Stiffness Mode Research Articles

Static and Fatigue Full-Scale Tests on a Lightweight Ship Balcony Overhang with Al/Fe Structural Transition Joints

Combination of lightweight and sustainable marine structures represents a crucial step to accomplish weight reduction and improve structural response. A key point when considering the reliability of innovative structural solutions, which should not be neglected, is represented by large-scale experimental investigations and not only by small-scale specimen analysis. The present research activity deals with the experimental assessment of a lightweight ship balcony overhang, which incorporates an aluminium honeycomb sandwich structure and Al/Fe structural transition joints obtained by means of the explosion welding technique. The ship balcony overhang was formerly designed with the aim of proposing the replacement of ordinary marine structures with green and lightweight options. Experimental investigations of a large-scale structure were performed to validate the design procedure and to evaluate the feasibility of the proposed solution. Large-scale bending tests of the ship balcony overhang were performed considering representative configurations of severe loading conditions. The experimental analysis allowed the evaluation of the structure’s strength, stiffness and failure modes. Comparisons with analogous structures reported in the literature were performed with the aim of assessing the benefits and drawbacks of the proposed lightweight structure. Fatigue tests were also performed in order to evaluate the hardening and the hysteresis loops. The collapse modes of the structure were investigated using X-ray radiography. The structural transition joints have experienced no cracks during the static and fatigue tests. The results clearly indicated that the proposed solution can be integrated in new and existing ships, even if made of steel, as the Al/Fe structural transition joints produced by explosion welding can be used to connect the ship structure to the Al honeycomb balcony. The systematic analysis of the experimental results gave valuable data to enhance the design methodology of such structures.

Output-Only Time-Varying Modal Parameter Identification Method Based on the TARMAX Model for the Milling of a Thin-Walled Workpiece.

The process parameters chosen for high-performance machining in the milling of a thin-walled workpiece are determined by a stability prediction model, which needs accurate modal parameters of the machining system. However, the in-process modal parameters are different from the offline modal parameters and are difficult to precisely obtain due to material removal. To address this problem, an accurate time-dependent autoregressive moving average with an exogenous input (TARMAX) method is proposed for the identification of the modal parameters in the milling of a thin-walled workpiece. In this process, a TARMAX model considering external force excitation is constructed to characterize the actual condition in the milling of a thin-walled workpiece. Then, recursive method and sliding window recursive method are used to identify TARMAX model parameters under time-varying cutting conditions. Subsequently, a three-degree of freedom (3-DOF) time-varying structure numerical model under theoretical milling forces and white-noise excitation is established, and the computational results show that the predicted natural frequencies using the proposed method are in close agreement with the simulated values. Finally, several experiments are designed and carried out to validate the effectiveness of the proposed method. The experimental results show that the predicted accuracy of the proposed method using actual cutting forces is 95.68%. Good agreement has been drawn in the numerical simulation and machining experiments. Our further research objectives will focus on the prediction of the damping ratios, modal stiffness, and modal mass.

Experimental assessment of dynamic and static based stiffness indices for RC structures

This paper presents the results obtained from experimental modal analysis and static load testing to examine the suitability of the dynamic parameters to estimate stiffness deterioration severity in RC structures. The study investigates the accuracy of different dynamic-based stiffness indices compared to static-based flexural and shear stiffness indices and how different damage levels and scenarios impact them. The flexural damage scenario was found to have the weakest effect on stiffness deterioration, indicated by the frequency-based stiffness index, while shear damage at quarter-span has the greatest influence. The frequency-based stiffness index indicates a stiffness increase when the damage level reaches 0.15, which is believed to be related to the composite effect in RC structures, as discussed by Abdul Razak and Fayyadh (2013). The combined modal parameters stiffness index revealed that the flexural damage scenario has the weakest influence on stiffness deterioration, while shear damage closer to the support has the highest effect on stiffness deterioration. The main difference between the combined parameters index and the other two dynamic indices is that the combined parameters index detects global stiffness changes through frequency, while the local stiffness changes are detected through the mode shape vectors. The general trend of the dynamic-based indices shows that the MAC-based stiffness index gives the lowest sensitivity for stiffness change for all damage levels, while the combined parameters-based stiffness index gives the highest sensitivity among the dynamic parameters indices to detect the stiffness deterioration for all damage levels. The static-based flexural stiffness index shows lower sensitivity to stiffness deterioration than the static-based shear stiffness index. The static-based stiffness indices show better sensitivity than the dynamic-based indices; however, the static load test is time-consuming, equipment and labour intensive, and causes significant disruptions to the existing use of the structures. Thus, the dynamic parameters-based stiffness indices are more suitable for existing structures.

Influential Article Review — The Effect of Seismic Excitation on the Passive Stiffness of a Seismically Isolated Bridge Deck

This paper examines the performance of modified bridge elements under simulated earthquake excitations. We present insights from a highly influential paper. Here are the highlights from this paper: This study is on the seismic performance assessment of a seismically isolated bridge structure with passive stiffness modification implementation. Performance of the passive stiffness modification approach is compared with semi-active control implementation on the bridge deck. El Centro NS (north-south) and Kobe EW(east￾west) earthquake excitations are used for the dynamic simulations of the passive case. Results are given in a comparative way for the uncontrolled bridge deck, passive stiffness modification implemented and semi￾actively controlled, seismically isolated bridge deck (Yanik & Aldemir, 2018). Frequency dependent acceleration, velocity and displacement response transmissibility ratios are defined to examine the results. For our overseas readers, we then present the insights from this paper in Spanish, French and German.

The Lower Limb Stiffness, Moments, and Work Mode During Stair Descent Among the Older Adults.

Lower limb stiffness strategies and work mode changes between young and older adults during stair descent are unclear. This study investigated the effect of aging on the lower limb stiffness, moments, and joint work mode during stair descent. Twenty young adults and 20 older adults were recruited from the local community for stair descent test. Kinematics and kinetics data were collected by Vicon system and Kistler force plate. The lower limb stiffness, moments, and work mode were calculated and assess between groups. No significant differences in gait parameters were detected between groups. Compared with young adults, older adults have decreased leg stiffness, knee and ankle stiffness, increased peak hip extension moment, hip stiffness, and ankle work contribution. The older adults actively reduce the lower limb stiffness to reduce the risk of injury during stair descent. The hip joint strategy reduces the risk of forwarding falls and ankle joint compensation work mode to make up for the lack of knee extension strength. This provides a reference for the focus of exercise intervention and rehabilitation strategies for older adults.

Tuning skyrmions in B20 compounds by 4d and 5d doping

Skyrmion stabilization in novel magnetic systems with the B20 crystal structure is reported here, primarily based on theoretical results. The focus is on the effect of alloying on the $3d$ sublattice of the B20 structure by substitution of heavier $4d$ and $5d$ elements, with the ambition to tune the spin-orbit coupling and its influence on magnetic interactions. State-of-the-art methods based on density functional theory are used to calculate both isotropic and anisotropic exchange interactions. Significant enhancement of the Dzyaloshinskii-Moriya interaction is reported for $5d$-doped FeSi and CoSi, accompanied by a large modification of the spin stiffness and spiralization. Micromagnetic simulations coupled to atomistic spin-dynamics and ab initio magnetic interactions reveal the spin-spiral nature of the magnetic ground state and field-induced skyrmions for all these systems. Especially small skyrmions $\ensuremath{\sim}50\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ are predicted for ${\mathrm{Co}}_{0.75}{\mathrm{Os}}_{0.25}\mathrm{Si}$, compared to $\ensuremath{\sim}148\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ for ${\mathrm{Fe}}_{0.75}{\mathrm{Co}}_{0.25}\mathrm{Si}$. Convex-hull analysis suggests that all B20 compounds considered here are structurally stable at elevated temperatures and should be possible to synthesize. This prediction is confirmed experimentally by synthesis and structural analysis of the Ru-doped CoSi systems discussed here, both in powder and in single-crystal forms.

Identification of Mistuning Based on Forced Response Measurements and Assessment for a Real Compressor Blisk

Abstract We develop a novel method for the identification of mistuned blisks. The method relies on forced response measurements in a certain target frequency band. Modal stiffness deviations and an overall structural damping coefficient are obtained using a least-squares fit, fully consistent with a component-mode-based reduced-order model. To achieve a high frequency resolution in a very short test, we carry out rapid frequency sweeps and use a noise-robust, wavelet-based estimation of the steady-state response. The forcing is eliminated from the equations governing the mistuning parameters, and it is identified separately to reconstruct the forced response if requested. We first validate the method numerically and then apply it experimentally to a compressor blisk with an alternating (B-A) mistuning pattern. While most previous works on mistuning identification are limited to the lowest 1-2 mode families, we assess the method for the seventh (1C) and eighth (3T) mode family. Measured and reconstructed frequency responses are in good to very good agreement. We conclude that the developed method is useful to obtain high-quality estimates of mistuning parameters, which can then be used for vibration prediction.

Degradation mechanism of steel/CFRP plate interface subjected to overloading fatigue and wetting/drying cycles

The bond strength of the interface between carbon fiber-reinforced polymer (CFRP) and steel is crucial for CFRP-reinforced steel structures. However, the influence of fatigue loading and the service environment on the bond durability of strengthened steel structures requires further investigation. This work focused on the bond degradation mechanism of the steel/CFRP interface subjected to overloading fatigue damage (OFD) and wetting/drying cycles (WDCs). A total of 12 CFRP/steel joints subjected to OFD and/or WDCs was tensioned to failure. Samples were collected from the debonding zone near the loading end for scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The SEM results revealed that cracks induced by OFD, and microcracks and hydrolysis caused by WDCs in the adhesive were the main reasons for interfacial bonding degradation. OFD introduced localized cracks in the adhesive, whereas WDCs caused widely distributed and staggered microcracks in the adhesive. OFD-induced cracks accelerated moisture intrusion and subsequently enhanced the generation of microcracks, leading to a further decrease in the interfacial bond strength. In addition, the EDS results demonstrated a content change in interfacial chemical elements and precipitates at different sampling locations, which also confirmed the above mechanical degradation mechanism. Furthermore, changes in the load capacity, interfacial stiffness, and failure mode were examined, and the aforementioned degradation mechanism was verified.

Compliant Cross-Axis Joints: A Tailoring Displacement Range Approach via Lattice Flexures and Machine Learning

Compliant joints are flexible elements that allow displacement due to the elastic deformations they experience under the action of external loading. The flexible parts responsible for these displacements are known as flexure hinges. Displacement, or motion range, in compliant joints depends on the stiffness of the flexure hinges and can be tailored through various parameters, including the overall dimensions, the base material, and the distribution within the hinge. Considering the distribution, we propose the stiffness modification of a compliant cross-axis joint via the use of lattice mechanical metamaterials. Due to the wide range of parameters that influence the stiffness of a lattice, different machine learning algorithms (artificial neural networks, support vector machine, and Gaussian progress regression) were proposed to forecast these parameters. Here, the machine learning algorithm with the best forecasting was the Gaussian progress regression; this algorithm has the advantage of well-tuning even with small regression databases, allowing these functions to more easily adjust to suit specific data, even if the dataset is small. Hexagonal, re-entrant, and square lattices were studied as flexure hinges. For each, the effect of the unit cell size and its orientation with respect to the principal axis on the effective stiffness were studied via computational and laboratory experiments on additively manufactured samples. Finite element predictions resulted in good agreement with the experimentally obtained data. As a result, using lattice-flexure hinges led to increments in displacement ranging from double to ten times those obtained with solid hinges. The most suitable machine learning algorithm was the Gaussian progress regression, with a maximum error of 0.12% when compared to the finite element analysis results.

Flutter prediction of its occurrence, amplitude and nonlinear behaviour

Flutter prediction of its occurrence, limit cycle amplitude and chaotic behaviour is investigated using a general binary (two degree of freedom) coupled modal model to provide new closed form analytical predictions and insight into the aeroelastic phenomenon. The generalised unsteady flutter model, based on flutter derivatives (for any cross-section) and an approximate continuous bilinear lift curve, is used in a mass decoupled form to formulate and develop analytical criteria for the onset and amplitude of flutter under damping and offset angle of attack. This is based on modal coupling through stiffness and damping terms, to clearly identify and clarify two forms of flutter based on stiffness and viscous mode coupling. Closed form analytical criteria for flutter onset are developed including a new necessary dynamic divergence criterion (analogous to spragging in brake squeal) based on flutter derivatives for any structure. The model is first validated with the well-known classical numerical and experimental airfoil results of Theodorsen and analytical results of Pines, which are also shown to correspond to stiffness mode coupling. In addition, the new modelling enables the detection and corresponding conditions of viscous mode coupling flutter which does not require closely spaced modes. The occurrence and conditions of both stiffness and viscous mode coupling flutter is verified numerically with results highlighting important structural parameters i.e. damping factors which could cause flutter outside classical conditions. In particular, the conditions for viscous mode coupling are newly identified and found to occur at flow velocities substantially lower (up to approximately 10%) than classical stiffness mode coupling values. Dynamic divergence is shown to be the lower bound for the critical flutter speed under a range of conditons. The viscous mode coupling is shown to occur under high levels of torsional damping (20 times the nominal 1% of critical), for the airfoil case, compared to vertical damping, but does not require the modal frequencies to be close. The efficient analytical prediction for flutter amplitude is then numerically verified for Theodorsen's airfoil. The results show that flutter amplitude is primarily dependent upon flow velocity and stall conditions without structural nonlinearity. Finally, a further nonlinear investigation of flutter identifies useful analytical criteria for the occurrence of chaotic flutter which is confirmed numerically. These results, based on a bilinear lift coefficient model, highlight that the occurrence of chaotic flutter can be due to simplified aerodynamics alone i.e. without nonlinear structural stiffness. The closed form analyses provide critical insight into; the occurrence and underlying mechanisms of flutter based on stiffness and viscous mode coupling, dynamic divergence as well as the nonlinear amplitudes and behaviour under an offset angle of attack and stall conditions.

Nonlinear Buckling and Postbuckling Analysis of Tow-Steered Composite Cylinders with Cutouts

The buckling and postbuckling behavior of two composite tow-steered shells with cutouts of different sizes is assessed using nonlinear finite element analysis and compared to experimental measurements. The cylindrical shells are manufactured using an automated fiber placement system, where the shells’ fiber orientation angles vary continuously around the shell circumference from on the crown and keel to on the sides. One shell features thickness variations due to tow overlaps that result from application of all 24 tows during each pass of the fiber placement system. The second shell uses the system’s tow drop/add capability to achieve a more uniform wall thickness without overlaps. Unreinforced cutouts of two different sizes—the first smaller cutout representing a passenger door on a commercial aircraft and the second larger cutout a cargo door—were machined into each of the two cylinders resulting in a total of four test cases. These cylinders were tested in axial compression and buckled elastically in previous work and are now analyzed using nonlinear finite element models to compare bifurcation buckling loads as well as the load–displacement response in the prebuckling and postbuckling regimes. For all four shells analyzed, the prebuckling stiffness, buckling load, and deformation mode sequence throughout the loading–unloading cycle is accurately reproduced by the models. In particular, the shells first buckle locally around the cutouts in a stable (supercritical) manner with only a slight decrease in axial stiffness, which occurs due to the favorable load redistribution facilitated by tow steering. The shells then buckle globally in an unstable (subcritical) manner with diamond-shaped buckles forming to the left and right (circumferential direction) of the cutouts. The buckling load of all shells with cutouts is at least 82% of the buckling load of the pristine shells without cutouts. Overall, the ability to sustain local buckling phenomena, and the relatively small reductions in global buckling load compared to pristine shells without cutouts, demonstrates the great potential of using tow steering to mitigate the adverse effects of cutouts in axially compressed shell structures.

INFLUENCE OF TRIATHLON ON AORTIC STIFFNESS: THE ATLAS STUDY

Objective: Arterial stiffness is a strong predictor of future cardiovascular events and all-cause mortality. Endurance exercise is efficacious in reducing arterial stiffness. Triathlon, i.e., swimming, cycling and running over long to ultra-long distances in a row, is one of the most demanding endurance sports disciplines inducing major cardiovascular adaptations. However, acute and long-term effects of triathlon on arterial stiffness are still unknown. Therefore, we investigated arterial stiffness modification during a triathlon season. Design and method: The ATLAS study is a prospective interventional controlled and age matched study. We included thirty-four 19–39-years-old male long-distance triathletes (Tri) and twenty-nine healthy sex- and age-matched volunteers who were insufficiently active (Ctrl) according to World Health Organization recommendations. All participants attended three visits (V1, V2, V3) each separated by five months intervals, spread over the course of the race season. Each visit included an aortic stiffness evaluation by applanation tonometry. Results: After a race season, aortic pulse wave velocity (aoPWV) was significantly lower in the triathletes group (aoPWV tri = 6,11 ± 0,95 versus aoPWV ctrl = 7,17 ± 1,24 m/s with p = 0,0009). Triathletes had central systolic blood pressure (cSBP) and central pulse pressure (cPP) significantly higher than in control group (respectively, cSBP tri = 122,60 ± 11,43 versus cSBP ctrl = 116,64 ± 6,88 mmHg with p = 0,01, and cPP tri = 54,18 ± 10,04 versus cPP ctrl = 45, 44 ± 7,55 mmHg with p = 0,0002). Triathlon was correlated with a statistically significant positive impact on subendocardial viability ratio (SEVR) (SEVR tri = 162,89 ± 26,55 versus SEVR ctrl = 133,22 ± 26,99 with p = 0,00009). No statistically significant longitudinal variation of aoPWV was observed in triathletes during the race season. Conclusions: These results suggest that triathlon has persistent beneficial effects on arterial stiffness, and even on myocardial perfusion. This improvement cardiovascular health for triathletes seems to reach its peak at the end of the sportive season.

Receptance-based natural frequency assignment of a real fluid-conveying pipeline system with interval uncertainty

The resonance problem of an industrial fluid-conveying pipeline system can be mitigated by shifting or assigning its natural frequencies. However, the desired natural frequencies are difficult to realize using typical numerical model-based parameter optimization technologies because of the modelling errors. Herein, the first theoretical and experimental study is reported on achieving desired natural frequencies of an industrial pipeline system by using measured receptances. This research involves the one-way fluid–structure interaction of the steady flow. On this basis, this paper considers the changeability of working conditions of the pipeline system and characterizes the uncertain flow speed as an interval uncertainty. The primary framework of the classic receptance method is employed, and a novel interval-based frequency assignment method is proposed. This method inherits the advantage of the receptance methodology, in which the determination of the optimal structural modifications entirely relies on the measured frequency response functions. More especially, the obtained stiffness modifications by using the proposed method can improve the robustness of the assignment results to uncertain flow speeds, and then actually achieved values of the assigned natural frequencies have a smaller perturbation. Several numerical examples demonstrate that the proposed method provides effective results. The application of the method to the modification of a real U-shaped fluid-conveying pipeline system gives experimental evidence of its effectiveness. The effective experimental results give the confidence to use the receptance-based structural modification method to improve the dynamical behaviour of real structures.

Research on reliability of mode coupling chatter of orthopedic surgery robot

In this paper, the reliability probability method is introduced into the prediction of mode coupling chatter of the orthopedic surgical robot milling to avoid the influence of the randomness of the input parameters on the chatter prediction results. The reliability of mode coupling chatter is defined to represent the probability of the stability (no mode coupling chatter occurs) of milling system. Probability model (reliability model) of mode coupling chatter is established to predict robotic milling chatter, in which the modal stiffness K x, K y and cutting force coefficient Ks involved in the prediction of chatter vibration during robotic milling are considered as random variables. The fourth moment method is adopted to solve the reliability model of milling system to obtain the reliability level. The mode coupling chatter reliability circle diagram (MCCRCD), which is a contour line with a specified reliability level as a function of the angle between the main stiffness direction and the cutting force and the depth of cut, is presented to identify the mode coupling chatter reliable and unreliable regions. An example studies the influence of random variables on the reliability of milling processing, and the time-domain simulation method and experiment method are used to verify the possibility and effectiveness of using MCCRCD to predict the stability of the robotic milling. It can be concluded that MCCRCD can be effectively used to predict the stability of robotic milling.

Multi-objective structural optimization of the aluminum alloy subway car body based on an approximate proxy model

To improve the comprehensive structural performance and optimize the structural quality of the aluminum alloy subway car body, a multi-objective structural optimization method based on a response surface approximate proxy model was proposed. On the basis of the sensitivity calculation of the objective response to the thickness changes of profiles in different regions of the car body, 21 groups of plate variables with the highest optimization potential were selected for optimization. The optimization experiments were performed on the design variables, and sufficient sample points were selected for the target analysis. Based on the calculated data, second-order response surface models for the bending modal frequency, bending stiffness, and complete mass of the subway car body were constructed considering the optimized profile thickness. After verifying the accuracy of the model, it was used to replace the finite element model for optimization analysis. To achieve feasible optimization results, two schemes with the profile thickness varied in steps of 0.1 and 0.5 mm in the design space were used. After the completion of the calculation, experimental evaluation of the two optimal schemes indicated that the structural performance was consistent with the numerical results.

TEBEM for springing responses of a container ship with forward speed and nonlinear effects in time domain

In this paper, springing responses of a container ship with forward speed in regular waves are studied by a time-domain TEBEM which is generalized to elastic modes. TEBEM is used principally to solve the steady and unsteady flow and the infinity radiation condition is achieved by the Damping Zone Method. In the structure part, the Transfer Matrix Method is used to get the generalized structural mass, damping, stiffness and principal mode of displacement and rotation of vertical oscillation modes which will participate in the subsequent hydrodynamic calculation. In the hydrodynamic part, the fourth-order Runge–Kutta method is used to solve the hydroelasticity equations. The effects of forward speed and 2nd order wave forces generated by 1st order velocity potential in head sea are analyzed. Meanwhile, the variation of different load components against wave direction for corresponding different stations and contribution of different part of ship for different load components in head sea are studied. Compared with the experimental results, the motions and loads of a 6750-TEU container ship with different forward speeds predicted by TEBEM show an acceptable agreement.

Flexural property evaluation of web reinforced GFRP-PET foam sandwich panel: Experimental study and numerical simulation

This work focus on the flexural property of polyethylene terephthalate (PET) foam-filled lattice composite sandwich panels subjected to four-point bending (FPB). The effects of different face sheet and core thicknesses on the flexural properties of the sandwich panels were analyzed. Experimental results indicated that the glass fiber-reinforced polymer (GFRP) webs can effectively prevent the sandwiches from catastrophic failure , the failure modes of the structures are dominated by foam shear, top face sheet compressive, and top face sheet-core debonding. Increasing the face sheet and PET foam thickness can effectively improve the ultimate load of the panels by 88.9% and 115.6%, respectively. An analytical model was employed to accurately predict the bending stiffness , ultimate load, and failure modes of the structures. The errors between theoretical predicted and experimental results are within 10% in terms of bending stiffness and ultimate load. Furthermore, a 3D numerical model was built to understand the failure modes and responses of the structures, a continuum damage material model for PET foam was imported into ABAQUS software by VUMAT subroutine, and Hashin failure criterion was used for GFRP. The simulated failure modes and load-displacement curves were in good agreement with experimental results.

Ex vivo biomechanical characteristics and effects on gap formation of using an internal fixation plate to augment primary three-loop pulley repair of canine gastrocnemius tendons.

To evaluate the effects of using an internal fixation plate to augment primary 3-loop pulley (3LP) repair of canine gastrocnemius tendons (GTs). 48 cadaveric GTs from 24 adult dogs. GTs were dissected free from other tissues, transected, and randomly assigned to 4 groups (n = 12/group). GTs were repaired with 2-0 polypropylene with a 3LP repair alone or a 3LP repair augmented with a 3-hole veterinary cuttable plate (3VCP), a 5-hole veterinary cuttable plate (5VCP), or a 7-hole veterinary cuttable plate (7VC P). Biomechanical loads, construct stiffness, gap formation, and failure modes were compared between groups. Yield, peak, and failure loads were all significantly increased for the 5VCP and 7VCP groups, compared with the 3LP alone group. Increasing plate length from 3VCP to 5VCP and from 3VCP to 7VCP increased yield, peak, and failure loads. No differences were found between the 3LP and 3VCP groups with regard to yield and peak loads, but failure load was increased in the 3VCP group. Loads to create 1-mm and 3-mm gaps were significantly greater for the 5VCP and 7VCP groups, compared with the 3LP alone and 3VCP groups. Mode of plate attachment failure differed among groups. Tendon plate augmentation may be a viable surgical option to increase the strength of the tenorrhaphy in dogs. However, in vivo studies evaluating the effects of plate augmentation on the tendon blood supply and progression of healing are needed prior to clinical application.

Predictions for the perturbed root mean square displacement of a vibrating structure using modal parameters

A harmonically vibrating structure is considered for this study and divided into segments. The objective of the proposed research is to use knowledge from the nominal structure to make predictions as to how the root mean squared (RMS) displacement of the structure will change when either the mass or stiffness of a segment is scaled. Typically, to calculate the value for the modified RMS displacement, a linear solve is required to determine the new displacement vector. Additionally, the Neumann series may be used. However, this approach requires computations of matrix-vector products, which may become expensive for larger Degrees of Freedom (DOF) systems. Here, a method is proposed to predict the RMS displacement for the modified system from scalar values of the nominal system. Since only scalar values from the nominal system are required, predictions for the perturbed system can be made cheaply. The proposed method is based on the modal displacement equation and the assumption that the system is forced near a natural frequency. This assumption is used in Rayleigh’s quotient to approximate the new natural frequency as a function of the approximated modal stiffness and mass. The limitations and accuracy will be explored. [Work supported by ONR Grant N00014-19-1-2100.]

Simulation of subcooled flow instability for high flux research reactors with ATHLET

Abstract The ATHLET code has been improved and validated for the safety analysis of high flux research reactors against experimental data of the Thermalhydraulic Test Loop of Oak Ridge National Laboratory (ORNL). Therefore, an extensive series of “stiff mode” experiments were recalculated. The comparison between experiments and ATHLET posttest calculations shows that the extended code simulates accurately the thermalhydraulic conditions and the flow instabilities in a wide range of inlet velocity, heat and mass flux.