force.

STRESS FIELD IN A SHEAR ZONE, AND FORMATION OF THE MAIN FAULT

Using the analytical approximation method, we calculated stress field parameters for cases with different relative positions of Riedel shears and loads required for shearing. Considering an internal friction angle of 30°, and the distance between adjacent shears exceeding 0.7 of the characteristic shear length, we estimated the Coulomb stress that can lead to fracturing. In the areas between the shears, it is below the shear strength value. This means that if an increase in the external load is lacking, there are no prerequisites for the formation of new fractures that may connect adjacent shears. If the shears are spaced closer to each other (i.e. at distances less than 0.7 of the shear length), the shear strength is exceeded in the areas between them, and new shears can occur there and connect the Riedel shears to each other. Therefore, in observations of a natural system of Riedel shears, it becomes possible to assess whether this system is sufficiently stable in its current status, or, in case of a critical increase in the Coulomb stress in the areas between adjacent shears, the equilibrium can be easily disturbed, and there is a possibility that the main fault forms in the strike-slip zone under study.

Identification of stress indices for screening of rice cultivars under high temperature

Rice is an important cereal crop and its productivity is being affected by many abiotic and biotic stresses. High temperature affects the rice yield and productivity. Thirty rice genotypes were evaluated in normal and under high temperature stress conditions. There was reduction in grain yield in all the tested genotypes. Based on yield recorded under normal and high temperature conditions, yield based indices were calculated. Stress Susceptibility Index (SSI), Geometric Mean Production (GMP), Mean Production (MP), Yield Index (YI), Modified stress tolerance (K1STI and K2STI) were significantly and positively correlated to yield under both stress and non-stress conditions and could be considered as good selection indices for screening for heat tolerance. Genotypes were ranked based on their tolerant or susceptibility indices and it was noted that among all IET 28412, IET 28397 and IET 28432 exhibited highest mean rank and a lower standard deviation of rank, hence they can be identified as heat tolerant genotypes. Genotypes IET 26468, IET 28393 and Gontrabidhan-3 were identified as highly susceptible to high temperature stress.

Mechanism of crack evolution in nano-indentation of single crystal silicon by atomistic simulations and theoretical analysis

The molecular dynamics (MD) model of nano-indentation process was established to study the crack evolution in single crystal during nano-indentation. Two workpieces with different cracks and one workpiece with no crack were selected for indentation simulation in this study. The parameters of atom displacement, coordination number (CN), temperature, potential energy and loading force in the indentation process are analyzed in detail. Cracks were found to close during nano-indentation. Two modes of crack closure are observed: cooperative displacement and indentation failure. The existence of cracks will affect the size of transformation zone and the coordination number of atoms after indentation. Besides, the existence of cracks will reduce the increase of temperature and potential energy, and the closing mode of cracks is found to affect the value of indentation load. In addition, the change of stress with indentation depth at crack tip is calculated by theoretical model. The calculated stress curves reveal the evolution trend of cracks during indentation. These results provide guidance for the production of silicon wafer with higher surface quality.