Dynamic Axial Loading Research Articles

Stability of a nonlinear second order equation under parametric bounded noise excitation

The motivation for the following work is a structural column under dynamic axial loads with both deterministic (harmonic transmitted forces from the surrounding structure) and random (wind and/or earthquake) loading components. The bounded noise used herein is a sinusoid with an argument composed of a random (Wiener) process deviation about a mean frequency. By this approach, a noise parameter may be used to investigate the behavior through the spectrum from simple harmonic forcing, to a bounded random process with very little harmonic content.The stability of both the trivial and non-trivial stationary solutions of an axially-loaded column (which is modeled as a second order nonlinear equation) under parametric bounded noise excitation is investigated by use of Lyapunov exponents. Specifically the effect of noise magnitude, amplitude of the forcing, and damping on stability of a column is investigated. First order averaging is employed to obtain analytical approximations of the Lyapunov exponents of the trivial solution. For the non-trivial stationary solution however, the Lyapunov exponents are obtained via Monte Carlo simulation as the stability equations become analytically intractable.

Reverse Engineering of a Tall Building and Buckling Analysis of Its Peripheral Columns

AbstractOn the basis of known geometry, loads, and materials but with incomplete design details, the structure of a certain tall building was recreated. The first phase was a series of hand calculations to initially estimate the necessary cross sections. Then, a global finite-element (FE) model of the building was constructed and subjected to gravity and wind forces. Of particular interest were the peripheral H-shaped columns located immediately inside the glass curtain. The detailed FE buckling models of those columns were created and subjected to moderately dynamic axial loading. The analyses were carried out for an average-sized column section, and its results are discussed here. Two types of models were used in the FE work, shell and solid models. Although the magnitude of the buckling force was obviously set by the design requirements, the postbuckling characteristic was of interest, because it could play a role under some extreme loading conditions. The properties of the column material, namely, hig...

Time Domain Dynamic Analysis of Floating Piles under Impact Loads

In this study, the differential quadrature method (DQM), a mathematically simple, accurate, and computationally efficient numerical tool, was employed to analyze the response of single piles under dynamic axial loading. The dynamic behavior of the piles and surrounding soil were modeled using the one-dimensional (1D) or three-dimensional (3D) axisymmetric elasticity theory, respectively. The governing equations subjected to the related boundary and initial conditions were discretized in both the spatial and temporal domains via DQM. The behavior of the floating piles embedded in a half-space layer of homogeneous and Gibson soils under uniform and sinusoidal impact loads was studied. The piles and the soil behavior were assumed to be linear elastic and linear viscoelastic, respectively. To compare and verify the results obtained from the DQM approach, the problems were also solved via a finite-element-based commercial software program in some cases. The approach was found to have high accuracy and numerical stability. Finally, the effects of the different parameters of the soil and pile on the pile-head settlement time history were investigated.

Crashworthiness design of horsetail-bionic thin-walled structures under axial dynamic loading

Bio-inspired engineering design has drawn increased attention in recent years for the excellent structural and mechanical properties of the biological systems. In this study, the horsetail-bionic thin-walled structures (HBTSs) were investigated for their crashworthiness under axial dynamic loading. Six HBTSs with different cross section configurations (i.e., number of cells) were evaluated using nonlinear finite element (FE) simulations. To obtain the optimal design of the HBTSs, an ensemble metamodel-based multi-objective optimization method was employed to maximize the specific energy absorption while minimizing maximum impact force of the HBTSs. Using the ensemble metamodeling, FE simulations and the NSGA-II algorithm, the Pareto optimum designs of all six HBTSs were obtained and the HBTS with 16 cells were found to have the best crashworthiness. An optimum design of the HBTS with 16 cells was verified using FE simulation and found to have good agreement with simulation results.

Effect of Aspect Ratio and Boundary Conditions in Modeling Shape Memory Alloy Nanostructures with 3D Coupled Dynamic Phase-Field Theories

The behavior of shape memory alloy (SMA) nanostructures is influenced by strain rate and temperature evolution during dynamic loading. The coupling between temperature, strain, and strain rate is essential to capture inherent thermomechanical behavior in SMAs. In this paper, we propose a new 3D phase-field model that accounts for two-way coupling between mechanical and thermal physics. We use the strain-based Ginzburg-Landau potential for cubic-to-tetragonal phase transformations. The variational formulation of the developed model is implemented in the isogeometric analysis framework to overcome numerical challenges. We have observed a complete disappearance of the out-of-plane martensitic variant in a very high aspect ratio SMA domain as well as the presence of three variants in equal portions in a low aspect ratio SMA domain. The dependence of different boundary conditions on the microstructure morphology has been examined energetically. The tensile tests on rectangular prism nanowires, using the displacement based loading, demonstrate the shape memory effect and pseudoelastic behavior. We have also observed that higher strain rates, as well as the lower aspect ratio domains, resulting in high yield stress and phase transformations occur at higher stress during dynamic axial loading.

Solidity effect on crashworthiness characteristics of thin-walled tubes having various cross-sectional shapes

ABSTRACTIn this paper, effects of solidity and cross-sectional shape on crashworthiness characteristics of thin-walled tubes under dynamic axial impact load are examined. For this purpose, explicit finite element analyses are conducted for tubes having triangular, square, circular, hexagonal, octagonal and multi-cell cross-sectional shapes. The FE model is validated with analytical and experimental results that were reported in literature. It is found that FE solutions agree well with analytical and experimental results presented in the literature. To compare the tubes having different geometric dimensions and shapes on the same ground, the tubes are selected such that they have the same mass and length, and the striker mass and velocity are identical for all tubes as well. By changing solidity of tubes between 0.05 and 0.25, the cross-sectional dimensions of tubes are specified. The crushing force efficiency, crushing strain, total efficiency, structural efficiency and dynamic energy absorbing effectiveness parameters are calculated for various tubes as a function of the solidity parameter. It is concluded that crashworthiness performance of thin-walled tubes is significantly influenced by the solidity and cross-sectional shape. Results about the most effective solidity values and cross-sectional shapes are presented.

Effects of coping designs on stress distributions in zirconia crowns: Finite element analysis

The purpose of this study was to evaluate the effects of coping designs on the stress distributions in posterior zirconia crowns by non-linear three-dimensional finite element analysis. Three-dimensional finite element models of a mandibular right first molar with layers of veneering porcelain, zirconia coping, cement, and abutment tooth were designed by computer software (HyperWorks 10.0). Ten zirconia crowns with different designs were produced according to various shoulder positions and heights. The shoulders (1-mm width) exhibited incremental height increases of 1mm, 2mm, and 3mm on the buccal, lingual, and proximal sides, respectively. An axial compressive dynamic load simulating the progressive load was applied until a stainless steel ball model (7mm in diameter) deepened the veneer surface to 0.7mm in depth. Loads were placed on the inner inclines of the mesiobuccal, distobuccal, and mesiolingual cusps. Residual maximum principal stresses (MPSs) at the veneer and coping under progressive loading were determined for each zirconia crown. Reinforcements with the shoulders on the buccal, lingual, and proximal axial walls resulted in lower MPSs in the veneering porcelain but higher MPSs in the zirconia coping. As the shoulder height increased, the tensile stresses decreased, while the compressive stresses increased in the veneering porcelains. It can be concluded that the shoulder height and position in the zirconia coping will affect the MPSs of the crown. Our findings conclusively reveal the critical role of the shoulder design of the coping in preventing veneer fracture on posterior zirconia restorations by reducing tensile stresses in veneering porcelain.

Residual Hole Orientation After Plate Removal: Effect on the Clavicle.

Clavicle fractures account for 2.6% to 4% of all fractures. Surgical stabilization of this type of injury is becoming more common. Anterior inferior plating and superior plating are 2 popular approaches to open reduction and internal fixation. Reports of plate removal have raised concerns about reinjury. The goal of the current study was to determine whether the orientation of screw holes in clavicles after removal of an anterior inferior plate vs a superior plate have different biomechanical effects on stiffness and load to failure. The medial and lateral ends of 28 matched pairs of fresh clavicles were potted. Pilot holes, 2.5 mm in diameter, were drilled and oriented anterior inferiorly or superiorly, simulating those left after removal of a plate for a middle-third fracture. The clavicles underwent dynamic axial compression and 3-point load to failure, replicating forces associated with reinjury. Clavicles with anterior inferior holes had a statistically significant higher median maximal load difference of 139 N compared with those with superior holes (P=.013). Anterior inferior holes showed a statistically significant median increase in stiffness of 16.3 N/mm compared with superior holes (P=.036). Clavicles with anterior inferior holes had a statistically significant increase in median maximal load to failure and an increase in median stiffness compared with those with superior holes. This finding is relevant for patients who undergo hardware removal and return to activities that put them at risk for repeat high-impact injuries to the clavicle.

Translational challenges for the development of a novel nucleus pulposus substitute: Experimental results from biomechanical and in vivo studies.

Nucleus pulposus replacement therapy could offer a less invasive alternative to restore the function of moderately degenerated intervertebral discs than current potentially destructive surgical procedures. Numerous nucleus pulposus substitutes have already been investigated, to assess their applicability for intradiscal use. Still, the current choice of testing methods often does not lead to efficient translation into clinical application. In this paper, we present the evaluation of a novel nucleus pulposus substitute, consisting of a hydromed core and an electrospun envelope. We performed three mechanical evaluations and an in vivo pilot experiment. Initially, the swelling pressure of the implant was assessed in confined compression. Next, we incorporated the implant into mechanically damaged caprine lumbar intervertebral discs to determine biomechanical segment behaviour in bending and torsion. Subsequently, segments were serially tested in native, damaged and repaired conditions under dynamic axial compressive loading regimes in a loaded disc culture system. Finally, nucleus pulposus substitutes were implanted in a live goat spine using a transpedicular approach. In confined compression, nucleus pulposus samples as well as implants showed some load-bearing capacity, but the implant exhibited a much lower absolute pressure. In bending and torsion, we found that the nucleus pulposus substitute could partly restore the mechanical response of the disc. During dynamic axial compression in the loaded disc culture system, on the other hand, the implant was not able to recover axial compressive behaviour towards the healthy situation. Moreover, the nucleus pulposus substitutes did not remain in place in the in vivo situation but migrated out of the disc area. From these results, we conclude that implants may mimic native disc behaviour in simple mechanical tests, yet fail in other, more realistic set-ups. Therefore, we recommend that biomaterials for nucleus pulposus replacement be tested in testing modalities of increasing complexity and in their relevant anatomical surroundings, for a more reliable prediction of clinical potential.

The energy absorption of bamboo under dynamic axial loading

The energy absorption characteristics and material parameters of the bamboo species Phyllostachys pubescens are studied by drop-weight and dynamic tensile tests. The dynamic tensile test shows that 1-year-old bamboo has the greatest tensile strength (251MPa), which is 1.85 times that of a 2A12 aluminium alloy. The drop-weight test shows that the energy absorption of nodal samples is greater than that of the internode samples, and the specific energy absorption (SEA) values of the nodal and internode samples are 11.85kJ/kg and 9.78kJ/kg, respectively. The SEA of nodal samples is close to that of aluminium alloy and steel tube, and the SEA of the internode samples is close to that of copper tube. The energy absorption increases with increasing moisture content and decreases with growth age. Analyses show that the bamboo nodes and the vascular bundle are the main factors affecting bamboo energy absorption. The results indicate that bamboo is a tubular structure with excellent mechanical and energy absorption properties.

Composite cylindrical shells under static and dynamic axial loading: An experimental campaign

The results of an experimental investigation performed at the Politecnico di Milano inside the European project DAEDALOS on three composite cylindrical shells are here presented. At first, static buckling tests were performed under axial compression. Then, two types of dynamic tests were carried out: modal tests at different load levels before buckling and dynamic buckling tests applying an axial shortening of short duration. At the end, one shell was statically tested until final failure. The tests allow to understand the behavior of thin-walled cylindrical shells subjected to axial compression both in static and dynamic conditions. The results show the strength capacity of these structures to work in the post-buckling range with a capacity to sustain a load that is about 40% of the buckling load. The modal tests at different load levels allowed to observe that an increase of the load determines a reduction of the modal frequency and an increase of the damping. Large deformations are obtained before the final failure with out-of-plane displacements of almost 40mm and a shortening equal to about 26 times the buckling shortening.

Experimental and numerical investigations of the impact behaviour of composite frontal crash structures

The present paper deals with the lightweight design and the crashworthiness analysis of a composite impact attenuator for a Formula SAE racing car, in order to pass homologation requirements. The analysed impact attenuator is manufactured by lamination of prepreg sheets in carbon fibres and epoxy matrix, particularly used for sporting applications, and has a very similar geometry to a square frusta, so as to obtain a progressive and controlled deformation. During the design, attention was focused on the material distribution and gradual smoothing, but also on the lamination process, which can heavily affect the energy absorption capability. To reduce the development and testing costs of a new safety design, computational crash simulations for early evaluation of safety behaviour under vehicle impact test were carried out. The dynamic analysis was therefore conducted both numerically, using an explicit finite element code such as LS-DYNA, and experimentally, by means of an appropriately instrumented drop weight test machine, in order to validate the model in terms of deceleration values during crushing. To assess the quality of the simulation results, a comparative analysis was initially developed on simple CFRP composite tubes subjected to dynamic axial loading. The numerical analysis was conducted using both shell and solid elements, in order to reproduce not only the brittleness of the composite structure but also the effective delamination phenomenon. Both the analyses show a good capacity to reproduce the crushing process; this is confirmed by the fact that model estimated displacements and accelerations are in close agreement with observed values for these variables. This confirms the quality of the methodology and approach used for the design of a racing car impact attenuator.

The effect of radial distance of concentric thin-walled tubes on their energy absorption capability under axial dynamic and quasi-static loading

This study investigates the effect of radial distance of concentric thin-walled tubes on their energy absorption capability through simulation by LS-Dyna finite element software and experimental tests. Concentric cylindrical tubes made of aluminum 1050 with different radii were exposed to axial dynamic and quasi-static loading and optimum radial distance in maximum energy absorption condition was obtained. In overall, 24 states for simulation and 8 states for experimental tests have been investigated. Dynamic loading has been applied in form of hitting of a 75kg mass with speed of 8m/s and quasi-static loading of the structures has been done with speed of 150mm/min and elongated to 70% of the initial length of tubes. Comparison of results of the two concentric tubes and single-tube system with equal mass of structures showed that energy absorption of two-tube system is more than single-tube system. Furthermore, in concentric tubes׳ system, in the state where the distance of both tubes has changed, the energy absorption is more than the state where the radius of the outer tube is constant and just the radius of the inner tube changes.

Mechanical behavior analysis for the determination of riser installation window in offshore drilling

The purpose of this paper is to figure out the safety operation window of riser in installation through taking three kinds of mechanical behavior. In this paper, the sea surface current velocity, surface wind velocity, wave height and wave period are taken as the evaluation factors of marine environment. The fleet angle at riser top section, the Mises equivalent stress on riser critical section, the axial dynamic load on riser bottom section and the riser transverse resonance are considered as the limiting factors which restrains the further complicacy of marine environment. Results show that the wave height and wave period mainly affect the lateral vibration frequency and the dynamic load. The sea surface wind velocity and sea surface current velocity determines the fleet angle and the Mises equivalent stress. However, compared with the fleet angle, the maximum equivalent stress is the main influence factor which limits the installation operation window. In general, under the circumstance of avoiding transverse resonance, smaller wave height and longer wave period are good for marine riser installation. The research has some reference value for safe and effective installation of marine riser.

Effect of filler on the impact characteristics of aluminium tubes

This paper presents the behaviour of empty and foam-filled aluminium alloy (AA6061-T6) tubes under dynamic axial loading. The influence of geometrical and various densities of fillers was investigated by employing a finite element code, LS-DYNA. The critical effective point for fillers of different densities is identified in a foam-filled tube based on a specific energy absorption value at which the weight effectiveness of a foam-filled tube exceeds that of the empty tube. The combination of an aluminium tube and an aluminium foam filler successfully induced significant increase in specific energy absorption, proving that the assessment of critical effective point is vital in identifying proper combination of tube-filler on the effectiveness of foam-filled tubes.

Closure to “Comprehensive Load Test on Prestressed Concrete Piles in Alluvial Clays and Marl in Savannah, Georgia” by Yong Tan and Guoming Lin

Tan and Lin are commended on presenting a detailed case history of the performance of prestressed concrete piles installed and tested for a liquefied natural gas (LNG) facility in Georgia, United States. The pile testing program was comprehensive and included high-strain dynamic tests, Statnamic tests, and static axial compression and reciprocal lateral load tests. To extract information about the mobilized skin friction (shaft resistance) from the tests, the authors made use of the readings of the strain gauges installed at various levels along the test piles. From the results, the authors concluded, among other findings, that “the potential degradation of pile concrete stiffness attributable to pile driving should be accounted for in pile capacity analysis” (p. 178), a point that this discusser also made in the paper by Lam and Jefferis (2011). However, in this discusser’s opinion, this study would benefit from a fuller discussion of the process involved in the derivation of the pile stiffness (Young’s modulus) values for the various load tests because the chosen values of this important parameter can have a significant effect on the accuracy of the interpretation. Specific discussion points are given subsequently.

Cervical spine injuries and flexibilities following axial impact with lateral eccentricity.

Determine the effects of dynamic injurious axial compression applied at various lateral eccentricities (lateral distance to the centre of the spine) on mechanical flexibilities and structural injury patterns of the cervical spine. 13 three-vertebra human cadaver cervical spine specimens (6 C3-5, 3 C4-6, 2 C5-7, 2 C6-T1) were subjected to pure moment flexibility tests (±1.5 Nm) before and after impact trauma was applied in two groups: low and high lateral eccentricity (1 and 150 % of the lateral diameter of the vertebral body, respectively). Relative range of motion (ROM) and relative neutral zone (NZ) were calculated as the ratio of post and pre-trauma values. Injuries were diagnosed by a spine surgeon and scored. Classification functions were developed using discriminant analysis. Low and high eccentric loading resulted in primarily bony fractures and soft tissue injuries, respectively. Axial impacts with high lateral eccentricities resulted in greater spinal motion in lateral bending [median relative ROM 3.5 (interquartile range, IQR 2.3) vs. 1.4 (IQR 0.5) and median relative NZ 4.7 (IQR 3.7) vs. 2.3 (IQR 1.1)] and in axial rotation [median relative ROM 5.3 (IQR 13.7) vs. 1.3 (IQR 0.5), p < 0.05 for all comparisons] than those that resulted from low eccentricity impacts. The developed classification functions had 92 % classification accuracy. Dynamic axial compression loading of the cervical spine with high lateral eccentricities produced primarily soft tissue injuries resulting in more post-injury spinal flexibility in lateral bending and axial rotation than that associated with the bony fractures resulting from low eccentricity impacts.

Fatigue Analysis of Large Diameter Threaded Connections Subjected to Dynamic Axial Loads

The large diameter threaded connections are components with high importance in the heavy mechanical systems. At the threaded elements subjected to variable loads one of the main causes of failure is represented by fatigue. This phenomenon is accentuated by the occurrence of high local stresses on the thread root. In this context, the correct evaluation of the local stresses and their correlation with the fatigue behaviour has a major importance in the correct assessment of remaining life of large diameter threaded connections.The paper is focused on the finite element analysis of the large diameter threaded bolts with two types of threads profiles: trapezoidal and square. For the two types of threads the simulations were performed using four nominal diameter and two different loading setups. The bolts were summited to static analysis in order to determine the local stresses distribution in the threaded area. The maximum values of Von Mises stresses obtained in the static analysis were exported to fatigue studies. The differences between the results obtained for the two types of threads are outlined.

English

Velocity of projectiles plays an important role in determining the response and behaviour of a target under impact. In this project metallic canister with a soft projectile, which is one of the critical dynamic components, is used to launch bird/ice pellet, to study its influence on the behaviour of aircraft and aero-engine parts. This is done by using conventional compressed air launcher which launches the projectile velocities up to 200 m/s. The canisters which house the test projectile collapse against canister arrester located at the delivery end of the launcher. The collapse characteristics of the canisters are dependent on the geometry of the tip of the arresters, material of the canisters, the shape of the canisters and the heat treatment process adapted for canisters. The impact load of the canister on the arrester can be up to several tonnes axially. This force is a function of the collapsibility of the canister at impact and in turn it is determined by the operating pressure of the launcher. It is therefore necessary to develop a velocity measuring system, which determines the dynamic axial loads on the launcher as well as the target plate, which in turn helps in studying the collapsibility parameter of the canister itself. In order to measure the high velocity of the projectile three measuring devices, namely, optical laser sensor, based on programmable logic device, copper wire and graphite rod based devices are developed and used. To provide adequate redundancy for the complex, highly dynamic and costly impact tests, two of these three velocity measuring devices are used simultaneously: (1) copper wire based sensing system and (2) graphite rod based sensing system. The measuring device input parameter is fabricated on a gun barrel with acrylic fixture at two positions at a distance of 1 m before the arrester where the canister strikes and collapses. At initial and final position, optical transmitter and receiver give the delay time when the canister passes through it by obstructing the sensing signal path. The copper wire circuit based device gives open circuit delay between the two points when canister breaks it. The time taken to travel 1m distance by the canister in the gun barrel is obtained from the device and the velocity of the canister or projectile is experimentally evaluated.

Dynamic Axial Crushing of Empty and Foam-Filled Conical Aluminium Tubes: Experimental and Numerical Analysis

This paper presents the crushing behaviour of empty and foam-filled conical tubes under axial dynamic loading. A nonlinear finite element (FE) model was developed and validated against experiments. The validated model was subsequently used to assess the beneficial of foam filling with regards to the variation in filler densities and tube materials. The results obtained were further analyzed and compared with straight tubes. We aim to evaluate the critical effective point for different density of fillers in foam-filled tubes based on specific energy absorption (SEA) value. The SEA value was highest for foam-filled conical aluminium tube with aluminium foam filler, followed by straight aluminium tube, straight carbon steel tube and conical carbon steel tube. Moreover, the initial peak force was found lower in aluminium tubes than carbon steel tubes and lower in conical tubes than that in straight tubes. The combination of conical aluminium tube and aluminium foam filler successfully convey the beneficial of foam filling and thus signify that proper combination and selection of tube and filler is vital in assessing the effectiveness of foam-filled tubes.