Axial Constraint Research Articles

Effect of Accelerated Aging on the Dynamic Mechanical Properties of RDX Based Explosives

Abstract To reveal the effect of accelerated aging on the mechanical properties of RDX based explosives, dynamic compressive experiments under passive confinement were conducted using a split Hopkinson pressure bar system. The stress-strain behavior of passively confined explosives at different strain rates (500/s∼1100/s) and different aging times (0day∼28days) was obtained. The results show that the compressive mechanical properties of RDX based explosives exhibits obvious strain rate hardening effect, especially the yield strength of unaged specimens. With the increase of aging time, the axial stress corresponding to a specified axial strain before yielding increases. The nonlinear relationship between axial yield stress and constraint ratio is established, and the fitting results are in good agreement with the experimental results for both unaged and aged explosives.

A thermal flexible rotor dynamic modelling for rapid prediction of thermo-elastic coupling vibration characteristics in non-uniform temperature fields

The flexible rotors within aero-engines operate in complex thermal environments, where temperature influences both the vibration frequency and amplitude. This study establishes a simple thermal flexible rotor dynamics model to rapidly and precisely predict thermo-elastic coupling vibration characteristics within a non-uniform temperature field. The thermal potential energy of the thermal rotor element is derived for any temperature field, and the motion equation is obtained using the Euler-Lagrange equation. Specifically, the generalized vector of an arbitrary point and cross-sectional non-uniform thermal stress of the thermal rotor element are considered in the thermal potential energy. The model's frequency error is <1 % under identical boundary conditions. Numerical findings indicate that thermal stress, temperature-dependent material properties, and the coupling effect collectively reduce the natural frequency (NF), with thermal stress having a more pronounced impact under axial constraint. Additionally, thermal stress and material decrease the amplitude across a broad range of rotation speeds, contrasting with thermal bending. This model will play a key role in the iterative calculation of thermo-elastic coupling vibration control due to its accuracy and simplicity.

Mechanism clarification and mitigation measures of radial deformation induced by girth welding of thin-walled pipes

Girth welding is extensively utilized in connecting pipeline structures. Excessive welding deformation may result in a decrease in ultimate bearing capacity and fatigue life. This research explores the characteristics and underlying mechanics of radial deformation in thin-walled girth welded pipes and proposes mitigation measures. Initially, several 304 stainless steel pipes were joined utilizing four distinct welding procedures. Welding deformation and residual stress distribution were measured using a 3D coordinate measuring machine (CMM) and the Cos α X-ray diffraction (XRD) method, respectively. Subsequently, numerical models were developed to simulate the welding process and verify the accuracy of the fusion zone, residual stress field, and radial deformation in the models. The mechanism of radial deformation was further elucidated based on the inherent strain method. The results show that the direction of radial deformation relies substantially on the hoop plastic strain. Hoop tensile plastic strain corresponds to radial outward deformation, while hoop shrinkage plastic strain corresponds to radial inward deformation. In addition, the influence of axial constraint on the formation process and results of hoop plastic strain, as well as the effect of heating zone width on radial deformation, was studied. Through simulation outcomes, the research suggests employing a regulated heat source method to mitigate radial deformation, which could potentially reduce deformation by 92.8%, offering considerable advantages in deformation mitigation and welding efficiency augmentation.

Quantum Dot-Based Three-Dimensional Single-Particle Tracking Characterizes the Evolution of Spatiotemporal Heterogeneity in Necrotic Cells.

Cell death is a fundamental biological process with different modes including apoptosis and necrosis. In contrast to programmed apoptosis, necrosis was previously considered disordered and passive, but it is now being realized to be under regulation by certain biological pathways. However, the intracellular dynamics that coordinates with cellular structure changes during necrosis remains unknown, limiting our understanding of the principles of necrosis. Here, we characterized the spatiotemporal intracellular diffusion dynamics in cells undergoing necrosis, using three-dimensional single-particle tracking of quantum dots. We found temporally increased diffusion rates in necrotic cells and spatially enhanced diffusion heterogeneity in the cell periphery, which could be attributed to the reduced molecular crowding resulting from cell swelling and peripheral blebbing, respectively. Moreover, the three-dimensional intracellular diffusion transits from strong anisotropy to nearly isotropy, suggesting a remodeling of the cytoarchitecture that relieves the axial constraint on intracellular diffusion during necrosis. Our results reveal the remarkable alterations of intracellular diffusion dynamics and biophysical properties in necrosis, providing insight into the well-organized nonequilibrium necrotic cell death from a biophysical perspective.

Effects of boundary conditions on RC beams subjected to accidental thermal load in nuclear power plants

Reinforced concrete (RC) beams in nuclear safety related structures should be designed for accidental thermal gradient combined with mechanical loads. Design capacity and structural behavior of those beams are affected by constraint boundary conditions at beam ends. In design approaches widely used in nuclear power plants (NPPs), it is commonly assumed that the thermal curvature of a RC member is fully constrained while axial constraint condition is often not specified. In this paper, RC beams with various constraint boundary conditions that are subjected to accidental thermal gradient combined with mechanical loads are studied using both experimental tests and nonlinear finite element (NLFE) models. Both rotational and axial stiffness ratios at boundary and their effects on thermal moments and ultimate flexural capacity of RC beam are evaluated. Results shows that the assumption of a fully constrained rotational boundary condition is practically accurate for most RC beams in NPPs subjected to thermal gradient. On the other hand, results also show that axial constraint boundary condition has considerable influence on both thermal moment and usable design flexural strength thus neglecting its effect may lead to insufficient design of RC beam. In addition, this study established relationship between axial constrain stiffness ratio and thermal moment, as well as relationship between axial constrain stiffness ratio and usable design flexural strength for different accidental temperature gradients and reinforcement ratios. That information can be used for correction of thermal moment and usable design flexural strength in design practice.

Theoretical model of energy absorption for square tubes under transverse loading with variable axial constraint condition

Thin-walled tubes in the actual engineering structure (e.g., autobody) are usually connected to other structure, and the boundary constraint condition at both ends of tubes are usually very complex when bending deformation occurs under transverse loading. This paper aims at the effect and mechanism of axial constraint on the bending energy absorption effect of thin-walled tube under limited and variable axial constraint condition at both ends. A typical of thin-walled tube, square tube, is taken as an example. Firstly, we compare the experiment results of square tube under transverse loading with three-point-bending condition and fully clamped condition, revealing that the axial constraint force has a significant effect on the energy absorption effect. After that, a FE model of the square tube under variable axial constraint condition at both ends is established and validated based on experiment. Based on the validated FE model, the effect of different sizes of axial constraint stiffness on the bending energy absorption of square tube are investigated, and it is found that it has a significant effect on the deformation mode and bending energy absorption. With the increase of axial constraint stiffness, it will mainly affect the latter segment of the force–displacement curve in the bending process, which increases the bending energy absorption. Finally, theoretical models are developed to predict the force–displacement curves and bending energy absorption of the square tube under three different types of variable axial constraint properties: linear axial constraint, unequal linear axial constraint and nonlinear axial constraint.

Failure mechanism of steel frames with angle steel-bolted connections exposed to fire under progressive collapse condition

Partially restrained connections comprise intricate systems of multiple components, each exhibiting varying degrees of performance degradation under fire conditions. Consequently, the damage patterns within these connections are reconfigured. Moreover, owing to the insulating effect of floor slabs, the thermal and mechanical states of remaining structures differ in hogging and sagging bending moment regions after removing critical members, thereby influencing the potential damage modes and collapse-resistant mechanisms. To elucidate this impact mechanism, a comprehensive analysis of the influence parameters on the failure mechanism and collapse-resistant action of a typical partially restrained connection (angle steel-bolted connection) steel frames exposed to fire was conducted based on multi-scale models. The results revealed that steel frame structures with different connection configurations all enter the compressive arching action (CAA) stage after the heating phase due to the thermal expansion behavior of beams. After the completion of load application, the deformations of all three connection models remain below 0.2 rad. The variation of beam load magnification factor α does not affect the structural failure mode, while a higher column load ratio causes the damage location to shift from the connection to the column under fire conditions and triggers a chain failure reaction. Compared to axial constraint kas, the rotational constraint krs is a critical factor influencing the damage mode. The negative axial force generated by the thermal expansion behavior of the fire-affected region can enhance the flexural action resistance in the unheated region by employing the CAA. Finally, a theoretical method is proposed to predict the failure mechanism of steel frames with angle steel-bolted connections exposed to fire under progressive collapse condition, providing a novel approach for identifying weak components of similar structures and improving anti-collapse capabilities.

Progressive collapse behaviour of composite substructure with large rectangular beam-web openings

This study explores the progressive collapse resistance of composite substructures with the fully-welded connection (FC) and fully-welded connection with rectangular web openings (FCRO). Quasi-static loading tests were conducted to analyze failure modes, internal forces, and resistance mechanisms. The effect of substructure's boundary constraints on its anti-collapse behaviour was investigated through validated numerical models. The results show that the damage mode of the FC specimen involves tensile beam flange fracture at the beam root, while the FCRO specimen experiences fracture of the rectangular web openings. The resistance development of the FCRO specimen includes bending, bending-catenary mixed, and catenary stages, whereas the FC specimen undergoes bending and bending-catenary mixed stages. In the bending-catenary mixed stage, the FCRO specimen exhibits a 69.2% higher maximum resistance and a 15.3% larger displacement compared to the FC specimen. Despite the reduction in flexural capacity, incorporating large rectangular openings in the beam web enables plastic hinge to shift outward at the beam root, promoting the effective evolution of the composite beam’s axial force. The study derived expressions for the boundary axial constraint coefficient and attenuation coefficient to the FCRO specimen. Traditional strengthening methods were found to have limited effectiveness in enhancing the collapse resistance of unreinforced composite substructures. Consequently, a novel and effective construction measure was proposed, resulting in a significant enhancement of 78% in maximum resistance and 85% in corresponding deformation of the substructure.

Cyclic loading test of earthquake-resilient steel frame joints with different connection forms

Three earthquake-resilient steel frame joints with different connection forms were investigated in this study. The three joints were first tested via cyclic loading tests. The test phenomena of the hinged joint were found to be similar to those of the bolted joint. Owing to the double-hinge connection, the instability of the energy dissipator occurred after its slippage. The cyclic performance of the bolted joint was the greatest among that of the three joints. Furthermore, the energy dissipation of the double-hinge joint was approximately identical with that of the hinged joint. The load–displacement relationships of the three joints were then deduced; a bilinear mechanical model was adopted for the joints. The theoretical results agreed with the test results. Thus, the monotonic load–displacement relationships can be used for evaluating the cyclic performance of the three joints. Ultimately, the test process was reproduced via finite element analysis, which was used to further analyze the difference of the three connection forms. The same amount of energy dissipation was observed for the energy dissipators. It was observed that the release of axial constraint has less effect on the cyclic performance of the joint.

Vibrations of columns in the thermal postbuckling state accounting for temperature effect on material properties

The article is concerned with natural frequencies of thermally buckled columns. The analysis accounts for the effect of temperature on the modulus of elasticity and the coefficient of thermal expansion of the material of the column. The columns are simply supported and two cases of axial boundary conditions include an elastic axial constraint and prescribed axial displacements. It is demonstrated that neglecting the effect of temperature on material properties leads to a significant quantitative mistake at low temperatures and a qualitative mistake at high temperatures. In flexible columns with axially constrained boundaries subject to a prescribed thermal loading the frequencies increase with an increasing temperature, but this trend is reversed at high temperatures due to the sharp drop of the modulus of elasticity. In the contrary, in the columns subject to a prescribed axial displacement in addition to thermal loading, frequencies remain relatively stable and subsequently decrease at high temperatures. It is demonstrated that the frequencies of columns with a low or moderate slenderness ratio are affected by the axial constraint to a much larger degree than those in more flexible columns.

Utilization of two‐equation model for analysis of flow over a moving needle in the presence of magnetic field

AbstractMany applications of the ocean and aerodynamic engineering, like the design of submarines, missiles, aircraft, rockets, and jet engines, involve flow over surfaces. Moreover, several experimental propulsion methods for seagoing ships, submarines and aerospace crafts are based on magnetohydrodynamics (MHD). This research explores the MHD boundary layer flow over a moving needle (shaped like a paraboloid of revolution). Governing equations are modelled in cylindrical coordinates under axial symmetry constraint, which is then simplified by employing order of magnitude analysis. In the absence of a magnetic field, these equations are self‐similar, and this analysis is already available in the literature. We have applied the local non‐similarity method to derive the boundary layer equations up to the second level of truncation to compute the solutions of non‐similar boundary layer equations. The built‐in MATLAB routine bvp5c is used to solve these equations numerically. This analysis will assist in predicting flow quantities that are important in designing the objects mentioned above.

A Generalized Variational Method and Its Applications in Design of the Single-Jack Flexible Nozzle

Abstract Nozzle facilities, which can generate high Mach number flows, are the core portions of the supersonic wind tunnel. Different from traditional fixed nozzles, a flexible nozzle can deform to designed contours and supply steady core flows in several Mach numbers. Due to the high-quality demands from the thermo-aerodynamic testing, the deformation of the flexible nozzle plate should be carefully designed. This problem is usually converted into the large deformation problem of a cantilever with movable hinge boundary conditions. In this paper, a generalized variational method is established to analyze the deformation behavior of the flexible nozzle. By introducing axial deformation constraint and Lagrange multiplier, an analytical model is derived to predict the deformed morphology of the flexible plate. Finite element analyses (FEA) of a single-jack flexible nozzle model is performed to examine the predicted deformations and reaction forces. Furthermore, the large deformation experiments of an elastic cantilever with a movable hinge connection are carried out to simulate the scenarios in supersonic flexible nozzle facility. Both the FEA and experimental results show high accuracy of current theoretical model in deformation predictions. This method can also serve as a general approach in the design of flexible mechanisms with movable boundaries.

An experimental study related to axial constraint of fuel rod under LOCA conditions

The fracture threshold of the fuel decreases if the oxidized Zr alloy cladding is strongly constrained by the spacer grid during quenching in a loss-of-coolant accident. Therefore, the estimation of realistic levels of the axial constraint has been a subject of significant interest on fuel safety. In this study, a test assembly consisting of a PWR-type simulated fuel segment and a 3 × 3 grid piece was heated in steam, cooled, and quenched, and the axial constraint force on the fuel segment was measured. The constraint force of the Zircaloy grid gradually decreased with temperature. Once the Zircaloy grid was heated to > 1060 K, the reduced constraint force had difficulty recovering, and thus the maximum constraint force during cooling and quenching was < 10 N. The constraint force was clearly reduced at > 1070 K during the tests with the Inconel grid. However, the reduced constraint force partially recovered during cooling. As a result, the maximum constraint force during cooling and quenching was 20–50 N for the Inconel grid. In conclusion, oxidation, ballooning, rupture, or eutectic formation would not generally cause an extremely strong constraint, as predicted by previous studies, at the grid position.

On the stability of thin-walled circular cylindrical shells under static and periodic radial loading

In this paper, we study the dynamic behavior of the cylindrical shells subjected to static and time-varying (periodic) radial loading. Flügge-Luré-Byrne shell theory has been used to derive dynamic equations in the form of nonlinear partial differential equations (PDEs). Linearization of PDEs about the steady-state periodic solutions, followed by Galerkin projection, yields a system of ordinary differential equations (ODEs) with time-varying periodic coefficients. Those are similar to the higher-order Mathieu-Hill equations and their stability analyses have been ascertained using Floquet theory. Dynamic instabilities of the cylindrical shells have been presented through stability charts in the plane of forcing parameters, namely, the static component of the pressure, the amplitude, and frequency of the fluctuating component of the pressure. Also, possible sub and super harmonic resonances have been explored through the stability charts. Four different boundary conditions, viz., simply supported, clamped–free, pinned–free (axial constraint at the pinned end) and pinned–free (axial constraint at the free end) have been considered in the present analysis. Presented charts can be very useful during the design of the shell structures subjected to static/dynamic loading.

Experimental–theoretical investigation of the short-term vibration response of uncracked prestressed concrete members under long-age conditions

The short-term vibration response of uncracked prestressed concrete (PC) members under long-age conditions was investigated. No study has examined long-term phenomena of concrete, such as its tendon relaxation, creep, curing, and shrinkage, under the aforementioned conditions. Therefore, a research sub-program was initiated at the National Center for Research on Earthquake Engineering on the basis of an experimental campaign on uncracked PC bridge members initiated in 2015. In particular, a simply supported PC beam composed of a parabolic-bonded tendon and high-strength concrete made in Taiwan was allowed to short and, consequently, long-term prestressing losses measured for approximately 9.5 months. During short-term prestressing losses and after long-term prestressing losses, free short-term transverse vibrations were induced in the PC beam characterized by a second-order initial curvature and an axial end constraint. The time-dependent Young’s modulus of the beam was determined by conducting compression tests on cylinders during the short-term prestressing losses. Moreover, compression tests were conducted on cores drilled along the PC beam’s span after free vibration tests. The theoretical time-dependent Young’s modulus of the beam was evaluated according to the compression test results. The experimental data were compared with a reference Euler–Bernoulli solution and finite-element (FE) model that describe the dynamics of PC girders by considering the effective prestressing force. The fundamental frequencies of the PC beam were mainly sensitive to the variation in its initial tangent Young’s modulus due to the consolidation or hardening of concrete. These frequencies were unaffected by the second-order initial curvature. In conclusion, the reference solution and FE model can accurately predict the short-term dynamics of uncracked PC members exposed for long times.

Torus-like solutions for the Landau–De Gennes model.

In this note we report on some recent progress [10, 11, 12] about the study of global minimizers of a continuum Landau–De Gennes energy functional for nematic liquid crystals in three-dimensional domains. First, we discuss absence of singularities for minimizing configurations under norm constraint, as well as absence of the isotropic phase for the unconstrained minimizers, together with the related biaxial escape phenomenon. Then, under suitable assumptions on the topology of the domain and on the Dirichlet boundary condition, we show that smoothness of energy minimizing configurations yields the emergence of nontrivial topological structure in their biaxiality level sets. Then, we discuss the previous properties under both the norm constraint and an axial symmetry constraint, showing that in this case only partial regularity is available, away from a finite set located on the symmetry axis. In addition, we show that singularities may appear due to energy efficiency and we describe precisely the asymptotic profile around singular points. Finally, in an appropriate class of domains and boundary data we obtain qualitative properties of the biaxial surfaces, showing that smooth minimizers exibit torus structure, as predicted in [16, 24, 25, 39].

Architected material analogs for shape memory alloys

Architected material analogs for shape memory alloys

A fully real-time spline interpolation algorithm with axial jerk constraint based on FIR filtering

Interpolators play a core and important role in CNC systems, and the scheduling of feedrate is a main task of CNC interpolators. It is routine to schedule the feedrate of linear/circular toolpaths because of their regular geometry; however, the feedrate scheduling of spline toolpaths remains challenging due to the freely and unpredictable geometric shape. Most of the existing methods require looking ahead or pre-processing to acquire sufficient toolpath information for generating drive-bounded smooth feedrate profile, which limits the integral machining efficiency. This paper presents a FIR (finite impulse response) filtering-based interpolation algorithm for spline toolpath, thus realizing the fully real-time capability without using time-consuming optimization such as looking ahead or pre-processing. The presented method merely utilizes the curvature radius information of the current interpolation point on the spline toolpath, and the scheduled feedrate can be generated within one step. Furthermore, the global tangential smoothness of the feedrate profile is guaranteed by directly applying convolution with a chain of two cascaded FIR filters. However, the FIR filtering induces interpolation error which plays as a negative cost during the one-step feedrate scheduling. To solve this issue, the principle of the filtering induced error is analyzed, so that it is computed by feed parameters and curvature radius, and the maximum error is added as an extra constraint beyond the commonly considered axial kinematic constraints. Effectiveness of the presented approach is demonstrated by both numerical illustration and experimental tests.

Experimental study on the geomechanical properties and failure behaviour of interbedded shale during SAGD operation

Interbedded shale (IBS) impedes the development of a steam chamber and poses a serious threat to the success of any steam-assisted gravity drainage (SAGD) operation in a heavy-oil reservoir. Breaking up IBS has been a continual challenge for the industry. This study summarizes experimental studies on the geomechanical properties and failure behaviour of IBS during thermal stimulation. IBS cores were collected by means of fully sealed coring in northwestern China. Mineral composition analysis, porosity, and permeability measurement, as well as imbibition tests, were initiated to evaluate the potential of imbibition. First, shear failure behaviour was studied through triaxial compression tests. Second, pore structure and failure processes of IBS were investigated through thermal stimulation experiments. Mudstone IBS was considered as the focus. Its porosity was approximately 7% and permeability was approximately 100 nD while in-situ stress was applied. Although IBS contained considerable smectite components, it didn’t disintegrate under high-effective stress. Last, shear failure behaviour of IBS was observed by computer tomography (CT) scanning when IBS was heated up to 180 °C under constant radial confining pressure and axial constraint of strain. While imbibition causes disintegration of IBS, it is unlikely when under in-situ stress conditions. Fortunately, shear failure can be achieved under uniaxial boundary conditions with rapid heating.

Chiral constrained stent: Effect of structural design on the mechanical and intravascular stent deployment performances

The high mortality of atherosclerosis-caused cardiovascular disease has promoted the requirement of designing novel intravascular stents with comprehensive robust mechanical performances, which is critical for re-constructing the blocked artery and recovering the biomechanical functions of artery vessels during vascular interventional therapy procedures. In this study, a chiral constrained structure with zero/negative Poisson's ratio and robust expansion transformation ability is proposed through combining the axial constraint effect of tetrachiral element and the good stretchability of oscillating wave element along circumferential direction. Afterwards, through applying the theoretical prediction, experimental tests and finite element analysis, the mechanical properties and stenting performances of the chiral constrained structure are explored. The modified tetrachiral topology shows strong orthotropic after integration of oscillating wave strut profile, indicating its capability of handling radial loads without great longitudinal recoil (foreshortening). Finally, stent deployment performances have been evaluated through a highly nonlinear coupling analysis of the stenosed vessels system, in terms of transformation characteristics, dogboning, radial recoil, foreshortening, and plaque prolapse ratio, etc. Especially, remarkable non-conventional performances of anti-dogboning and anti-foreshortening are identified in chiral constrained stent. Synthetically index is generated to compensate the competing behaviors among the above terms, and to improve the comprehensive mechanical performances of chiral constrained stent. These results show the potential in generating a new family of the intravascular stent with chiral mechanical metamaterials and provide a promising clinical application in curing atherosclerosis.