Low Dynamic Loading Research Articles

Kinetics of the cervical spine in pediatric and adult volunteers during low speed frontal impacts

Previous research has quantified differences in head and spinal kinematics between children and adults restrained in an automotive-like configuration subjected to low speed dynamic loading. The forces and moments that the cervical spine imposes on the head contribute directly to these age-based kinematic variations. To provide further explanation of the kinematic results, this study compared the upper neck kinetics – including the relative contribution of shear and tension as well as flexion moment – between children (n=20, 6–14yr) and adults (n=10, 18–30yr) during low-speed (<4g, 2.5m/s) frontal sled tests. The subjects were restrained by a lap and shoulder belt and photo-reflective targets were attached to skeletal landmarks on the head, spine, shoulders, sternum, and legs. A 3D infrared tracking system quantified the position of the targets. Shear force (Fx), axial force (Fz), bending moment (My), and head angular acceleration (θ̈head) were computed using inverse dynamics. The method was validated against ATD measured loads. Peak Fz and θ̈head significantly decreased with increasing age while My significantly increased with increasing age. Fx significantly increased with age when age was considered as a univariate variable; however when variations in head-to-neck girth ratio and change in velocity were accounted for, this difference as a function of age was not significant. These results provide insight into the relationship between age-based differences in head kinematics and the kinetics of the cervical spine. Such information is valuable for pediatric cervical spine models and when scaling adult-based upper cervical spine tolerance and injury metrics to children.

A modified Kolsky bar system for testing ultrasoft materials under intermediate strain rates

A 25 mm aluminum Kolsky bar is modified to test ultrasoft materials under intermediate strain rates (10-100 s(-1)). In this modified system, an ultralong loading pulse (5-50 ms) is generated using an 800 mm steel striker in combination with a pulse shaper made of 2.5 mm thick soft rubber. For such long loading pulses, the traditional data reduction scheme based on strain gauge measurements is not possible. We use a laser gap gauge to measure the deformation of the sample directly and monitor the low amplitude dynamic loading stresses on the sample with a pair of piezoelectric transducers that are embedded in the bars. A commercial foam rubber is tested to demonstrate the feasibility of our modified Kolsky bar system.

Noise enhances the rapid nitric oxide production by bone cells in response to fluid shear stress

Stochastic resonance is exhibited by many biological systems, where the response to a small stimulus is enhanced with the aid of noise. This intriguing possibility provides a novel paradigm for understanding previously reported osteogenic benefits of low amplitude dynamic loading. However, it is unknown whether bone cell mechanosensitivity is enhanced by noise as an alternative mechanism for an amplified response to small stresses. We studied whether noise of varying intensities enhanced the mechanosensitivity of MC3T3-E1 cells. Nitric oxide (NO) production was measured as the parameter for bone cell activation. Dynamic fluid shear stress stimulated bone cells provided an initial-stress kick was implemented. Without the initial stress-kick bone cells did not release a significant amount of NO demonstrating an essential non-linearity to bone cell responses to stress and the possibility of stochastic resonance in bone cell mechanosensitivity. The rapid NO response of MC3T3-E1 cells to a small periodic fluid shear stress was increased with the addition of noise compared to the response to stress with only noise. This confirms the possibility of stochastic resonance enhancement of NO production by bone cells. Since NO regulate bone formation as well as resorption, our results suggest that noise enhances the activity of bone cells in driving the mechanical adaptation of bone.

Stiff Electrohydraulic Drives using a Hybrid Valve

One of the main problems of electrohydraulic drives is relatively low dynamic load stiffness due to compressibility of oil. Sudden load variations can cause significant deviations of the position of the drive and, consequently, a reduced precision of the controlled process. This has led to a partial substitution of electrohydraulic drives by electromechanical drives in a low power range recently. One of the approaches to increase dynamic stiffness of a cylinder is to raise valve dynamics. Thus, the system can react to a control deviation and provide the cylinder with a compensating flow faster. The objective of this paper is to show the development of a stiff cylinder drive using a new high-response servovalve the hybrid valve. In contrast to a conventional servovalve with an unmovable sleeve, the idea of the hybrid valve is to actuate both the spool and the sleeve simultaneously in opposite directions. Using a piezo-actuator as sleeve drive provides the hybrid valve with very high dynamics in the range of small valve opening, which should be sufficient to increase the disturbance rejection of the cylinder. The performance of the valve prototype and of the cylinder will be shown, and application fields will be discussed in this work.

Pile response and free field vibrations due to low strain dynamic loading

This paper presents the results of in situ measurements during dynamic pile testing at a construction site in Louvain-la-Neuve. Main objectives are the investigation of the pile response and the free field vibrations due to low strain dynamic loading on a single cast in situ pile with a 5.5 kg hammer impact on the pile head. Whereas low strain testing is usually performed to assess the integrity of the pile as a structural member, this study focuses on both pile and ground vibrations. The pile head response and ground motions are measured with accelerometers during different blows with the impact hammer. The dynamic characteristics of the soil are determined with a SASW test. Experimental results are compared with predictions obtained with a coupled finite element–boundary element model. The computed pile head and free field response show a good correspondence with the measured response. In addition, the static stiffness of the pile estimated by means of the mobility function shows a very good agreement with the value calculated by an analytical formulation.

Using seismic isolation columns for retrofitting buildings with soft stories

AbstractBase isolation is known as en effective way to improve structural seismic response and reduce damages that may be caused to structures by earthquakes. The objective of seismic isolation is to decouple the structure from the earthquake motion input, preventing the structure from absorbing the earthquake energy. A new engineering solution for seismic isolation columns is presented in this study. It is aimed first of all at retrofitting existing buildings with a soft storey by replacing weak conventional columns, having low static and dynamic load carrying capacity, by the proposed ones. The idea is based on a well known friction pendulum principle. The proposed seismic isolation column is a kinematic system. Its main joint has high load carrying capacity as well as all properties of a three‐dimensial hinge connection. No special materials are required for its manufacturing. No special lifting equipment is required to replace the conventional columns in existing structures by the proposed ones. An additional advantage is that manufacturing of such seismic isolation columns is relatively easy and does not require high accuracy and testing of each unit, like in other types of base isolators. A method for design of buildings, incorporating the proposed devices, was developed. The developed design procedure was applied for a multi‐storey RC frame with infill walls in a numerical example. It was shown that using the proposed solution yields significant improvement in structural response to earthquakes. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Modelling of complex structures using superelements

The object of this investigation is a transient analysis of a complex structure. The modelling method using superelements is presented first. The working arm of a hydraulic excavator is then taken as a vehicle to illustrate the method, by employing substructuring (superelements) in a sample working cycle, where a pre-defined potential cutting force taken as a function of time is the major external load. The forces in all the joints of the working arm and the stress distribution in all the sub-structures have been obtained. Using the obtained results recommendations are given for the digging cycle that will produce lower dynamic loads.

Dynamic Road Load Simulation of Transportable Cranes

This paper develops a general tool for understanding the dynamic road loads generated by mobile cranes that use a boom support dolly. The focal point is the dynamic road loads generated by the vehicle, which is a primary cause of pavement damage. A pitch plane model of the crane is developed and a finite element model is used to characterize the flexible motion of the boom. The simulation can characterize both the traditional and modern styles of mobile cranes. The traditional style uses a walking beam suspension on the carrier and a leaf spring suspension on the dolly. The modern cranes implement a hydropneumatic carrier suspension and an air ride suspension on the dolly. Additionally, other suspension configurations are implemented. The simulation results show that modern cranes generate significantly lower dynamic loads than do traditional cranes. This indicates that there is a need to regulate axle load limits in a manner that compensates for different suspension systems.

Rupture simulation of 3D elastoplastic structures under dynamic loading

A numerical approach of the rupture phenomenon in metallic bodies under low dynamic loading is presented in order to develop a numerical prediction tool. The implementation of an elastoplastic behaviour and the introduction of a new rupture criterion by means of the bifurcation equilibrium theory in a finite-element code (ABAQUS/Explicit) are the two major parts of this work. Numerical and experimental results (rupture-limit velocity and fracture location) are compared for the rupture of a steering knuckle.

Development of New Profile-Based Truck Dynamic Load Index

A new roughness index called the dynamic load index (DLI) is developed for the purpose of identifying pavement profiles that are likely to generate high dynamic truck-axle loads. The DLI is calculated as a weighted index of variances of the profile elevation in the frequency ranges of 1.5 to 4 Hz and 8 to 15 Hz. The first frequency range corresponds to truck body bounce, and the second frequency range corresponds to axle bounce. The analysis showed a very good relationship between the DLI and dynamic load. The DLI was tested on a range of road profiles from in-service pavements, and it was found that for any particular value of ride quality index (RQI), the DLI can cover a wide range of values, and this variation in DLI was found to correlate well with dynamic load, as predicted by a truck simulation program. This was not the case for the international roughness index, which gave a low coefficient of correlation with dynamic load for the same range of profiles. Therefore, the new index can differentiate between profiles that generate high dynamic loads and those having the same RQI but generating low dynamic loads. Most importantly, the use of the DLI negates the need for running a truck simulation program. This makes it possible for a state highway agency to decide whether a particular pavement with a given surface profile needs smoothing (to extend its service life) based on the DLI value.

Application of Adaptive Analysis to Reinforced Concrete Frames

This is the second of two papers concerned with the application of adaptive techniques to the nonlinear analysis of reinforced concrete frames. The first paper presented a new nonlinear elastic formulation capable of representing an entire reinforced concrete member with one element, and the present paper discusses the use of such a formulation within the framework of adaptive inelastic analysis. This is followed by a description of an inelastic cubic formulation based on the layered approach, which complements the elastic formulation within the proposed methodology. The concept of automatic mesh refinement is then outlined, and comments are made regarding the applicability of the elastic formulation under low amplitude dynamic or cyclic loading. Finally, the accuracy and efficiency of adaptive analysis is verified through static and dynamic analyses using the nonlinear analysis program ADAPTIC. Comparisons are made where appropriate with the results of existing analysis methods for reinforced concrete frames to illustrate the advantages of the proposed methodology.

Scalable Parallel and Distributed Expert Database Systems with Predictive Load Balancing

Much prior work in AI on various attempts to speed up rule-based systems by parallel processing has been reported. Unfortunately, many of these results indicate that there is limited parallelism to be found when rules are applied to relatively small amounts of data. Thus, one can predict that much greater parallelism can be extracted when rules are applied to large amounts of data. However, traditional compile-time parallelization strategies as developed for main-memory based systems do not scale to large databases. We propose a scalable strategy for the efficient parallel implementation of rule-based systems operating upon large databases. We concentrate on load balancing techniques in a synchronous model of rule execution, where the variance in runtime of the distributed sites is minimized per cycle of rule processing, thus increasing utilization and speedup. We demonstrate that static load balancing techniques are insufficient, and thus low overhead dynamic load balancing is the key to successful scaling. We present a form of dynamic load balancing that is based upon predicting future system loads, rather than conventional demand-driven approaches that monitor current system state. We analyze a number of possible predictive dynamic load balancing protocols by isoefficiency analysis to guide the design of a parallel database rule processing system.

Dynamic Characteristics of Prestressed Waffle Slabs

The dynamic response and fatigue behavior of rectangular and skew prestressed concrete waffle slabs are investigated. Such slabs have several advantages over the more conventional slabs when used as decks for concrete bridges and as process floors in semiconductor manufacturing plants. A solution for the dynamic response is derived based on the classical orthotropic plate theory. The theoretical results are verified and substantiated by experimental results obtained from tests on two prestressed concrete waffle slab models. A parametric study is also reported. It is shown that prestressing enhances the natural frequencies and increases the fatigue life of orthotropic slab structures, such as the waffle slab. This enables the designer to avoid resonance under low‐frequency excited sources of vibration, thus using a low dynamic load allowance and hence achieving economy in his design.

Musculoskeletal load of municipal employees aged 44 to 58 years in different occupational groups

This study is part of a comprehensive multidisciplinary project aimed at determining criteria for retirement ages in municipal occupations in Finland. Musculoskeletal load at work was studied in eight different occupational groups to see what differences between occupational groups exist. A job analysis of the work of 116 subjects was made with the ergonomic job description analysis method (AET). The work of 116 men and women, mean age 52 years, was studied in the work place. The study revealed three levels of overall musculoskeletal load. The groups with high musculoskeletal load worked in installation, auxiliary, home care and transport groups, with an assessed load of on average about one third of the maximal on the AET scale. Both quantitative and qualitative loading problems were concentrated in these groups. In contrast, teaching and administration groups had low musculoskeletal load averaging about 10% of the maximal. The nursing and office groups were between these two levels with a load of approximately 20% of the maximal. However, with the same overall quantity, the quality of the musculoskeletal load varied. In the transport group, monotonous static work with both arms (about 60% of workshift) and legs (30%) was most common, while the musculoskeletal load of the auxiliary and home care groups consisted of heavy dynamic muscular work with arms (60% of workshift) and legs (70%). On the other hand, the administration group alone had a static load of the arms (about 15% of workshift) and legs (2%) and also a low dynamic load with arms (0%) and legs (33%), which could have hypokinetic effects. In conclusion, there are significant differences between the profile groups in both the quantitative and qualitative load of the musculoskeletal system. The differences are even more pronounced when aging employees are studied. These facts support different retirement ages for the profile groups, at least when the musculoskeletal system is concerned.

Strain estimation for external event loads

It is common knowledge that structural design on the basis of stress limitation leads to an overestimation of the damage potential of low cycle dynamic loads (e.g. seismic events). Until now the high effort needed for step-by-step numerical evaluation of nonlinear dynamic structural response is a serious obstacle for a more realistic design based on strain limitation. The concept of energy dissipation and structural hardening as functions of strain range leads to the definition of a set of 1-DOF-substitute structures gained by a limited number of quasi-static non linear computations or by a certain knowledge of the yield behaviour of structural members. Step-by-step numerical computations are restricted to the substitutes and thus the effort for a strain limitation design is reduced to a bearable level.

Dynamic behaviour of PMMA in low-pressure range

The dynamic stress-strain relation of poly(methyl methacrylate) (PMMA) under low dynamic loading has been measured up to a stress of about 1.5 GPa. Shock compression of specimens is performed by an one-dimensional impact experiment using an aluminium flyer plate accelerated by a light gas gun. The impact velocities u g , shock velocities U s , and peak pressure p of the shock waves are determined. From the U s − p and the U s − u p relations obtained ( u p is the velocity of particles in medium) an abrupt change of slope was found in the region u g=108m/s( U s=3020m/s, p=0.385GPa). The Hugoniot elastic limit (HEL) is therefore deduced to be 0.385±0.05GPa; that is only half the value which has been stated previously. The results demonstrate that in spite of the existence of a HEL an elastic precursor cannot be expected.

Behaviour of multilayer stacks of thin layers under dynamic loads

Although considerable attention has been given to solids in static contact, very little information is available concerning vibrated contacts. Stacks of thin metallic layers were subjected to high and low dynamic loads (⩽ 156 + 1290 sin 2 (2800 πt) N, t being in min). Freshly assembled stacks show an initial fall in resistance, (which was not found for run-in stacks), the rate of fall being greater the harder the material. For run-in stacks under high dynamic loads, the asperity tips tended to shear, resulting in a resistance rise, while under low dynamic loads the asperities tended to deform so that the resistance decreased. Also under low dynamic loads, an exponential rate of fall of the electrical resistance of stacks of thin layers of soft material has been observed.