Effect Of Prestress Research Articles

Decoupling Uniaxial Tensile Prestress and Waveguide Effects From Estimates of the Complex Shear Modulus in a Cylindrical Structure Using Transverse-Polarized Dynamic Elastography.

Dynamic elastography, whether based on magnetic resonance, ultrasound, or optical modalities, attempts to reconstruct quantitative maps of the viscoelastic properties of biological tissue, properties altered by disease and injury, by noninvasively measuring mechanical wave motion in the tissue. Most reconstruction strategies that have been developed neglect boundary conditions, including quasi-static tensile or compressive loading resulting in a nonzero prestress. Significant prestress is inherent to the functional role of some biological tissues currently being studied using elastography, such as skeletal and cardiac muscle, arterial walls, and the cornea. In the present article a configuration, inspired by muscle elastography but generalizable to other applications, is analytically and experimentally studied. A hyperelastic polymer phantom cylinder is statically elongated in the axial direction while its response to transverse-polarized vibratory excitation is measured. We examine the interplay between uniaxial prestress and waveguide effects in this muscle-like tissue phantom using computational finite element simulations and magnetic resonance elastography measurements. Finite deformations caused by prestress coupled with waveguide effects lead to results that are predicted by a coordinate transformation approach that has been previously used to simplify reconstruction of anisotropic properties using elastography. Here, the approach estimates material viscoelastic properties that are independent of the nonhomogeneous prestress conditions without requiring advanced knowledge of those stress conditions.

Effect of Membrane Load on the Stability of an Electrostatically Actuated Initially Curved Circular Micro Plate

Abstract The effects of membrane load on the axisymmetric bistable behavior of circular curved micro plates are studied via an axisymmetric reduced order (RO) model, incorporating radial prestress. The model is based on Kirchhoff’s hypothesis and Föppl-von-Kármán’s kinematics. The model is first validated for a “mechanical” (displacement-independent) load, against an RO model with 20 degrees-of-freedom (DOF), a finite differences (FD) solution, and finally, a finite elements (FE) model, serving as the reference. All solutions implement the “Riks” continuation method to track unstable branches, which can swerve in a complex form due to the presence of higher buckling modes. A convergence study is carried out for the snap-through location and load, as well as for the critical elevation and prestress required for bistability. Based on the validated results of the mechanical analysis, the reliability of the model for predicting the effect of prestress on the plate behavior under nonlinear (displacement-dependent) electrostatic load is then investigated while using FD as the reference. The study furnishes an expended RO model for curved plates, which includes the effect of prestress. The resulting model can further be used to estimate the value of residual membrane load present in electrostatically actuated curved plates, as well as predict the threshold for bistability.

Influence of the prestressed layer on spherical transducer in sound radiation performance

To improve the acoustic radiation performance of the spherical transducer, a prestressed layer is formed in the transducer through fiber winding. The influence of the prestressed layer on the transducer is studied from the effects of the radial prestress ([Formula: see text][Formula: see text]) and acoustic impedance, respectively. First, a theoretical estimation of [Formula: see text][Formula: see text] is established with a thin shell approximation of the prestressed layer. Then, the acoustic impedance is measured to evaluate the efficiency of sound energy transmission within the prestressed layer. Further, the ideal effects of [Formula: see text][Formula: see text] on the sound radiation performances of the transducer are analyzed through finite element analysis (FEA). Finally, four spherical transducers are fabricated and tested to investigate their dependence of actual properties on the prestressed layer. The results show that with the growth of [Formula: see text][Formula: see text], the acoustic impedance of the prestressed layer grows, mitigating the enormous impedance mismatch between the piezoelectric ceramic and water, while increasing attenuation of the acoustic energy, resulting in a peak value of the maximum transmitting voltage response ([Formula: see text]) at 1.18 MPa. The maximum drive voltage increases with [Formula: see text][Formula: see text], leading to a steady growth of the maximum transmitting sound level ([Formula: see text]), with a noticeable ascend of 3.9 dB at a 3.44 MPa [Formula: see text][Formula: see text]. This is a strong credibility that the prestressed layer could improve the sound radiation performance of the spherical transducer.

Biaxial Tensile Prestress and Waveguide Effects on Estimates of the Complex Shear Modulus Using Optical-Based Dynamic Elastography in Plate-Like Soft Tissue Phantoms.

Dynamic elastography attempts to reconstruct quantitative maps of the viscoelastic properties of biological tissue, properties altered by disease and injury, by noninvasively measuring mechanical wave motion in the tissue. Most reconstruction strategies that have been developed neglect boundary conditions, including quasi-static tensile or compressive loading resulting in a nonzero prestress. Significant prestress is inherent to the functional role of some biological tissues, such as skeletal and cardiac muscle, arterial walls, and the cornea. In the present article a novel configuration, inspired by corneal elastography but generalizable to other applications, is studied. A polymer phantom layer is statically elongated via an in-plane biaxial normal stress while the phantom's response to transverse vibratory excitation is measured. We examine the interplay between biaxial prestress and waveguide effects in this plate-like tissue phantom. Finite static deformations caused by prestressing coupled with waveguide effects lead to results that are predicted by a novel coordinate transformation approach previously used to simplify reconstruction of anisotropic properties. Here, the approach estimates material viscoelastic properties independent of the nonzero prestress conditions without requiring advanced knowledge of those stress conditions.

Compression-only behavior: Effect of prestress and shell rheology on bifurcation diagrams and parametric stability of coated microbubbles in an unbounded flow

Compression only is a counterintuitive response pattern of lipid coated microbubbles that are strain softening by nature and normally exhibit a bias towards expansion during volume pulsation. It is a result of shell viscoelastic behavior and arises for shells with low bending resistance and shear viscosity. During sonication such microbubbles buckle when the shell is prestressed, leading to oscillations around deformed shapes of lower volume. The latter are energetically favored and arise due to parametric mode excitation. Lower values of shear shell viscosity enhance this pattern that can be used to improve protocols for the acoustic characterization of shell mechanical properties.

Prey localization in spider orb webs using modal vibration analysis

Spider webs are finely tuned multifunctional structures, widely studied for their prey capture functionalities such as impact strength and stickiness. However, they are also sophisticated sensing tools that enable the spider to precisely determine the location of impact and capture the prey before it escapes. In this paper, we suggest a new mechanism for this detection process, based on potential modal analysis capabilities of the spider, using its legs as distinct distributed point sensors. To do this, we consider a numerical model of the web structure, including asymmetry in the design, prestress, and geometrical nonlinearity effects. We show how vibration signals deriving from impacts can be decomposed into web eigenmode components, through which the spider can efficiently trace the source location. Based on this numerical analysis, we discuss the role of the web structure, asymmetry, and prestress in the imaging mechanism, confirming the role of the latter in tuning the web response to achieve an efficient prey detection instrument. The results can be relevant for efficient distributed impact sensing applications.

Weakly nonlinear two dimensional wave scattering in incompressible pre-stressed materials: Far field approximation

We develop a series expansion approach to the calculation of the scattering coefficients of in-plane transverse elastic waves. The waves are scattered by a cylindrical obstacle as they propagate through an incompressible nonlinear material which is inhomogeneously pre-stressed. We deal with the calculation using a distorted wave Born approximation in the far field. A neo-Hookean strain energy function is considered for the incompressible elastic material, and a rigid cylinder is considered as an obstacle. From the static equilibrium configuration, a small-on-large approach is constructed via a series expansion of the wave equation which allows us to compute both the scattered wave and the induced pressure field, and their dependence on the applied pre-stressed. In the far field limit, the pre-stress effect on the scattering coefficients is calculated via partial waves. The formalism can be expanded directly to three-dimensional media with different obstacle symmetries.

Effects of pre-stress on the mechanical properties and microstructure of neutron-irradiated high-purity aluminum

Although structural materials in nuclear reactors are usually serviced under pre-stressed conditions, the irradiated mechanical properties and microstructures for prestressed metals have not been well understood due to the experimental challenges. In this study, the annealed high-purity aluminum was subjected to a low temperature (313 K) neutron irradiation under a stress of 1.5 MPa, which was ten percent of its unirradiated yield strength. Post-irradiation tensile tests were carried out at 300 K with a strain rate of 1.1 × 10−3 s−1. A strong radiation-induced hardening as well as a loss of uniform ductility was observed on all the specimens, and the pre-stress could defer irradiation hardening and embrittlement and decrease plastic instability stress, which was involved with a possible asymmetry distribution of voids induced by the pre-stress. Transmission electron microscopy (TEM) was conducted on the samples to quantify the void size and density as a function of pre-stress and dose. It was shown that the pre-stress could accelerate the void nucleation, increase the density but decrease the size of voids. Although previous studies have reported that the dislocation channel could be probably attributed to the micro-mechanism for irradiation embrittlement, none of dislocation channels were discovered in the deformed irradiated high-purity aluminum samples, which implied that the dislocation channel is unnecessary for irradiation embrittlement. In addition, a neutron irradiation embrittlement model for metals, which had been built on the basis of Johnson-Cook constitutive model, was found applicable to metals irradiated with pre-stress. The stimulated results also well agreed with the experimental data.

Frequency domain analysis of pre-stressed elastomeric vibration isolators

Two types of elastomeric vibration isolators used for equipment vibration isolation in aerospace vehicles are considered for the present study. These isolators are constructed using elastomers mounted in steel encasings. These isolators are initially deformed statically and dynamic loads are applied on the deformed configuration. To capture the static deformation, equivalent static load corresponding to its load rating and specified displacements are created. Static deformation is computed using Finite Element methods with four node axi-symmetric element which include the geometric non-linear effect for steel and with standard Yeoh hyper-elastic material model for elastomers (Muhammed and Zu, 2012) [1]. Yeoh material constants are derived from uni-axial tension test data of the elastomer specimen. These isolators are subjected to harmonic and random excitations in the pre-deformed state. For numerical analysis, elastomeric constants at dynamic conditions are obtained as complex function of frequency using Dynamic Mechanical Analyzer (DMA) for a range of frequencies. The standard material model of Yeoh is modified incorporating frequency dependant material characteristics and damping in the range of frequencies of interest. A multiplicative non-separable variables law is derived for Yeoh material model to include the effect of static pre-stress, based on the methodology given in literature (Nashif et al., 1985; Beda et al., 2014) [2,3]. The modifications of Yeoh model suitable for frequency domain analysis is the novelty in the present study. In the analysis, while dynamic loads are applied, the configuration is updated considering initial static loading. The frequency response of the isolators is computed using material properties evaluated at progressive dynamic strains until a match in natural frequency is observed. Appropriate damping corrections are then incorporated to match the test observed transmissibility. Then updated material properties are used to compute the random response which showed good agreement with results of experiments, validating the approach taken for the development of this model.

Supershear surface waves reveal prestress and anisotropy of soft materials

Surface waves play important roles in many fundamental and applied areas from seismic detection to material characterizations. Supershear surface waves with propagation speeds greater than bulk shear waves have recently been reported, but their properties are not well understood. Here we describe theoretical and experimental results on supershear surface waves in rubbery materials. We find that supershear surface waves can be supported in viscoelastic materials with no restriction on the shear quality factor. Interestingly, the effect of prestress on the speed of the supershear surface wave is opposite to that of the Rayleigh surface wave. Furthermore, anisotropy of material affects the supershear wave much more strongly than the Rayleigh surface wave. We offer heuristic interpretation as well as theoretical verification of our experimental observations. Our work points to the potential applications of supershear waves for characterizing the bulk mechanical properties of soft solid from the free surface.

Dynamic progressive fracture behavior of axially confined sandstone specimens containing a single flaw

Rock masses are frequently subjected to dynamic disturbance in addition to pre-stress, which plays a key role in the stability and safety of underground projects. In this paper, the effects of flaw inclination angle and pre-stress on the mechanical properties and fracture behavior of sandstone specimens under repetitive impact loadings were investigated through a series of laboratory tests using a modified split Hopkinson pressure bar (SHPB) system. The results show that the dynamic strength decreases gradually with the increasing repetitive impact number. A larger flaw inclination angle produces fewer repetitive impacts for a given axial pre-stress. The repetitive impact counts and the cumulative absorbed energy peak at an axial pre-stress of 20–40 MPa. By the digital image correlation (DIC) technique, high strain localizations around the flaw periphery can be observed after the application of pre-stress. During the repetitive impact loadings, wing and anti-wing cracks occupy the dominant position. Compressive cracks also contribute to the ultimate failure if the specimen is subjected to solely repetitive impacts. Except for 90° specimens, shear cracks make a difference as well in the cracking process as the pre-stress increases.

Blast energy absorption in topological interlocking elastic columns

This study investigates blast dynamics of prestressed segmented columns assembled of osteomorphic interlocking elastic blocks without any binding materials or mechanical connectors. The unique ability of the interlocking structures to absorb and dissipate energy through the independent movement of their segments is analyzed using the nonlinear finite element method. The effects of the number of blocks, the axial prestress, the friction, and the level of interlocking between the contact surfaces on the flexural response and energy absorption capability of the segmented column are investigated parametrically. It is demonstrated that the detachments between blocks control the energy absorption capability of these structures. Therefore, the number of blocks, the axial prestress, and the level of interlocking (i.e., the parameters influencing the available detachment between blocks) define the response of this column to blast loads. Columns segmented into a greater number of blocks with lower prestress levels exhibit a higher damping capability but reduced column stiffness, which is usually undesirable in structural applications. A higher level of interlocking between the contact surfaces of blocks increases the damping capacity of the column without reducing its stiffness. The effect of friction is found to be insignificant for all cases considered. The effect of interlocking is small relative to the effects of prestress and the number of blocks. However, it provides an independent design parameter for the passive mitigation of blast loads.

Parametrical Analysis for Symmetrical Loading of a Single-Span Composite String Steel Structure

The article analyzes the string and composite string bridge behavior and compositional parameters. The derived parameters are calculated and presented by selecting the appropriate limit parameters. The main parameters for suspended structures are the cross-sectional area of the tensile element and the initial sag of the structure. The selection of different component parameters and assigning different prestressing to the string leads to the analysis: which cross-sectional areas meet the defined conditions, what is the effect of prestressing, of the lower cable initial sag and the axial stiffness ratios of the string and the lower cable on the weight of the structure.

Closed-Form Solution of Fundamental Frequency of Steel-Concrete Hybrid Wind Turbine Tower

With the development of wind power energy, the steel-concrete hybrid tower has been gradually popularized as the onshore tall wind turbine tower. To avoid the resonant response that brings violent damage to the wind turbine tower, the fundamental frequency of the hybrid tower must be carefully analyzed. This study develops a closed-form solution of the fundamental frequency of the hybrid tower, where the abrupt changes of cross-section and material properties, the effect of prestressing, and the mass and rotary inertia of the wind turbine are taken into account. It provides an effective way to obtain the fundamental frequency in the preliminary design of the hybrid tower. The effectiveness of the proposed solution is validated by comparing it with the results of finite element analysis, showing good agreement. By using the proposed solution, a parametric analysis is conducted to investigate the variation rules of the fundamental frequency to the design parameters. It indicates that with the variation of the proportion of the concrete/steel segments, the correlations between the fundamental frequency and the design parameters also change significantly. The parametric analysis conducted herein may provide instructive guidelines to adjust the parameters in the preliminary design of the hybrid tower.

Enhancement of compressive strength and strain ductility of SMA fiber reinforced concrete considering fiber’s aspect ratios

In this study, the effect of recovery stress on the toughness of cementitious materials was investigated using randomly distributed SMA fibers in concrete. The composite was reinforced by cold drawn crimped SMA fibers due to their great potential in developing a prestressing effect as well as providing sufficient bond resistance. To this end, considering the same volumetric fraction, different aspect ratios of the SMA fibers were embedded into the matrix and then molded into different reinforced concrete cylinders. Compression tests were performed where several parameters, including compressive strength, ductility, axial, and volumetric strain were examined before and after inducing the prestressing effect. Furthermore, the present study dealt with fracture modes, as well as internal and surface crack analysis via X-ray and digital image correlation (DIC) equipment to investigate the effect of prestressing with different aspect ratios. The results showed that not only the current geometry of SMA fibers in concrete could improve the ductility of the composite, but also elevate the compressive strength demonstrating a considerable rise in energy absorption capacity.

Identification and assessment of seismic damage for RC containment structures considering prestressing effect

Containment structures shall remain viable even after strong ground motions to save lives and valuables. Therefore, the current study exploits the prestressing effect to mitigate seismic damage and prevent function and structural failure during seismic events. The seismic response of the studied reinforced concrete containment (RCC) structure is well captured and investigated. The results reveal that the prestressing could significantly mitigate the seismic damage and the RCC structure’s bottom is the most vulnerable region. Hence, the prestressing value might be designed to mitigate the seismic damage of the RCC structure not only to keep its integrity under overpressure. Moreover, damage assessment using different damage indices has been presented, and a modification for the Park-Ang damage index has been formulated. Furthermore, a new formula has been proposed to predict the RCC structure’s damage ratio accurately, and a new classification for the damage states has been developed to identify the performance level.

Experimental investigation on cantilever square CFST columns under lateral continuous impact loads

Concrete-filled steel tube (CFST) columns can be used in passive flexible protective engineering to improve the bearing capacity of the overall protective system. This paper conducted the lateral continuous impact resistance of cantilever square weathering concrete-filled steel tube (WCFST) columns and compared it with that of the normal CFST. Considering the effect of steel tube wall thickness and prestress on the lateral impact resistance of such components, a total of 10 specimens were subjected to impact tests. The damage form and failure mode of each member were studied in detail, and the effects of the different impact mass and the height of the impact point were considered. The dynamic strain, displacement, and acceleration of each member under different impact loads were analyzed and compared. It was found that tested members show local damage at the impact point, bent failure through whole members, and bulged or fractured at the root. Slippage occurs between the internal concrete and the steel pipe wall of the member without prestressing, and this phenomenon always occurred at the second impact. Results show that when the height of the impact point of members is 0.30 h, the position of the peak displacement point is 0.76 h, where h stands for column height. The application of prestressing can significantly reduce the dynamic strain, displacement, and acceleration of members. Under the same circumstances, applying to the prestress of cantilever members can control the final deformation of it more than increasing the wall thickness. Moreover, the smaller the wall thickness of the component, the better the effect of applying to prestress to control the plastic displacement. The final cumulative plastic displacement of the member is positively correlated with its stiffness degradation after each impact.

Breakdown Voltage of Polymeric HVDC Insulation at DC Stress and Superimposed Lightning Impulse Voltages

Space charge formation makes it difficult to predict the resulting electric field distribution within HVDC cable insulation.The main purpose of the work presented here has been to experimentally study how the lightning impulse breakdown strength of thin (0.1 mm thick) insulation foils of polypropylene (PP) and Polyethylene Terephthalate (PET) are affected by DC prestress, polarity of the impulse voltage and the temperature.Tests were performed using parallel plate electrodes and prior to DC prestressing the lightning impulse breakdown strengths were found to be approximately 17.5 kV for samples of both PP and PET. Prior to superimposing lightning impulse breakdown voltages, a DC voltage of 8 kV was applied for 60 seconds. The effect of DC prestressing was to significantly increase the impulse breakdown strength during aiding and reduce the breakdown strength during opposing impulse polarity. This strongly support the assumption of homo-charge formation close to the metal electrodes during DC stress.The results showed higher and more temperature dependent charge accumulation in samples of PET. It is indicated that at 22 °C the homo-charge formed during DC stressing approximately reduced the electric field at the electrodes from 100 to 48 kV/mm in the PP samples and from 80 to 22 kV/mm in the PET samples, respectively

Distributed displacement estimation of pre-stressed concrete beams with random cracks using long-gauge strain sensing

Large pre-stressed concrete structures are usually confronted with random cracks and down-warping. Aiming to these structure diseases, a strategy for estimating distributed displacement of pre-stressed concrete beams with random cracks based on long-gauge strain sensing is presented in this article. First, an indirect method of calculating displacement of beams considering the presence of random cracks and pre-stress effect is derived. The method only relies on the collected strain responses and material parameters, and is irrelevant to external load. Then, a pre-stressed beam subjected to static load is designed for prove the effectiveness of the strategy. In the experiment, variable neutral axis position, curvature, concrete stress and vertical displacement are extracted under different load steps. Experiment results indicate that the estimated displacement has large relative error if random cracks are neglected. When the influence of random cracks is taken into account, the identified displacement agrees well with the directly monitored one by displacement sensors. Meanwhile, the results using different sensor arrangement are also compared. The presented strategy without the necessity of reference point could be an alternative for displacement monitoring of pre-stressed structures.

Experimental Study on Flexural Performance of the Prestressed Glulam Continuous Beam after Long-Term Loading

The study of the long-term behavior of the prestressed continuous beam is vital for the design and the appliance of wood structures in engineering. In this study, long-term experiments were first conducted to determine the long-term creep behavior. Afterward, the prestress of the long-term beam was regulated to the initial state, and we carried out short-term flexural experiments to explore the effect of prestressing regulation. The influences of prestressed value and the number of prestressed steel wires on the mechanical properties of the continuous beam were investigated and discussed. The experimental results demonstrate that the creep reduced the stress in the steel wires, weakened the effect of prestressing, and increased the tensile stress at the bottom of the beam, which led to a reduction in the bearing capacity of the beam. The prestressing regulation could increase the moment arm, so the bearing capacity of the beam was improved.