Impulsive Loading Research Articles

Mechanical performance of Continuous/Short carbon Fiber-Reinforced Poly(phenylene sulfide) composites

A method for improving the fracture resistance and mechanical strength behavior of polymer composites by incorporating short carbon fibers into continuous fiber/poly(phenylene sulfide) laminate was explored in this manuscript. This method involves the incorporation of short carbon fibers into continuous carbon fiber thermoplastic laminates, by using the hot compression molding technique. Characterization of a continuous and discontinuous carbon fiber composite material with poly(phenylene sulfide) matrix (S-CCF/PPS laminate) was performed by using tensile, V-Notched Iosipescu, combined load compression and impulse excitation tests. According to the found results for this composite material, it was observed that the tensile strength and the elastic modulus values were 41% and 48% lower than a composite reinforced only by continuous fiber, respectively. The shear strength of the mixed material was found to be 32% lower than a continuous carbon fiber composite. These results were expected since it was used a combination of 50% of continuous and 50% of short carbon fiber, in volume. However, the compression strength of the mixed composite was found to be only 11.9% lower than a laminate reinforced with solely continuous carbon fiber, showing a synergic gain in this case. In addition, the impulse excitation results show that the material mechanical properties are within the expected range, but a high dispersion of the values was observed due probably to the random nature of the discontinuous fiber. Furthermore, it was observed that the failure modes for the composite evaluated in this work are similar to those found for composites processed only with continuous reinforcements and that the models used during the simulations presented similar results to what was found experimentally.

Experimental investigation on the damping characteristics in dry and saturated sands

An experimental study of the behavior of sandy soils having different densities was conducted under the action of a single impulsive load. Dry silica sands of different densities and saturated sand were tested. Different falling masses from different heights were conducted using the falling weight deflectometer to provide the single pulse energy. Variation of damping with different parameters was examined, these are; footing embedment, depth to diameter ratios, diameter of the impact plate, the applied energy and soil density (dense, medium, and loose) in addition to moisture content (dry or saturated sands). The variation of soil density leads to increase the damping ratio increases by about 20-80% when the soil density is varied from loose to dense.

The effect of flow speed on the bubble dynamics: A numerical study

The Naiver-Stokes equations are utilized to investigate the bubble dynamics subjected to the flow speed in the vicinity of a rigid wall. The predictions are compared with the Gilmore method and available experimental results, and a reasonable agreement is obtained. The important flow properties, e.g., bubble morphology, collapse time, jet development, pressure wave propagation, and vorticity evolution are studied in detail. The results demonstrate that the combined effect of the flow speed and rigid wall results in the generation of oblique jet, which may contribute to the development of the fish-scale pits on the surface of the hydraulic machinery. Moreover, our results show that the impulse loads caused by the bubble collapse in moving water are lower than those in stationary water. Finally, it is indicated that the stretching term plays a major role in the vorticity both inside and outside the bubble, while the dilatation term mainly influences the vorticity inside the bubble.

Improved theory of the elastic-plastic response of spherical containment vessels subjected to internal blast loading and its application

Baker built the displacement equations of motion for a thin spherical shell to spherically symmetric internal blast loading, and further provided the analytical solution of the elastic-plastic response under a triangular decaying pressure, neglecting shell thinning and radius variation [1], which was widely used in the fields of analysis and design of explosion containment vessels. However, it appears that some improvements and corrections are needed when we revisit this classic problem. In this paper, the elastic-plastic response of thin spherical shells is investigated mathematically. The complete equations of motion for three models are developed. Then the main ideas of the analytical and numerical methods are presented, as well as some important procedures and results by using Laplace transform techniques, which are convenient to use and generalize. Three typical examples are solved to exhibit differences between our improved theory and Baker’s [1]. Furthermore, our models are employed to gain some insights into response regularities, including the effect of various degrees of strain-hardening on maximum displacements under impulsive loading, and the effect of loading duration on the shell displacement for a perfectly plastic material.

Interactional Aerodynamics of the XV-15 Tiltrotor Aircraft during Conversion Maneuvers

The high-fidelity computational fluid dynamics (CFD) tool CREATETM -AV HELIOS is used to investigate the interactional aerodynamics of the XV-15 tiltrotor in this work. The full vehicle is studied in a quasi-static manner with various airspeeds, nacelle angles, and vehicle attitudes to simulate hover-to-forward flight transition. The rotor is trimmed using CFD/CSD coupling with CAMRAD II. Significant wing-on-rotor interactions are observed where the thickness and loading effects of the wing create an impulsive doublet loading on the rotor as it passes over the wings. Furthermore, the wing is shown to alter the blade–vortex interactions of the rotor at high nacelle angles. The rotor in turn noticeably alters the lift and drag characteristics of the wing. At moderate to low nacelle angles, the rotor downwash enhances the dynamic pressure primarily on the upper surface of the wing, increasing the total wing lift by up to 14%. The effect on the drag varies depending on the nacelle angle. At high nacelle angles, the rotor is shown to decrease both lift and drag of the wing by 15% and 20%, respectively. Overall, this work serves to illuminate and quantify some of the complex aerodynamic interactions that occur during the conversion maneuver of tiltrotor aircraft.

Effect of Explosive Shapes (in Sand Buried Condition) on the Failure of a Circular Clamped Plate of Protective Vehicle

Protective vehicles like armoured personnel carriers (APCs) require assessment of failure of structural elements subjected to impulsive load resulting from explosive blast under sand buried conditions. The explosive shape and location of detonation affect the failure in near field region. In the present study, a circular clamped Rolled Homogenous Armour (RHA) steel plate has been modelled using JC strength & damage model and explosive using JWL equation. Initially, the reflected pressure and specific impulse for a fixed quantity of explosive (3.75 kg) of various shapes i.e. sphere, hemisphere, cylinders with Length to Diameter (L/D) ratio varying from 0.1 to 1 were studied for sand buried at a standoff distance of 118.1 mm. Further, studies were extended for cylindrical charges of φ 213.77 mm with conical 120°-150° and hemispherical cavities with radius of R1.2-R1.8. It was observed that, reflected pressure and specific impulse is much higher for hemispherical cavity of R1.2. The permanent deformation obtained using non-dimensional impulse is valid for explosives without cavities. However, the cavity charges produce failure of plate in the central region of the charge. The critical impulse emerges as an important parameter for assessing failure due to cavity charges. In addition, the scale down experiment is conducted to validate the effectiveness of cavity charges. It can be concluded that cavity charge with hemispherical radius of R1.2 can provide highest damage to RHA plates in close standoff distance.

Ground reaction forces and muscle activities during anteriorly-loaded overground walking: Preliminary results

Anterior-load carriage is common and has been identified as a fall risk factor in occupations. However, there is a lack of investigation of how anterior-load carriage influences kinetics and muscle activities. This study examined how anterior-loading affects ground reaction forces (GRFs) and leg/trunk electromyography (EMG) during level overground walking among young adults. Thirty young adults were randomly distributed into three groups: no load, 10% bodyweight (bw), and 20% bw. Each participant walked on a walkway at self-selected speed while holding the assigned load (assembled by a fitness sandbag) with both hands and anterior to the abdomen. Dependent variables included the kinetics (the peak GRF in vertical, forward, backward directions, rate of loading, and braking and propulsive impulses) and EMG magnitudes of six leg/trunk muscles (tibialis anterior, gastrocnemius, vastus lateralis, biceps femoris, gluteus maximus, and oblique). The results did not show significant group-related differences in any kinetic parameters or the EMG magnitudes except for the gluteus maximus. Both loaded groups displayed significantly higher gluteus maximus EMG magnitude than the unloaded group during anteriorly-loaded walking. Greater activation of the gluteus maximus may reflect the body's effort to stabilize the posteriorly leaned trunk, which was the main strategy to offset the anteriorly shifted body-load system's center of mass caused by the anterior load and to maintain dynamic balance during gait. The findings of this study could contribute to reducing the risk of falls in the industrial environment by providing insight into the kinetic control of the human body during anteriorly-loaded walking.

A Novel VSG Control Strategy for UPS Voltage Source Inverter with Impulsive Load

This paper proposes a novel control strategy to alleviate the impulsive current for Uninterruptible Power Supply (UPS), which is designed based on the theory of virtual synchronous generator (VSG) and dynamic switching method. The power quality of UPS will become distortion when the instantaneous impulsive load is imposed on UPS. Especially, the output current of UPS will increase quickly in short time. In order to maintain the uninterrupted output voltage and reduce the distortion of output voltage of UPS under the action of instantaneous impulse load, Dynamic switching method is proposed to resist impulse load with large scale impulse current, Meanwhile, the VSG control strategy is adopted to deal with the slow power switching and provide virtual inertia for the system, so that the output voltage of UPS can remain uninterrupted and stable. Finally, simulation and experimental results show that this method can effectively suppress the impulse current, reduce the output voltage distortion of UPS inverter, and ensure the good performance of On-line UPS under impulse load.

Analytical optimization of the tuned viscous mass damper under impulsive excitations

The inerter-based damper has proven as an efficient vibration suppression device for steady responses in a broad frequency band. However, the inerter-based structures may experience an impulsive loading process, in which the dynamic performance and pertinent design method remain unknown. Dealing with this, this study proposes a time-decay rate-based optimal design methodology for a typical inerter-based damper, tuned viscous mass damper (TVMD), to suppress the impulse-induced vibration, successively deriving the analytical formula to facilitate the demand-oriented design. Considering the impulsive circumstance, the mechanical model of the TVMD-based structure is established, and a pole-based analysis is performed to quantify the functionality of TVMDs for the transient response attenuation. Inspired by the pole-based analysis result, a time-decay rate-based optimal design method and corresponding analytical formula are proposed to maximize the attenuation rate of TVMDs. Through the implementation of TVMD-based structures, the effectiveness of the pole-based design method is demonstrated by the dimensionless analysis, and then its advantages are illustrated through a comparison with the previous design method. Furthermore, a series of maximum response spectra are established to quantify the dynamic performances of TVMDs. The results show that the pole-inspired design method guarantees the TVMD improved transient attenuation rate and degrees, in comparison with the widely used fixed-point method. In addition, the provided transient response spectra can be adopted as a guideline or reference for the performance evaluation of the TVMD-equipped structure and the preliminary design of TVMDs to meet the expected vibration suppression demand for impulsive excitations.

Pre- and Post-Failure Experimental Bending Analysis of Glass Elements Coated by Aged Anti-Shatter Safety Films

The main goal of Anti-Shatter Films (ASFs) applications for structural glass is to create a barrier able to keep together fragments and minimize risk after any impulsive or static load that could lead glass to cracking. The influence of ASF properties on the flexural strength of coated glass elements is thus a relevant topic for safe design purposes, but still little investigated. To this aim, an experimental material investigation is presented in this paper, in order to achieve a good knowledge of common ASFs from a chemical point of view. Moreover, the deterioration of mechanical and adhesion characteristics for ASF samples subjected to different environmental conditions and accelerated ageing is also investigated, so as to simulate the effects of long-term exposure to high humidity (HU) or high temperature (HT). An experimental campaign carried out on 20 small scale ASF-coated glass specimens is finally presented, based on a three-point bending (3PB) test setup. The out-of-plane bending response of unaged or aged samples is performed by taking into account two different displacement-rate levels, to assess their performance and bending capacity under steady-static or impulsive loads. In both cases, the attention is given to the characterization of elastic and post-failure performances. Finally, support for the interpretation of experimental outcomes is derived from a simplified theoretical model of composite beam with partial connection, in order to estimate the shear stiffness of ASF adhesive components in the elastic stage.

Impact response of inclined self-weighted functionally graded porous beams reinforced by graphene platelets

This study presents a theoretical analysis on the low-velocity impact response of inclined porous nanocomposite beams under various impulsive loads. The laminated beam model consists of multiple layers modelled as closed-cell cellular solids with identical thickness, where each layer contains uniformly distributed internal pores and is reinforced by dispersing graphene platelets into the matrix. The layer-wise continuous variations in both internal pore size/density and graphene fraction result in functionally graded lightweight beams with controllable density distributions and varying elastic moduli across the thickness direction. The material properties of each layer are determined according to Halpin–Tsai micromechanics model and the extended rule of mixture. The governing equations of the inclined beam are derived based on Timoshenko beam theory then solved by employing Ritz method for space domain and Newmark method for time domain. The static bending due to the self-weight of the beam is examined first, and then imported into the dynamic analysis as the initial stress state for the beam under impulsive impacts with six different pulse shapes. A comprehensive parametric study is conducted with a special focus on the combined effects of graded material distributions and inclined angle on the beam behaviour. Results show that a larger inclined angle reduces the beam deflection, and the rectangular impulsive load can lead to the largest mid-span deflection of fully clamped graded beams that may reach over 70% more than those under some of other impact load types. This study should provide insights into the design of lighter and stiffer inclined structural components subjected to various impulsive loading conditions.

Bolt Loosening Detection of Rocket Connection Structure Based on Variational Modal Decomposition and Support Vector Machines

This paper designed a bolt-loosening Support Vector Machines’ conduct detection method with feature vectors comprising eigenvalue decomposition based on Variational Modal Decomposition (VMD) and Singular Value Decomposition (SVD), combined with permutation entropy. Particle Swarm Optimization-Support Vector Machines (PSO-SVMs) are used for small-sample machine learning and can effectively identify and judge the state of bolt preload. The effectiveness of the proposed method is verified in a typical example of a connection structure under random-amplitude impulse loads and Gaussian white noise with different signal-to-noise ratios. The effect of other bolt numbers being arranged is also discussed in the results. This method’s bolt-loosening identification rate is close to 90% under both equal-amplitude and variable-amplitude loads. Following the interference, with a signal-to-noise ratio of 20 dB, the method also has a recognition rate higher than 70% under various working conditions and bolt equipment schemes. The effectiveness of the method was verified by experiments.

A systematic review on CFST members under impulsive loading

Concrete-filled steel tubes (CFST) have been increasingly used in the construction industry due to their favourable structural performance. With the increase of accidental events in the past two decades, researchers paid more attention to the response of CFST members under impulsive (lateral impact/blast) loading. Research projects conducted aim to better understand the structural performance of CFST members through experimental, numerical, and analytical approaches. This paper aims to systematically characterise the state-of-the-art research on the structural behaviour of CFST subjected to lateral impulsive loading. The reviewed studies include circular and rectangular/square cross-sections, conventional members and those strengthened with fibre reinforced polymer (FRP). Members with normal strength concrete (NSC), high strength concrete (HSC), as well as ultra-high-strength concrete (UHSC) infill are all included. Following the literature review, the current research trends are identified, and potential future research directions are discussed based on the identified knowledge gaps. A consensus of the results shows that CFST members have superior resistance to impulsive loading compared to their hollow counterparts and reinforced concrete (RC) members.

Flexure and shear response of an impulsively loaded rigid-plastic beam by enhanced linear complementarity approach

The linear complementarity approach has been utilized as a systematic and unified numerical process for determining the response of a rigid-plastic structure subjected to impulsive loading. However, the popular Lemke Algorithm for solving linear complementarity problems (LCP) encounters numerical instability issues whilst tracing the response of structures under extreme dynamic loading. This paper presents an efficient LCP approach with an enhanced initiation subroutine for resolving the numerical difficulties of the solver. The numerical response of the impulsively loaded structures is affected by the initial velocity profile, which if not found correctly can undermine the overall response. In the current study, the initial velocity profile is determined by a Linear Programming (LP) subroutine minimizing the energy function. An example of a uniform impulsively loaded simply supported beam is adduced to show the validity and accuracy of the proposed approach. The beam is approximated with bending hinges having infinite resistance to shear. Comparison of the numerical results to the available closed-form solution confirms the excellent performance of the approach. However, a subsequent investigation into a beam having the same support conditions and the applied loading, but with bending and shear deformation, results in numerical instability despite optimizing the initial velocity profile. Thus a more generic description of kinetics and kinematics is proposed that can further enhance the numerical efficiency of the LCP formulation. The ensuing numerical results are compared with the available close form solution to assess the accuracy and efficiency of the developed approach.

Anti-Shock Foundation of Naval Engines for Naval Vessels

The foundation of engines for naval vessels ensures multi-axis stiffness values and optimal isolation of harmonic vibrations on all axes. Another factor is the shock impulses caused by the battle actions of the ships. Dynamic impulse loads result from underwater or surface detonation of sea mines, a hit by a missile or an artillery shell. Such an impact is a high-energy step impulse. The paper presents the problem of selecting the physical parameters of materials used for shock absorbers. The presented models of material properties ena-ble the presentation of physical and mathematical shock absorbers models for both impulse and harmonic interactions.

Experimental parametric study on hydroelastic behaviour of a 15000-TEU container ship based on segmented ship model

ABSTRACT Large-scale ships with high elasticity and low natural frequency, behave high dynamic response amplitude under impulsive wave loads (whipping) and short period waves (springing), which could result in serious ultimate loads and fatigue damage. The segmented model test is a more powerful approach for the ship hydroelastic analysis compared with ship seakeeping test. However, the effect of some test parameters on the hydroelastic responses is still not a clear statement. Therefore, the number of segments, propulsion mode and backbone form are chosen for the parametric study on ship hydroelastic behaviour. A three-dimensional (3-D) nonlinear time-domain hydroelastic method is developed, in which the nonlinearity of instantaneous wetted surface and slamming force are considered. Through the analysis of natural frequencies and vertical bending moments (VBM), the impact on hydroelastic behaviour is discussed. Results show that self-propelling models with variable cross-section backbones are more suitable and reliable in studying nonlinear hydroelastic behaviour.

NONLINEAR STATIONARY OSCILLATIONS OF BEAM GIRDER STRUCTURES FROM THE ACTION OF PERIODIC PULSES

Purpose. The aim of this work is to construct an approximate solution of stationary oscillations of a nonlinear equation with a cubic characteristic. The design scheme under consideration best corresponds to the operating conditions of the girder structure in the event of defects of the supporting parts. In this case, longitudinal forces may appear in the girder of the girder structure. Methods. The paper investigates nonlinear oscillations of girder structures of small bridges. The undamped oscillations of the beam resting on hinged fixed support parts on both sides from the action of periodic unilateral impulse loading are considered. The design scheme differs from the statically defined beam scheme, which takes into account defective support parts. In the article, the possible transition to the beam system on moving support parts is considered. To solve the nonlinear equation of the second order of a single-span beam taking into account the longitudinal force, a method of variable time scale used, developed by Academician of the Academy of Sciences of the USSR, professor Bondar M. G. The method of variable scale gives fairly approximate solutions to this class of problems. Results. An approximate analytical solution is performed by the method of variable time scale of stationary nonlinear oscillations of the girder structure taking into account the longitudinal force caused by one-sided pulses that are periodically repeated. According to the obtained formulas of the solution of the nonlinear equation with cubic characteristic it is easy to construct amplitude-frequency characteristics and forms of oscillations. Originality. For the first time the solution of the equation of oscillations of a beam taking into account longitudinal force is executed. The criteria of impact resonance for the system without friction, as well as taking into account the viscous friction. An expression for the amplitude-frequency characteristic of the system under consideration is obtained. Practical meaning. Studies of beam structure oscillations taking into account the longitudinal force gave practical value in the possibility of analytical calculation of oscillation forms, shock resonance criteria and amplitude-frequency characteristics of the system from the action of periodic unilateral impulse loads. The system of girder beams takes into account defective supporting parts (jamming of movable supporting parts) of girder structures of small and medium railway bridgesd.

Tibiofemoral contact force differences between flat flexible and stable supportive walking shoes in people with varus-malaligned medial knee osteoarthritis: A randomized cross-over study.

To compare the effect of stable supportive to flat flexible walking shoes on medial tibiofemoral contact force (MTCF) in people with medial knee osteoarthritis and varus malalignment. This was a randomized cross-over study. Twenty-eight participants aged ≥50 years with medial knee osteoarthritis and varus malalignment were recruited from the community. Three-dimensional full-body motion, ground reaction forces and surface electromyograms from twelve lower-limb muscles were acquired during six speed-matched walking trials for flat flexible and stable supportive shoes, tested in random order. An electromyogram-informed neuromusculoskeletal model with subject-specific geometry estimated bodyweight (BW) normalized MTCF. Waveforms were analyzed using statistical parametric mapping with a repeated measures analysis of variance model. Peak MTCF, MTCF impulse and MTCF loading rates (discrete outcomes) were evaluated using a repeated measures multivariate analysis of variance model. Statistical parametric mapping showed lower MTCF in stable supportive compared to flat flexible shoes during 5-18% of stance phase (p = 0.001). For the discrete outcomes, peak MTCF and MTCF impulse were not different between the shoe styles. However, mean differences [95%CI] in loading impulse (-0.02 BW·s [-0.02, 0.01], p<0.001), mean loading rate (-1.42 BW·s-1 [-2.39, -0.45], p = 0.01) and max loading rate (-3.26 BW·s-1 [-5.94, -0.59], p = 0.02) indicated lower measure of loading in stable supportive shoes compared to flexible shoes. Stable supportive shoes reduced MTCF during loading stance and reduced loading impulse/rates compared to flat flexible shoes and therefore may be more suitable in people with medial knee osteoarthritis and varus malalignment. Australian and New Zealand Clinical Trials Registry (12619000622101).

MODEL OF BLOCK MEDIA TAKING INTO ACCOUNT INTERNAL FRICTION

The block medium is modeled by a discrete-periodic spatial lattice of masses connected by elastic springs and viscous dampers. To describe the viscoelastic behavior of the interblock layers, a rheological model of internal friction with two Maxwell elements and one Voigt element with the quality factor of the material as the determining parameter is proposed. Numerical experiments show that, within the framework of this interlayer model, it is possible to select the viscosity and stiffness of the Maxwell and Voigt elements so that the quality factor of the material differs from the given constant value by no more than 5%. In the one-dimensional case, within the framework of the proposed model, the influence of the quality factor on the dispersion properties of a block medium is studied and it is shown that the greatest effect of the quality factor on the dispersion is observed in the low-frequency part of the spectrum. In the three-dimensional case, within the framework of the proposed model, some geomechanical problems are numerically studied for a block half-space under the action of a surface concentrated vertical load. Namely, the attenuation of the velocity amplitudes of surface blocks was studied depending on the Q-factor under step action and under the action of a Gaussian pulse. In addition, we study a layer on the surface of a half-space under the action of a concentrated vertical impulse load in the case when both the layer and the half-space are block media but have different properties.

Self-sensing composite material based on piezoelectric nanofibers

Recently, efforts have been made to manufacture self-sensing smart composites by integrating piezoelectric sensors with laminates. However, the interleaving of pressure sensors, such as piezoelectric polymeric films, dramatically reduces the impact resistance of the hosting laminates, and consequently, delamination can occur. This study aimed to fabricate a self-sensing composite material by embedding piezoelectric nanofibers of poly(vinylidenefluoride-trifluoroethylene) (PVDF-TrFE) in a polymeric elastic matrix and carbon black-based electrodes to detect a piezoelectric signal. The mechanical and electrical properties of the self-sensing laminate were maintained after 106 fatigue test cycles. By appropriately tuning the parameters of the acquisition circuit, the sensor could measure not only impulsive loads but also low-frequency loads as low as 0.5 Hz. A piezoelectric model with lumped parameters for the polarization process and piezoelectric response of the nanofibers is proposed and validated by experimental results. As a proof of the model, the piezoelectric nanofiber sensors were embedded in a prosthetic carbon fiber sole, and the piezoelectric signal response closely followed the ground reaction force with a sensitivity of 0.14 mV/N.