Wide Variety Of Loading Conditions Research Articles

Bidirectional graphics-based digital twin framework for quantifying seismic damage of structures using deep learning networks

Tremendous effort has been devoted toward developing automated post-earthquake inspection techniques, including automated image collection and damage identification. However, few studies have attempted to establish the complex relationship between visible damage and structural conditions. Moreover, the lack of training data further hinders the potential use of deep learning algorithms. This paper proposes a framework, termed Bidirectional Graphics-based Digital Twin (Bi-GBDT), that allows for assessment of the structural condition based on post-earthquake photographic surveys. The procedure contains two parts: (i) A GBDT of a target structure, containing a computer graphics model and a finite element (FE) model, is developed (ii) Synthetic data subsequently is generated from the GBDT to train neural networks to predict the damage measures and structural conditions. To demonstrate the proposed approach, a seismically-designed reinforced concrete shear wall is considered. First, the GBDT of the shear wall is constructed and calibrated so that the associated damage patterns simulated by the corresponding FE model match well with the experimental results. Next, synthetic images of the damage patterns are created from the validated GBDT, along with the corresponding structural damage measures, and used to train Residual Neural Network and Conditional Generative Adversarial Networks to determine the maximum drift, the stress/strain fields and the structural condition. The neural networks trained on synthetic data are shown to perform well for the experimental data, confirming the proposed approach. Subsequently, the neural networks are tested on the synthetic data for a wide variety of loading conditions to demonstrate the robustness of the approach. In addition, the realistic images rendered from the GBDT are utilized as input to predict the structural condition, showcasing the comprehensive Bi-GBDT framework. These results demonstrate the efficacy of the proposed approach to generate accurate digital twins and point toward its future application for development of automated post-earthquake assessment strategies.

Structural-time nature of the dynamic instability of the fracture process

Principal effects of dynamic instability of the fracture process are investigated. Particularly an issue of a dependence of stress intensity factor (K) on crack velocity (v) is discussed. Uniqueness, related to the given material, and even existence of this dependence has been a matter of discussion among researchers, since contradicting experimental results have been reported in the literature over last decades. In this paper results of numerical simulations of the crack propagation process are presented. A numerical scheme based on finite element method and incubation time fracture criterion was developed. Scattering of the K values during the propagation process is considered to be a feature principally related to a spatial-temporal nature of the fracture process. It is found that quasistatic loading is characterized by a small scatter of the K values and fitting of the data can be performed in order to obtain some well-known K - v curve. In the case of a pulse loading the scatter is much higher and it can be concluded that a wide range of K values correspond to a particular crack velocity and no unique continuous K - v curve can be associated with the process. These results are supported by well-known experimental observations. Thus, it is proved that the incubation time fracture criterion makes it possible to investigate dynamic crack propagation for a wide variety of loading conditions (quasistatic, high rate and short pulse loading) and only one extra material parameter – the incubation time is needed to predict a big variety of effects of the dynamic instability of the fracture process. This is a huge advantage comparing to a widespread approach, which involves complicated experimental determination of material strain rate dependencies and an a priori given K - v relationship.

Partial slip contact of a rigid pin and a linear viscoelastic plate

This paper analyzes partial slip contact problems in the theory of linear viscoelasticity under a wide variety of loading conditions, including cyclic (fretting) loads, using a semi-analytical method. Such problems arise in applications like metal-polymer contacts in orthopedic implants. By using viscoelastic analogues of Green’s functions, the governing equations for viscoelastic partial-slip contact are formulated as a pair of coupled Singular Integral Equations (SIEs) for a conforming (pin-plate) geometry. The formulation is entirely in the time-domain, avoiding Laplace transforms. Both Coulomb and hysteretic effects are considered, and arbitrary load histories, including bidirectional pin loads and remote plate stresses, are allowed. Moreover, the contact patch is allowed to advance and recede with no restrictions. Viscoelasticity necessitates the application of the stick-zone boundary condition in convolved form, and also introduces additional convolved gap terms in the governing equations, which are not present in the elastic case. Transient as well as steady-state contact tractions are studied under monotonic ramp-hold, unload-reload, cyclic bidirectional (fretting) and remote plate loading for a three-element solid. The contact size, stick-zone size, indenter approach, Coulomb energy dissipation and surface hoop stresses are tracked during fretting.Viscoelastic fretting contacts differ from their elastic counterparts in notable ways. While they shakedown just like their elastic counterparts, the number of cycles to attain shakedown states is strongly dependent on the ratio of the load cycle time to the relaxation time. Steady-state cyclic bulk hysteretic energy dissipation typically dominates the cyclic Coulomb dissipation, with a more pronounced difference at slower load cycling. However, despite this, it is essential to include Coulomb friction to obtain accurate contact stresses. Moreover, while viscoelastic steady-state tractions agree very well with the elastic tractions using the steady-state shear modulus in load-hold analyses, viscoelastic fretting tractions in shakedown differ considerably from their elastic counterparts. Additionally, an approximate elastic analysis misidentifies the edge of contact by as many as 7 degrees in fretting, showing the importance of viscoelastic contact analysis. The SIE method is not restricted to simple viscoelastic networks and is tested on a 12-element solid with very long time scales. In such cases, the material is effectively always in a transient state, with no steady edge-of-contact. This has implications for fretting crack nucleation.

Ultimate compressive strength of deteriorated steel web plate with pitting and uniform corrosion wastage

Steel structural members are likely exposed to corrosive environments, and thus corrosion is one of the dominant life-limiting factors of steel structures. Extensive studies on the effectsof pitting and uniform corrosion on the strength performance of steel structural members under a wide variety of loading conditions have been undertaken to assess the relationship between pitting corrosion intensity and residual strength. The aim of this study is to investigate the ultimate compressive strength characteristics of steel web plate elements with pit and uniform corrosion wastage. A series of ABAQUSnonlinear elastic-plastic large deformation finite element analyses are carried out on I-shapedsection steel girder models with varying pittingcorrosion intensities. Artificial pitting of different intensities is considered on the web plates and a uniform loading applied vertically on the upper flange section. The ultimate load-carrying capacity of deteriorated models with different levels of uniform thickness loss is also studied. The results are applied to assessing the ultimate compressive strength of web plates with different pitting corrosion intensities and a uniform loss thicknessby developing design formulae that represent the average loss thickness versus the ultimate load-carrying capacity.

The design and validation of a magnetic resonance imaging-compatible device for obtaining mechanical properties of plantar soft tissue via gated acquisition.

Changes in the mechanical properties of the plantar soft tissue in people with diabetes may contribute to the formation of plantar ulcers. Such ulcers have been shown to be in the causal pathway for lower extremity amputation. The hydraulic plantar soft tissue reducer (HyPSTER) was designed to measure in vivo, rate-dependent plantar soft tissue compressive force and three-dimensional deformations to help understand, predict, and prevent ulcer formation. These patient-specific values can then be used in an inverse finite element analysis to determine tissue moduli, and subsequently used in a foot model to show regions of high stress under a wide variety of loading conditions. The HyPSTER uses an actuator to drive a magnetic resonance imaging-compatible hydraulic loading platform. Pressure and actuator position were synchronized with gated magnetic resonance imaging acquisition. Achievable loading rates were slower than those found in normal walking because of a water-hammer effect (pressure wave ringing) in the hydraulic system when the actuator direction was changed rapidly. The subsequent verification tests were, therefore, performed at 0.2 Hz. The unloaded displacement accuracy of the system was within 0.31%. Compliance, presumably in the system's plastic components, caused a displacement loss of 5.7 mm during a 20-mm actuator test at 1354 N. This was accounted for with a target to actual calibration curve. The positional accuracy of the HyPSTER during loaded displacement verification tests from 3 to 9 mm against a silicone backstop was 95.9% with a precision of 98.7%. The HyPSTER generated minimal artifact in the magnetic resonance imaging scanner. Careful analysis of the synchronization of the HyPSTER and the magnetic resonance imaging scanner was performed. With some limitations, the HyPSTER provided key functionality in measuring dynamic, patient-specific plantar soft tissue mechanical properties.

A comparative analysis of numerical approaches to the mechanics of elastic sheets

Numerically simulating deformations in thin elastic sheets is a challenging problem in computational mechanics due to destabilizing compressive stresses that result in wrinkling. Determining the location, structure, and evolution of wrinkles in these problems has important implications in design and is an area of increasing interest in the fields of physics and engineering. In this work, several numerical approaches previously proposed to model equilibrium deformations in thin elastic sheets are compared. These include standard finite element-based static post-buckling approaches as well as a recently proposed method based on dynamic relaxation, which are applied to the problem of an annular sheet with opposed tractions where wrinkling is a key feature. Numerical solutions are compared to analytic predictions of the ground state, enabling a quantitative evaluation of the predictive power of the various methods. Results indicate that static finite element approaches produce local minima that are highly sensitive to initial imperfections, relying on a priori knowledge of the equilibrium wrinkling pattern to generate optimal results. In contrast, dynamic relaxation is much less sensitive to initial imperfections and can generate low-energy solutions for a wide variety of loading conditions without requiring knowledge of the equilibrium solution beforehand.

Characteristic failure initiation sites in plain weave textile composites

Failure initiation locations were determined in the tows and matrix pockets of a plain weave textile composite for a wide variety of loading conditions in an attempt to identify characteristic failure initiation sites. Failure was found to initiate in a limited number of locations in the composite for a wide variety of loading conditions. It was also found that the different degrees of weave undulation studied shared similar characteristic failure initiation sites. Additionally, it was observed that some idealized features of the textile geometry led to fictitious stress concentrations that could artificially bias the predicted failure initiation locations. Steps were taken to reduce the influence of such stress concentrations, including modifying the textile geometry and excluding certain problematic regions of the textile from consideration when searching for failure initiation locations.

Computational geomechanics: The heritage of Olek Zienkiewicz

AbstractGeotechnical engineering design is based on mathematical, constitutive and numerical models implemented in computer codes which are able to predict the behaviour of geostructures and foundations under a wide variety of loading conditions, sometimes demanding or extreme. Prediction is fundamental in areas such as seismic engineering, marine foundations or landslide hazard analysis. No computer code is better than the mathematical, constitutive and numerical models which it implements. This paper is devoted to (i) reviewing the decisive contributions of Professor Olek Zienkiewicz to the development of mathematical, constitutive and numerical models in geotechnical engineering and (ii) presenting some extensions based on his work which have been proposed by the authors. The extensions to be described here are the following: (i) a particularization of the u − prmw Zienkiewicz model to the case of fast catastrophic landslides (ii) an extension of the generalized plasticity model proposed by him and (iii) an SPH numerical model for the propagation of fast catastrophic landslides. Copyright © 2011 John Wiley & Sons, Ltd.

Modeling, design and analysis of low frequency platform for attenuating micro-vibration in spacecraft

One of the most important factors that affect the pointing of precision payloads and devices in space platforms is the vibration generated due to static and dynamic unbalanced forces of rotary equipments placed in the neighborhood of payload. Generally, such disturbances are of low amplitude, less than 1 kHz, and are termed as ‘micro-vibrations’. Due to low damping in the space structure, these vibrations have long decay time and they degrade the performance of payload. This paper addresses the design, modeling and analysis of a low frequency space frame platform for passive and active attenuation of micro-vibrations. This flexible platform has been designed to act as a mount for devices like reaction wheels, and consists of four folded continuous beams arranged in three dimensions. Frequency and response analysis have been carried out by varying the number of folds, and thickness of vertical beam. Results show that lower frequencies can be achieved by increasing the number of folds and by decreasing the thickness of the blade. In addition, active vibration control is studied by incorporating piezoelectric actuators and sensors in the dynamic model. It is shown using simulation that a control strategy using optimal control is effective for vibration suppression under a wide variety of loading conditions.

Homogenization-based constitutive models for porous elastomers and implications for macroscopic instabilities: II—Results

In Part I of this paper, we developed a homogenization-based constitutive model for the effective behavior of isotropic porous elastomers subjected to finite deformations. In this part, we make use of the proposed model to predict the overall response of porous elastomers with (compressible and incompressible) Gent matrix phases under a wide variety of loading conditions and initial values of porosity. The results indicate that the evolution of the underlying microstructure—which results from the finite changes in geometry that are induced by the applied loading—has a significant effect on the overall behavior of porous elastomers. Further, the model is in very good agreement with the exact and numerical results available from the literature for special loading conditions and generally improves on existing models for more general conditions. More specifically, we find that, in spite of the fact that Gent elastomers are strongly elliptic materials, the constitutive models for the porous elastomers are found to lose strong ellipticity at sufficiently large compressive deformations, corresponding to the possible onset of “macroscopic” ( shear band-type) instabilities. This capability of the proposed model appears to be unique among theoretical models to date and is in agreement with numerical simulations and physical experience. The resulting elliptic and non-elliptic domains, which serve to define the macroscopic “failure surfaces” of these materials, are presented and discussed in both strain and stress space.

Effect of pitting corrosion on strength of web plates subjected to patch loading

Pitting corrosion is typical corrosion observed on coated hold frames in way of cargo holds of bulk carriers, which exclusively carry coal and iron ore. Extensive survey on the effect of pitting corrosion on structural strength under a wide variety of loading conditions is necessary to clarify the relationship between pitting intensity and residual strength in detail. In the present study, a series of tests has been conducted on structural models which consist of web, shell and face plates to investigate the effect of pitting corrosion on strength of web plates subjected to patch loading. In these tests, artificial pitting was made on the web plates and two equal concentrated loads have been applied vertically at the one third points of simply supported models. It was found that web crippling behavior is strongly affected by the pit distribution on the web plates. According to the FE-analyses following the experiment, ultimate strength of the web plates with pitting under patch loading is a little smaller than or almost the same as that of the web plates with uniform corrosion in terms of average thickness loss.

Active capacitor multiplier in Miller-compensated circuits

A technique is presented whereby the compensating capacitor of an internally compensated linear regulator, Miller-compensated two-stage amplifier, is effectively multiplied. Increasing the capacitance with a current-mode multiplier allows the circuit to occupy less silicon area and to more effectively drive capacitive loads. Reducing physical area requirements while producing the same or perhaps better performance is especially useful in complex systems where most, if not all, functions are integrated onto a single integrated circuit. Die area in such systems is a luxury. The increasing demand for mobile battery-operated devices is a driving force toward higher integration. The enhanced Miller-compensation technique developed in this paper helps enable higher integration while being readily applicable to any process technology, be it CMOS, bipolar, or BiCMOS. Furthermore, the technique applies, in general, to amplifier circuits in feedback configuration. Experimentally, the integrated linear regulator (fabricated in a 1-/spl mu/m BiCMOS process technology) proved to be stable for a wide variety of loading conditions: load currents of up to 200 mA, equivalent series resistance of up to 3 /spl Omega/, and load capacitors ranging from 1.5 nF to 20 /spl mu/E The total quiescent current flowing through the regulator was less than 30 /spl mu/A during zero load-current conditions.

Real-time emulation for power equipment development. Part 2: The virtual machine

Power electronic control of electromechanical systems has become common. These systems employ electronic components which switch at high frequency and have very complex interactions. The load which they supply is often intricate in itself and difficult (or often impossible) to create in an experimental environment. It would be advantageous to replace the electromechanics with a solid-state equivalent which can be flexibly programmed to emulate the real system. The paper is concerned with describing this idea, illustrating the concept by emulating an electric motor and its associated mechanical load. The phrase ‘virtual machine’ has been coined to describe the system. It gives/takes power from the electronic converter to match as closely as possible the real electrical machine and load. The virtual machine is effectively a dynamically controllable source/sink which is capable of providing a bidirectional power level interface to a power electronic converter. Using the virtual machine, a power electronic converter can be tested in diverse applications and under a wide variety of loading conditions without the need for any electromechanics. Test results show that close agreement between the virtual and real systems is possible. The limits imposed by practical implementation of the concept are also examined.

Analysis of short-latency reflexes in human elbow flexor muscles.

1. A motor and digital controller have been developed to apply rapid stretches to the human elbow joint. The digital controller returns the forearm to the initial position before the reflex contraction. Thus short-latency reflex responses can be cleanly separated in time from the mechanical effects of the stretch under a wide variety of loading conditions. 2. The reflex force varies linearly with the velocity of stretch over nearly 2 orders of magnitude. The reflex force also varies linearly with the tonic level of force over the entire range of forces studied (0-100 N). This contrasts sharply with, for example, the human ankle joint, which shows a very limited linear range. 3. As the digital controller is made more compliant (less stiff), reflex shortening increases dramatically and becomes more prolonged, whereas the reflex force becomes somewhat smaller and shorter. With compliant loads and the brief stretches we applied, the reflex shortening is approximately equal to the stretch that generated it. 4. Simulations of the results confirm that the dependence of reflex shortening and force on the stiffness of the load is mainly determined by the mechanics of the limb and muscles. The simulations also indicate that 1) the gain of the reflex is as high as it can be without causing instability and 2) the presence of a rectification nonlinearity (e.g., lengthening the muscle produces a reflex, but shortening the muscle does not) is mainly responsible for preserving the stability of the elbow system.

A New Constitutive Model for Several Metals Under Arbitrary Temperature and Loading Conditions

A new theory of viscoplasticity is described that models yielding as a random phenomenon. A circle on the deviatoric stress plane represents the intensity of yielding with the radius equal to the random yielding microstress. This random model does not utilize a yield surface; yielding intensity is quantified by expected values defined in the deviatoric stress plane. The circle in the deviatoric stress plane with a random radius is a simple way to model multi-axial loading. Approximations for stress, strain energy density, and plastic strain energy density are used to improve the computational efficiency of parameter selection and to quantify the flow criterion. The exact state equations are derived, which can be manipulated to describe a wide variety of loading conditions for a broad temperature range. Reversed loading, stress relaxation, creep, and nonproportional loading are all natural properties of the model, which require little additional elaboration. Material properties were specified for five metals, three at room temperature and two over a wide temperature range.

Extensive modeling of high temperature deformation for cyclically softening material

A unified inelastic constitutive equation is proposed to describe the cyclic inelastic deformation of 2 1 4 Cr-1Mo steel at an elevated temperature of 550°C. In the present study, the evolution of overstress is discussed which is shown to lead to a time-independent term in the inelastic strain rate. This term is proportional to the stress rate. Another significant point proposed in this study is the distinction of the inelastic deformation depending on the evolution of overstress. A mathematical model has been developed based on these assumptions. Many numerical experiments were conducted to examine the validity of the present model, and in comparison with the experimental results, the present model has been shown to give good descriptions for a wide variety of loading conditions, including rate-dependent plasticity, stress relaxation, and cyclic softening.

Finite-element solutions of crack growth in incompressible elastic-plastic solids with various yielding extents and loadings: Detailed comparisons with analytical solutions

We have conducted numerical finite element studies of plane strain quasistatic crack growth in elastic-plastic material for a wide range of applied loading conditions and yielding extents, especially general yielding. To facilitate precise comparisons with previous analytical results, we have employed a fully incompressible, nonhardening material model. A reduced/selected integration procedure is successfully used to enforce material incompressibility. For crack growth under bending-dominant conditions, we employ an experimentally-measured applied load versus crack length history for a compact tension specimen that experiences crack growth from small-scale yielding through general yielding conditions. A constant crack tip opening angle crack growth criterion is employed to investigate crack growth under tension-dominant loadings in the same geometry. We have also conducted a small-scale yielding crack growth simulation employing a highly refined mesh, and several additional general yielding stationary crack solutions to further explore the effects of different far-field loading combinations. Detailed comparisons of the finite element results with Drugan and Chen's [1] ‘m-family’ of asymptotic analytical solutions are made in an effort to assess the latter's accuracy and range of applicability, and to identify their asymptotically indeterminate parameters m and R as functions of crack growth history. Among several interesting results, we find that Drugan and Chen's near-tip characterizing parameter has a nearly constant value of m ≈ 1.23 for the entire crack growth process from small-scale yielding through general yielding conditions under bending-dominant loading when specimens have traction-free sides. However, we find m to vary significantly from that value as general yielding conditions are approached in tension-dominant loading situations, and whenever specimen sides are subjected to uniform applied loading. The numerical solutions confirm that Chen and Drugan's [2] global approximate analytical solutions for general yielding crack growth are remarkably accurate to substantial distances from the crack tip under a wide variety of loading conditions. The fully incompressible material model employed also facilitates great physical insight into the global stress and deformation fields accompanying general yielding crack growth: numerous figures display the slip lines (which are characteristics for both the stress and velocity fields) throughout the plastically deforming regions.

Fabric‐reinforced cement, folded plate

Vistarp Karbhari and Robert Nicholls, Professor of Civil Engineering at the University of Delaware, show that sandwich panels, having fabric‐reinforced cement mortar faces with triangularly folded cores, can be designed to maximize flexural stiffness per unit material cost. The authors suggest that a wide variety of loading conditions could be satisfied from stock sheet, folded to structural engineers’ individual designs.

Damping Measurements of a Controlled Composite Material

Increased use of filamentary materials as reinforcements in structural components has necessitated obtaining information on the response of such material systems to a wide variety of loading conditions. One such technically important problem area is that associated with the dynamic response of composite materials to impact-type loading situations. In order to understand the phenomenon of wave trial in material systems, specific knowledge of the damping coefficient and phase velocity as a function of circular frequency is necessary. In the present paper, tests have been conducted on beam-type specimens of a steel-epoxy composite having controlled volume fractions and varying wire sizes. Measurements are reported on the damping coefficient as a function of circular frequency, a(ω), and the real part of the complex modulus, E1. The latter parameter has been further used to calculate the phase velocity, and data are presented on phase velocity, c(ω), as a function of frequency. Detailed tabular and graphical data are presented for the various specimens tested.